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Reply to comments on “ordering in CuAu”
Scripta METALLURGICA
Vol. 4, pp. 865-868, 1970 Printed in the United States
Pergamon Press,
REPLY TO COMMENTS ON "ORDERING IN CuAu" B. Ramaswami Department of Metallurgy and Materials Science, University of Toronto, Toronto 5, Ontario, Canada.
(Received September
24, 1970)
I appreciate the comments of Arunachalam (1) on the paper "Ordering in CuAu" by Chandra and Ramaswami (2). Due to an oversight, we missed the work of Arunachalam and Cahn (3) on this subject. I would like to make the following points on the questions raised by Arunachalam (1). The first point of controversy is on the variation of hardness with ordering time during isothermal ordering of CuAu. T h e r e is general agreement (1-4) that the hardness increases with ordering time on isothermal ordering of CuAu at 150°C. With increasing ordering time, hardness has been observed to decrease after reaching a maximum on isothermal ordering at 300°C by Nowack (4) and at 240"C and 340°C by Arunachalam and Cahn (i, 3). Such a decrease in hardness has not been observed by Chandra and Ramaswami (2) at long ordering times on isothermal ordering at 220 ° and 300°C. The reason for this discrepancy in the variation of hardness with ordering time in the temperature range 220"C to 340°C is not clear at present. As our study was confined to electrical resistivity measurements and a limited amount of metallographic investigation, we do not have a satisfactory explanation for this discrepancy in experimental results. The second point of controversy is on the time required for complete ordering at 300"C. On the basis of microhardness and electrical resistivity measurements it was claimed (2) that there is complete ordering after fifty hours at 300°C for a prior disordering temperature of 650°C. The discrepancy in experimental results may be due to the effect on ordering of the prior disordering temperature. The rate of ordering increases with increasing prior disordering temperature (5, 6). For example, on isothermal ordering at 300"C an electrical resistivity of 5.00 micro-ohm-cm at which stage ordering is almost complete - is reached after ordering times of 11 hours, 15 hours and 40 hours respectively after prior disordering at 850"C, 650"C and 450°C. Thus, the time for complete ordering can vary over a wide range depending on the prior disordering temperature. These results are in agreement with those of Kuczynski et al (6). The third point of controversy is on the proposed model (2) for ordering. Step I in the ordering sequence was considered to be the nucleation of ordered plate-like CuAu I coherent with the disordered matrix (2, 7). However, Mannan and Arunachalam (8) have shown that at 150°C ordering occurs most probably by a homogeneous mode and at 250°C ordering starts by a homogeneous mode
865
Inc
866
REPLY TO COMMENTS ON "ORDERING
IN CuAu"
Vol.
4, No.ll
and changes over to a heterogeneous mode very quickly. But, the activation energy for ordering in the initial stages of ordering in the temperature range where the possibility of ordering by the homogeneous mode exists, viz around 150°C (8), is not available at present. Kuczynski et al (6) have determined the activation energy for the initial stages of ordering in the temperature range 240 ° to 360°C where ordering occurs by the heterogeneous mode. Kuczynski et al find that the initial stage of ordering is affected by the supersaturation of vacancies and short range order (6) present in the quenched material. They report an activation energy of 0.44 to 0.65 eV/atom depending on the prior disordering temperature. The discussion in reference (2) was to this aspect of ordering in CuAu. The proposed model suggests step (4) for the ordered region growing at the expense of the disordered matrix. The disordered region was identified by optical metallographic studies and microhardness measurements. The disordered region was characterized as the region free from "ripples" and "cross-ripples" (9) when viewed under an optical microscope. The microhardness of such regions was the same as the microhardness on the samples quenched from the disordering temperature (5). X-ray diffraction techniques were used to identify the disordered region without much success. Although quantitative metallography was not used in this study (2) the metallographic observations indicate that the volume fraction of the disordered regions decrease with increasing ordering time on isothermal ordering at 300"C. I agree with Arunachalam's comment (i) that the above observations contradict the conclusions of Smith and Bowles (i0) that "the tetragonal phase does not form at the expense of cubic phase by the advancement of a well-defined interface'. I agree that our observations have to be substantiated by more refined experiments. The activation energy for later stages of ordering where either step III or step IV of the proposed model would be ratecontrolling has been determined by measurements of electrical resistivity over the temperature range 150°C to 3 0 0 " C . It has b e e n shown that the activation energy for the later stages of ordering is determined by a single r a t e - c o n t r o 1 ~ - ~ p r o c e s s over this temperature range and is independent of the prior disordering temperature. This result indicates that the process occurring in the initial stages of ordering, viz, whether the ordering occurs by the homogeneous mode or the heterogeneous mode, do not affect the activation energy of the rate processes occurring in the later stages of ordering. There also appears to be some evidence at - ~ t h e activation energy of 0.4 eV/atom reported in reference (2) is for step III rather than for step IV of the sequence in the proposed model. Khobaib and Gupta (ii) have determined the activation energy for diffusion of copper in ordered CuAu to be 0.46 eV. This would indicate that the activation energy of 0.4 eV/atom determined from the kinetics of ordering during the later stages of ordering maybe for step III in the propsoed model, v l z , ~ n t i p h a s e domain growth. Finally, the comments of Arunachalam (i) do not alter the main conclusions of our paper (2), i.e. "there are three ordering
Vol. 4, No. II
REPLY TO COMMENTS ON "ORDERING IN CuAu"
867
stages with activation energies 0.4 eV/atom (Step I ), 2 eV/atom (Step Xl), 0.4 eV/atom (step fix or IV), for the disorder--~ order transformation in CuAu'
REFERENCES
(1)
V.S. Arunachalam, Scrlpta Met. 4,
(1970).
(2)
T. Chandra and B. R~maswami, Scripta Met. 4, 175
(3)
V.S. Arunachalam and R.W. Cahn, J. Mat. Science, 2, 160
(4)
L. Nowack, Zeit. Metallk. 22., 94
(5)
T. Chandra, M.A. Sc. Thesis, University of Toronto
(6)
G.C. Kuczynski, R.F. IIochman, and M. Doyama, J. Appl. Phys. 26, 871 (1955).
(7)
M. Hirabayashi and S. Weissmann, Acta. Met. 100, 25 (1962).
(8)
J.L.O'Brien and G.C. Kuczynski, Acta. ~et. 7, 803
(9)
D. Harker, Trans. ASM, 32, 210
(1970). (1967).
(1930). (1969).
(1959).
(1944).
(10)
R. Smith and J.S. Bowles, Acta Met. 8, 405
(1960).
(11)
M. Khobaib and K.P. Gupta, Scripta Met. 4, 605
(1970).
Vol. 4, pp. 865-868, 1970 Printed in the United States
Pergamon Press,
REPLY TO COMMENTS ON "ORDERING IN CuAu" B. Ramaswami Department of Metallurgy and Materials Science, University of Toronto, Toronto 5, Ontario, Canada.
(Received September
24, 1970)
I appreciate the comments of Arunachalam (1) on the paper "Ordering in CuAu" by Chandra and Ramaswami (2). Due to an oversight, we missed the work of Arunachalam and Cahn (3) on this subject. I would like to make the following points on the questions raised by Arunachalam (1). The first point of controversy is on the variation of hardness with ordering time during isothermal ordering of CuAu. T h e r e is general agreement (1-4) that the hardness increases with ordering time on isothermal ordering of CuAu at 150°C. With increasing ordering time, hardness has been observed to decrease after reaching a maximum on isothermal ordering at 300°C by Nowack (4) and at 240"C and 340°C by Arunachalam and Cahn (i, 3). Such a decrease in hardness has not been observed by Chandra and Ramaswami (2) at long ordering times on isothermal ordering at 220 ° and 300°C. The reason for this discrepancy in the variation of hardness with ordering time in the temperature range 220"C to 340°C is not clear at present. As our study was confined to electrical resistivity measurements and a limited amount of metallographic investigation, we do not have a satisfactory explanation for this discrepancy in experimental results. The second point of controversy is on the time required for complete ordering at 300"C. On the basis of microhardness and electrical resistivity measurements it was claimed (2) that there is complete ordering after fifty hours at 300°C for a prior disordering temperature of 650°C. The discrepancy in experimental results may be due to the effect on ordering of the prior disordering temperature. The rate of ordering increases with increasing prior disordering temperature (5, 6). For example, on isothermal ordering at 300"C an electrical resistivity of 5.00 micro-ohm-cm at which stage ordering is almost complete - is reached after ordering times of 11 hours, 15 hours and 40 hours respectively after prior disordering at 850"C, 650"C and 450°C. Thus, the time for complete ordering can vary over a wide range depending on the prior disordering temperature. These results are in agreement with those of Kuczynski et al (6). The third point of controversy is on the proposed model (2) for ordering. Step I in the ordering sequence was considered to be the nucleation of ordered plate-like CuAu I coherent with the disordered matrix (2, 7). However, Mannan and Arunachalam (8) have shown that at 150°C ordering occurs most probably by a homogeneous mode and at 250°C ordering starts by a homogeneous mode
865
Inc
866
REPLY TO COMMENTS ON "ORDERING
IN CuAu"
Vol.
4, No.ll
and changes over to a heterogeneous mode very quickly. But, the activation energy for ordering in the initial stages of ordering in the temperature range where the possibility of ordering by the homogeneous mode exists, viz around 150°C (8), is not available at present. Kuczynski et al (6) have determined the activation energy for the initial stages of ordering in the temperature range 240 ° to 360°C where ordering occurs by the heterogeneous mode. Kuczynski et al find that the initial stage of ordering is affected by the supersaturation of vacancies and short range order (6) present in the quenched material. They report an activation energy of 0.44 to 0.65 eV/atom depending on the prior disordering temperature. The discussion in reference (2) was to this aspect of ordering in CuAu. The proposed model suggests step (4) for the ordered region growing at the expense of the disordered matrix. The disordered region was identified by optical metallographic studies and microhardness measurements. The disordered region was characterized as the region free from "ripples" and "cross-ripples" (9) when viewed under an optical microscope. The microhardness of such regions was the same as the microhardness on the samples quenched from the disordering temperature (5). X-ray diffraction techniques were used to identify the disordered region without much success. Although quantitative metallography was not used in this study (2) the metallographic observations indicate that the volume fraction of the disordered regions decrease with increasing ordering time on isothermal ordering at 300"C. I agree with Arunachalam's comment (i) that the above observations contradict the conclusions of Smith and Bowles (i0) that "the tetragonal phase does not form at the expense of cubic phase by the advancement of a well-defined interface'. I agree that our observations have to be substantiated by more refined experiments. The activation energy for later stages of ordering where either step III or step IV of the proposed model would be ratecontrolling has been determined by measurements of electrical resistivity over the temperature range 150°C to 3 0 0 " C . It has b e e n shown that the activation energy for the later stages of ordering is determined by a single r a t e - c o n t r o 1 ~ - ~ p r o c e s s over this temperature range and is independent of the prior disordering temperature. This result indicates that the process occurring in the initial stages of ordering, viz, whether the ordering occurs by the homogeneous mode or the heterogeneous mode, do not affect the activation energy of the rate processes occurring in the later stages of ordering. There also appears to be some evidence at - ~ t h e activation energy of 0.4 eV/atom reported in reference (2) is for step III rather than for step IV of the sequence in the proposed model. Khobaib and Gupta (ii) have determined the activation energy for diffusion of copper in ordered CuAu to be 0.46 eV. This would indicate that the activation energy of 0.4 eV/atom determined from the kinetics of ordering during the later stages of ordering maybe for step III in the propsoed model, v l z , ~ n t i p h a s e domain growth. Finally, the comments of Arunachalam (i) do not alter the main conclusions of our paper (2), i.e. "there are three ordering
Vol. 4, No. II
REPLY TO COMMENTS ON "ORDERING IN CuAu"
867
stages with activation energies 0.4 eV/atom (Step I ), 2 eV/atom (Step Xl), 0.4 eV/atom (step fix or IV), for the disorder--~ order transformation in CuAu'
REFERENCES
(1)
V.S. Arunachalam, Scrlpta Met. 4,
(1970).
(2)
T. Chandra and B. R~maswami, Scripta Met. 4, 175
(3)
V.S. Arunachalam and R.W. Cahn, J. Mat. Science, 2, 160
(4)
L. Nowack, Zeit. Metallk. 22., 94
(5)
T. Chandra, M.A. Sc. Thesis, University of Toronto
(6)
G.C. Kuczynski, R.F. IIochman, and M. Doyama, J. Appl. Phys. 26, 871 (1955).
(7)
M. Hirabayashi and S. Weissmann, Acta. Met. 100, 25 (1962).
(8)
J.L.O'Brien and G.C. Kuczynski, Acta. ~et. 7, 803
(9)
D. Harker, Trans. ASM, 32, 210
(1970). (1967).
(1930). (1969).
(1959).
(1944).
(10)
R. Smith and J.S. Bowles, Acta Met. 8, 405
(1960).
(11)
M. Khobaib and K.P. Gupta, Scripta Met. 4, 605
(1970).