- Email: [email protected]
Deformation behaviour of Mg–Li alloys at elevated temperatures
Materials Science and Engineering A 410–411 (2005) 148–151
Deformation behaviour of Mg–Li alloys at elevated temperatures Z. Trojanov´a ∗ , Z. Drozd, P. Luk´acˇ , F. Chmel´ık Charles University, Faculty of Mathematics and Physics, Department of Metal Physics, Ke Karlovu 5, 121 16 Prague, Czech Republic Received in revised form 26 January 2005
Abstract The mechanical properties of three magnesium alloys Mg4Li, LA43 and LA45 have been investigated in compression tests at temperatures between 25 and 200 ◦ C. The yield stress and the maximum stress are sensitive to the testing temperature. The deformation behaviour of the specimens can be attributed to the occurrence of hardening and softening during deformation. Additions of aluminium to Mg4Li alloy increase the alloy strength. After stress relaxation of LA43 and LA45 alloy an increase in the flow stress is observed. © 2005 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Compression tests; Work-hardening; Softening
1. Introduction It is a great honour to write this paper to Professor Terence G. Langdon, an outstanding scientist. Terry has made many excellent contributions to a better understanding of the deformation behaviour of metallic and ceramic materials at elevated temperatures. His papers on creep and superplasticity have always been an inspiration to many scientists. Magnesium alloys are promising structural materials because they exhibit a high specific strength, i.e. the ratio of the yield stress (hardness) to density, at room temperature. The strength decreases rapidly with increasing temperature. On the other hand, Mg alloys have a poor ductility, especially at room temperature. In the recent years, the mechanical properties have been the subject of many investigations. The testing temperature above room temperature influences their deformation behaviour. It should be pointed out that the room temperature (293 K = 0.32TM , where TM is the absolute melting temperature) is relative high temperature for Mg alloys [1]. Among Mg alloys, magnesium–lithium alloys, as the lightest metallic materials, are attractive for a large amount of applications [2]. They are of great importance also for medicine purposes. Therefore, it is important to investigate mechanical properties at different temperatures and to estimate the deformation mechanisms responsible for the deformation behaviour of Mg–Li alloys at elevated temperatures. ∗
Corresponding author. Tel.: +420 221 911 357; fax: +420 221 911 490. E-mail address: [email protected] (Z. Trojanov´a).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.088
The aim of the present work is to study the influence of temperature on compressive properties of Mg4Li alloy, to estimate the strengthening effect of Al atoms and to discuss possible hardening and softening mechanisms. 2. Material and experimental procedure The experiments were conducted using the following cast alloys: Mg4Li (Mg–4 wt.% Li), LA43 (Mg–4 wt.% Li–3wt.% Al) and LA45 (Mg–4 wt.% Li–5 wt.%Al). Test specimens with diameters of 5 mm and gauge lengths of 7 mm were machined from bars. Compression tests were performed in an Instron machine at temperatures between 25 and 200 ◦ C and at a constant crosshead speed giving an initial strain rate of 2.4 × 10−4 s−1 . The argon atmosphere was used as a protecting atmosphere in the furnace at elevated temperatures. Stress relaxation tests were carried out in order to estimate parameters of a possible thermally activated process. At a certain stress (strain) the machine was stopped and the stress was allowed to relax during 300 s. Subsequently, the specimen was reloaded to a higher strain and the test was repeated. In order to estimate the activation volume of the thermally activated processes from the stress relaxation (SR) the following relationship between stress drop rate, ∂σ/∂t, and stress, σ, was used: −∂σ ln = C + n ln σ (1) ∂t where C is a constant and n is the stress sensitivity parameter. A simple relation between the parameter n and the activation
Z. Trojanov´a et al. / Materials Science and Engineering A 410–411 (2005) 148–151
149
volume V is given by: nkT = Vσ
(2)
where k is Boltzmann’s constant and T is the absolute temperature. 3. Experimental results Fig. 1 shows the temperature variation of the yield stress, σ 02 , as well as the maximum stress, σ max , of Mg4Li alloy. It is apparent from Fig. 1 that the temperature variation of σ 02 is complex. It seems that the temperature variation of σ 02 exhibits a small local maximum at 100 ◦ C (the measurements were repeated several times). The maximum stress of Mg4Li alloy decreases rapidly with increasing temperature. The differences between σ max and σ 02 exhibit a rapid decrease with increasing temperature. The values of σ 02 for Mg4Li, LA43 and LA45 alloys at selected temperatures are plotted in Fig. 2. It is seen that a small addition of aluminium enhances the mechanical strength. The maximum stress of Mg4Li as well as LA43 and LA45 decreases rapidly with increasing temperature. It is in agreement of observation of Haferkamp et al. [2] who estimated that Al content substantially increases the yield stress and maximum stress as well as the ductility of Mg–Li alloys at room temperature.
Fig. 3. Stress dependence of the activation volume for LA45 alloy at different temperatures.
Fig. 4. Strain dependence of the increase in the flow stress after stress relaxation for LA43 and LA45 alloys.
Fig. 1. Temperature variation of the yield stress and maximum stress.
The stress dependences of the activation volume calculated from the stress relaxation curves obtained for LA45 are shown in Fig. 3. It should be mentioned that the activation volume at a certain stress decreases rapidly with temperature. The flow stress after SR, σ 1 , was higher than the flow stress at the beginning of SR, σ 0 . The values of σ = σ 1 − σ 0 are plotted against strain for LA43 and LA45l alloys at room temperature (Fig. 4). It can be seen that the values of σ at small strains increase with strain and then they decrease with strain. 4. Discussion
Fig. 2. Influence of Al on the yield stress at selected temperatures.
It is well known that the dominant slip system in Mg and Mg alloys at room temperature is the basal one. To fulfil von Mises criterion, a non-basal slip system should be active. The activity of non-basal slip systems plays an important role in dynamic recovery (softening). During deformation of magnesium alloy polycrystals, the motion not only a (basal) dislocations but also c + a (pyramidal) dislocations is assumed. Screw components of the c + a dislocations can move to the parallel slip planes by double cross-slip and then annihilate, which causes a decrease in the work-hardening rate; softening is observed. Addition of Li may
Z. Trojanov´a et al. / Materials Science and Engineering A 410–411 (2005) 148–151
150
result in higher activity of non-basal slip [3]. Screw component of a dislocations in the basal planes may move on the prism planes. Therefore, the free path of dislocations increases and work-hardening rate decreases. The activity of non-basal slip systems and cross-slip of dislocation increases with temperature. It means that the work-hardening rate should decrease and the elongation to failure should increase with increasing temperature. Both are observed experimentally. The role of the non-basal c + a slip mode in understanding mechanical behaviour of Mg solid solution alloys containing Li has been demonstrated by Agnew et al. [4]. They revealed that the c + a dislocation improve the ductility of Mg–Li alloys, Pawełek et al. [5] estimated, using acoustic emission, a high level of acoustic emission intensity in Mg4Li as a result of non-basal slip in the prismatic and pyramidal systems. The main strengthening effect of Mg4Li alloy is solid solution hardening. The flow stress, σ, of the alloy depends on the average dislocation density, ρ, as: σ = σy + αGbρ1/2
(3)
where σ y is the yield stress and it is a function of the concentration of solute atoms, grain size and temperature, α a constant, G the shear modulus and b is the magnitude of the Burgers vector. The dislocation structure changes during deformation and therefore, the flow stress also changes with strain and temperature. The change of the flow stress with strain is characterised by the work-hardening rate Θ = ∂σ/∂ε, which decreases with strain. Additions of Al atoms to Mg4Li cause the formation of precipitates (Al2 Li and Mg17 Al12 ), which leads to remarkable age hardening as observed also in [6–8]. Precipitates also influence the storage of dislocations during plastic deformation. In the LA43 and LA45 alloys, there are obstacles of non-dislocation type and forest dislocations. On the other hand, processes such as cross-slip and climb of dislocations contribute to softening. For simplicity, we can consider that the total dislocation density is the characteristic parameter of the evolution of microstructure during deformation. According to the model of Luk´acˇ and Bal´ık [9], the stress dependence of the work-hardening rate for polycrystals can thus be expressed in the following form: Θ=
A + B − C(σ − σy ) − D(σ − σ)3 σ − σy
(4)
Parameter A is connected with the interaction of dislocations with the non-dislocation obstacles. Parameter B characterises the work-hardening due to interaction with the forest dislocations. Both parameters A and B do not change with temperature, except at higher temperatures, when the activity of non-basal slip increases and it may change the forest dislocation density. The parameter C relates to recovery due to cross-slip, the parameter D is connected with a local climb of dislocation. Both parameters should increase with temperature. At higher temperatures, cross-slip becomes a significant recovery process responsible for softening. The result may be a quasi steady state character of the flow stress curves at higher temperatures, which is experimentally observed. The values of the activation volume and its stress dependence indicate thermally activated processes
during deformation. The motion of jogged dislocations is very probably the thermally activated process at room temperature. The forest dislocation (dislocations in the pyramidal systems) density increases during straining. Therefore, a decrease in the activation volume can be explained by a decrease in the mean jog spacing due to the intersection of moving dislocations with the forest dislocations. Jogs may also be produced by cross-slip of screw dislocations. The very low values of the activation volume at 200 ◦ C may be explained assuming that the rate-controlling mechanism is dislocation climb. A local maximum in the temperature variation of σ 02 at 100 ◦ C and the post-stress relaxation effect indicate dynamic strain ageing. The post-stress relaxation effect – an increase or a decrease in the flow stress after the SR – may be described by a complex partial differential equation [10]. Solute atoms locking dislocations cause the stress increase after SR, which depends on strain and on temperature. An increase in the flow stress is needed to move the dislocations after stress relaxation. The local solute concentration increment, c, can be expressed as: −tw p c = c − c0 = cM 1 − exp , (5) t0 where c is the local solute concentration in the dislocation core, c0 the nominal solute concentration in the matrix and cM is the maximum concentration increment. The exponent p is typically 2/3 and 1/3 for bulk and pipe diffusion, respectively. The relaxation time t0 depends on the binding energy between a dislocation and a solute atom, on solute concentration, and on the diffusion coefficient of solute atoms. t0 is inversely proportional to the diffusion coefficient in the case of bulk diffusion, whereas for pipe diffusion 1/t0 ∼ Dρ3/2 [11], where ρ is the density of forest dislocations. The mean waiting time for successful activation, tw , is connected with the strain rate by the Orowan equation: ε˙ =
bρm Λ bΩ = tw tw
(6)
where ρm is the mobile dislocation density and Λ = 1/ρ1/2 is the mean free path of dislocations. The elementary plastic strain per activation event Ω = bρm ρ−1/2 is strain dependent; it may have a local maximum at a certain strain [12,13]. σ is simply related to the kinetic law c(tw ) Eq. (5) as: σ = (f1 + f2 )c(tw ).
(7)
The first term f1 corresponds to the dislocation–dislocation interaction influenced by dynamic strain ageing and the second term f2 results from the solute atoms-dislocation interaction influenced by dynamic strain ageing. It is reasonable to consider that σ is proportional to the number of impurities on the dislocation lines. The dislocation density increases with strain. But the concentration of solute atoms is constant. At a certain strain, the Ω starts to decrease with strain [12,13] and hence, tw also decreases. This leads to a decrease in σ with strain for strains higher than a critical one, which is observed.
Z. Trojanov´a et al. / Materials Science and Engineering A 410–411 (2005) 148–151
151
5. Conclusions
References
The deformation behaviour of Mg4Li alloy depends significantly on the testing temperature. At and above a certain temperature, softening processes influence the course of the work-hardening curve. Increasing activity of softening processes with increasing temperature causes the differences in the deformation behaviour. Cross-slip and climb of dislocations may be responsible for the observed softening. In LA43 and LA45, dynamic strain ageing causes the post-stress relaxation effect—the flow stress at the beginning of plastic deformation after SR is higher than that at the beginning of the SR.
[1] M.O. Pekguleryuz, A.A. Kaya, Adv. Eng. Mater. 5 (2003) 866. [2] H. Haferkamp, C. Jaschik, P. Juchmann, V. Kaese, M. Niemeyr, P. Tai, Mat. Wiss. Werkstofftech 32 (2001) 25. [3] S. Kamado, Y. Kojima, Metall. Sci. Technol. 16 (1998) 45. [4] S.R. Agnew, M.H. Yoo, N. Tom´e, Acta Mater. 49 (2001) 4277. [5] A. Pawełek, Z. Jasie´nski, S. K´udela, A. Pi˛atkowski, P. Ranachowski, F. Rejmund, in: J. Jerz (Ed.), Proceedings of the International Conference Advanced Metallic Materials, Smolenice, Slovakia, 2003, p. 225. [6] A. Alamo, A.D. Banchik, J. Mater. Sci. 15 (1980) 222. [7] K. Matsuzawa, T. Koshihara, S. Ochiai, Y. Kojima, J. Jpn. Inst. Light Met. 30 (1989) 45. [8] T. Aida, H. Hatta, C.S. Ramesh, S. Kamado, Y. Kojima, J. Jpn. Inst. Light Met. 47 (1997) 195. [9] J. Bal´ık, P. Luk´acˇ , Key. Eng. Mater. 97–98 (1998) 307. [10] A. Dlouh´y, P. Luk´acˇ , Z. Trojanov´a, Kovove Mater. 26 (1984) 688. [11] J. Friedel, Dislocations, Pergamon Press, Oxford, 1964. [12] L.P. Kubin, Y. Estrin, Acta Metall. Mater. 38 (1990) 697. [13] J. Bal´ık, P. Luk´acˇ , Acta Metall. Mater. 41 (1993) 1447.
Acknowledgements The authors appreciate support from Grant Agency of the Czech Republic (Grant No. 106/03/0843) and the Grant Agency of the Academy of Sciences of the Czech Republic under Grant No. A2112303.
Deformation behaviour of Mg–Li alloys at elevated temperatures Z. Trojanov´a ∗ , Z. Drozd, P. Luk´acˇ , F. Chmel´ık Charles University, Faculty of Mathematics and Physics, Department of Metal Physics, Ke Karlovu 5, 121 16 Prague, Czech Republic Received in revised form 26 January 2005
Abstract The mechanical properties of three magnesium alloys Mg4Li, LA43 and LA45 have been investigated in compression tests at temperatures between 25 and 200 ◦ C. The yield stress and the maximum stress are sensitive to the testing temperature. The deformation behaviour of the specimens can be attributed to the occurrence of hardening and softening during deformation. Additions of aluminium to Mg4Li alloy increase the alloy strength. After stress relaxation of LA43 and LA45 alloy an increase in the flow stress is observed. © 2005 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Compression tests; Work-hardening; Softening
1. Introduction It is a great honour to write this paper to Professor Terence G. Langdon, an outstanding scientist. Terry has made many excellent contributions to a better understanding of the deformation behaviour of metallic and ceramic materials at elevated temperatures. His papers on creep and superplasticity have always been an inspiration to many scientists. Magnesium alloys are promising structural materials because they exhibit a high specific strength, i.e. the ratio of the yield stress (hardness) to density, at room temperature. The strength decreases rapidly with increasing temperature. On the other hand, Mg alloys have a poor ductility, especially at room temperature. In the recent years, the mechanical properties have been the subject of many investigations. The testing temperature above room temperature influences their deformation behaviour. It should be pointed out that the room temperature (293 K = 0.32TM , where TM is the absolute melting temperature) is relative high temperature for Mg alloys [1]. Among Mg alloys, magnesium–lithium alloys, as the lightest metallic materials, are attractive for a large amount of applications [2]. They are of great importance also for medicine purposes. Therefore, it is important to investigate mechanical properties at different temperatures and to estimate the deformation mechanisms responsible for the deformation behaviour of Mg–Li alloys at elevated temperatures. ∗
Corresponding author. Tel.: +420 221 911 357; fax: +420 221 911 490. E-mail address: [email protected] (Z. Trojanov´a).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.088
The aim of the present work is to study the influence of temperature on compressive properties of Mg4Li alloy, to estimate the strengthening effect of Al atoms and to discuss possible hardening and softening mechanisms. 2. Material and experimental procedure The experiments were conducted using the following cast alloys: Mg4Li (Mg–4 wt.% Li), LA43 (Mg–4 wt.% Li–3wt.% Al) and LA45 (Mg–4 wt.% Li–5 wt.%Al). Test specimens with diameters of 5 mm and gauge lengths of 7 mm were machined from bars. Compression tests were performed in an Instron machine at temperatures between 25 and 200 ◦ C and at a constant crosshead speed giving an initial strain rate of 2.4 × 10−4 s−1 . The argon atmosphere was used as a protecting atmosphere in the furnace at elevated temperatures. Stress relaxation tests were carried out in order to estimate parameters of a possible thermally activated process. At a certain stress (strain) the machine was stopped and the stress was allowed to relax during 300 s. Subsequently, the specimen was reloaded to a higher strain and the test was repeated. In order to estimate the activation volume of the thermally activated processes from the stress relaxation (SR) the following relationship between stress drop rate, ∂σ/∂t, and stress, σ, was used: −∂σ ln = C + n ln σ (1) ∂t where C is a constant and n is the stress sensitivity parameter. A simple relation between the parameter n and the activation
Z. Trojanov´a et al. / Materials Science and Engineering A 410–411 (2005) 148–151
149
volume V is given by: nkT = Vσ
(2)
where k is Boltzmann’s constant and T is the absolute temperature. 3. Experimental results Fig. 1 shows the temperature variation of the yield stress, σ 02 , as well as the maximum stress, σ max , of Mg4Li alloy. It is apparent from Fig. 1 that the temperature variation of σ 02 is complex. It seems that the temperature variation of σ 02 exhibits a small local maximum at 100 ◦ C (the measurements were repeated several times). The maximum stress of Mg4Li alloy decreases rapidly with increasing temperature. The differences between σ max and σ 02 exhibit a rapid decrease with increasing temperature. The values of σ 02 for Mg4Li, LA43 and LA45 alloys at selected temperatures are plotted in Fig. 2. It is seen that a small addition of aluminium enhances the mechanical strength. The maximum stress of Mg4Li as well as LA43 and LA45 decreases rapidly with increasing temperature. It is in agreement of observation of Haferkamp et al. [2] who estimated that Al content substantially increases the yield stress and maximum stress as well as the ductility of Mg–Li alloys at room temperature.
Fig. 3. Stress dependence of the activation volume for LA45 alloy at different temperatures.
Fig. 4. Strain dependence of the increase in the flow stress after stress relaxation for LA43 and LA45 alloys.
Fig. 1. Temperature variation of the yield stress and maximum stress.
The stress dependences of the activation volume calculated from the stress relaxation curves obtained for LA45 are shown in Fig. 3. It should be mentioned that the activation volume at a certain stress decreases rapidly with temperature. The flow stress after SR, σ 1 , was higher than the flow stress at the beginning of SR, σ 0 . The values of σ = σ 1 − σ 0 are plotted against strain for LA43 and LA45l alloys at room temperature (Fig. 4). It can be seen that the values of σ at small strains increase with strain and then they decrease with strain. 4. Discussion
Fig. 2. Influence of Al on the yield stress at selected temperatures.
It is well known that the dominant slip system in Mg and Mg alloys at room temperature is the basal one. To fulfil von Mises criterion, a non-basal slip system should be active. The activity of non-basal slip systems plays an important role in dynamic recovery (softening). During deformation of magnesium alloy polycrystals, the motion not only a (basal) dislocations but also c + a (pyramidal) dislocations is assumed. Screw components of the c + a dislocations can move to the parallel slip planes by double cross-slip and then annihilate, which causes a decrease in the work-hardening rate; softening is observed. Addition of Li may
Z. Trojanov´a et al. / Materials Science and Engineering A 410–411 (2005) 148–151
150
result in higher activity of non-basal slip [3]. Screw component of a dislocations in the basal planes may move on the prism planes. Therefore, the free path of dislocations increases and work-hardening rate decreases. The activity of non-basal slip systems and cross-slip of dislocation increases with temperature. It means that the work-hardening rate should decrease and the elongation to failure should increase with increasing temperature. Both are observed experimentally. The role of the non-basal c + a slip mode in understanding mechanical behaviour of Mg solid solution alloys containing Li has been demonstrated by Agnew et al. [4]. They revealed that the c + a dislocation improve the ductility of Mg–Li alloys, Pawełek et al. [5] estimated, using acoustic emission, a high level of acoustic emission intensity in Mg4Li as a result of non-basal slip in the prismatic and pyramidal systems. The main strengthening effect of Mg4Li alloy is solid solution hardening. The flow stress, σ, of the alloy depends on the average dislocation density, ρ, as: σ = σy + αGbρ1/2
(3)
where σ y is the yield stress and it is a function of the concentration of solute atoms, grain size and temperature, α a constant, G the shear modulus and b is the magnitude of the Burgers vector. The dislocation structure changes during deformation and therefore, the flow stress also changes with strain and temperature. The change of the flow stress with strain is characterised by the work-hardening rate Θ = ∂σ/∂ε, which decreases with strain. Additions of Al atoms to Mg4Li cause the formation of precipitates (Al2 Li and Mg17 Al12 ), which leads to remarkable age hardening as observed also in [6–8]. Precipitates also influence the storage of dislocations during plastic deformation. In the LA43 and LA45 alloys, there are obstacles of non-dislocation type and forest dislocations. On the other hand, processes such as cross-slip and climb of dislocations contribute to softening. For simplicity, we can consider that the total dislocation density is the characteristic parameter of the evolution of microstructure during deformation. According to the model of Luk´acˇ and Bal´ık [9], the stress dependence of the work-hardening rate for polycrystals can thus be expressed in the following form: Θ=
A + B − C(σ − σy ) − D(σ − σ)3 σ − σy
(4)
Parameter A is connected with the interaction of dislocations with the non-dislocation obstacles. Parameter B characterises the work-hardening due to interaction with the forest dislocations. Both parameters A and B do not change with temperature, except at higher temperatures, when the activity of non-basal slip increases and it may change the forest dislocation density. The parameter C relates to recovery due to cross-slip, the parameter D is connected with a local climb of dislocation. Both parameters should increase with temperature. At higher temperatures, cross-slip becomes a significant recovery process responsible for softening. The result may be a quasi steady state character of the flow stress curves at higher temperatures, which is experimentally observed. The values of the activation volume and its stress dependence indicate thermally activated processes
during deformation. The motion of jogged dislocations is very probably the thermally activated process at room temperature. The forest dislocation (dislocations in the pyramidal systems) density increases during straining. Therefore, a decrease in the activation volume can be explained by a decrease in the mean jog spacing due to the intersection of moving dislocations with the forest dislocations. Jogs may also be produced by cross-slip of screw dislocations. The very low values of the activation volume at 200 ◦ C may be explained assuming that the rate-controlling mechanism is dislocation climb. A local maximum in the temperature variation of σ 02 at 100 ◦ C and the post-stress relaxation effect indicate dynamic strain ageing. The post-stress relaxation effect – an increase or a decrease in the flow stress after the SR – may be described by a complex partial differential equation [10]. Solute atoms locking dislocations cause the stress increase after SR, which depends on strain and on temperature. An increase in the flow stress is needed to move the dislocations after stress relaxation. The local solute concentration increment, c, can be expressed as: −tw p c = c − c0 = cM 1 − exp , (5) t0 where c is the local solute concentration in the dislocation core, c0 the nominal solute concentration in the matrix and cM is the maximum concentration increment. The exponent p is typically 2/3 and 1/3 for bulk and pipe diffusion, respectively. The relaxation time t0 depends on the binding energy between a dislocation and a solute atom, on solute concentration, and on the diffusion coefficient of solute atoms. t0 is inversely proportional to the diffusion coefficient in the case of bulk diffusion, whereas for pipe diffusion 1/t0 ∼ Dρ3/2 [11], where ρ is the density of forest dislocations. The mean waiting time for successful activation, tw , is connected with the strain rate by the Orowan equation: ε˙ =
bρm Λ bΩ = tw tw
(6)
where ρm is the mobile dislocation density and Λ = 1/ρ1/2 is the mean free path of dislocations. The elementary plastic strain per activation event Ω = bρm ρ−1/2 is strain dependent; it may have a local maximum at a certain strain [12,13]. σ is simply related to the kinetic law c(tw ) Eq. (5) as: σ = (f1 + f2 )c(tw ).
(7)
The first term f1 corresponds to the dislocation–dislocation interaction influenced by dynamic strain ageing and the second term f2 results from the solute atoms-dislocation interaction influenced by dynamic strain ageing. It is reasonable to consider that σ is proportional to the number of impurities on the dislocation lines. The dislocation density increases with strain. But the concentration of solute atoms is constant. At a certain strain, the Ω starts to decrease with strain [12,13] and hence, tw also decreases. This leads to a decrease in σ with strain for strains higher than a critical one, which is observed.
Z. Trojanov´a et al. / Materials Science and Engineering A 410–411 (2005) 148–151
151
5. Conclusions
References
The deformation behaviour of Mg4Li alloy depends significantly on the testing temperature. At and above a certain temperature, softening processes influence the course of the work-hardening curve. Increasing activity of softening processes with increasing temperature causes the differences in the deformation behaviour. Cross-slip and climb of dislocations may be responsible for the observed softening. In LA43 and LA45, dynamic strain ageing causes the post-stress relaxation effect—the flow stress at the beginning of plastic deformation after SR is higher than that at the beginning of the SR.
[1] M.O. Pekguleryuz, A.A. Kaya, Adv. Eng. Mater. 5 (2003) 866. [2] H. Haferkamp, C. Jaschik, P. Juchmann, V. Kaese, M. Niemeyr, P. Tai, Mat. Wiss. Werkstofftech 32 (2001) 25. [3] S. Kamado, Y. Kojima, Metall. Sci. Technol. 16 (1998) 45. [4] S.R. Agnew, M.H. Yoo, N. Tom´e, Acta Mater. 49 (2001) 4277. [5] A. Pawełek, Z. Jasie´nski, S. K´udela, A. Pi˛atkowski, P. Ranachowski, F. Rejmund, in: J. Jerz (Ed.), Proceedings of the International Conference Advanced Metallic Materials, Smolenice, Slovakia, 2003, p. 225. [6] A. Alamo, A.D. Banchik, J. Mater. Sci. 15 (1980) 222. [7] K. Matsuzawa, T. Koshihara, S. Ochiai, Y. Kojima, J. Jpn. Inst. Light Met. 30 (1989) 45. [8] T. Aida, H. Hatta, C.S. Ramesh, S. Kamado, Y. Kojima, J. Jpn. Inst. Light Met. 47 (1997) 195. [9] J. Bal´ık, P. Luk´acˇ , Key. Eng. Mater. 97–98 (1998) 307. [10] A. Dlouh´y, P. Luk´acˇ , Z. Trojanov´a, Kovove Mater. 26 (1984) 688. [11] J. Friedel, Dislocations, Pergamon Press, Oxford, 1964. [12] L.P. Kubin, Y. Estrin, Acta Metall. Mater. 38 (1990) 697. [13] J. Bal´ık, P. Luk´acˇ , Acta Metall. Mater. 41 (1993) 1447.
Acknowledgements The authors appreciate support from Grant Agency of the Czech Republic (Grant No. 106/03/0843) and the Grant Agency of the Academy of Sciences of the Czech Republic under Grant No. A2112303.