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Deformation behavior of Al–Li single crystals
March 1998
Materials Letters 34 Ž1998. 383–386
Deformation behavior of Al–Li single crystals B.H. Tian
a,)
, Y.G. Zhang b, C.Q. Chen
b
a
b
PO Box 81-2, Beijing Institute of Aeronautical Materials, Beijing 100095, People’s Republic of China Department of Materials Science and Engineering, Beijing UniÕersity of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China Received 18 April 1997; revised 22 July 1997; accepted 23 July 1997
Abstract Deformation behavior of binary Al–Li single crystals was investigated in the temperature range of 200–453 K at a strain rate of 5.6 = 10y3 sy1. Flow stress increases with increasing temperature in the crystals both in solid solution and underaged conditions. Serrated flow was observed in the crystals in the underaged state, but not in the solid solution. Variations in stress drops and elapsed time of individual serrations indicate that the shearing of d X particles is important in the serrated flow of Al–Li single crystals. q 1998 Elsevier Science B.V. Keywords: Deformation; Aluminum–lithium alloy; Single crystals
1. Introduction Flow stress generally decreases with increasing temperature ŽT . during the deformation of most materials. Under some circumstances flow stress can abnormally increase with T because of the predomination of thermal activation processes within a certain temperature range, then the deformation is referred to abnormal flow behavior. Abnormal flow behavior may be accompanied by the occurrence of the Portevin–Le Chatelier ŽPLC. effect, that is usually ascribed to the dynamic interaction between solute atoms and mobile dislocations during deformation, named as dynamic strain aging ŽDSA. effect
)
Corresponding author.
w1x. In binary Al–Li single crystals, the critical resolved shear stress ŽCRSS. has been reported to increase with temperature w2,3x, while the temperature dependence of the flow curve character remains to be investigated. However, serrated flow in Al–Li alloys appears strongly in artificially aging state in comparison with that in solid solution condition w4x, which is contradictory to DSA theory w1x. Nevertheless, critical strains for the occurrence of serrated flow and the strain rate sensitivity of flow stress in Al–Li alloys appear similar to the behavior associated with DSA of solute atoms w5,6x. The mechanism behind serrated flow in Al–Li alloys is still unclear. In this note, the effect of the deformation temperature on the flow stress and serrated flow in binary Al–Li single crystals is investigated in the temperature range of 200–453 K. The adoption of binary Al–Li single crystals is to avoid the complicated influencing factors met with commercial alloys.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 0 0 - 0
384
B.H. Tian et al.r Materials Letters 34 (1998) 383–386
2. Experimental method Two binary Al–Li single crystals were prepared for the present study. The crystal of 0.90 wt% Li with a w134x orientation is referred to crystal A, while the other of 2.10 wt% Li with a w612x orientation is crystal B. After a homogenization treatment, the crystals were cut into samples of 6.2 mm = 3.4 mm = 3.4 mm, with the loading axes parallel to the growth orientations. Then, the samples were solid solution treated in a salt bath at 793 K for 30 min, followed by water quenching at room temperature. Crystal A was tested in the as-quenched state when its Li content was below the solid solubility at room
Fig. 2. Stress drops of individual serrations versus strain Ža. and elapsed time of the serrations Žb. in crystal B. T s 293 K, ´˙ s 5.6=10y3 sy1 .
Fig. 1. True stress–true strain curves under different temperatures for crystal A Ža. and B Žb..
temperature. Crystal B was artificially aged in an oil bath at 448 K for 20 h to an underaged condition. Compressive deformation tests were performed with a MTS-880 machine in the temperature range of 200–453 K at a strain rate Ž ´˙ . of 5.6 = 10y3 sy1. The load–displacement curves were recorded with an X–Y recorder. The compression can be regarded as uniform deformation under the condition that the data were obtained within a strain of 0.15. The resolution of the testing system was 0.5 MPa, meaning that no serrations could be detected when stress drops of serrations were smaller than this magnitude. Elapsed time Ž d t . of individual serrations was defined as the time elapsed in one cycle of serrations, while stress drop Ž ds . as the true stress from the highest to the lowest point of individual serrations. All distinguishable serrations in the load–displacement curves were selected to get the values of the defined elapsed time and stress drops.
B.H. Tian et al.r Materials Letters 34 (1998) 383–386
3. Results The dependence of flow stresses on temperature for crystal A and B is shown in Fig. 1Ža, b.. The level of the true stress–true strain curve Ž s – ´ . is higher at 293 K than that at 200 K for crystal A for strains Ž ´ . beyond 0.03, but decreases with temperature above 293 K. No serrations were found for the crystal deformed in the temperature range of 200–453 K. Flow stress increases with T for crystal B except at 293 K for ´ - 0.085. Serrated flow was observed explicitly except when the crystal deformed at 200 K. It can been seen that abnormal flow behavior appears both in crystal A in the temperature range of 200–293 K; and in crystal B for 293–453 K. However, serrated flow is found only in crystal B deformed between 293 and 453 K. Examples of stress drops Ž ds . of individual serrations versus strain and elapsed time Ž d t . are shown in Fig. 2Ža, b., respectively. ds appears independent of increasing strain. The same ds corresponds to several different d t, while the same d t corresponds to several different ds . ds does not show monotonic variations with strain or elapsed time of the serrations.
4. Discussion 4.1. Abnormal flow behaÕior The plastic flow behavior of the crystal at the as-quenched state is considered to be affected by two factors, one is the short range order of solute atoms, the other is the dynamic strain aging of solutes. Increase of the flow stress with T in crystal A can be correlated with the DSA effect of solute Li atoms that is most manifested at ambient temperature w7x, while the short range order of Li atoms decreases with T. DSA of solutes could enhance the friction stress to the movement of mobile dislocations, or delay the dynamic recovery of dislocation structures w8x. As a result, the flow stress increases with T until vanishing of the DSA effect at high temperature. The abnormal flow behavior in crystal B appears in a higher temperature range compared with that in crystal A. Dependence of CRSS on T has been explained by Miura et al. with the role of the Kear–
385
Wilsdorf ŽK–W. lock at T below 323 K, and with the shearing of d X particles by unit dislocations at T above 323 K w3x. In the present investigation, it seems that the K–W lock and break of dislocation pairs not only account for the variations of CRSS with T, but also for increasing flow stresses, thus give rise to abnormal flow behavior in crystal B with T above 293 K. The DSA of solute Li atoms could be responsible for the partial abnormal flow behavior at 293 K in crystal B for the effect of DSA may not be neglected after dissolution of d X particles at large deformation w9x. 4.2. Occurrence of the serrated flow Though the effect of DSA on flow stress was obvious in the temperature range between 200 and 293 K for both crystals A and B. Serrations were observed explicitly in the underaged state with shearable d X particles, not in the solid solution condition, even if the crystal was deformed over a wide temperature and strain rate range w10x. It seems difficult for the occurrence of serrated flow in Al–Li single crystals to be ascribed to DSA of solute atoms. In addition, stress drops of the serrations change irregularly with strain during deformation, whilst for the PLC effect in Al–Mg single crystals, stress drops attain a maximum at the end of the second hardening stage w11x. It is indicated that the mechanisms of serrated flow in Al–Li and Al–Mg single crystals may be different. Serrated flow in Al–Li alloys has been explained by shearing of d X particles itself which could induce dissolution of the particles and a negative strain rate sensitivity of flow stress, followed by the occurrence of the serrated flow w9x. The present results on Al–Li single crystals tend to support shearing processes inducing serrated flow. Stress drops for individual serrations bear no relation to elapsed time, which is abnormal for the PLC effect where variations in stress drops as well as elapsed time Žor frequency. can be explained by diffusivity of solute atoms w12,13x. Variations in stress drops and elapsed time may be related to d X particles. It could be suggested that stress drops or elapsed time during serrated flow may be related to particle size, distribution, distance among the particles etc., but they are probably determined by two independent processes that need to be investigated further.
386
B.H. Tian et al.r Materials Letters 34 (1998) 383–386
5. Conclusions Investigation of the deformation behavior of Al–Li single crystals shows that abnormal behavior of flow stress appears in the crystals both in as-quenched and underaged conditions, but in different temperature ranges. Shearing of d X particles is required for the occurrence of serrated flow. Stress drops of individual serrations appear not to be relevant to strain and elapsed time of the serrations.
w3x
w4x w5x w6x w7x w8x w9x w10x
References w1x P.G. McCormick, Acta Metall. 6 Ž1972. 351. w2x Y. Miura, A. Matsui, M. Furukawa, M. Nemoto, in: Baker et
w11x w12x w13x
al. ŽEds.., Proc. of 3rd Int. Conf. on Aluminum–Lithium Alloys, London, 1986, p. 427. Y. Mirua, K. Yusu, N. Furukawa, M. Nemoto, in: G. Champier et al. ŽEds.., Proc. of 4th Int. Conf. on Aluminum–Lithium Alloys, Paris, 1987, p. 549. N. Behnood, J.T. Evans, Acta Metall. 37 Ž1989. 687. J.C. Huang, G.T. Gray, Scr. Metall. Mater. 24 Ž1990. 85. S. Kumar, E. Pink, Scr. Metall. Mater. 32 Ž1995. 749. N. Diego, J. del Rio, D. Segers, L. Dorikens-Vanpraet, M. Dorkens, Phys. Rev. B 48 Ž1993. 6781. U.F. Kock, Metall. Trans. A 16 Ž1985. 2109. Y. Brechet, Y. Estrin, Scr. Metall. Mater. 31 Ž1994. 185. C.Q. Chen, B.H. Tian, K.G. Wang, Y.G. Zhang, in: J. Dryer et al. ŽEds.., Proc. of 5th Int. Conf. on Aluminium Alloys, Grenoble, France, 1996, p. 1043. H. Fujita, T. Tabata, Acta Metall. 25 Ž1977. 793. E. Pink, A. Grinberg, Acta Metall. 30 Ž1982. 2153. K.T. Hong, S.W. Name, Acta Metall. 37 Ž1989. 31.
Materials Letters 34 Ž1998. 383–386
Deformation behavior of Al–Li single crystals B.H. Tian
a,)
, Y.G. Zhang b, C.Q. Chen
b
a
b
PO Box 81-2, Beijing Institute of Aeronautical Materials, Beijing 100095, People’s Republic of China Department of Materials Science and Engineering, Beijing UniÕersity of Aeronautics and Astronautics, Beijing 100083, People’s Republic of China Received 18 April 1997; revised 22 July 1997; accepted 23 July 1997
Abstract Deformation behavior of binary Al–Li single crystals was investigated in the temperature range of 200–453 K at a strain rate of 5.6 = 10y3 sy1. Flow stress increases with increasing temperature in the crystals both in solid solution and underaged conditions. Serrated flow was observed in the crystals in the underaged state, but not in the solid solution. Variations in stress drops and elapsed time of individual serrations indicate that the shearing of d X particles is important in the serrated flow of Al–Li single crystals. q 1998 Elsevier Science B.V. Keywords: Deformation; Aluminum–lithium alloy; Single crystals
1. Introduction Flow stress generally decreases with increasing temperature ŽT . during the deformation of most materials. Under some circumstances flow stress can abnormally increase with T because of the predomination of thermal activation processes within a certain temperature range, then the deformation is referred to abnormal flow behavior. Abnormal flow behavior may be accompanied by the occurrence of the Portevin–Le Chatelier ŽPLC. effect, that is usually ascribed to the dynamic interaction between solute atoms and mobile dislocations during deformation, named as dynamic strain aging ŽDSA. effect
)
Corresponding author.
w1x. In binary Al–Li single crystals, the critical resolved shear stress ŽCRSS. has been reported to increase with temperature w2,3x, while the temperature dependence of the flow curve character remains to be investigated. However, serrated flow in Al–Li alloys appears strongly in artificially aging state in comparison with that in solid solution condition w4x, which is contradictory to DSA theory w1x. Nevertheless, critical strains for the occurrence of serrated flow and the strain rate sensitivity of flow stress in Al–Li alloys appear similar to the behavior associated with DSA of solute atoms w5,6x. The mechanism behind serrated flow in Al–Li alloys is still unclear. In this note, the effect of the deformation temperature on the flow stress and serrated flow in binary Al–Li single crystals is investigated in the temperature range of 200–453 K. The adoption of binary Al–Li single crystals is to avoid the complicated influencing factors met with commercial alloys.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 0 0 - 0
384
B.H. Tian et al.r Materials Letters 34 (1998) 383–386
2. Experimental method Two binary Al–Li single crystals were prepared for the present study. The crystal of 0.90 wt% Li with a w134x orientation is referred to crystal A, while the other of 2.10 wt% Li with a w612x orientation is crystal B. After a homogenization treatment, the crystals were cut into samples of 6.2 mm = 3.4 mm = 3.4 mm, with the loading axes parallel to the growth orientations. Then, the samples were solid solution treated in a salt bath at 793 K for 30 min, followed by water quenching at room temperature. Crystal A was tested in the as-quenched state when its Li content was below the solid solubility at room
Fig. 2. Stress drops of individual serrations versus strain Ža. and elapsed time of the serrations Žb. in crystal B. T s 293 K, ´˙ s 5.6=10y3 sy1 .
Fig. 1. True stress–true strain curves under different temperatures for crystal A Ža. and B Žb..
temperature. Crystal B was artificially aged in an oil bath at 448 K for 20 h to an underaged condition. Compressive deformation tests were performed with a MTS-880 machine in the temperature range of 200–453 K at a strain rate Ž ´˙ . of 5.6 = 10y3 sy1. The load–displacement curves were recorded with an X–Y recorder. The compression can be regarded as uniform deformation under the condition that the data were obtained within a strain of 0.15. The resolution of the testing system was 0.5 MPa, meaning that no serrations could be detected when stress drops of serrations were smaller than this magnitude. Elapsed time Ž d t . of individual serrations was defined as the time elapsed in one cycle of serrations, while stress drop Ž ds . as the true stress from the highest to the lowest point of individual serrations. All distinguishable serrations in the load–displacement curves were selected to get the values of the defined elapsed time and stress drops.
B.H. Tian et al.r Materials Letters 34 (1998) 383–386
3. Results The dependence of flow stresses on temperature for crystal A and B is shown in Fig. 1Ža, b.. The level of the true stress–true strain curve Ž s – ´ . is higher at 293 K than that at 200 K for crystal A for strains Ž ´ . beyond 0.03, but decreases with temperature above 293 K. No serrations were found for the crystal deformed in the temperature range of 200–453 K. Flow stress increases with T for crystal B except at 293 K for ´ - 0.085. Serrated flow was observed explicitly except when the crystal deformed at 200 K. It can been seen that abnormal flow behavior appears both in crystal A in the temperature range of 200–293 K; and in crystal B for 293–453 K. However, serrated flow is found only in crystal B deformed between 293 and 453 K. Examples of stress drops Ž ds . of individual serrations versus strain and elapsed time Ž d t . are shown in Fig. 2Ža, b., respectively. ds appears independent of increasing strain. The same ds corresponds to several different d t, while the same d t corresponds to several different ds . ds does not show monotonic variations with strain or elapsed time of the serrations.
4. Discussion 4.1. Abnormal flow behaÕior The plastic flow behavior of the crystal at the as-quenched state is considered to be affected by two factors, one is the short range order of solute atoms, the other is the dynamic strain aging of solutes. Increase of the flow stress with T in crystal A can be correlated with the DSA effect of solute Li atoms that is most manifested at ambient temperature w7x, while the short range order of Li atoms decreases with T. DSA of solutes could enhance the friction stress to the movement of mobile dislocations, or delay the dynamic recovery of dislocation structures w8x. As a result, the flow stress increases with T until vanishing of the DSA effect at high temperature. The abnormal flow behavior in crystal B appears in a higher temperature range compared with that in crystal A. Dependence of CRSS on T has been explained by Miura et al. with the role of the Kear–
385
Wilsdorf ŽK–W. lock at T below 323 K, and with the shearing of d X particles by unit dislocations at T above 323 K w3x. In the present investigation, it seems that the K–W lock and break of dislocation pairs not only account for the variations of CRSS with T, but also for increasing flow stresses, thus give rise to abnormal flow behavior in crystal B with T above 293 K. The DSA of solute Li atoms could be responsible for the partial abnormal flow behavior at 293 K in crystal B for the effect of DSA may not be neglected after dissolution of d X particles at large deformation w9x. 4.2. Occurrence of the serrated flow Though the effect of DSA on flow stress was obvious in the temperature range between 200 and 293 K for both crystals A and B. Serrations were observed explicitly in the underaged state with shearable d X particles, not in the solid solution condition, even if the crystal was deformed over a wide temperature and strain rate range w10x. It seems difficult for the occurrence of serrated flow in Al–Li single crystals to be ascribed to DSA of solute atoms. In addition, stress drops of the serrations change irregularly with strain during deformation, whilst for the PLC effect in Al–Mg single crystals, stress drops attain a maximum at the end of the second hardening stage w11x. It is indicated that the mechanisms of serrated flow in Al–Li and Al–Mg single crystals may be different. Serrated flow in Al–Li alloys has been explained by shearing of d X particles itself which could induce dissolution of the particles and a negative strain rate sensitivity of flow stress, followed by the occurrence of the serrated flow w9x. The present results on Al–Li single crystals tend to support shearing processes inducing serrated flow. Stress drops for individual serrations bear no relation to elapsed time, which is abnormal for the PLC effect where variations in stress drops as well as elapsed time Žor frequency. can be explained by diffusivity of solute atoms w12,13x. Variations in stress drops and elapsed time may be related to d X particles. It could be suggested that stress drops or elapsed time during serrated flow may be related to particle size, distribution, distance among the particles etc., but they are probably determined by two independent processes that need to be investigated further.
386
B.H. Tian et al.r Materials Letters 34 (1998) 383–386
5. Conclusions Investigation of the deformation behavior of Al–Li single crystals shows that abnormal behavior of flow stress appears in the crystals both in as-quenched and underaged conditions, but in different temperature ranges. Shearing of d X particles is required for the occurrence of serrated flow. Stress drops of individual serrations appear not to be relevant to strain and elapsed time of the serrations.
w3x
w4x w5x w6x w7x w8x w9x w10x
References w1x P.G. McCormick, Acta Metall. 6 Ž1972. 351. w2x Y. Miura, A. Matsui, M. Furukawa, M. Nemoto, in: Baker et
w11x w12x w13x
al. ŽEds.., Proc. of 3rd Int. Conf. on Aluminum–Lithium Alloys, London, 1986, p. 427. Y. Mirua, K. Yusu, N. Furukawa, M. Nemoto, in: G. Champier et al. ŽEds.., Proc. of 4th Int. Conf. on Aluminum–Lithium Alloys, Paris, 1987, p. 549. N. Behnood, J.T. Evans, Acta Metall. 37 Ž1989. 687. J.C. Huang, G.T. Gray, Scr. Metall. Mater. 24 Ž1990. 85. S. Kumar, E. Pink, Scr. Metall. Mater. 32 Ž1995. 749. N. Diego, J. del Rio, D. Segers, L. Dorikens-Vanpraet, M. Dorkens, Phys. Rev. B 48 Ž1993. 6781. U.F. Kock, Metall. Trans. A 16 Ž1985. 2109. Y. Brechet, Y. Estrin, Scr. Metall. Mater. 31 Ž1994. 185. C.Q. Chen, B.H. Tian, K.G. Wang, Y.G. Zhang, in: J. Dryer et al. ŽEds.., Proc. of 5th Int. Conf. on Aluminium Alloys, Grenoble, France, 1996, p. 1043. H. Fujita, T. Tabata, Acta Metall. 25 Ž1977. 793. E. Pink, A. Grinberg, Acta Metall. 30 Ž1982. 2153. K.T. Hong, S.W. Name, Acta Metall. 37 Ž1989. 31.