Alloys as Solutions 
 
 BY - 
 
 John Alexander Mathews, Ph. D. 
 
 Barnard Fellow of Columbia University 
 and Carnegie Research Scholar of 
 the Iron and Steel Institute 
 of Great Britain 
 
 (Reprinted from The Engineering and Mining Journal, 
 December 21st and 28th, 1901) 
 
ALLOYS AS SOLUTIONS 
 
 By John Alexander Mathews 
 
 It is truly remarkable that any class of substances 
 which have been so long known and studied as the 
 alloys have been should have hidden the secrets 
 of their inner molecular constitution so completely 
 until within a little more than a decade. The art 
 of metallurgy is a venerable one, and wonder¬ 
 ful achievements were accomplished by the earliest 
 civilized peoples in practicing metallurgy as an 
 empirical art and with the crudest of appliances. 
 The Chinese are supposed to have had metallic 
 currency for at least 4,000 years; the Lydians de¬ 
 picted nymphs, deities and animals upon metallic 
 coins in the eighth century B. C., and the coins of 
 Greece dating 300 B. C. show a skill of workman¬ 
 ship not excelled by modern engravers. We may, 
 from these evidences of the advances in civiliza¬ 
 tion, conclude that the knowledge of metals and 
 how to obtain and utilize them was of very early 
 origin. 
 
 Historical Review. —Six metals are mentioned in 
 the Old Testament scriptures, and from this and 
 other sources we know that brass, bronze, solders 
 and other alloys were all well known before the 
 Christian era. We have records of the metal¬ 
 lurgical processes of cupellation, amalgamation 
 and smelting of an equally early date. During all 
 the centuries from that time to the present, men 
 have been discovering new metails and mixing 
 them in new proportions hoping either by design 
 or chance to produce improved materials suitable 
 co for special purposes. The shrewd observers of all 
 ages seem to have been aware of the profound 
 changes which the properties of metals undergo 
 
4 
 
 ALLOYS AS SOLUTIONS. 
 
 when mixed; even though one metal may be add¬ 
 ed in but minute quantities the result is often prac¬ 
 tically a new metal, and to these early students 
 these were considered in very truth new metals. 
 The ancients did not recognize brass as an alloy. 
 They only knew that when copper was melted with 
 a certain earth (zinc ore) it became more yellow 
 and golden in appearance. - This single experiment 
 no doubt had much to do with the development 
 of the alchemistic idea of the transmutability of 
 metals. It must be remembered that although the 
 speculative philosophy of the ancient Greeks did 
 much to hinder the development of modern scien¬ 
 tific methods and imposed strong opposition to 
 physical research, yet it did much to stimulate 
 thought about natural law. The teachings of Aris¬ 
 totle and Plato must certainly have inculcated a 
 deep feeling of respect for the dignity of science, 
 but this spirit was held in check for many centuries 
 by reason of the evolution of the rigid dogmatism 
 of the Christian era. Encouraging signs of im¬ 
 provement in the way of a revival of learning dur¬ 
 ing the time and through the influence of Alfred 
 the Great were offset by the more than equal re¬ 
 vival in ecclesiasticism at the same period. 
 
 The work of two or three men left a lasting im¬ 
 pression upon scientific metallurgy in spite of all 
 opposition. Geber, in the eighth century, and Agri¬ 
 cola and Biringuccio, in the fifteenth century, re¬ 
 corded many discoveries as the results of their 
 studies and experiments upon the properties of 
 metals. These men, who were in advance of their 
 times, lived during the days when the church heap¬ 
 ed contempt upon all investigations of nature be¬ 
 cause of a belief in their futility. Men of inquiring 
 minds had to work in secret, and thus the belief in 
 the occult, magic and witchcraft were mightily 
 stimulated. These were the days when papal bulls 
 still roared that the earth was not round, because 
 in Holy Writ the “corners of the earth” are men¬ 
 tioned. But even after the domination of scholasti- 
 
ALLOYS AS SOLUTIONS. 
 
 5 
 
 cism had in great part ceased, no one seems to have 
 advanced any idea as to the nature of metallic al¬ 
 loys. Oxidation and reduction had long been sub¬ 
 jects for study, and one of the very earliest of the 
 Royal Society’s investigations was into the cause 
 of the gain in weight of lead when heated in air, 
 and it was discovered that it was due to the ab¬ 
 sorption of some constituent of the atmosphere. It 
 is not strange that this problem attracted such 
 early attention, for the behavior of lead in the 
 fire had been utilized in the refining of gold and 
 silver for sixteen hundred years and cupellation 
 for the sake of quantitatively determining the rich¬ 
 ness of gold and silver alloys had been used in 
 England from the time of Roger, Bishop of Salis¬ 
 bury, in the reign of Henry I, to whom Du Cange, 
 the historian, gives the credit for “inventing” as¬ 
 saying. The bad practices in which the coiners 
 of money had engaged during the times of William 
 the Conqueror and his son, Rufus, were thus cor¬ 
 rected by Henry, and of fifty coiners examined by 
 Roger only four escaped punishment. 
 
 In the phlogistic period, which may be said to 
 comprise the end of the seventeenth and most of 
 the eighteenth centuries, metallurgy made some 
 decided advances. Bergman advanced an explana¬ 
 tion of the distinctions between wrought iron, cast 
 iron and steel as early as 1781, and even stated the 
 percentages of carbon (which he called phlogiston) 
 that each class contained. Still earlier than this, 
 Reaumur (1722) explained the hardening of steel by 
 assuming that the metal was constituted of parti¬ 
 cles which might be likened to piles of shot. These 
 particles contained “sulphur and salts,” which, by 
 reheating, were driven into the intersticial spaces, 
 and by quenching the cooling was so quick that 
 they could not get back into the molecule. At 
 the end of the eighteenth century the discussion 
 of free or combined carbon had begun; Clouet car¬ 
 bonized iron by means of a diamond in 1798; and 
 a year later it was stated that carbon and iron form 
 
6 
 
 ALLOYS AS SOLUTIONS. 
 
 a true chemical compound. Notwithstanding these 
 great advances, no very decided opinions upon the 
 constitution of intermetallic mixtures seem to have 
 been recorded. . 
 
 One could scarcely expect that the hard prob¬ 
 lems of molecular physics, which a study of alloys 
 involves, would be solved before the announcement 
 of a molecular theory of the constitution of mat¬ 
 ter, nor could it have made any progress before 
 the period of chemical reform begun by Lavoisier, 
 the period of quantitative investigation. But only 
 recently, even in this period in which we still la¬ 
 bor, have the relations of physical properties to 
 chemical constitution attracted both chemists and 
 physicists to concerted activity, and in this field of 
 investigation alloys were last to yield fruitful re¬ 
 sults. Physical and chemical methods of research, 
 which were entirely satisfactory when applied to 
 bodies of other kinds, failed utterly to afford a ra¬ 
 tional explanation of the molecular conditions 
 existing in alloys. The solution of these problems 
 required new methods and new apparatus. Fortu¬ 
 nately, these are at hand; they have been slowly 
 evolving and the past decade has been most fruit¬ 
 ful of results. Problems of the greatest practical 
 importance and of extreme difficulty have been 
 solved. Yet much remains to be done, as we have 
 but entered upon this new field of applied science. 
 Let us review the work so recently accomplished 
 by a host of investigators which, I am sorry to say, 
 does not include many Americans. We have not 
 done our part in this work, for, with few excep¬ 
 tions, the great leaders in it belong to the 
 school of English and French physicists, chemists 
 and metallurgists. A resume of their achievements 
 may be of interest at this time, and this is given 
 in the pages that follow. 
 
 A most potent factor in this recent progress has 
 been the official recognition of the necessity of 
 study along these lines. More than ten years ago 
 the Institution of Mechanical Engineers of Great 
 
ALLOYS AS SOLUTIONS. 
 
 7 
 
 Britain appointed an Alloys Research Committee, 
 which receives financial assistance from the insti¬ 
 tution, and five reports have been published by it, 
 and a sixth report is, I believe, about to appear. 
 It is in connection with the work of this committee 
 that Sir William Robert-Austen, his pupils and the 
 co-workers of his staff have produced such excel¬ 
 lent work, especially in pyrometry and in explain¬ 
 ing the complicated changes, allotropic and other¬ 
 wise, which steel and cast iron undergo. The prac¬ 
 tical operations of annealing, tempering, harden¬ 
 ing, etc., can now be conducted upon a scientific 
 and rational basis. The effects of thermal opera¬ 
 tions can be predicted accurately and desired re¬ 
 sults obtained unfailingly. It would be too much 
 to say that all this is due to this committee alone; 
 but if we include also the committee of the British 
 Association, appointed to investigate the nature of 
 alloys, of which Mr. Neville is chairman and the 
 “Commission des Alliages, ,, of the Societe d’En- 
 couragement lTndustrie Nationale, as also sharing 
 the honors of recent great advances in both our 
 theoretical and practical knowledge of alloys, then 
 we have recognized the three most prolific contribu¬ 
 tors to our knowledge of metallic mixtures. While 
 giving these official bodies full credit for their dis¬ 
 coveries, we would not detract in any way from 
 the honor due to many individual workers not con¬ 
 nected with committees who are scattered through 
 many lands. It is safe to say, however, that the 
 larger results of the concerted work of commit¬ 
 tees has had a stimulating effect upon the private 
 worker. One of the greatest evidences of apprecia¬ 
 tion of the necessity for research in connection with 
 alloys is that nearly every railroad in France and 
 many metallurgical concerns contributed to sus¬ 
 tain the work of the “Commission des Alliages.” 
 
 The purely scientific reasons which have made 
 possible the recent advances in alloys research 
 seem to me to be : (i) An increased knowledge of 
 solutions. For this we are undoubtedly indebted 
 
8 
 
 ALLOYS AS SOLUTIONS. 
 
 in very great measure to the great Dutch and 
 German physical chemists—Vant Hoff, Rooze- 
 boom, Ostwald, Nernst and others. (2) The 
 development of metallography. This seems to have 
 originated with Dr. Sorby, of Sheffield. Although 
 he wrote about the microstructure of meteoric iron 
 as early as 1864, the progress of metallography was 
 very slow. Only within the past ten or fifteen years 
 have even metallurgists begun to recognize what a 
 valuable aid to research it is and unfortunately the 
 manufacturers have not yet given it just recogni¬ 
 tion. Recent progress in this branch of applied 
 sciences is most closely attached to the names of 
 Stead, Martens, Osmond, Le Chatelier, Charpy, Rob- 
 erts-Austen, Howe and Sauveur. (3) Improvements 
 in pyrometry. The study of metals, alloys, furnace re¬ 
 actions, etc., was retarded for many years because of 
 the lack of the suitable means of measuring accurately 
 very high temperatures. Such means as have been 
 available for these purposes in past years were not 
 suitable for general use around shops and metal¬ 
 lurgical works and were inconvenient even for the 
 laboratory. Now, however, there are at least two 
 reliable pyrometers which are conveniently mani¬ 
 pulated and lend themselves readily to accurate 
 measurements of high temperatures. The Siemens 
 electrical resistance pyrometer, as perfected by Cal- 
 lendar and Griffiths, gives wonderfully accurate 
 readings within certain limits. For use both in the 
 laboratory and in the factory it does not seem as 
 popular—in the United States at least—as the Le 
 Chatelier thermo-electric couple. The very accu¬ 
 rate work of Messrs. Heycock and Neville is per¬ 
 haps as good a recommendation for the possibilities 
 of the electrical resistance pyrometer as can be 
 cited. The particular advantages of the thermo¬ 
 electric couple as applied to alloys research are— 
 its accuracy at high temperatures, its ability to be 
 used with very small masses of heated substances, 
 and, indirectly, its ability to be used in conjunc¬ 
 tion with a photo-autographic recorder, as has been 
 
ALLOYS AS SOLUTIONS. Q 
 
 so well demonstrated by Sir William Roberts- 
 Austen. 
 
 Crystalline Growth in Metals .—Before we enter 
 upon the consideration of alloys, it. may be well to 
 note a few general properties of metals, particular¬ 
 ly the mode of crystalline growth and the effects of 
 strain. To both of these subjects Professor Ewing, 
 of Cambridge, has given much attention and has 
 explained the phenomena of these operations very 
 clearly. He states that all solid metals are crystal¬ 
 line, even though when examined by the micro¬ 
 scope the individual crystals or crystalline grains 
 may not present a simple geometrical outline. The 
 essential point is that the particles composing the 
 mass of a crystal lie in one direction, i.e., have the 
 same plane of orientation. Mr. G. T. Beilby read 
 a paper at the meeting of the British Association 
 held in Glasgow, September, 1901, upon “The Minute 
 Structure of Metals,” and cites evidence to show 
 that he has identified minute spicules or scales 
 showing remarkable uniformity of size and appear¬ 
 ance in metals of all the leading groups. These 
 under mechanical treatment may be converted into 
 transparent glass-like substance and may again re¬ 
 appear as spicules unchanged in size. The diam¬ 
 eter of the scales is estimated to range from 1-300 
 to 1-400 of a millimeter. The metals examined by 
 Mr. Beilby included gold, silver, platinum, cobalt, 
 nickel, chromium, iron, copper, lead, bismuth, anti¬ 
 mony, tin, cadmium, magnesium, aluminum, zinc 
 and sodium. Brasses and bronzes were also ex¬ 
 amined. The author says that the crystalline faces 
 and cleavages of such metals as antimony, bismuth 
 and zinc exhibit the same features as the softer 
 and more malleable metals, being covered with a 
 film of transparent metal, while in fractures at right 
 angles to cleavage planes, scales are distinctly seen. 
 Mr. Beilby’s conclusions are: “The persistence of 
 these minute scales or spicules under all kinds of 
 mechanical and thermal treatment, the remarkable 
 uniformity of their size and appearance in metals 
 
10 
 
 ALLOYS AS SOLUTIONS. 
 
 of all the leading groups, their disappearance into 
 the transparent form and their reappearance again, 
 apparently unchanged in size or otherwise, seem to 
 afford fair ground for the conjecture that they 
 are in some way definite units in the structure of 
 metals.” 
 
 It may be that these are the “units” whose ar¬ 
 rangement in certain definite orders constitute 
 crystals, and this similarity of disposition of par¬ 
 ticles is what constitutes orientation. Professor 
 Ewing explains crystallization in metals in this way: 
 the formation of crystals must be assumed to start 
 simultaneously at many points. The crystals grow 
 until they touch one another, and, although indi¬ 
 vidual crystals are similar, it is not necessary that 
 the corresponding axes of any two of them lie in 
 the same plane. No doubt each crystal is of per¬ 
 fect form so long as it is free to grow in all direc¬ 
 tions; when the crystals touch, their symmetrical 
 growth is hindered, but the orientation of the sub¬ 
 sequent particles which attach themselves is not 
 changed. It is for this reason that orientation 
 rather than geometrical outline is considered the 
 essential feature of crystalline growth. In the case 
 of commercial metals, more or less impure, the im¬ 
 purities are cast out by the growing crystals, and 
 being in fact alloys of the admixed metals with the 
 still fluid principal metal, they have a lower melt¬ 
 ing point than the pure metal. Therefore, they 
 solidify last and form an investing cement which 
 holds together the primary crystals. 
 
 The Effect of Strain .—When a polished metal is 
 subjected to strain there is no change noticeable 
 until the stress exceeds the elastic limit. When, 
 however, the plastic stage is reached, there will 
 appear dark lines more or less perpendicular to the 
 direction of stress. This, Professor Ewing says, 
 is not due to Assuring, for by changing the direc¬ 
 tion of the rays which illuminate the specimen 
 under observation for vertical to oblique, it will 
 be seen that the erstwhile dark lines appear light, 
 
ALLOYS AS SOLUTIONS. 
 
 II 
 
 while the background has changed from light to 
 dark. Fissures would not have reflected light in 
 any case; the permanent elongation is due to slip¬ 
 ping of the components of the crystals past each 
 other. That is, the surface which was originally 
 plain, presents, after stress has been applied beyond 
 the elastic limit, the appearance of minute steps. 
 It is as though one should take a pack of playing 
 cards with their ends presenting a smooth surface. 
 Now if the thickness of the pack of cards were one 
 inch, and the pack were bent, the end surface of 
 the pack would measure more than an inch and 
 would consist of little steps, not only the ends, but 
 also a part of the flat surfaces of the cards being ex¬ 
 posed. 
 
 In the case of metals, the slip-lines need not 
 necessarily lie in one plane. As many as four sets 
 of parallel slip lines have been noticed by Professor 
 Ewing. Mr. William Campbell worked upon this 
 phenomenon independently at about the time that 
 Professor Ewing was making these interesting ob¬ 
 servations. He experimented with tin, while Pro¬ 
 fessor Ewing worked chiefly with lead, and it is 
 interesting to note how completely their results 
 agree. 
 
 The growth of crystals from a solidifying molten 
 mass is easy to comprehend, but why is it that 
 crystals grow in a solid mass of metal not only by 
 annealing but even at ordinary temperatures? Big 
 crystals actually consume the little ones. A bit of 
 rolled tin which was allowed to stand for eighteen 
 months at ordinary temperatures in the laboratory 
 showed a marked change in appearance, due to 
 growth of its crystals. If a specimen of tin is put 
 upon a hot plate and heated to, say, 200° C. for 
 ten days, a growth such as is shown in Fig. 1 is 
 obtained, which shows the tin before and after 
 annealing. This photograph is one of Mr. 
 Campbell’s, and is a little under actual size. It 
 also shows very well the different orientation of 
 the crystals. In fact, the growth of these crystals 
 
12 
 
 ALLOYS AS SOLUTIONS. 
 
 was so rapid that a microsection through them 
 reveals slip-lines, probably due to this rapid growth. 
 The fact that the slip lines are only parallel within 
 the same crystal seems to confirm this view, for, 
 had the stress been applied longitudinally by artifi¬ 
 cial means all the slip-lines would have been more 
 or less normal to the direction of the stress. 
 
 With regard to the growth of crystals in strained 
 metals when they are subsequently warmed, Mr. 
 Rosenhain has offered this explanation: In strained 
 metals the layer of enveloping alloy surrounding 
 the separate crystals becomes broken, and the dif- 
 
 FIG. i. 
 
 ference of potential between the metal and the al¬ 
 loy set up by electrolytic action causes a solution 
 of the crystal on one side of the alloy and a de¬ 
 posit on the adjoining side. This he verified by the 
 following experiment: Two pieces of lead were 
 scraped clean and welded by the application of great 
 pressure. A micro-section through the weld show¬ 
 ed that the crystals in neither piece extended ac- 
 cross the weld. When, however, between two such 
 surfaces a little powdered tin was sprinkled and 
 pressure applied, a micro-section revealed that 
 electrolytic action had taken place and that crystals 
 from either side extended across the weld. In¬ 
 genious as is this explanation, yet I am not certain 
 that the proof is conclusive. If I mistake not, at 
 
ALLOYS AS SOLUTIONS. 
 
 13 
 
 the last Cantor lectures before the Society of Arts, 1 
 Roberts-Austen and Rose showed crystals of 
 gold extending across a weld, although the purest 
 standard gold was used in the experiment. In fact, 
 long annealing of this gold obliterated the weld al¬ 
 most completely. 
 
 Freezing Point of Metals .—With water and many 
 other liquids we know that it is quite possible to 
 lower the temperature to a considerable amount 
 below their freezing-point without the separation 
 of any solid. On the other hand, it is not known 
 that any solid may be heated above its melting- 
 point without becoming liquid. The property first 
 mentioned is known as surfusion. On account of 
 this phenomenon it would be theoretically prefer¬ 
 able to determine the melting rather than the freez¬ 
 ing point of metals, but practically this is a very 
 difficult task, while freezing-points are in most 
 cases easily determined and surfusion need not in¬ 
 terfere with their accuracy in the least, for when 
 solid matter begins to separate from a pure liquid 
 thus cooled, the temperature rises rapidly to the 
 true freezing point and remains constant until the 
 whole mass is solid. Sir William Roberts-Austen 
 cites an instance in which tin was cooled 20° C. be¬ 
 low its freezing point without solidification, and I 
 have frequently observed the same result in tin and 
 some other metals, but to a less degree. In work¬ 
 ing with metals the first point at which solids sepa¬ 
 rate is taken as the freezing point, and surfusion 
 need not obscure this point, particularly if an 
 autographic record of the cooling curve is taken. 
 If we plot a typical cooling curve of a pure metal, 
 using temperature and time as the co-ordinates, 
 we obtain a curve represented by A B, Fig. 2, hav¬ 
 ing two very distinct angles, while a portion is 
 horizontal, i. e., the temperature remains constant 
 during the whole period of solidification. If sur¬ 
 fusion takes place, and it is more common in pure 
 metals than in alloys, the curve A' B' represents 
 
 0 ) J. Soc. of Arts. (1901) p. 851. 
 
14 ALLOYS AS SOLUTIONS. 
 
 what happens. The temperature falls below the 
 real freezing point, and then rises abruptly to that 
 point and remains constant as in A B until solidifi¬ 
 cation is complete. In impure metals—and since 
 the impurities are usually metallic, they are really 
 alloys—the temperature does not remain constant 
 at all, but a decided change in direction is noticed 
 as shown in A" B". The greater the amount of 
 impurity, the more rounded will appear the cool¬ 
 
 ing curve. Since these are really the curves of 
 alloys and solid solutions, they will be explained 
 at greater length subsequently. This third type, 
 A" B" is introduced here because it shows that 
 the nature of the cooling curve gives us a clue as 
 to the purity of the metal under examination. To 
 explain surfusion, we need only remember that 
 fusion is always attended by absorption of heat and 
 solidification by evolution of heat. When a :< sur- 
 
ALLOYS AS SOLUTIONS. 
 
 15 
 
 fused” substance begins to solidify it disengages 
 heat and the temperature rises until the melting 
 point is reached. No further rise takes place be¬ 
 cause at this temperature only, liquid and solid 
 are in equilibrium. Any further change of liquid 
 to solid or sodid to liquid must be effected by ab¬ 
 stracting or adding heat. In practice, either heat 
 is lost by radiation and the whole mass solidifies, 
 or is supplied by artificial warming, and the whole 
 mass becomes liquid. 
 
 Binary Alloys .—Having now noted some of the 
 properties of pure metals we may next consider 
 what happens when two metals are melted together. 
 We shall not at this time consider ternary or more 
 complex alloys. The first thing noticed upon melt¬ 
 ing together two metals is that (1) they mix in all 
 proportions, or (2) they do not. In the first in¬ 
 stance, under proper conditions an approximately 
 homogeneous solid mass is likely to result when the 
 molten alloy is cooled. In the second case, on cool¬ 
 ing it will be found that the metals have separated 
 into layers, the lighter metal on top. Each layer, 
 however, will be found upon analysis to contain a 
 certain amount of the other metal in solution, just 
 as ether and water dissolve small quantities each 
 of the other. As between certain metals and solid 
 non-metallic elements it seems as though they were 
 absolutely insoluble in one another at ordinary 
 temperatures, although they may be quite soluble 
 at high temperatures. For example, M. Moissan 
 tells us that metals of the platinum group readily 
 dissolve several per cent of carbon at the temper¬ 
 ature of the electric furnace, but cast it all out as 
 graphite on cooling. It has not yet been deter¬ 
 mined whether the graphite, which separates in 
 cast iron, contains iron or not. It seems to me 
 that we shall come nearer the truth if we consider 
 such cases as solutions of almost infinite dilution, 
 rather than as absolutely pure metal or non-metal. 
 
 We have just spoken of metals dissolving metals, 
 and it is this conception of alloys as solutions— 
 
l6 ALLOYS AS SOLUTIONS. 
 
 solid solutions—that has been steadily progressing 
 in favor in recent years and which we hope both 
 to explain and to confirm by citing the results of 
 the experiments of many investigators. I remember 
 hearing a teacher, after explaining the nature of the 
 elements and their tendency to combine in definite 
 propositions, assure the pupils that “alloys are al¬ 
 loys’’—a delightfully simple statement, but future 
 pupils as well as their teachers will be expected to 
 know a little more about them. 
 
 Very striking analogies exist between ordinary 
 solutions and alloys; in fact, to point out these 
 similarities is to give our present conception of 
 the constitution of alloys. ( Van’t .) 
 
 Solid Solutions .—This expression is due to Vant 
 Hoff, but Roberts-Austen has done most to make 
 it familiar to metallurgists. Our text-books upon 
 physical chemistry contain much information upon 
 the subjects of solutions of gases in liquids, li¬ 
 quids in liquids, and solids in liquids, but are for 
 the most part silent upon the subject of the solu¬ 
 tions of solids in solids. The extension of our 
 knowledge upon this subject is very recent and 
 mostly confined to technical periodical literature of 
 recent dates. Professor Roozeboom and his pu¬ 
 pils have carried on researches upon the nature of 
 fused mixtures of salts and their behavior upon 
 cooling. This work throws considerable light up¬ 
 on the problems of chemical equilibrium in alloys. 
 As defined by Sir William Roberts-Austen a Solid 
 solution is “a homogeneous mixture of two or 
 more substances in the solid state.” In metals 
 no one has worked as yet with non-crystalline mix¬ 
 tures, and solid solutions of metals when crystal¬ 
 line are solid “isomorphous mixtures,” or “mixed 
 crystals.” The use of these three terms to denote 
 one condition is confusing and unnecessary, and as 
 objections exist to the last two synonyms, their 
 use should be discontinued. The term “mixed crys¬ 
 tals” is particularly apt to convey a wrong im¬ 
 pression to one who sees this name for the first 
 
ALLOYS AS SOLUTIONS. 1 7 
 
 time, especially if it is unaccompanied by a defini¬ 
 tion or explanation. 
 
 Mr. Stead, in the Journal of the Iron and Steel 
 Institute , 1900, classifies solid solutions as follows: 
 
 (1) Those in which one constituent of an alloy 
 in crystallizing retains a portion of the other homo¬ 
 geneously diffused through its whole crystalline 
 mass. 
 
 (2) Those in which during crystallization the cen¬ 
 tral portion of the crystals contains less of the dis¬ 
 solved substance than their external boundaries. 
 
 (3) Those in which the metals form a definite 
 compound, which is retained in solid solution in 
 the excess of metal or metals. I should like to 
 add that the “compound” may be the “solvent,” in 
 the sense in which those terms are here used. 
 
 The condition described in (2) is that of an unsta¬ 
 ble system. Crystallization has taken place more rap¬ 
 idly than diffusion, and in such cases where crystals 
 are not of uniform composition from center to out¬ 
 side complete homogeneity could probably be ob¬ 
 tained by annealing. Mr. Stead adds a fourth class. 
 
 (4) Those in which the non-metallic elements 
 form definite compounds with a portion of the dis¬ 
 solving metal that remain in solid solution. This 
 class would include the property of metals, such 
 as iron and copper, to dissolve a portion of their 
 oxides. Such solutions have been little studied. 
 
 In the case of metals which do not mix in all 
 proportions, the two layers formed on cooling con¬ 
 sist of solid solutions belonging to one of the 
 types just mentioned. Some of the pairs of metals 
 which do not mix in all proportions at ordinary 
 temperatures are Zn-Pb, Zn-Bi, Pb-Al, Bi-Al, Cd-Al. 
 These have all been studied by Dr. Alden Wright,* 
 and Mr. Campbell and myself have repeated and 
 verified a part of Dr. Wright's work upon the 
 aluminum alloys. In all these pairs of metals the 
 relative solubility is a function of the temperature; 
 
 *(Proc. Roy. Soc. 1898-1893) 
 
ALLOYS AS SOLUTIONS. 
 
 therefore, it appears likely that at some temper¬ 
 ature solubility is complete. Accordingly we may 
 represent the condition existing between these and 
 any other similar pairs of metals by a “critical 
 curve (Fig. 3). A and B are the metals which are 
 not entirely miscible at ordinary temperatures, but 
 which form layers. The metal A contains the quan¬ 
 tity of B represented by the space A x on the com¬ 
 position line. Similarly the metal B contains B y 
 per cent of A. With increasing temperature the 
 mutual solubilities increase, and at a temperature 
 z there is but one solution. Alloys represented 
 by percentage compositions, which fall outside the 
 
 A Composition £ 
 
 curve x z y are homogeneous. Those whose com¬ 
 position is represented by percentages within x z y 
 may be considered as similar to emulsions at all 
 temperatures below the curve at which they are 
 liquid. They are incapable of remaining homo¬ 
 geneous. To such alloys, Sir George Gabriel 
 Stokes gives the name of ideal (i.e., unreal, imagi¬ 
 nary) alloys while those falling outside the area 
 x z y he calls real alloys. The distinction between 
 such pairs of metals as form separate layers on 
 cooling, and the more or less laminated or striated 
 structure produced upon solidification of eutectic 
 alloys, will be pointed out later. 
 
ALLOYS AS SOLUTIONS. 19 
 
 Molecular Freedom in Solids .—It must not be 
 supposed that because certain bodies are desig¬ 
 nated “solid” that entire absence of molecular mo¬ 
 bility is implied. Such an idea is far from the truth; 
 the difference in mobility of the molecules in “sol¬ 
 ids” and “liquids” is one of degree only. On the 
 boundary between solids and liquids substances are 
 said to be viscous or plastic. Just as crystals of a 
 soluble salt placed in contact with water tend to 
 become dissolved and uniformly distributed 
 throughout the solvent; so metals, even at nor¬ 
 mal temperatures, tend to form uniform solutions; 
 this tendency, though slight, is none the less real. 
 Metals even tend to evaporate and to be surround¬ 
 ed by their own vapor. Robert Boyle thought 
 that “even gold” had its “little atmosphere.” From 
 analogy with salt solutions we may suppose 
 that one metal dissolved in another shows a small 
 expansive tendency to escape from solution; in 
 other words, exhibits osmotic pressure. 
 
 Professor W. Spring, of Liege, demonstrated the 
 fluidity of solids, if I may use that expression, in 
 his famous experiments upon the behavior of metals 
 under exceedingly high pressures—ten thousand at¬ 
 mospheres, more or less. He showed that by pres¬ 
 sure alone: 
 
 (1) Metals can be made to flow. The familiar 
 operation of striking coins illustrates this property 
 of metals and alloys. A medal has recently been 
 struck in steel. 
 
 (2) Solids, both metals and salts, like liquids and 
 gases, possess perfect elasticity and suffer no per¬ 
 manent diminution in yolume by pressure. Strain, 
 torsion or bending may produce permanent de¬ 
 formation in a metal or alloy, but there is no limit 
 of elasticity in regard to diminution in volume, 
 for when the pressure is removed the solid as¬ 
 sumes its original volume. An exception to this 
 is cited by Professor Spring in his next generaliza¬ 
 tion, viz.: 
 
 (3) Allotropic changes may result from pressure. 
 
20 ALLOYS AS SOLUTIONS. 
 
 Prismatic sulphur becomes octahedral sulphur and 
 amorphous arsenic becomes crystalline. In both 
 these cases the second allotropic form is denser 
 than the first, and from these and other experi¬ 
 ments Professor Spring argues that allotropic 
 tranisformaitio,ns by pressure show that f matter 
 takes the state which corresponds to the volume 
 it is obliged to occupy. Moissan’s brilliant experi¬ 
 ments upon the production of artificial diamonds il¬ 
 lustrates this principle. 
 
 (4) Metallic filings are converted by pressure 
 alone into solid masses, just as if they had been 
 fused. Similarly mixed filings yield alloys; e.g., 
 brass was produced by compressing clean, flat sur¬ 
 faces of zinc and copper. Reactions of double 
 decomposition between dry salts and the produc¬ 
 tion of the highly colored regulus of Venus (Cu 2 
 Sb) show that not only intimate agglutination, but 
 also chemical combination may take place between 
 bodies in the solid state. I have confirmed some 
 of Professor Spring’s results, but was unable to 
 produce several typical intermetallic compounds 
 as Au Al 2 , Sb Sn, Sn Cu 4 , etc. 
 
 Professor Roberts-Austen has demonstrated the 
 fact of molecular mobility among solids in a pro¬ 
 longed series of very beautiful experiments upon 
 the diffusion of gold into lead at temperatures 
 from 250° C. down to the ordinary temperature. 
 Graham showed long ago that gold diffusing into 
 molten lead, tin, etc., obeyed Fick’s law, 
 d v d 2 v 
 
 d't ~~ ^ d x 2 
 
 where x is the distance through which dif¬ 
 fusion takes place (against gravity), v is the 
 concentration of the diffusing metal, t = time, and 
 k is the diffusion constant, i.e., the quantity of 
 metal in grams diffusing in unit of area (1 cm 2 ) in 
 unit of time (1 day) when unit difference of con¬ 
 centration (in grams per cm 3 ) is maintained be¬ 
 tween the sides of a layer 1 cm. thick.* That is, 
 
 *See Roberts-Austen, Graham Lecture before the Philosophi¬ 
 cal Society of Glasgow, 1901. 
 
ALLOYS AS SOLUTIONS. 
 
 21 
 
 metals dissolve in metals, just as salts dissolve in 
 water. Were it not for the operation of this 
 law it would not be such an easy matter to pre¬ 
 pare homogeneous alloys. For instance, with 
 relatively little stirring and relying upon diffusion, 
 it is very easy, according to Sir William Roberts- 
 Austen to make 1,200 ounces of coin gold alloy, 
 the first and last pourings of which shall not differ 
 by more than 1-10,000 part in fineness. The idea 
 that interchange of matter can take place between 
 solids is not a new one, as is shown by the cemen¬ 
 tation process of the ancient Hebrews by which 
 gold was purified. 
 
 The experiments upon the diffusion of gold into 
 solid lead, which Roberts-Austen has recently re¬ 
 ported to the Royal Society, may be summarized 
 as follows: 
 
 Gold placed in the bottom of a tube filled with 
 lead and maintained at 250° C. (79 0 below 
 the melting point of lead) appeared at the top in 
 notable quantities in a month. At ioo° C. the rate 
 of diffusion was 1-100,000 of that in fluid lead, and 
 in solid lead at the ordinary temperature, allowed 
 to stand in contact for four years, unmistakable 
 signs of diffusion took place, but its rate was such 
 that in one thousand years the diffusion would 
 equal that taking place in one day in lead just 
 molten. 
 
 The figure 4 shows another sort of change which 
 may take place in solid materials. An alloy con¬ 
 taining 88.7 per cent Cu and 11.3 per cent Al. 
 corresponds closely to the compound Cu 3 Al. It 
 should, therefore, appear almost homogeneous, and 
 so it does when quickly cooled. When slowly cool¬ 
 ed, however, it changes at 500° C., or nearly 
 6oo° below its freezing point, and presents the 
 variegated appearance shown in Fig. 4, which no 
 metallographist would care to call a typical struc¬ 
 ture for an intermetallic compound. 
 
 We apply practically the phenomenon of molecu¬ 
 lar mobility in solids in all annealing operations. 
 
22 
 
 ALLOYS AS SOLUTIONS. 
 
 By this means the ill effects of strain due to sud¬ 
 den cooling are remedied, and equilibrium is estab¬ 
 lished in articles of metal, glass, enamel and other 
 materials. The rate of diffusion of gold in lead 
 at the various temperatures is an indication of the 
 rapid increase in molecular mobility which results 
 by very moderate increments in temperature. This 
 knowledge is applied whenever we resort to moder¬ 
 ately elevated temperatures in all sorts of anneal¬ 
 ing operations. Steel is very sensibly annealed at 
 
 fig. 4. 
 
 temperatures as much as i,ooo° C. below its melt¬ 
 ing point. 
 
 Behavior of Binary Alloys During Cooling from 
 Fused State .—It has been stated that metals may 
 or may not mix in all proportions. If they do mix, 
 we may further distinguish those cases in which (1) 
 chemical combination does not take place, and (2) 
 those in which chemical combination does take 
 place, and one or more intermetallic compounds 
 result. The reality of the existence of intermetal¬ 
 lic compounds is undoubted, yet, so far as I know, 
 no mention is made of them in the standard books 
 on general chemistry. Indeed very little is known 
 in regard to the nature of the affinity between 
 metals. It does not seem necessary to suppose 
 that the laws applying to combinations between 
 elements or radicals which are relatively electro¬ 
 positive and electro-negative be equally applica- 
 
ALLOYS AS SOLUTIONS. 
 
 23 
 
 ble here; and our conceptions of valency are seri¬ 
 ously shattered if we attempt to reconcile them 
 to intermetallic compounds. The difficulties in 
 studying intermetallic compounds are very great, 
 because they are troublesome to isolate, and in 
 many of their properties they resemble the metals. 
 Frequently, too, they are quite unstable and show 
 marked dissociation. Chemical methods of study¬ 
 ing them are of very limited application; filtering 
 off solidified intermetallic compounds from a still 
 fluid metallic mother liquor at high temperatures 
 presents extreme difficulties and volatilization of 
 the excess of solvent metal is rarely applicable. 
 In a few cases the heat of formation can be de¬ 
 termined and electrical methods by which the po¬ 
 tential of binary alloys is compared with that of 
 the more positive metal in the alloy may yield im¬ 
 portant evidence as to the solubility of one metal 
 in another in the solid state. The microscopic evi¬ 
 dence and that of the freezing point curves require 
 some amplification, for they are not wholly satis¬ 
 factory in this regard. (See Herschkowitz,, Zeits. 
 Phys. Chemie, XXVII., p. 113, and Laurie, Trans. 
 Chem. Soc. (1888), p. 104.) One great difficulty 
 in studying intermetallic compounds by chemical 
 means is that even if definite crystals can be iso¬ 
 lated from an alloy by using suitable solvents it is 
 found that their compositions vary according to 
 the percentage composition of the alloy from which 
 they were obtained. Apparently identical crystals 
 isolated from alloys of different composition, 
 though of the same crystalline form, will usually 
 differ on analysis. In other words, crystals need 
 not be composed solely of a definite intermetallic 
 compound. This seems to have been the general 
 idea, however, in times past and every worker who 
 succeeded in isolating definite crystals from an 
 alloy promptly assigned to them a formula. Unless 
 a compound is indicated by the appearance of a 
 summit in the freezing-point curve, and unless the 
 alloy corresponds in percentage composition exact- 
 
24 
 
 ALLOYS AS SOLUTIONS. 
 
 ly to this summit, it is more than likely that the 
 crystals isolated, though isomorphous with those 
 of the compound, are not of exactly the same com¬ 
 position. This, in general, no doubt accounts for 
 the “discoveries” from time to time of possibly 
 ten Al-Cu compounds and nearly as many Al- 
 Mo compounds of the types Rx-R'y, where x and 
 y may be any number from one to twenty. To the 
 same category belong the numerous carbides of 
 iron. I have discovered six in the literature, but 
 not in iron, the existence of all but one of which 
 is denied. For this very reason, namely, the im¬ 
 probable existence of most of the intermetallic com¬ 
 pounds mentioned in technical literature of the past 
 ten years, I hesitate to give the formula, W Ah, to 
 some beautiful hexagonal crystals, which are so re¬ 
 fractory that neither aqua regia, nor aqua regia and 
 sulphuric acid, nor fused sodium carbonate scarcely 
 attacks them, and whose composition agrees very 
 closely with that formula. 
 
 Cooling Curves .—During the cooling of a molten 
 alloy, various constituents may crystallize out suc¬ 
 cessively; definite compounds which are stable only 
 at high temperatures may split up into simple con¬ 
 stituents; or, new combinations, impossible at high 
 temperatures, may be formed as the temperature 
 falls. All such molecular changes are accompanied 
 by corresponding thermal effects, such as the dis¬ 
 engagement or the absorption of heat. By the accu¬ 
 rate measurement of the temperature at which these 
 changes take place we obtain most valuable in¬ 
 formation in regard to the molecular movements 
 in the mass. The Le Chatelier pyrometer used in 
 conjunction with an auto-photographic recording 
 device seems best suited to measure and record 
 these changes. The general principles of thermo¬ 
 electric pyrometry are too well known to require 
 discussion here. For reference, however, I could 
 not do better than to refer to H. Le Chatelier and 
 O. Boudouard’s volume, “Mesure des Tempera¬ 
 tures Elevees,” or to its English translation with 
 supplement by George K. Burgess. 
 
ALLOYS AS SOLUTIONS. 
 
 25 
 
 The instruments used by Sir William Roberts- 
 Austen in his private laboratory have been de¬ 
 scribed fully in the Reports of the Alloys Research 
 Committee of the Institution of Mechanical Engi¬ 
 neers. During several months that it was my privi¬ 
 lege to work in his laboratory I made use of a record¬ 
 er designed by Dr. Stansfield and constructed by the 
 Cambridge Instrument Company. For a description 
 of this, with illustrations, see the Journal of the 
 Franklin Institute, January, 1902. Neither of these 
 forms of recorder is perfect, and many improvements 
 will be required to produce an instrument that is en¬ 
 tirely satisfactory. Fortunately it is not the accuracy 
 of these instruments, but certain inconveniences of 
 manipulation that are at fault. In using any thermo¬ 
 couple, with or without a recorder, it is necessary 
 to calibrate frequently. The temperatures usually 
 taken as “fixed points” are the boiling points of 
 water (ioo°), naphthaline (218°), mercury (356.7°), 
 and sulphur (444.5 0 ); the melting points of tin 
 (232°), lead (329 0 ), aluminum (655°), gold (1064°), 
 and copper (1083°). Not all of these need be de¬ 
 termined for a single calibration; three or four 
 points if determined with great accuracy will suffice. 
 Other points may be determined by way of verifi¬ 
 cation, and it is convenient as well to have a large 
 variety of materials to select from so that for any 
 special work the curve may be calibrated by means 
 of substances whose melting or boiling points lie 
 near the temperatures at which we desire to oper¬ 
 ate. For instance, one would not ordinarily use 
 the temperatures ioo°, 218° and 232° in construct¬ 
 ing a curve for use above iooo°, but would choose 
 655°, 1064° and 1083°, or other temperatures in this 
 region which are well established as the melting 
 point of potassium sulphate (1084°), or sodium car¬ 
 bonate (850°). 
 
 In using the thermo-couple to obtain a cooling- 
 curve of a metal or alloy it is only necessary to 
 insert the couple, suitably protected by a fire-clay 
 or porcelain tube, into the molten mass. A cur¬ 
 rent of electricity is generated whose electro mo- 
 
26 
 
 ALLOYS AS SOLUTIONS. 
 
 tive force is approximately proportional to the tem¬ 
 perature, or rather the current gives us a measure of 
 the difference in temperature between the hot and 
 cold junctions of the couple. If now this current 
 be passed through a reflecting galvanometer, the 
 beam of light moves rapidly from left to right 
 until it attains a deflection proportional to the 
 temperature measured. This beam of light falls 
 upon the scale which has been calibrated by the 
 establishment of certain fixed points as just ex¬ 
 plained. If the beam passes through a narrow 
 horizontal slit in a recording device, behind which 
 a photographic plate is rising or falling at a uni¬ 
 form rate, we obtain a cooling-curve whose co-or¬ 
 dinates are time and temperature. The type of 
 curves given by a pure metal, by surfusion and im¬ 
 pure metals has already been shown in Fig 2. 
 Two other classes of alloys give a cooling-curve 
 identical with that of a pure metal, viz.: eutectic 
 alloys and intermetallic compounds, that is these 
 three classes of substances freeze at a single tem¬ 
 perature. The nature of a eutectic will be dis¬ 
 cussed later; suffice it here to define it as that alloy 
 of a series which has the lowest freezing-point, 
 which is constant in composition, and which is 
 not a chemical compound of the metals which 
 compose it. In Fig. 5, the curve A B represents 
 that of a homogeneous solid solution. It shows 
 but one “break,” though the temperature does 
 not remain constant during the whole period of 
 solidification, as in the case of pure metals, com¬ 
 pounds and eutectics. In A' B' is shown a curve 
 with two “breaks/’ the upper one resembling that 
 of a solid solution and the lower one shows the 
 form of eutectic. The first abrupt change in di¬ 
 rection in either of these curves indicates the tem¬ 
 perature at which solidification begins. As the 
 excess of one metal solidifies the concentration of 
 the residual fluid is increased with respect to the 
 metal which is not in excess. The ultimate cause 
 of the change in direction is the liberation of the 
 latent heat of solution or of fusion. 
 
ALLOYS AS SOLUTIONS. 
 
 27 
 
 As is well known, when concentrated hydro¬ 
 chloric acid (solution of hydrochloric acid gas in 
 water) is boiled, the hydrochloric acid gas evap¬ 
 orates faster than the water. If dilute hydro¬ 
 chloric acid is boiled, the water evaporates relative¬ 
 ly faster than the gas. The inevitable result is 
 
 that at a certain concentration hydrochloric acid 
 and water vapor leave the liquid at a rate pro¬ 
 portional to their concentration; thereafter the 
 composition of the liquid remains constant and 
 the boiling point fixed. 
 
 A somewhat analogous condition is that of the 
 solidification of two metals, M and N. Either of 
 them may be considered as the solvent. If M is 
 the solvent and N the dissolved substance, then 
 on cooling such alloy, M crystallizes first re¬ 
 taining some N in solid solution. Similarly an 
 alloy containing much N and little M begins to 
 solidify by the separation of crystals of nearly pure 
 
28 
 
 ALLOYS AS SOLUTIONS. 
 
 N containing M in solid solution. As M 
 or N begin to separate out in solid form the re¬ 
 maining liquid portions become concentrated in 
 respect to N and M respectively. The freezing- 
 point of the remaining liquid is thus continually 
 lowered. Eventually either of these classes of al¬ 
 loys reaches a concentration greater than that of 
 a saturated solid solution and then there crys¬ 
 tallizes out simultaneously two solid solutions, one 
 of which is a saturated solid solution of M in N 
 and the other a saturated solid solution of N in 
 M. In any one series of metals this takes place 
 at a definite fixed temperature which is the lowest 
 freezing point in the series and no matter what 
 the original composition of the alloy, the part 
 solidifying at this constant temperature is uni¬ 
 form in composition. This is the “eutectic” al¬ 
 loy; it will be considered more fully later in this 
 paper. 
 
 Some students of alloys consider that at tem¬ 
 peratures between the first separation of solid and 
 the point at which the residue solidifies as a whole, 
 there does not exist a homogeneous condition in 
 the still liquid portions. They liken alloys within 
 this range to an emulsion, or conjugate solutions, 
 etc., simply because ultimately the mass solidifies 
 in a banded or laminated mass characteristic of 
 eutectics. It is not necessary, however, to con¬ 
 sider that the two solid solutions constituting the 
 ordinary eutectic existed as such in an emulsified 
 state above the eutectic point. To be sure, the 
 microscope reveals a very marked separation of 
 the alloy into two constituents (when there is any 
 eutectic), usually with characteristically laminated 
 structure, but these two components show no 
 tendency to separate into layers when the alloy is 
 kept for a long time just above its eutectic point; 
 on the other hand, the magnitude of the eutectic 
 structure is markedly influenced by the slowness 
 of cooling. Furthermore there is always an evo¬ 
 lution of heat during the freezing of a eutectic 
 
ALLOYS AS SOLUTIONS. 
 
 29 
 
 which is very decided and may well be accounted 
 for by such a marked molecular rearrangement as 
 the formation of a laminated structure from a 
 homogeneous mass would involve, and lastly, we 
 might cite some concrete evidence on this point which 
 was brought to my attention by Mr. William Camp¬ 
 bell. As is well known the constituent of steel 
 known as martensite which is a solid solution of 
 iron carbide in iron is capable of splitting up at 
 about 690° C. or nearly 8oo° C. below its freezing- 
 point into the beautifully characteristic structure 
 known as pearlite; accompanying this change is a 
 marked evolution of heat, the critical point An 
 of Osmond. If such a decided transformation 
 can take place in solid steel, it surely ought not 
 to be thought improbable that a similar one could 
 take place in an alloy at the instant of final solidi¬ 
 fication when the molecular freedom of the par¬ 
 ticles is doubtless many times as great as in the 
 example cited. 
 
 Molecular Depression of the Freezing-Point of 
 Metals .—In 1889, Prof. Ramsey determined the 
 molecular weight of many of the metals by the 
 method of measuring the change in vapor pressure 
 when certain known weights of solid metals were 
 dissolved in mercury. He also made determinations 
 at the boiling point of mercury. From these ex¬ 
 periments he states “that it would appear legiti¬ 
 mate to infer that in solution, as a rule, the atom 
 of a metal is identical with its molecule.” 
 
 Messrs. Heycock and Neville found that when 
 two metals were melted together, considering M 
 as solvent and N as the dissolved metal, then 
 
 (1) The freezing-point of M is lowered,—the 
 most frequent result. 
 
 (2) The freezing-point of M is raised, e. g., silver 
 in cadmium, and antimony in tin. 
 
 (3) The freezing-point is unchanged, e. g., thal¬ 
 lium in lead, and, I believe, within narrow limits, 
 silver in gold. 
 
 But these skillful investigators, to whom we are 
 
30 
 
 ALLOYS AS SOLUTIONS. 
 
 so much indebted for their careful researches upon 
 alloys, went further and showed that none of these 
 cases contradicted Van’t Hoff’s theory of solution. 
 This they did in each case by filtering off the part 
 first solidifying. In the first case the more fusible 
 filtrate was found to be richer in the dissolved 
 metal than were the first crystals. In the second 
 instance the crystals were richer in the dissolved 
 metal; that is, the excess of solvent was not first to 
 separate in the solid form but what was probably 
 a definite compound of the two metals having a 
 higher melting point than the metal M, which was 
 assumed to be the solvent. In the third case there 
 was absolutely no separation of the two metals 
 during cooling; that is, the first crystals and the 
 
 fluid part were identical in composition. That is, 
 the two metals in these instances torm Isomor- 
 phous mixtures. In all their experiments, Messrs. 
 Heycock and Neville were dealing with dilute so¬ 
 lutions in which, as in the case of salts dissolved in 
 water, it might be expected that the molecules of 
 the dissolved substance would obey the laws of 
 gases. When their experiments were conducted 
 quantitatively, Heycock and Neville found that two 
 of the empirical laws of Coppet and Raoult hold 
 good for alloys, viz: 
 
ALLOYS AS SOLUTIONS. 
 
 31 
 
 (1) For moderate concentration the fall of 
 freezing point is proportional to the weight of dis¬ 
 solved substance present in a constant weight of 
 solvent, and 
 
 i 7 
 
 (2) When the falls produced in the same solvent 
 by different metals are compared, it is found that 
 a molecular weight of a dissolved metal produces 
 
 + the same fall whatever the metal is. 
 
 In these experiments tin was the solvent and it 
 was assumed that the metals are monatomic in 
 solution or that their molecules are of one type,— 
 Rn^^when n is constant and probably equal to 1. 
 
 The third law is probably incorrect, for it as¬ 
 
 sumes that if a constant number of molecules of sol¬ 
 vent be employed, the fall is independent of the 
 nature of the solvent. The solvent in the case of 
 metals often tends to chemical combination with 
 the other metal. Thus gold in 100 atoms of sodium 
 gave 4 0 C. depression in the freezing-point; in tin, 
 3° C. and in potassium only i.8° C. 
 
 Freezing-point Curves .—One must not confuse 
 cooling-curves and freezing-point curves. The 
 
32 ALLOYS AS SOLUTIONS. 
 
 former, as already pointed out, result during the 
 cooling of a single metal or alloy and time and 
 temperature are the co-ordinates of such curves. 
 When a series of such curves are obtained for any 
 pair of metals mixed in all proportions from 
 o to ioo per cent M in ioo to o per cent N, 
 and the critical points are plotted in a diagram of 
 which the co-ordinates are temperature and com¬ 
 position we get what is known as a freezing-point 
 curve. That is, the points at which all possible 
 combinations of the two metals freeze are indicated 
 more or less accurately. The more points in the 
 curve actually determined, the more likely becomes 
 the accuracy of the other points. Subsidiary 
 points in the cooling-curves may also be plotted, 
 though they are not strictly speaking freezing- 
 
 Ax By Ax By' 
 
 points. Figure 6 is taken from a paper upon “La 
 Constitution des Alliages Metalliques ,, by Rob- 
 erts-Austen and Stansfield which was presented at 
 the Physical Congress, Paris, 1900. The vertical 
 co-ordinate is temperature and the horizontal one 
 represents both time (for cooling-curves) and 
 composition (for the freezing-point curve). It 
 shows graphically how a complete freezing-point 
 curve is constructed from cooling-curves. It will 
 be noticed that the metals A and B forming the 
 alloy give cooling-curves of the type of pure met¬ 
 als. In the curves showing a eutectic break, it 
 
ALLOYS AS SOLUTIONS. 
 
 33 
 
 will be noticed that the magnitude of the eutectic 
 break as compared with the upper or freezing- 
 point break becomes relatively greater as we ap¬ 
 proach the composition of the alloy c which is pure 
 eutectic and gives a cooling-curve identical in 
 form with those of A and B. In fact this ratio 
 gives a rough approximation of the proportion of 
 the alloy which consists of solid solution separating 
 at the first break and eutectic alloy crystallizing 
 at the fixed temperature indicated by the horizon¬ 
 tal part of the cooling-curve; or, in other words, 
 if the alloy consists mostly of solid solution and a 
 little eutectic, it will take the former longer to 
 solidify than it does the latter and the cooling- 
 curve indicates this roughly. Sometimes more 
 than two breaks occur in a cooling-curve, indicat¬ 
 ing changes, allotropic or otherwise, occurring in 
 the solid. 
 
 Freezing-point curves conform in general to three 
 types, as shown in Fig. 7. These curves are not 
 drawn exactly to scale, but are approximately cor¬ 
 rect. Gold and silver alloys form perfectly isomor- 
 phous mixtures, and their freezing points give almost 
 a straight line joining the freezing points of the pure 
 metals. Copper and silver give the typical curve of 
 two metals, which mix in all proportions, but do not 
 unite chemically. Antimony and copper unite to give 
 the highly colored compound known as regulus of 
 Venus, Cu 2 Sb. *This compound is indicated in the 
 cooling-curve by the intermediate summit. In gen¬ 
 eral such a summit will be found to occur at a for¬ 
 mula percentage, and indicates an intermetallic com¬ 
 pound. To explain such a curve, we have only to 
 consider it as made up of separate sections, as 
 indicated in the figure by a dotted line. The inter¬ 
 metallic compound has a melting point of its own, 
 quite independent of that of the constituent metals. 
 It may be higher, lower or intermediate as com- 
 
 *The formula usually given in the past was Cu 5 Sb2, but 
 Stead says the purple constituent is Cu 2 Sb. 
 
lioo a 
 
 34 ALLOYS AS SOLUTIONS. 
 
 100/ c 
 
ALLOYS AS SOLUTIONS. 
 
 35 
 
 pared with the constituent metals which compose it'. 
 In such a curve as the Sb-Cu one, we may consider 
 that one series of alloys is composed of mixtures of 
 Sb and Sb Cu 2 , and the other of Sb Cu 2 and Cu. 
 We are virtually dealing with two distinct series of 
 alloys, each of which taken separately is of the simple 
 type illustrated by the Ag-Cu curve. In the Sb-Sb- 
 Cu 2 portion of the curve there is no free copper, and 
 in Cu-Sb Cu 2 portion of the curve there is no free 
 antimony. Dissociation of the compound might make 
 some modification of the above statement necessary, 
 and, indeed, it has been suggested that dissociation 
 causes a summit to appear rounded instead of an¬ 
 gular. Curves containing more than one summit 
 may be similarly resolved into series of two com¬ 
 ponents, which may be respectively a metal and an 
 intermetallic compound, or, if between two summits, 
 the components are both intermetallic compounds. 
 An ideal representation of such a curve is shown in 
 Fig. 8. In this figure we see that alloys whose com¬ 
 positions fall between i and 2 consist of metal A and 
 compound Ax By; alloys between 2 and 3 consist of 
 components Ax By, and Ax' By'—they contain no 
 free A or B; alloys between 3 and 4 consist of com¬ 
 pound Ax' By' and metal B. Each of these pairs of 
 components has its melting point depressed by the 
 presence in it of the adjacent component. Our Cu- 
 A1 curve (Fig. 9) may be explained in this way: 
 Two summits occur at the compositions correspond¬ 
 ing to Cu 2 Ah and A 1 Cu 3 (48.49% Cu, and 87.6% Cu 
 respectively). Le Chatelier thinks he has detected 
 with the microscope at least four compounds. Our 
 curve gives no indication of them. Our own 
 microscopic study of these alloys is not yet com¬ 
 pleted. Our Al-Sb curve (Fig. 9) shows the pres¬ 
 ence of a compound whose melting point is more than 
 400° C. above that of either constituent. Its formula 
 is Sb A 1 (81.6% Sb). In this series of alloys we are 
 dealing with two pairs of constituents, Al-Al Sb and 
 A 1 Sb-Sb. The compound seems to be almost insolu¬ 
 ble in either of the pure metals, but on theoretical 
 
36 # ALLOYS AS SOLUTIONS. 
 
 grounds we should expect to find a slight depression 
 in the freezing point of each metal as indicated in 
 the curve. These points have not as yet been experi¬ 
 mentally detected. The tin-aluminum curve shows a 
 fall of 3 0 or 4 0 C. by the addition of 0.5% Al, while 
 with increased additions of aluminum the freezing 
 point is raised 300° C. at a concentration of 10% Al, 
 90% Sn. 
 
 FIG. 10. 
 
 If in a certain series of alloys we get a cooling 
 curve with a summit occurring at a formula percent¬ 
 age, and examine the alloys whose freezing points 
 give rise to that summit, we shall find that in general 
 an intermetallic compound in the pure state presents 
 under the microscopic a homogeneous mass made up 
 of crystals, all of which are the same. As we de- 
 
ALLOYS AS SOLUTIONS. 37 
 
 scend from the summit on either branch of the curve 
 we find these crystals becoming less and less in num¬ 
 ber and size, and at the next angle in the curve they 
 disappear entirely. This is illustrated by the accom¬ 
 panying photographs of antimony-aluminum alloys. 
 These microphotographs were made by Mr. Camp¬ 
 bell. 
 
 Fig. io.—20 per cent Sb, 80 per cent A 1 X 33 diam¬ 
 eters, oblique illumination, shows crystals of Sb A 1 
 in granular ground mass of nearly pure Al. 
 
 FIG. ii. 
 
 Fig. ii.—50 per cent Sb, 50 per cent Al X 33 diam¬ 
 eters, oblique. Crystals of Sb Al increasing in size 
 and amount, ground mass decreasing. 
 
 Fig. 12.—82 per cent Sb, 18 per cent Al X 16 diam- 
 
ALLOYS AS SOLUTIONS. 
 
 38 
 
 eters, verticle illumination. This photograph shows 
 nearly pure Sb Al. 
 
 Fig. 13.—85 per cent Sb, 15 per cent Al X 33 diam¬ 
 eters, vertical illumination. We have passed the sum¬ 
 mit of the curve; the ground mass of aluminum has 
 disappeared and some free antimony is seen. 
 
 Fig. 14.—95 per cent Sb, 5 per cent Al X 33 diam¬ 
 eters, oblique. Sb Al crystals diminishing in quan¬ 
 tity, and ground mass of Sb—probably containing a 
 little Sb Al in solid solution—is conspicuous. 
 
 Regarding this microscopic evidence, supporting 
 
 FIG. 12. 
 
 the pyrometric evidence, Mr. Neville says: ‘These 
 criteria taken together, (1) the occurrence of a sum¬ 
 mit at a formula percentage, (2) the presence of large 
 
ALLOYS AS SOLUTIONS. 39 
 
 crystals of the same kind, decreasing in amount as 
 we descend the branch on either side, are an absolute 
 proof of the reality of a compound.” 
 
 The Nature of Eutectics .—In several portions of 
 this paper eutectic alloys have been mentioned, a 
 term for which we are indebted to Dr. Guthrie, who 
 used it to designate the most fusible alloy of a series 
 —the one which freezes last. There are certain points 
 in which eutectics may differ, and upon these differ¬ 
 ences they may admit of classification. Every eutec¬ 
 tic, however, possesses the following properties: 
 
 1. It is of uniform composition in any one series 
 of alloys. 
 
 2 . Its freezing point is constant throughout any one 
 series. 
 
 3. The freezing point is the lowest in the series. 
 
 4. It is not a chemical compound. 
 
 The composition of a eutectic may and occasion¬ 
 ally does correspond to some simple atomic ratio. 
 This coincidence, though striking, does not prove the 
 presence of a compound as the sole constituent of a 
 eutectic. Usually it possesses a laminated micro¬ 
 structure, and often requires very high magnifica¬ 
 tion to detect the two constituents. Mr. Stead, in his 
 splendid paper upon iron and phosphorus, gives his 
 ideas upon the subject of eutectics, and in addition 
 to what has just been said as to the essentials of a 
 eutectic, he adds some remarks upon what a eutec¬ 
 tic may be: (i) “It may consist of two or more 
 metals which do not unite chemically, or (2) of a 
 metal and a definite compound (containing that 
 metal), or (3) possibly of two or more definite com¬ 
 pounds. (4) It may consist of a mixture of a solid 
 solution of one metal in another and a free metal. 
 (5) It may contain a solid solution of a definite 
 metallo-metallic salt (intermetallic compound) and 
 that same metallo-metallic salt in the free state. (6) 
 It may possibly consist of two solid solutions.” The 
 last of these statements, which he qualified by the 
 word “possibly,” seems to me to be the most impor¬ 
 tant of all; and since I am unwilling to admit that any 
 
40 
 
 ALLOYS AS SOLUTIONS. 
 
 metal or intermetallic compound ever separates ab¬ 
 solutely pure, those conditions in which Mr. Stead 
 speaks of the separation of pure substances seem to 
 me to be inaccurate. The form of the freezing point 
 curves and the explanation of them according to 
 Prof. Roozeboom renders it highly improbable that 
 any strictly pure material separates at the freezing 
 
 FIG. 13. 
 
 point of the eutectic. The most that we can say of 
 the constituents of a eutectic is that in certain cases 
 they are solutions of extreme dilution. In cases 
 where the freezing point curve lies almost entirely 
 above the melting point of either or both constituents, 
 but in which there is a slight depression of the freez¬ 
 ing point by very small additions of one metal to the 
 
ALLOYS AS SOLUTIONS. 
 
 41 
 
 other, as in the case of tin-aluminum, previously 
 mentioned, and when this depression is further indi¬ 
 cated in a well-marked eutectic break in other alloys 
 of the series, it may be possible that the eutectic is a 
 single solid solution. The quantity of dissolved metal 
 in such a eutectic might not be enough to change the 
 type of cooling curve seriously, and probably exceed¬ 
 ingly high magnification would fail to resolve such 
 an eutectic into two components, for it crystallizes 
 isomorphously with the pure tin, hence it seems that 
 no two juxtaposited constituents are there. This con¬ 
 ception of a eutectic is not at variance with the four 
 essentials already enumerated. While not speaking 
 
 FIG. 14. 
 
 positively on this point, I think it reasonable, and am 
 not sure but that in such a curve as the Sb-Sn curve, 
 where all the points seem to lie above the melting 
 point of tin, we must consider pure tin as the eutectic 
 —it melts lowest for the series, it is of constant com¬ 
 position, and it is not a chemical compound. 
 
 We have now indicated many points of resemblance 
 between metallic mixtures and ordinary solutions. 
 One metal diffuses into another like a salt into water; 
 like two liquids, they may be perfectly miscible or 
 form layers; the layers are not pure, but each con¬ 
 tains a little of the other in solution; in general, the 
 solubility increases with the temperatures. They will 
 
42 
 
 ALLOYS AS SOLUTIONS. 
 
 flow under pressure; they may or they may not react 
 chemically when brought into intimate association by 
 fusion or pressure; the molecular mobility increases 
 with the temperature; upon cooling of binary alloys 
 we observe phenomena strongly suggestive of the 
 freezing of salt solutions; the depression of the freez¬ 
 ing point of one metal when another is added follows 
 the laws of Coppet and Raoult, and the eutectic re¬ 
 minds us very much of the “cryohydrates” of ordi¬ 
 nary solutions. Scarcely another point of resem¬ 
 blance is needed but one is at hand, and a very im¬ 
 portant one, viz., the phase rule applies quite as well 
 to the explanation of conditions of equilibrium in 
 alloys as it does to the explanation of similar problems 
 in regard to liquid solutions. The classification of 
 the systems, however, requires some modification be¬ 
 fore the generalizations of Trevor apply, and it is 
 practically somewhat difficult to ensure complete 
 equilibrium in an alloy—the cooling is usually much 
 
 too rapid to allow of the establishment of equilibrium 
 in the solid mass. If equilibrium has been estab¬ 
 lished, then the number of distinct substances in the 
 mass will depend upon the number of constituents 
 which enter into the composition of the mass. In my 
 paper already referred to in the Journal of the Frank¬ 
 lin Institute, February, 1902, I have shown how the 
 
ALLOYS AS SOLUTIONS. 
 
 43 
 
 adaptation of the phase rule to alloys is accomplished, 
 and as this is more a chemical conception than one of 
 direct interest to engineers, it need not be dwelt 
 upon here. The phase rule does not tell us the num¬ 
 ber of phases in which one or two components may 
 exist, but how many of them may exist simultaneous¬ 
 ly in equilibrium. Thus pure iron may exist in three 
 solid, and one liquid phase, but never in all four at 
 one time. Again the iron-carbon alloys, according to 
 Roozeboom, may exhibit at least seven phases, viz., 
 carbon, alpha-, beta-and gamma-iron, liquid solution, 
 solid solution of carbon in gamma-iron, and cementite 
 or iron-carbide. The provisions of the phase rule, 
 however, tell us that only three of these may exist in 
 equilibrium at any given temperature and concentra¬ 
 tion. Professor Roozeboom’s explanation of a freez¬ 
 ing-point curve considered as an equilibrium curve is 
 of great interest in helping to make clear the series 
 of phenomena which are exhibited during the actual 
 solidification of molten alloys. His original explana¬ 
 tion was given in connection with the highly compli¬ 
 cated iron-carbon curves of Roberts-Austen. It has 
 seemed to me that a simpler figure would lend itself 
 more readily to this explanation and the accompany¬ 
 ing figure shows the type of curves which gold-copper 
 and copper-silver alloys give, i. e., pairs of metals 
 soluble in each other in all proportions but not form¬ 
 ing any compounds. In Fig. 15 the curve a e c repre¬ 
 sents the temperatures at which for each concentra¬ 
 tion solidification begins. The curve a b d c shows 
 the temperature at which solidification is complete. 
 Except for the straight portion of this curve, b d, the 
 exact position of it is a matter of conjecture. Accord¬ 
 ing to Stansfield, its position may be calculated with 
 some degree of accuracy upon theoretical grounds, if 
 the latent heat of fusion of the solvent is known and 
 assuming that the dissolved substance is monatomic 
 in the liquid state. The upper line, a e c Roozeboom 
 calls the “liquid” curve, and the lower line, a b e d c 
 he calls the “solid” curve. When in a series of alloys 
 
44 
 
 ALLOYS AS SOLUTIONS. 
 
 the liquid and solid curves have a maximum or 
 minimum point formed by the branches of the curve, 
 they touch at this point as in the curve before us. 
 The point in our diagram where the liquid and solid 
 curves meet is the eutectic point. It means that the 
 liquid represented by the composition corresponding 
 to that point solidifies as a whole at a single tempera- 
 ature. Curves similar to this may be used to explain 
 such complicated freezing-point curves as that of the 
 Al-Cu series, remembering that the terminii of the 
 liquid and solid curves are not necessarily pure met¬ 
 als, but may be a metal and a compound, or perhaps 
 two compounds. The more or less triangular areas, 
 a b e and c d e represent mixtures of liquid and solid 
 phases. The areas a b r f and c d g s are solid solu¬ 
 tions, and in the area b d f g we have a conglomerate 
 of eutectic alloy, with varying amounts of crystals of 
 solid solution depending upon the composition of the 
 alloy from which they were derived. At the single 
 composition e, however, we have an exception to this 
 statement, for this alloy consists of eutectic only, 
 while to the left of this the conglomerate consists of 
 eutectic plus solid solutions of N in M, and in alloys 
 represented by compositions to the right of e we 
 have eutectic (always constant in composition) plus 
 more or less of a solid solution of M in N. 
 
 Above the liquid curve we have only liquid phases; 
 below the solid curve we have only solid phases, while 
 intermediate areas correspond to a mixture of liquid 
 and solid phases. 
 
 Any alloy whose composition falls within the areas 
 marked solid solution gives but one break in the 
 cooling curve. Alloys represented by compositions 
 falling within the area of the eutectic (except alloy 
 e) give two breaks in the cooling-curves. The eutec¬ 
 tic itself consists of two solid solutions, whose com¬ 
 positions are indicated by compositions correspond¬ 
 ing to the extreme ends of the eutectic line, viz., 
 b and d. However, since the solubility of one metal 
 in another is a function of the temperature below 
 
ALLOYS AS SOLUTIONS. 45 
 
 its freezing point as well as above, we may assume 
 that the solubility in the two components of the eu¬ 
 tectic decreases slightly even after solidification, and 
 therefore the lines b f and d g are represented as 
 slanting rather than perpendicular to the composition 
 line. These lines indicate the maximum solubility of 
 either metal in the other at temperatures below the 
 freezing point of the eutectic. 
 
 Let us consider the phenomena of solidification of 
 an alloy represented by a composition on a e, as ex¬ 
 plained by Roozeboom. Any solution represented by 
 the composition m begins to solidify at that tempera¬ 
 ture by the separation of crystals of the composition 
 n. They therefore contain less of N than the liquid 
 alloy m. The residual alloy has therefore become en¬ 
 riched in N, and its freezing point is lowered; its 
 composition passes thus from m to p, at which point 
 solidification is complete, for at this point the tem¬ 
 perature has fallen till it encounters the solid curve 
 at the point o. The point o, moreover, indicates the 
 composition of the last crystals to solidify. That is, 
 while the liquid solution is changing from m to p, the 
 composition of the crystals has changed from n to o. 
 Complete solidification has thus taken place through 
 an interval, m o. Now there is an alloy whose final 
 solidifying point is b ; and according to Roozeboom’s 
 explanation it is an alloy of composition, e, which is 
 freezing. At the same temperature, however, occurs 
 the solidification of the solid solution of composition 
 d. Finally, then, all alloys represented by composi¬ 
 tions between i and 2, after separating out crystals of 
 solid solutions of N in M, or M in N, represented by 
 compositions along the lines 3, 1 and 2, 4, respectively, 
 become concentrated to the composition e, where sat¬ 
 urated solid solutions of compositions b and d solid¬ 
 ify side by side, constituting the eutectic. That is, 
 an alloy the vertical projection of whose composition 
 upon the solid curve does not cut the eutectic line, 
 consists of a single solid solution. An alloy whose 
 composition projected vertically intersects the eutec¬ 
 tic line, consists of a conglomerate of eutectic of con- 
 
ALLOYS AS SOLUTIONS. 
 
 46 
 
 stant composition e and a solid solution, whose com¬ 
 position is represented by the horizontal projection of 
 its composition upon the corresponding solid curve. 
 At temperatures and concentrations lying within the 
 areas enclosed by the liquid and solid curves there 
 exists in equilibrium solid and liquid solutions, e. g., 
 at a temperature and concentration represented on the 
 line m n there exists simultaneously liquid solution 
 m and solid solution n. 
 
 Great as has been the progress in alloys research 
 within the past few years, yet much remains to be 
 done. When many sets of complete freezing-point 
 curves have been determined, when all the alloys of 
 these series have been examined metallographically, 
 and when their ordinary physical tests, tensile strength, 
 electrical and heat conductivity, specific gravities, etc., 
 have been accurately worked out, then we may be able 
 to generalize and to predict properties of new pairs of 
 metals as well as to set out intelligently to 
 produce new and useful alloys. It must be admitted 
 that the production of alloys in the past has been 
 by the very wasteful method of chance. I hope to see 
 the manufacture of alloys become an exact science and 
 that ultimately even ternary or more complex alloys 
 may be brought within the range of applicability of 
 “natural laws,” which as yet remain undiscovered. 
 We are making progress in the right direction, but 
 American scientists have not yet assumed their due 
 share of this task. It is time that we followed the 
 example of the British association, the Societe d’En- 
 couragement pour l’lndustrie Nationale, the Institu¬ 
 tion of Mechanical Engineers and the National Physi¬ 
 cal Laboratory of Great Britain, in promoting the 
 scientific study of problems in connection with alloys. 
 
rS 
 
 'r 
 
 
*