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Comments on “An investigation of the precipitation-hardening process in aluminum alloy 2219 by means of sound wave velocity and ultrasonic attenuation”
Materials Science and Engineering, 57 (1983) 261-262
261
Correspondence C o m m e n t s o n " A n investigation o f the precipitation-hardening process in aluminum alloy 2 2 1 9 b y m e a n s o f s o u n d wave v e l o c i t y and ultrasonic a t t e n u a t i o n "
P. MERLE, J. MERLIN and F. FOUQUET Groupe d'Etudes de Mdtallurgie Physique et de Physique des Matdriaux, Laboratoire associd au CNRS 341, lnstitut National des Sciences Appliqudes de Lyon, Bdtiment 502, 69621 Villeurbanne Cddex (France)
(Received October 8, :1982)
Rosen e t al. [1] have assessed that the m a x i m u m of hardness is obtained during the precipitation of the 0' phase when the degree of c o h e r e n c y between the precipitates and the matrix begins to decrease. Such an assessment is in c o n t r a d i c t i o n to experimental observations realized on binary A1-Cu alloys where similar maxima in hardness are observed [2]. Figure 1 shows the evolution of the hardness of a reverted A1-4wt.%Cu alloy for different aging temperatures between 175 and 275 °C.
'"
I [
k m~,R\ . ~, ". "~ ",,
8()
\
200 "c l'#~J"c
t
0.1
1
10
(h)
100
Fig. 1. The evolution of hardness during the precipitation of the 0' phase in an A1-4wt.%Cu aged at different temperatures after reversion ( ) and the evolution of the hardness of the domains occupied by the 0' precipitation in the heterogeneous stage of precipitation ( - - - ) . 0025-5416/83/0000-0000/$03.00
To explain the m a x i m u m of hardness we have shown [ 2, 3 ] that it is necessary to take into account the heterogeneous character of the 0' precipitation. The evolution of the Vickers hardness of the sample is given by a simple rule-of-mixtures equation H v = Hvo' + (1 -- ~ ) H v s s
where Hvo' is the Viekers hardness of the regions occupied by the 0' precipitation, the volume fraction of the specimen where this precipitation occurs and Hw~ the Vickers hardness of the residual solid solution. H v o ' always decreases with time (even in the heterogeneous stage of precipitation); this is due to the const ant increase in the interprecipitate spacing. The m a x i m u m of hardness always occurs when the heterogeneous precipitation of 0' begins to occupy the whole matrix (~2 = 1). The same observations have been made on samples aged after different initial thermomechanieal treatments and, among them, on specimens aged after a reversion t r e a t m e n t followed by a plastic d e f o r m a t i o n of 5%; this is a t r e a t m e n t giving an initial microstructure rather similar to that of the specimens used by Rosen e t al. [1]; the only differences lie in the fact that kinetics of precipitation are faster than after a reversion treatment. This is correlated to a faster increase in ~2. O t her studies [4] have shown that the previous precipitation of 0" modifies the shape of the hardness curves but that the m a x i m u m of hardness c a n n o t be, in any case, at t ri but ed to the loss of coherency of precipitates. This p o i n t of view is support ed by the fact t hat the apparent activation energy Q3 f o u n d by Rosen e t al. (approximately equal to 27 kcal mo1-1) is, owing to experimental uncertainties, of the same order as the activation energy of the diffusion of copper in an aluminium matrix (about 30 kcal mol-1). This seems to prove that the m a x i m u m of hardness is only related to a diffusion phen o m e n o n and n o t to the loss of coherency of precipitates which may involve ot her parameters than diffusion. Moreover, electron © Elsevier Sequoia/Printed in The Netherlands
262
microscope studies [ 5] have shown that, at low aging temperatures such as those used by Rosen et al., the loss of coherency of 0' precipitates occurs at times much greater than that corresponding to the maximum of hardness. For example, in specimens of A1-4wt.%Cu aged at 200 °C after a revision treatment, it is not possible to observe an appreciable loss of coherency of the 0' precipitates even for thermal treatments of up to 500 h, while the hardness maximum occurs after an aging of a b o u t 8 h. Measurements of the elastic modulus of the same specimens realized by resonant methods ( f ~ 1 kHz) have shown that the elastic properties of this material were effectively sensible to the loss of coherency of 0' precipitates [6] but have also proved that this phenomenon appears after aging times much longer than that of the hardness maximum. Peaks 3 of the ultrasonic attenuation curves of Rosen e t al. cannot thus be attributed to "an interaction between the acoustic vibrations of the ultrasonic waves propagating through the material and the interphase dislocations surrounding the grown 0' precipitates". There is no doubt, however, that the influence on this peak of various parameters (the measuring frequency and the prior plastic deformation of the specimen)
indicate that this peak is related to a size effect of the precipitates. We must also point out that the reference to the theory of coarsening of Lifshitz and Slyuzov [ 7] to explain the precipitation of 0' is inadequate as this theory was established for a nearly constant volume fraction of precipitates; this is not the case for the experiments of Rosen e t al. Conclusions drawn from such an analysis are thus not valid and recent studies [ 5, 8] have shown that a detailed analysis of the stages of 0' precipitation must be done before trying to draw conclusions on the growth law of 0' precipitates. REFERENCES 1 M. Rosen, E. Horowitz, S. Fick, R. C. Reno and R. Mehrabian, Mater. Sci. Eng., 53 (1982) 163. 2 P. Merle, F. Fouquet, J. Merlin and P. F. Gobin, Mater. ScL Eng., 26 (1976) 277. 3 P. Merle, F. F o u q u e t and J, Merlin, Scr. Metall., 15 (1981) 373. 4 F. Fouquet, Th~se, Lyon, 1977. 5 P. Merle and F. Fouquet, Acta Metall., 29 (1981) 1919. 6 F. Fouquet, P. Merle, M. Kohen, J. Merlin and P. F. Gobin, Acta Metall., 27 (1979) 315. 7 I . M . Lifshitz and V. Slyuzov, J. Phys. Chem. Solids, 19 (1961) 35. 8 P. Merle and J. Merlin, Acta Metall., 29 (1981) 1928.
261
Correspondence C o m m e n t s o n " A n investigation o f the precipitation-hardening process in aluminum alloy 2 2 1 9 b y m e a n s o f s o u n d wave v e l o c i t y and ultrasonic a t t e n u a t i o n "
P. MERLE, J. MERLIN and F. FOUQUET Groupe d'Etudes de Mdtallurgie Physique et de Physique des Matdriaux, Laboratoire associd au CNRS 341, lnstitut National des Sciences Appliqudes de Lyon, Bdtiment 502, 69621 Villeurbanne Cddex (France)
(Received October 8, :1982)
Rosen e t al. [1] have assessed that the m a x i m u m of hardness is obtained during the precipitation of the 0' phase when the degree of c o h e r e n c y between the precipitates and the matrix begins to decrease. Such an assessment is in c o n t r a d i c t i o n to experimental observations realized on binary A1-Cu alloys where similar maxima in hardness are observed [2]. Figure 1 shows the evolution of the hardness of a reverted A1-4wt.%Cu alloy for different aging temperatures between 175 and 275 °C.
'"
I [
k m~,R\ . ~, ". "~ ",,
8()
\
200 "c l'#~J"c
t
0.1
1
10
(h)
100
Fig. 1. The evolution of hardness during the precipitation of the 0' phase in an A1-4wt.%Cu aged at different temperatures after reversion ( ) and the evolution of the hardness of the domains occupied by the 0' precipitation in the heterogeneous stage of precipitation ( - - - ) . 0025-5416/83/0000-0000/$03.00
To explain the m a x i m u m of hardness we have shown [ 2, 3 ] that it is necessary to take into account the heterogeneous character of the 0' precipitation. The evolution of the Vickers hardness of the sample is given by a simple rule-of-mixtures equation H v = Hvo' + (1 -- ~ ) H v s s
where Hvo' is the Viekers hardness of the regions occupied by the 0' precipitation, the volume fraction of the specimen where this precipitation occurs and Hw~ the Vickers hardness of the residual solid solution. H v o ' always decreases with time (even in the heterogeneous stage of precipitation); this is due to the const ant increase in the interprecipitate spacing. The m a x i m u m of hardness always occurs when the heterogeneous precipitation of 0' begins to occupy the whole matrix (~2 = 1). The same observations have been made on samples aged after different initial thermomechanieal treatments and, among them, on specimens aged after a reversion t r e a t m e n t followed by a plastic d e f o r m a t i o n of 5%; this is a t r e a t m e n t giving an initial microstructure rather similar to that of the specimens used by Rosen e t al. [1]; the only differences lie in the fact that kinetics of precipitation are faster than after a reversion treatment. This is correlated to a faster increase in ~2. O t her studies [4] have shown that the previous precipitation of 0" modifies the shape of the hardness curves but that the m a x i m u m of hardness c a n n o t be, in any case, at t ri but ed to the loss of coherency of precipitates. This p o i n t of view is support ed by the fact t hat the apparent activation energy Q3 f o u n d by Rosen e t al. (approximately equal to 27 kcal mo1-1) is, owing to experimental uncertainties, of the same order as the activation energy of the diffusion of copper in an aluminium matrix (about 30 kcal mol-1). This seems to prove that the m a x i m u m of hardness is only related to a diffusion phen o m e n o n and n o t to the loss of coherency of precipitates which may involve ot her parameters than diffusion. Moreover, electron © Elsevier Sequoia/Printed in The Netherlands
262
microscope studies [ 5] have shown that, at low aging temperatures such as those used by Rosen et al., the loss of coherency of 0' precipitates occurs at times much greater than that corresponding to the maximum of hardness. For example, in specimens of A1-4wt.%Cu aged at 200 °C after a revision treatment, it is not possible to observe an appreciable loss of coherency of the 0' precipitates even for thermal treatments of up to 500 h, while the hardness maximum occurs after an aging of a b o u t 8 h. Measurements of the elastic modulus of the same specimens realized by resonant methods ( f ~ 1 kHz) have shown that the elastic properties of this material were effectively sensible to the loss of coherency of 0' precipitates [6] but have also proved that this phenomenon appears after aging times much longer than that of the hardness maximum. Peaks 3 of the ultrasonic attenuation curves of Rosen e t al. cannot thus be attributed to "an interaction between the acoustic vibrations of the ultrasonic waves propagating through the material and the interphase dislocations surrounding the grown 0' precipitates". There is no doubt, however, that the influence on this peak of various parameters (the measuring frequency and the prior plastic deformation of the specimen)
indicate that this peak is related to a size effect of the precipitates. We must also point out that the reference to the theory of coarsening of Lifshitz and Slyuzov [ 7] to explain the precipitation of 0' is inadequate as this theory was established for a nearly constant volume fraction of precipitates; this is not the case for the experiments of Rosen e t al. Conclusions drawn from such an analysis are thus not valid and recent studies [ 5, 8] have shown that a detailed analysis of the stages of 0' precipitation must be done before trying to draw conclusions on the growth law of 0' precipitates. REFERENCES 1 M. Rosen, E. Horowitz, S. Fick, R. C. Reno and R. Mehrabian, Mater. Sci. Eng., 53 (1982) 163. 2 P. Merle, F. Fouquet, J. Merlin and P. F. Gobin, Mater. ScL Eng., 26 (1976) 277. 3 P. Merle, F. F o u q u e t and J, Merlin, Scr. Metall., 15 (1981) 373. 4 F. Fouquet, Th~se, Lyon, 1977. 5 P. Merle and F. Fouquet, Acta Metall., 29 (1981) 1919. 6 F. Fouquet, P. Merle, M. Kohen, J. Merlin and P. F. Gobin, Acta Metall., 27 (1979) 315. 7 I . M . Lifshitz and V. Slyuzov, J. Phys. Chem. Solids, 19 (1961) 35. 8 P. Merle and J. Merlin, Acta Metall., 29 (1981) 1928.