:: :.. W . . PS ... FEE 64 UNCLASSIFIED ORNL -- : W 556 2 1 . G 2 ie 1, 47 *! i'o O © ©n? oooooo ORNL-P o5r6 CONF-711-3 0716: 3 6472 : OCT 30 Note: This 18 & draft of a paper which was presented at the International Conference on Diffusion in Body-Centered Cubic Materials, Gatlinburg, Tennessee, on September 18, 1964. Contents of this paper: should not be quoted nor referred to without permission of the author. -I1OAL NOTICI - the mount werd und wo rd met helt , ww , wo wote Online A. W o rsten, die warm the meer murheten, mert min men w ho a more , and more od Hope meg med at tratta di un an Where the modern dem with a , o mo wo d erne per t er. THE.EFFECT OF IRRADIATION ON PRECIPITATION OF NITRIDES IN IRON J. T. Stanley SOLID STATE DIVISION OAK RIDGE NATIONAL LABORATORY Operated by UNION CARBIDE CORPORATION for the U. S. Atomic Energy Commission Oak Ridge, Tennessee July, 1964 TIE EFFECT OF IRRADIATION ON PRECIPITATION OF NITRIDES IN IRON* J. T. Stanley Solid State Division, Oak Ridge National Laboratory Oak Ridge, Tennessee ABSTRACT Internal friction measurements were used to study the effect of irradiation on the precipitation of nitrogen from solid solution in Q-Fe. Although irradiation does not change the jump rate of nitrogen in a-Fe, the rate of precipitation is attributed to the formation of nucleation sites during the irradiation. The enhancement effect saturates after a fast neutron dose of 3 x 2015 neutrons/cm. 2 Bombardmerit with 2 MeV.electrons to a dose of 18x 2016 electrons/cm? did not produce any enhanced nucleation. At the present time it 16 believed that the radiation-produced nucleation sites are vacancy of interstitial clusters. - -- - Research sponsored by the U. 3. Atomic Energy Commission under contract with Union Carbide Corporation. . ; www.**.***. CA . '? WIN. R , . I .. " . . .. . . .. . . . . .2. THE EFFECT OF IRRADIATION ON PRECIPITATION OF NITRIDES IN IRON J. T. Stanley Solid State Division, Oak Ridge National Laboratory Oak Ridge, Tennessce Wagor.blast and Demask have shown that neutron irradiation increases the rate at which carbon 16. removed from supersaturated solid solution. - According to their interpretation two different reactions occur; (1) carbon atoms are trapped by individual point defects produced by the Irradiation, (2) irradiation produces nucleation sites for precipitation of the metastable carbide. In order to observe the first effect the concentration of point defects must be comparable to the concentration of carbon atoms in solution which is of the order of 10/cm”. Wagonblas't and Damask irradiated to high doses at low temperature and studied the trapping of carbon atoms at vacancies. They also made a few measurements which indicated that the second effect predominates when the number of primary knock on events is comparable to the number of particles in the usual aging experiment, i.e. 10°/cm. . Our experiments on the effect of irradiation on precipitation of nitrides in iron have been carried out at doses such that the number of . point defects produced by the irradiation is much smaller than the number of nitrogen atoms so that only the enhanced nucleation of precipitates should be observed. Research sponsored by the U. 3. Atomic Energy Commission under contract with the Union Carbide Corporation, .. . .. - : - - . . ., -3- · Experimental Procedure and Results i Specimens used in this investigation were cut from 0.005 in. thick Ferrovac E foil which had been cold rolled from 1 1/2 in. diameter bar. The specimens were de carburized by heating in moist hydrogen at 720ºC for 72 hr. The de carburized specimens were then heated to 950°C for 1 hr. in a dry hydrogen atmosphere and slow cooled (180°C per bour) to 590°C to obtain a large grain size. The specimens were nitrided at 590°C using a mixture of armonia and hydrogen gas. The gas mixture was adjusted, with the aid of data given by Darken and Gurry!, to give a nitrogen content of about 0.015 wt% nitrogen. The nitrided specimens were quenched in water from 590°C to retain the nitrogen in solid solutior.. : The amount of nitrogen in solution was de termined by measuring the height of the nitrogen internal friction peak. We assumed the constant T. relating nitrogen concentration in weight per cent to internal friction peak height to be unity. The internal friction measurements were made on specimens 1 1/2 in. x 3/16 in. x .005 in. vibrating in flexare at approximately 33 c.p.s. The nitrogen peak occurs at 60°C for this vibration frequency. The apparatus used for the neutron irradiations was designed to fit into the low temperature irradiation facility in Hole 50 of the Oak Ridge this apparatus the specimen could be cooled rapidly enough to retain nitrogen in solution after annealing at 400°C. This treatment will be referred to as a ra-solution anneal' as it restores the original nitrogen .: Internal friction peak after an aging treatment at some lower temperature. ::: The re-solution anneal 18 carried out by beating the specimed to 400°C j maroon ... in vacuo and then cooling rapidly by simultaneously turning off the heater current and admitting hellum gas into the chamber. Out-of-reactor experiments in a mockup of the reactor facility showed that essentially all of the nitrogen could be retained in solution if the specimen cooled from 400°C to 50°C in about 6 minutes. To achieve this cooling rate in the mockup it was necessary to cool the chamber walls with dry ice. 'In the low temperature irradiation facility the chamber walls were kept cool by the flow of cold helium gas through the heat exchanger, Even though the specimen 18 cooled fast enough to retain all of the nitrogen in solution as detected by the internal friction peak, it is still possible for there to be differences in the number of nucleation sites for precipitation for different cooling rates. However, studies of the aging of the nitrogen peak at 65°C showed little difference between the specimen as water quenched from 590°C and gas quenched from 400°C as described above. These two sets of data are shown in Fig. 2 along with the data for this specimen after various amounts of irradiation at low temperature, . .. . The results presented in Fig. 2 represent measurements made on two . . . . - - . * * specimens of about the same nitrogen content. The aging run after the 23 day irradiation was made on one specimen which will be called Specimen A and the rest of the aging runs were for another specimen which will be called Specimen B. The measurements for Specimen B were made in the order listed in the caption, and, except for the initial water quench, each aging run was preceded by a 400°C re-solution anneal as described above. The results presented in Higi 2 show that itradiation does increase the rate I ' ......amina mamilionit -5. of nitrogen precipitation, and that the 400°C re-solution anneal removes most of the effect of the irradiation. A comparison of the aging runs made after the 23 day irradiation and the 3 day irradiation shows that the time for half completion of the reaction 18 about the same in both cases. Thus there 18 a saturation of the enhanced precipitation effect after a relatively low dose. This saturation occurs we believe after a l day low temperature irradiation, which corresponds to a fast neutron dose of 3 x 10+) neutrons/cm². This conclusion 18 based on results obtained on Specimen A and shown in Fig. 3. In this case internal friction measurements were made at a series of temperatures upon warming after a 1 day irradiation at low temperature. It was noted that considerable precipitation of nitrogen occurs during the time required to warm up and cool down in the temperature range of the nitrogen peak. It is possible to construct an effective isothermal aging curve from these data using the known shape of the nitrogen internal friction peak, the known activation energy for nitrogen diffusion and the time versus temperature curve for the experiment. This was done and the results are shown in Fig. 4. Thus it seems that a 1 day irradiation 16 just as effective as a 23 day irradiation in increasing the rate of nitride precipitation. In addition to the above neutron irradiations, the aging of a specimen was studied after irradiation with 2-Mev electrons. In this case the specimen was irradiated at a temperature of about o°C for 30 minutes at a current density of 2 uamp/cm and then placed in a flexure pendulum apparatus for measurement. The aging of the nitrogen internal friction peak for this sample before and after Irradiation 18 shown in Nig. 5. . . We see that the electron irradiation did not change the aging kinetics. The calculated number of point defects produced by this irradiation would give a concentration of 1.3 x 106 atomic fraction. This is approximately the same concentration of defects that would be produced by the 1 day neutron irradiation. Discussion Transmission electron microscope observations by Ken and Wreidt have shown that nitrides precipitate preferentially on dj.slocations, but for high supersaturations nitride particles also precipitated uniformly in the matrix. It is not known if the nitride particles which precipitated in the matrix did so on some lattice defect which was not visible in the electron microscope, but Leslie* who found similar effects for precipita- tion of carbides in iron suggested that individual lattice vacancies might act as nucleation sites for carbide precipitation. Various impurity atoms in the iron matrix might also act as nucleation sites for nitride pre- cipitation since it has been shown that certain elements added to iron in small amounts can greatly change the rate of nitride and carbide precipi- tation.",° The irons used for the precipitation studies typically contain about 10 ppm of various impurities. This amount of impurity would be sufficient to account for the particle density usually observed in the precipitation experiment. The point of the preceding discussion 18 just to show that carbide and nitride particles require some sort of defect for nucleation and so it is not surprising to find that the defects produced by irradiation can . form structures which act as nucleation sites. . The surprising feature of S . * our data is that the enhanced nucleation saturates after a very low : !. . . irradiation dose of about 1075 neutrons/cm2. This 18 far below the dose at which radiation damage as measured by other properties shows saturation effects. The lack of erlianced nucleation after electron irradiation 18 a very significant fact as it immediately narrows down the field of possible radi- was 'n ation produced nucleation sites. As we know from the present theories of radiation damage, neutron irradiation produces highly damaged regions in the lattice with clusters of many vacancies anu Interstitials whereas electron irradiation produces only isolated interstitial vacancy pairs. It is known that the fraction of interstitials and vacancies which com o . a recombine is higher for electron irradiation as compared to reutron irradiation, but even so, a significant number of vacancies and inter- stitials are left to migrate to other traps. Since we did nut observe enhanced nucleation after electron irradiation, we are forced to the - m - conclusion that the irradiation produced nucleation sites are clusters of A vacancies or interstitials. :. The electron irradiation experiment eliminated from consideration as - - nucleation sites several possibilities which seemed capable of explaining the saturation at low doses. These potential nucleation sites would be produced by the combination of an existing lattice defect with a radiation produced defect. Some examples of these possibilities are jogs on dis- locations and the combination of vacancies with substitutional impurity atoms. Since the experiments indicate that the nucleation sites are clusters of vacancies' or interstitials, we must conclude that the saturation erfect 18 related to some property of the solid solution and the precipi. tate that prevents finer division of the precipitate. *** - TV1,'. -8. J. W. Cahn' assumed a model for nucleation on a dislocation in which the free energy of the nucleus consisted of the sum of three terms, a strain energy term, a surface energy term, and a volume energy term. According to this model, growth of a particle on the dislocation lowers the strain energy of the dislocation so that for small particle diameters the free energy decreases rapidly with increasing diameter. As the particle grows the surface energy of the particle increases and may cause the total free energy to rise again. Finally, at very large diameters, the volume energy term predominates and the free energy decreases again. Whether or not there 18 a minimum in the free energy versus particle diameter curve depends on the relative magnitudes of the three energy terms. Now we can imagine that in a system with a large number of nucleation sites, particles at first start growing on all of the sites because of the rapid decrease in the strain energy. It may be that because of the large number of such sites the solution becomes depleted of solute atoms to such an extent that the volume free energy term is substantially reduced. In this case the free energy versus particle diameter curve will have a bump whereas for a smaller initial number of particles it would not have iad a bump. At this time we have not been able to carry the model to the point of making calculations to determine 18 it could account for our results so it is offered just as a suggestion to explain the peculiar saturation effect. . , CONCLUSION Neutron irradiation of supersaturated iron nitrogen solutions causes :::: an enhancement in the rate of precipitation of nitrides from solution. . . The enhanced rate of precipitation is caused by nucleation sites produced by the irradiation. Lack of an enhanced rate of precipitation after electron Irradiation shows that individual lattice defects cannot produce the nucleation sites for precipitation. ACKNOWLEDGMENTS It is a pleasure to acknowledge the help of W. E. Brundage in making some of the internal friction measurements and in building parts of the apparatus. Much thanks 18 due Monroe Wechsler for many suggestions and discussion relating to this work. . ..... -•. -.-.-.-.-.-.-.-. -..-.-.-. cer --------- -- - -10 REFERENCES lo H. Wagonblast and A. C. Damask, "Kinetics of Carbon Precipitation in Irradiated Iron," J. Phys. Chem. Solids 33, 221 (1962). 2. Darken, L. S., and R. W. Gurry, Physical Chemistry of Metals, p. 377, McGraw Hill, New York (1953). 3. Keh, A. S. and Wriedt, H. A., "An Electron Transmission Study of Nitride Precipitation in Alpha Iron" Trans. A.I.M. E. 224, 560 (1962). ..... Leslie, W. C. 'The Quench-Aging of Low-Carbon Iron and Iron-Manganese Alloys: An Electron Transmission Study; " Acta Met. 2, 1004 (1961). ...... Dijkstra, L. J., and R. J. Sladek, "Effect of Alloying Elements on the Behavior of Nitrogen in Alpha Iron," Journal of Metalo 5, 69 (1953). 6. Wert, C. A., "Diffusion and Precipitation of Carbon in Some Alloys of i Iron, " Journal of Metals 4, 602 (1952). 7. Cann, J. W., "Nucleation on Dislocations," Acta Met. 3, 169 (1957). -11. FIGURE CAPTIONS Figure 1 Approximate Scale Drawing of In-Reactor Internal Friction Apparatus. Figure 2 Erfect of various Meutron Irradiations on Aging of the Nitrogen Peak at 65°C. Figure 3 Figure 4 In-Reactor Flexure Pendulum Measurements at 33 cps. Comparison of the Effect of 1 and 23 Day Irradiailons on Aging of the Nitrogen Peak at 65°C. Figure 5 Effect of Electron Irradiation on the Aging of the Nitrogen Peak at 65°c. ------ ---- . -------------- ofig: 1 UNCLASSIFIED ORNL- DWG 63-7541 WA . w ere CAPSULE SOLENOID WINDING HEATER WINDING- ........ SPECIMEN- UNIT TI . 11 . . nr. . VANE ..... .. DRIVING ELECTRODE — INSULATOR .... CAPACITANCE PLATES / METAL SPACER : INSULATORS Fig. 2 UNCLASSIFIED ORNL-DWG 64-813A . O UNIRRADIATED, WATER QUENCH FROM 590°C O UNIRRADIATED, IN SITU GAS QUENCH FROM 400°C A IRRADIATED 7 hr AT <-120°C AND AGED DURING IRRADIATION : • IRRADIATED 3 DAYS AT <-120°C VIRRADIATED 23 DAYS AT <-120° C AND AGED DURING IRRADIATION . • POST IRRADIATION ANNEAL AT 400°C AND IN SITU GAS QUENCH - FRACTION OF NITROGEN REMAINING IN SOLUTION -- - - - . - 101 -------- 2 5 10² 2 .5 103 2 5 ... 109 , AGING TIME AT 65°C (min) *** Aging of the Nitrogen peak in Iron - Nitrogen Alloy at. 65°C. UNCLASSIFIED ORNL-DWG 63-4538R 4* 10 20 TEST TEMPERATURE (°C) .. 30 40 50 60 70 80 90 100 0.016 0.014 0.012 0.010 0-INTERNAL FRICTION A A SPECIMEN QUENCHED FROM 590 °C, IRRADIATED 1 DAY AT T<-120 °C, AND TESTED WITH REACTOR ON o o SAME SPECIMEN AFTER 6 DAY IRRADIATION HEATED TO 400 °C, • FAST COOLED, AND TESTED WITH REACTOR OFF O A HEATING . A COOLING 0.008 0.006 : 0.004 0.002 - A. 3.8 3.6 3.4 IK) .... .............. ......... ...... OLO 3.2 2.8 2.6 ...... .... . 100% (OK) ............. Internal Friction vs Test Temperature for Iron-Nitrogen Alloy. In-Pile Flexure Pendulum Measurements at 33 cps. . I'. ! UNCLASSIFIED ORNL-DWG 63-6902 tomone FRACTION OF NITROGEN REMAINING IN SOLUTION وعمم LO UNIRRADIATED A IRRADIATED 1 day AT TS-120°C LO IRRADIATED 23 days AT TS-120°C امممممم 1000 ... .......... ...- ................... 10 . 100.. . TIME AT TEST TEMPERATURE ( min ) -- Aging of the Nitrogen Peak in Iron-Nitrogen Alloy at 65°C. . . . . . . .. . ... .. . UNCLASSIFIED ORNL-DWG 64-5915 0.024 0.020 0.016 - Q" ** -- 1-A ** . - . 0.012 . - .- ------- iyo 0.008 O ELECTRON IRRADIATED. 1.8 X1016 2 Mev ELECTRONS/cm? 0.004 LO UNIRRADIATED tito 10. 1000 100 7, AGING TIME AT 65°C (min) i.ax sol :. HA S * 13 ENT . : wie 1. I - . - KUR IN 2.1 11. NA . .- ,' . . . . 6 c w XN . DATE FILMED 16/23/65 -LEGAL NOTICE – This report was prepared as an account of Government sponsored work. Neither the United Statas, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or yohulness of the information contained in this report, or that the wae of any information, apparatus, method, or procars Colound in this roport may not infringe privately owned rights; or B. Assum.. nay liabilities with roroct to the use of, or for damage resulting from the use of cay Information, apparatus, mothod, or procesu discloud in this report. As used in the above, “por son noting on beball of the Commission" includes any om- ploys or contractor of the Commission, or oployee of such contructor, to the extent that such employs or contractor of the Commission, or employs of such contractor preparos, dissemiastou, or provides access to, any information pursuaat to bio employmeat or contract with the Commission, or his employment with such contractor, END ........ 16 . . . 1