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Study on dynamic strain aging phenomenon of 3004 aluminum alloy
Materials Science and Engineering A 415 (2006) 53–58
Study on dynamic strain aging phenomenon of 3004 aluminum alloy Kaiping Peng ∗ , Wenzhe Chen, Kuangwu Qian College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350002, People’ Republic of China Accepted 17 August 2005
Abstract 3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10−5 to 5.56 × 10−3 s−1 ) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, Tt , was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than Tt . According to the activation energy, it is believed that the process at the temperature region lower than Tt is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations. © 2005 Elsevier B.V. All rights reserved. Keywords: Dynamic strain aging; Aluminum alloy; Serration flow; Dislocation
1. Introduction Within a certain regime of temperature and strain rate, serrations in the flow stress–strain curves occur during plastic deformation of alloys containing interstitial or substitutional solutes. The development of these serrations is generally referred to as the Portevin-Le Chatelier effect (PLE). The phenomenon of the PLE is common in various alloy systems [1–7]. The so called normal behaviour of the onset strain at which serrated flow starts (εc ), i.e. εc decreases with the test temperature or increases with the strain rate, is well explained by the commonly accepted dynamic strain aging model of serrated flow [8,9]. However, an inverse behaviour, i.e. εc increases with the test temperature or decreases with the strain rate, is also observed in several alloys, especially in some precipitation alloys, above a certain temperature [10–15]. Brechet and Estrin [16] have developed a theoretical model for some concentrated solid solutions or in alloys in which precipitation occurs, demonstrating how the shearing of the precipitates can give rise to a negative strain rate sensitivity, and consequently to serrated flow. 3004 Aluminum alloy for can body stock is of considerable economic importance in making two-piece beverage cans. ∗
Corresponding author. Tel.: +86 591 8789 3540; fax: +86 591 8789 3593. E-mail address: [email protected] (K. Peng).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.216
According to the forecasting of Alcem Company in Canada [17], the production of can body stock is about 4.2 billion kilograms in 2001, and it will be about 4.8 billon kilograms in the coming 5 years. The present intention is to reduce the thickness of the body stock gage from 0.32 to 0.25 mm. This remarkable decrease in gage requires the further improvement of the strength and formability of the currently used aluminum alloys. Most of the investigations of date, however, have concentrated on the effect of thermomechanical treatment on microstructure and earing behaviour of aluminum alloy during the plastic deformation process, and in rare cases touched the serrated yielding phenomenon. The current work is carried out to examine the characteristics of the serrated flow in aluminum alloy, mainly on the onset plastic strain for serrations and to investigate the temperature dependence of this critical strain εc . 2. Experimental procedure The chemical composition of the alloy is (in wt%): Mg 0.8; Mn 0.7; Fe 0.4; Si 0.2; Cu 0.15; Ti 0.03 and Al rem. The as received rods were machined into tensile specimens with 30 mm gauge length and 6 mm diameter. The specimens were annealed at 723 ± 3 K, and the average grain size is 0.08 mm. Tensile tests were carried out on an Instron 1185 machine in the temperature range of 233–573 K within the strain rate range
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of 5.56 × 10−5 to 5.56 × 10−3 s−1 . The specimens were cooled in the liquid nitrogen at the temperature of 233–273 K; the specimens were heated in the water at the temperature of 323–373 K; for tests at elevated temperature, a resistance-heating furnace was used with a temperature control of ±2 K. The accuracy of the test temperature in the whole parallel section of the specimen was controlled within ±2 K. The strain rate sensitivity (SRS) of flow stress was determined by strain rate change tests in which the initial strain rate of 5.56 × 10−4 s−1 was instantaneously increased to 5.56 × 10−3 s−1 and computed as the ratio of the change in flow stress (σ) at a constant strain (0.02) to the difference in the logarithm of the strain rate ( ln ε˙ ). At least three samples were tested under each temperature–strain rate condition and the average value of the property was calculated. The JEM-2010EX TEM with INCA-energy spectrometer was used to investigate the metallic foils of the specimens with the accelerate voltage of 200 kV.
Fig. 2. The relationship between σ 0.2 , σ u.t.s. and temperature at a strain rate of 5.56 × 10−4 s−1 .
3. Experimental results and discussion 3.1. The serrated flow of the material Table 1 summarized the tensile test results of the 3004 aluminum alloy under various loading conditions. It can be clearly seen that the serrated flow does appear in a certain temperature region under different strain rates. Fig. 1 shows the relationship between the strain rate sensitivity and deformation temperature. It can be seen from this figure that the strain rate sensitivity value is negative within Table 1 The occurrence of serrated flow phenomenon Strain rate (s−1 )
Temperature range (K)
5.56 × 10−5 2.78 × 10−4 5.56 × 10−4 5.56 × 10−3
253–373 253–373 253–423 273–423
the temperature region of 253–453 K, where the phenomenon of serrated flow (DSA) appears as shown in Table 1. This is reasonable and can be explained as follows: the mobile dislocations carrying by the plastic deformation are temporarily arrested at some localized obstacles, for example, the forest dislocations in the slip path. During the waiting time spent at obstacles, glide dislocations are subjected to an additional pinning by solute atoms diffusing towards the dislocations, such that the obstacle strength increases as the waiting times increase. As shorter waiting times correspond to larger average dislocation glide velocities and macroscopic strain rates, an inverse rate sensitivity of the flow stress may arise (strain rate softening). The aging by mobile solute atoms on the one hand, and the thermally activated unpinning of glide dislocations from their solute clouds on the other hand, represent a competing processes, which are entirely dynamic, local in nature, and, most importantly, alternating and repetitive. Then, dynamic strain aging (DSA) may give rise to a negative SRS, which manifests itself in repeated plastic oscillations. Fig. 2 shows the yield strength (YS) and ultimate strength (UTS) of the material as a function of the testing temperature at a strain rate of 5.56 × 10−4 s−1 . Because of DSA, the YS and UTS decreased slowly with increasing temperature in the temperature range of DSA. 3.2. The dependence of the critical plastic strain for the onset of serrations upon temperatures
Fig. 1. Relationship between strain rate sensitivity and temperature.
It is widely accepted that a critical plastic strain, εc , is necessary for the occurrence of DSA phenomenon under certain temperature and strain rate conditions, and εc is a function of temperature under constant applied strain rate. Fig. 3 shows the dependence of εc upon temperature under a constant strain rate of 5.56 × 10−4 s−1 . The result shows that the relationship between εc and tensile temperature T is different in different temperature regions. Within the lower temperature regions, εc has a negative temperature coefficient, i.e. εc decreases with increasing temperature. Within the higher temperature region, on the other hand,
K. Peng et al. / Materials Science and Engineering A 415 (2006) 53–58
Fig. 3. Dependence of the critical strain of the onset of serrations, εc , upon temperature T under a strain rate of 5.56 × 10−4 s−1 .
the coefficient changes to positive. We define the temperature Tt , where the temperature coefficient of εc transits from negative to positive as the ‘transition temperature’. (In the present case, it is about 322 K.) 3.3. The normal PLC effect The existing literature shows that for most FCC alloys εc has a negative temperature coefficient in general. This behaviour has been modelled in terms of the enhancement of solute diffusion by excess vacancies produced during deformation [18,19] or by refining the relation between dislocation mobility and waiting time and taking into account the strain dependence of the densities of mobile and forest dislocations [20–22]. Evidently, the higher the temperature, the stronger the mobility of solute atoms, and the smaller the critical strain. The present experimental result is consistent with the existing physical models, which attribute the serrated flow to originate from the pinning of dislocations by solute atoms of the alloy. In such serrated flow models, the critical strain εc for the onset of serrated flow has been related to the stain rate (˙ε) and temperature by an equation of the form [23] ε˙ kTb C1 3/2 Qm m+β εc = exp αCo NKUm Do kT where Co is the initial solute concentration in the bulk alloy, C1 the local solute concentration required for locking the dislocation, Um the solute-dislocation binding energy, Qm the activation energy for solute migration, Do the diffusion frequency factor, b the Burger’s vector, N, m, β and α are constants and T is the temm+β perature (in K). A plot of ln(εc /T ) versus 1/T for a given strain rate should yield an activation energy Q = slope × R, where R is the gas constant. However, the method needs evaluation of m+β (m + β) to compute εc at various temperatures where serrated flow is observed. This method has been employed to identify the elements responsible for serrated flow by determining activation energy for a large number of alloy systems [24–27]. Within the
55
Fig. 4. Relationship between strain rate and critical strain at 273 K.
lower temperature region of the present study (lower than Tt , 322 K), for example, the slope of 273 K plot on Fig. 4 yields the value of (m + β) as 2.13. This value of (m + β) is used to calculate the activation energy for the onset of serration from the m+β slope of the plot of ln(εc /T ) versus 1/T, as shown in Fig. 5. The activation energy at lower temperature range is 48.2 kJ/mol. This value falls within the range of the activation energy values (35–55 kJ/mol) measured for the onset of serrations in Al–Mg alloy [1,28]. This also is in reasonable agreement with the value of 53.1 kJ/mol, which is the activation energy for the migration of Mg atoms in Al while the activation energy for vacancy migration and the binding energy between Mg atom and vacancy are taken to be 72.4 kJ/mol [29] and 19.3 kJ/mol [30], respectively. The possibility of the migration of Mn atoms as the governing factor in serrated flow can be ruled out because the activation energy for the diffusion of Mn in Al is much higher (212 kJ/mol) [31]. Other solute atom will not cause PLC effects due to either low solid solubility or low solute diffusivity. Therefore, Mg is
m+β
Fig. 5. A plot of ln(εc /T ) as a function inverse of test temperature (1/T) at a strain rate of 5.56 × 10−4 s−1 .
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Fig. 7. EDS of the matrix of 3004 aluminum alloy.
Fig. 6. The microstructure of original state.
responsible for the effects at lower temperature range in this study. 3.4. The ‘inverse’ PLC effect Some previous investigations [11,12] and our own experimental results show that in some alloys, in which precipitationaging strengthening exists, the temperature coefficient of εc may be positive under certain conditions. The data shown in Fig. 2 indicating the effect of strain temperature on εc under a constant applied strain rate of 3004 aluminum alloy is another example. From Fig. 2 one can see clearly that εc decreases with increasing deformation temperature while it is lower than the transition temperature Tt (about 322 K), but εc increases with increasing deformation temperature while T is higher than Tt . In order to study the influence of precipitation on the PLC effect, the microstructure of various pre-treated specimens has been observed. Fig. 6 shows the electron-micrography of the original microstructure of the 3004 aluminum alloy. The dislocation density is very low with only a few visible dislocation lines and there is some second phase on the matrix. Fig. 7, Table 2 and Fig. 8, Table 3 is the energy spectrum results of the matrix and the second phase, respectively. According to the energy spectrum analysis, it is found that the matrix is composed of Al and the other element cannot be detected because
Fig. 8. EDS of second phase.
of the lower amount. The second phase is mainly composed of Al and Mn. Besides, the dislocation structure of the specimens by various pre-strain within the temperature range of “reverse PLC effect” at 398 K are shown in Fig. 9. When the pre-strain (1%) is smaller than the critical strain (2.7%), the dislocation density of the specimen is low and dose not appears aggregation (as shown in Fig. 9a). When the pre-strain is equal to the critical strain, the dislocation density is increased and shows a tendency of becoming dislocation tangle structure (as shown in Fig. 9b). When the pre-strain (4%) is larger than the critical strain, the dislocation of the specimen forms a typical tangle structure, as if it is pinned by something (as shown in Fig. 9c). The local energy spectrum analysis shows the existence of Mg element in that location (as shown in Fig. 10, Table 4) even if we did not find the visible second phase precipitation in the microstructure. This strongly confirms that Mg atoms certainly play an important role in pinning the moving dislocation and causes the serration flow. We have also found that the original existing second phase in 3004 aluminum alloy which does not contain Mg does not Table 3 EDS of second phase Element
Table 2 EDS of the matrix of 3004 aluminum alloy Element
wt%
at%
Al K
100.00
100.00
Total
100.00
wt%
Al K Mn K Si K Fe K Cu K
64.99 22.97 6.32 3.64 2.08
Total
100.00
at% 76.47 13.27 7.14 2.07 1.04
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Fig. 9. Dislocation structure after pre-strain at 398 K: (a) 1% pre-strain, (b) 2.7% pre-strain and (c) 4% pre-strain.
affect the PLC effect. The serrated flow of this alloy is caused by Mg atoms solely. In this alloy, Mg precipitates out in the form of Mg2 Si precipitates. The precipitation process starts from about 323 K but forms visible second phase at higher temperature. (It was reported that the visible semi-coherent precipitation of Mg2 Si takes place in the temperature range of
423–523 K [32,33].) Therefore, Mg cannot precipitate out during the tensile test. Hence, during the current tests, the cause of the inverse behaviour in this alloy cannot be the precipitation process. During tensile test, when deformation strain reaches εc , the aggregation of solute atoms at the dislocation increases the solute atom concentration locally. This aggregation will lead to a decrease of the “effective” concentration of Mg (which can freely diffuse to the moving dislocations and lock them) in the solid solution, it is necessary certainly, therefore, to increase further the plastic deformation and let more Mg atoms move to
Table 4 EDS of tangle dislocation Element
Fig. 10. EDS of tangle dislocation.
wt%
Al K CK Mg K
86.38 12.69 0.93
Total
100.00
at% 74.51 24.60 0.89
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the dislocations. It means that in this case, the critical strain, εc , increases and the temperature coefficient of εc become positive. 4. Conclusions (1) The serrated flow behaviour of 3004 aluminum alloy has been studied in the temperature range of 253–423 K at the strain rate range of 5.56 × 10−5 to 5.56 × 10−3 s−1 . Within these temperature–strain rate combinations, an increase in σ 0.2 , σ u.t.s. and negative strain rate sensitivity have been observed. These observations uncover the presence of DSA phenomenon in the material. (2) There is a critical transition temperature Tt . Within the T < Tt region the critical plastic strain εc of the onset of serrated yielding of the material has a negative temperature coefficient; within the T > Tt region the temperature coefficient of εc changes to positive. (3) The process at the temperature region lower than Tt is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, the aggregation of Mg atoms and precipitation of second phase decrease the amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations. Acknowledgements The present work was supported by the National Natural Science Foundation (Grants 50441013) and the Education Department of Fujian Province (Grants JA03015), China.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
D.M. Riley, P.G. McCormick, Acta Metall. 25 (1977) 181. L.J. Cuddy, W.C. Leslie, Acta Metall. 20 (1972) 1157. A. Van den Beukel, Acta Metall. 28 (1980) 965. E. Pink, A. Grinberg, Mater. Sci. Eng. 51 (1981) 1. M.C. Chaturvedi, D.J. Lloyd, Mater. Sci. Eng. 58 (1983) 107. V. Radmilovic, D. Drosnjak, M. Jovanovic, Mater. Sci. Technol. 5 (1989) 908. S. Venkadesan, C. Phaniraj, P.V. Sivaprasad, P. Rodrigues, Acta Metall. 40 (1992) 569. E. Rizzi, P. Hahner, Int. J. Plast. 20 (2004) 121. P. Hahner, Acta Mater. 45 (1997) 3695. S. Kumar, Scripta Metall. Mater. 33 (1995) 81. S. Kumar, J. Korl, E. Pink, Scripta Mater. 35 (1996) 775. J. Balrk, P. Lukac, L.P. Kubin, Scripta Mater. 42 (2000) 465. S. Kumar, E. Pink, Acta Mater. 45 (1997) 5295. E. Pink, Acta Metall. 37 (1989) 1773. Y. Brechet, Y. Estrin, Acta Metall. Mater. 43 (1995) 955. Y. Brechet, Y. Estrin, Scripta Metall. Mater. 31 (1994) 185. Z. Wang, Light Met. 4 (2001) 23 (in Chinese). P.G. McCormick, Acta Metall. Mater. 36 (1988) 3061. P. Hahner, E. Rizzi, Acta Mater. 51 (2003) 3385. L.P. Kubin, Y. Estrin, Acta Metall. Mater. 38 (1990) 697. S.J. Mesarovic, J. Mech. Phys. Solid 43 (1995) 671. L.P. Kubin, K. Chihab, Y. Estrin, Acta Metal. 36 (1988) 2707. P.G. McCormick, Acta Metall. 20 (1972) 351. I.S. Kim, Mc. Chaturvedi, Mater. Sci. 12 (1979) 691. K.W. Qian, R.E. Reed-Hill, Acta Metall. 31 (1983) 87. I.S. Kim, E.O. Hall, Met. Forum 1 (1978) 95. Mc. Chaturvedi, J. Logel, Mater. Sci. Eng. 58 (1983) 107. E. Pink, A. Grinberg, Mater. Sci. Eng. 51 (1981) 1. J. Bass, Philos. Mag. 15 (1967) 717. A.J. Perry, K.M. Entwistle, J. Inst. Met. 96 (1968) 344. G.M. Hood, R.J. Schultz, Philos. Mag. 23 (1971) 1479. G. Thomas, J. Inst. Met. 90 (1961) 57. L. Chen, J.G. Morris, Scripta Metall. 18 (1984) 1365.
Study on dynamic strain aging phenomenon of 3004 aluminum alloy Kaiping Peng ∗ , Wenzhe Chen, Kuangwu Qian College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350002, People’ Republic of China Accepted 17 August 2005
Abstract 3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10−5 to 5.56 × 10−3 s−1 ) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, Tt , was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than Tt . According to the activation energy, it is believed that the process at the temperature region lower than Tt is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations. © 2005 Elsevier B.V. All rights reserved. Keywords: Dynamic strain aging; Aluminum alloy; Serration flow; Dislocation
1. Introduction Within a certain regime of temperature and strain rate, serrations in the flow stress–strain curves occur during plastic deformation of alloys containing interstitial or substitutional solutes. The development of these serrations is generally referred to as the Portevin-Le Chatelier effect (PLE). The phenomenon of the PLE is common in various alloy systems [1–7]. The so called normal behaviour of the onset strain at which serrated flow starts (εc ), i.e. εc decreases with the test temperature or increases with the strain rate, is well explained by the commonly accepted dynamic strain aging model of serrated flow [8,9]. However, an inverse behaviour, i.e. εc increases with the test temperature or decreases with the strain rate, is also observed in several alloys, especially in some precipitation alloys, above a certain temperature [10–15]. Brechet and Estrin [16] have developed a theoretical model for some concentrated solid solutions or in alloys in which precipitation occurs, demonstrating how the shearing of the precipitates can give rise to a negative strain rate sensitivity, and consequently to serrated flow. 3004 Aluminum alloy for can body stock is of considerable economic importance in making two-piece beverage cans. ∗
Corresponding author. Tel.: +86 591 8789 3540; fax: +86 591 8789 3593. E-mail address: [email protected] (K. Peng).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.08.216
According to the forecasting of Alcem Company in Canada [17], the production of can body stock is about 4.2 billion kilograms in 2001, and it will be about 4.8 billon kilograms in the coming 5 years. The present intention is to reduce the thickness of the body stock gage from 0.32 to 0.25 mm. This remarkable decrease in gage requires the further improvement of the strength and formability of the currently used aluminum alloys. Most of the investigations of date, however, have concentrated on the effect of thermomechanical treatment on microstructure and earing behaviour of aluminum alloy during the plastic deformation process, and in rare cases touched the serrated yielding phenomenon. The current work is carried out to examine the characteristics of the serrated flow in aluminum alloy, mainly on the onset plastic strain for serrations and to investigate the temperature dependence of this critical strain εc . 2. Experimental procedure The chemical composition of the alloy is (in wt%): Mg 0.8; Mn 0.7; Fe 0.4; Si 0.2; Cu 0.15; Ti 0.03 and Al rem. The as received rods were machined into tensile specimens with 30 mm gauge length and 6 mm diameter. The specimens were annealed at 723 ± 3 K, and the average grain size is 0.08 mm. Tensile tests were carried out on an Instron 1185 machine in the temperature range of 233–573 K within the strain rate range
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of 5.56 × 10−5 to 5.56 × 10−3 s−1 . The specimens were cooled in the liquid nitrogen at the temperature of 233–273 K; the specimens were heated in the water at the temperature of 323–373 K; for tests at elevated temperature, a resistance-heating furnace was used with a temperature control of ±2 K. The accuracy of the test temperature in the whole parallel section of the specimen was controlled within ±2 K. The strain rate sensitivity (SRS) of flow stress was determined by strain rate change tests in which the initial strain rate of 5.56 × 10−4 s−1 was instantaneously increased to 5.56 × 10−3 s−1 and computed as the ratio of the change in flow stress (σ) at a constant strain (0.02) to the difference in the logarithm of the strain rate ( ln ε˙ ). At least three samples were tested under each temperature–strain rate condition and the average value of the property was calculated. The JEM-2010EX TEM with INCA-energy spectrometer was used to investigate the metallic foils of the specimens with the accelerate voltage of 200 kV.
Fig. 2. The relationship between σ 0.2 , σ u.t.s. and temperature at a strain rate of 5.56 × 10−4 s−1 .
3. Experimental results and discussion 3.1. The serrated flow of the material Table 1 summarized the tensile test results of the 3004 aluminum alloy under various loading conditions. It can be clearly seen that the serrated flow does appear in a certain temperature region under different strain rates. Fig. 1 shows the relationship between the strain rate sensitivity and deformation temperature. It can be seen from this figure that the strain rate sensitivity value is negative within Table 1 The occurrence of serrated flow phenomenon Strain rate (s−1 )
Temperature range (K)
5.56 × 10−5 2.78 × 10−4 5.56 × 10−4 5.56 × 10−3
253–373 253–373 253–423 273–423
the temperature region of 253–453 K, where the phenomenon of serrated flow (DSA) appears as shown in Table 1. This is reasonable and can be explained as follows: the mobile dislocations carrying by the plastic deformation are temporarily arrested at some localized obstacles, for example, the forest dislocations in the slip path. During the waiting time spent at obstacles, glide dislocations are subjected to an additional pinning by solute atoms diffusing towards the dislocations, such that the obstacle strength increases as the waiting times increase. As shorter waiting times correspond to larger average dislocation glide velocities and macroscopic strain rates, an inverse rate sensitivity of the flow stress may arise (strain rate softening). The aging by mobile solute atoms on the one hand, and the thermally activated unpinning of glide dislocations from their solute clouds on the other hand, represent a competing processes, which are entirely dynamic, local in nature, and, most importantly, alternating and repetitive. Then, dynamic strain aging (DSA) may give rise to a negative SRS, which manifests itself in repeated plastic oscillations. Fig. 2 shows the yield strength (YS) and ultimate strength (UTS) of the material as a function of the testing temperature at a strain rate of 5.56 × 10−4 s−1 . Because of DSA, the YS and UTS decreased slowly with increasing temperature in the temperature range of DSA. 3.2. The dependence of the critical plastic strain for the onset of serrations upon temperatures
Fig. 1. Relationship between strain rate sensitivity and temperature.
It is widely accepted that a critical plastic strain, εc , is necessary for the occurrence of DSA phenomenon under certain temperature and strain rate conditions, and εc is a function of temperature under constant applied strain rate. Fig. 3 shows the dependence of εc upon temperature under a constant strain rate of 5.56 × 10−4 s−1 . The result shows that the relationship between εc and tensile temperature T is different in different temperature regions. Within the lower temperature regions, εc has a negative temperature coefficient, i.e. εc decreases with increasing temperature. Within the higher temperature region, on the other hand,
K. Peng et al. / Materials Science and Engineering A 415 (2006) 53–58
Fig. 3. Dependence of the critical strain of the onset of serrations, εc , upon temperature T under a strain rate of 5.56 × 10−4 s−1 .
the coefficient changes to positive. We define the temperature Tt , where the temperature coefficient of εc transits from negative to positive as the ‘transition temperature’. (In the present case, it is about 322 K.) 3.3. The normal PLC effect The existing literature shows that for most FCC alloys εc has a negative temperature coefficient in general. This behaviour has been modelled in terms of the enhancement of solute diffusion by excess vacancies produced during deformation [18,19] or by refining the relation between dislocation mobility and waiting time and taking into account the strain dependence of the densities of mobile and forest dislocations [20–22]. Evidently, the higher the temperature, the stronger the mobility of solute atoms, and the smaller the critical strain. The present experimental result is consistent with the existing physical models, which attribute the serrated flow to originate from the pinning of dislocations by solute atoms of the alloy. In such serrated flow models, the critical strain εc for the onset of serrated flow has been related to the stain rate (˙ε) and temperature by an equation of the form [23] ε˙ kTb C1 3/2 Qm m+β εc = exp αCo NKUm Do kT where Co is the initial solute concentration in the bulk alloy, C1 the local solute concentration required for locking the dislocation, Um the solute-dislocation binding energy, Qm the activation energy for solute migration, Do the diffusion frequency factor, b the Burger’s vector, N, m, β and α are constants and T is the temm+β perature (in K). A plot of ln(εc /T ) versus 1/T for a given strain rate should yield an activation energy Q = slope × R, where R is the gas constant. However, the method needs evaluation of m+β (m + β) to compute εc at various temperatures where serrated flow is observed. This method has been employed to identify the elements responsible for serrated flow by determining activation energy for a large number of alloy systems [24–27]. Within the
55
Fig. 4. Relationship between strain rate and critical strain at 273 K.
lower temperature region of the present study (lower than Tt , 322 K), for example, the slope of 273 K plot on Fig. 4 yields the value of (m + β) as 2.13. This value of (m + β) is used to calculate the activation energy for the onset of serration from the m+β slope of the plot of ln(εc /T ) versus 1/T, as shown in Fig. 5. The activation energy at lower temperature range is 48.2 kJ/mol. This value falls within the range of the activation energy values (35–55 kJ/mol) measured for the onset of serrations in Al–Mg alloy [1,28]. This also is in reasonable agreement with the value of 53.1 kJ/mol, which is the activation energy for the migration of Mg atoms in Al while the activation energy for vacancy migration and the binding energy between Mg atom and vacancy are taken to be 72.4 kJ/mol [29] and 19.3 kJ/mol [30], respectively. The possibility of the migration of Mn atoms as the governing factor in serrated flow can be ruled out because the activation energy for the diffusion of Mn in Al is much higher (212 kJ/mol) [31]. Other solute atom will not cause PLC effects due to either low solid solubility or low solute diffusivity. Therefore, Mg is
m+β
Fig. 5. A plot of ln(εc /T ) as a function inverse of test temperature (1/T) at a strain rate of 5.56 × 10−4 s−1 .
K. Peng et al. / Materials Science and Engineering A 415 (2006) 53–58
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Fig. 7. EDS of the matrix of 3004 aluminum alloy.
Fig. 6. The microstructure of original state.
responsible for the effects at lower temperature range in this study. 3.4. The ‘inverse’ PLC effect Some previous investigations [11,12] and our own experimental results show that in some alloys, in which precipitationaging strengthening exists, the temperature coefficient of εc may be positive under certain conditions. The data shown in Fig. 2 indicating the effect of strain temperature on εc under a constant applied strain rate of 3004 aluminum alloy is another example. From Fig. 2 one can see clearly that εc decreases with increasing deformation temperature while it is lower than the transition temperature Tt (about 322 K), but εc increases with increasing deformation temperature while T is higher than Tt . In order to study the influence of precipitation on the PLC effect, the microstructure of various pre-treated specimens has been observed. Fig. 6 shows the electron-micrography of the original microstructure of the 3004 aluminum alloy. The dislocation density is very low with only a few visible dislocation lines and there is some second phase on the matrix. Fig. 7, Table 2 and Fig. 8, Table 3 is the energy spectrum results of the matrix and the second phase, respectively. According to the energy spectrum analysis, it is found that the matrix is composed of Al and the other element cannot be detected because
Fig. 8. EDS of second phase.
of the lower amount. The second phase is mainly composed of Al and Mn. Besides, the dislocation structure of the specimens by various pre-strain within the temperature range of “reverse PLC effect” at 398 K are shown in Fig. 9. When the pre-strain (1%) is smaller than the critical strain (2.7%), the dislocation density of the specimen is low and dose not appears aggregation (as shown in Fig. 9a). When the pre-strain is equal to the critical strain, the dislocation density is increased and shows a tendency of becoming dislocation tangle structure (as shown in Fig. 9b). When the pre-strain (4%) is larger than the critical strain, the dislocation of the specimen forms a typical tangle structure, as if it is pinned by something (as shown in Fig. 9c). The local energy spectrum analysis shows the existence of Mg element in that location (as shown in Fig. 10, Table 4) even if we did not find the visible second phase precipitation in the microstructure. This strongly confirms that Mg atoms certainly play an important role in pinning the moving dislocation and causes the serration flow. We have also found that the original existing second phase in 3004 aluminum alloy which does not contain Mg does not Table 3 EDS of second phase Element
Table 2 EDS of the matrix of 3004 aluminum alloy Element
wt%
at%
Al K
100.00
100.00
Total
100.00
wt%
Al K Mn K Si K Fe K Cu K
64.99 22.97 6.32 3.64 2.08
Total
100.00
at% 76.47 13.27 7.14 2.07 1.04
K. Peng et al. / Materials Science and Engineering A 415 (2006) 53–58
57
Fig. 9. Dislocation structure after pre-strain at 398 K: (a) 1% pre-strain, (b) 2.7% pre-strain and (c) 4% pre-strain.
affect the PLC effect. The serrated flow of this alloy is caused by Mg atoms solely. In this alloy, Mg precipitates out in the form of Mg2 Si precipitates. The precipitation process starts from about 323 K but forms visible second phase at higher temperature. (It was reported that the visible semi-coherent precipitation of Mg2 Si takes place in the temperature range of
423–523 K [32,33].) Therefore, Mg cannot precipitate out during the tensile test. Hence, during the current tests, the cause of the inverse behaviour in this alloy cannot be the precipitation process. During tensile test, when deformation strain reaches εc , the aggregation of solute atoms at the dislocation increases the solute atom concentration locally. This aggregation will lead to a decrease of the “effective” concentration of Mg (which can freely diffuse to the moving dislocations and lock them) in the solid solution, it is necessary certainly, therefore, to increase further the plastic deformation and let more Mg atoms move to
Table 4 EDS of tangle dislocation Element
Fig. 10. EDS of tangle dislocation.
wt%
Al K CK Mg K
86.38 12.69 0.93
Total
100.00
at% 74.51 24.60 0.89
58
K. Peng et al. / Materials Science and Engineering A 415 (2006) 53–58
the dislocations. It means that in this case, the critical strain, εc , increases and the temperature coefficient of εc become positive. 4. Conclusions (1) The serrated flow behaviour of 3004 aluminum alloy has been studied in the temperature range of 253–423 K at the strain rate range of 5.56 × 10−5 to 5.56 × 10−3 s−1 . Within these temperature–strain rate combinations, an increase in σ 0.2 , σ u.t.s. and negative strain rate sensitivity have been observed. These observations uncover the presence of DSA phenomenon in the material. (2) There is a critical transition temperature Tt . Within the T < Tt region the critical plastic strain εc of the onset of serrated yielding of the material has a negative temperature coefficient; within the T > Tt region the temperature coefficient of εc changes to positive. (3) The process at the temperature region lower than Tt is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, the aggregation of Mg atoms and precipitation of second phase decrease the amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations. Acknowledgements The present work was supported by the National Natural Science Foundation (Grants 50441013) and the Education Department of Fujian Province (Grants JA03015), China.
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