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An investigation of serrated yielding in 5000 series aluminum alloys
Materials Science and Engineering A354 (2003) 279 /285 www.elsevier.com/locate/msea
An investigation of serrated yielding in 5000 series aluminum alloys Wei Wen *, J.G. Morris Department of Chemical and Materials Engineering, University of Kentucky, 177 Anderson Hall, Lexington, KY 40506, USA Received 18 October 2002; received in revised form 20 December 2002
Abstract The effect of different thermal treatment temperatures (from 472 to 783 K) on the characteristics of serrated yielding of three commercial aluminum alloys, AA5052, AA5754 and AA5182, was investigated. In the high temperature treatment range, the stress drop (Ds ) decreases with increasing thermal treatment temperature. This is the result of an increase in grain size as the thermal treatment temperature increases and can be explained from the solute /dislocation interaction model. For these alloys without any thermal heat treatment after cold rolling and those with a 472 K heat treatment after cold rolling, there is a critical strain before the onset of serrated yielding. The critical strain is larger for those after 472K heat-treatment as compared to those without heattreatment. This result can be explained by the precipitation of Mg atoms from the solid solution when heat-treated at 472 K. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Aluminum alloys; Serrated yielding; Dynamic strain ageing; Critical strain
1. Introduction Serrated yielding, also known as the PLC effect, has been under intensive study for the past few decades. Till now, a few models have evolved to account for the different phenomena related to it. The two main branches of models are the dislocation-solute atom interaction models [1 /5] and dislocation-precipitate interaction models [6]. In the first models, researchers disagree at what point dynamic strain ageing occurs. Dynamic strain ageing is thought to be the underlying mechanism that causes the PLC effect. Solute atoms form atmospheres around the mobile dislocations and the dislocations then have to break away from the atmospheres and continue their motion. In one of the dislocation /solute atom interaction models [1 /3], dynamic strain ageing is thought to occur when the bulk diffusion of solute atoms has acquired a speed (through vacancies created during the deformation) high enough
* Corresponding author. Tel.: /1-859-257-4433; fax: /1-859-2321929. E-mail address: [email protected] (W. Wen).
to drag the moving dislocations. In the other model [4,5], the solute/dislocation interaction occurs mainly at the obstacles where dislocations are temporarily held. The solute atoms can then diffuse to these dislocations and cause atmospheres to form around them. To continue deformation, the dislocations have to break away from these atmospheres and multiply. This leads to serrated yielding. Our research results tend to favor the latter model. However, the results indicate that the obstacles arresting mobile dislocations are mainly grain boundaries since as the grain size increases, the grain boundaries */the main source of obstacles */decrease. Accordingly, the magnitude of serrations, the main measure of serrated yielding, decreases, which means dynamic strain ageing is not so intense. In addition, Mg atoms have long been thought to account for serrated yielding. This is also confirmed in our study. The three commercial materials have different Mg concentrations, and in accordance, the serration magnitude increases with increasing Mg content. At low test temperatures, when a large number of Mg atoms are precipitated out of solid solution, not only is the magnitude of the serrations decreased, but the serrated phenomenon is delayed.
0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00017-0
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2. Experimental Tests were carried out on three commercial Al /Mg alloys, i.e. AA5052, AA5754, and AA5182. The chemical compositions in weight percent are listed in Table 1. The materials were received in sheet form of 0.160 in. for AA5052 and AA5754, and 0.100 in. for AA5182. The as-received materials were then solution treated at 769 K for 4 h, cold rolled to 80% reduction and then heattreated at different temperatures ranging from 472 to 783 K for 4 h. Metallographic specimens were electropolished and anodized before examining on the optical microscope. The longitudinal-transverse sections were examined. Electrical resistivity tests were carried out for all the final conditions. For each sample, eight positions were measured and the average value was calculated. Tensile specimens with a gauge length of 31.7 mm and a width of 6 mm were cut from these samples. The cold rolled samples were also prepared for tensile tests for comparison with the other conditions. Tensile tests were conducted on an Instron tensile machine at room temperature with different initial strain rates ranging from 2.1 /103 to 3.29 /105 s1.
Fig. 1. The grain structure of AA5052 after homogenization at 769 K for 4 h followed by 80% cold rolling and then heat-treatment at (a) 472 K, (b) 589 K, (c) 727 K and (d) 783 K for 4 h.
3. Results and discussion 3.1. Microstructure Figs. 1 /3 show the grain structures for all three alloys under different heat-treatment conditions. With a 472 K heat-treatment, all three alloys possess a deformation structure with the grains elongated in the rolling direction. As temperature increases to 589 K, all three alloys are recrystallized. AA5182 alloy possesses an equiaxed grain structure while the other two alloys still possess partly elongated grains. With a further increase in temperature, the grains grow in all three alloys. Though there are some larger grains after the relatively low temperature heat-treatment, there are also a lot of newly nucleated grains which have a very small grain size. The average grain size is therefore smaller for the lower temperature heat-treated samples than for the higher temperature heat-treated samples. When the temperature increases to 783 K, AA5182 undergoes abnormal grain growth. Only one or two grains can be Table 1 Chemical compositions of the aluminum alloys used in this work (wt.%) Alloys
Mg
Si
Fe
Cu
Mn
Cr
AA5052 AA5754 AA5182
2.470 2.854 4.351
0.196 0.095 0.084
0.472 0.239 0.218
0.032 0.028 0.044
0.075 0.316 0.327
0.203 0.011 0.001
Fig. 2. The grain structure of AA5754 after homogenization at 769 K for 4 h followed by 80% cold rolling and then heat-treatment at (a) 472 K, (b) 589 K, (c) 727 K and (d) 783 K for 4 h.
observed in the metallographic specimens. This abnormal grain growth has a great effect on the serrated yielding as will be seen later in the discussion of the tensile results. 3.2. Electrical resistivity Fig. 4 shows the electrical resistivity change for the different heat-treatments. From room temperature to 472 K heat-treatment, there is a large decrease in electrical resistivity in AA5182 alloy, while there are only modest decreases in AA5052 and AA5754 alloys. According to earlier researchers’ results [7], Mg atoms in solid solution cause an average increase in resistivity of 0.54 mV cm per wt.%, while there is only a 0.22 mV cm per wt.% increase when the atoms are out of solid
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Fig. 5. Stress drop as a function of strain for all three alloys homogenized at 769 K for 4 h, then cold rolled to 80% reduction followed by annealing at 589 K for 4 h (/o˙/ /2.63/10 4 s 1).
Fig. 3. The grain structure of AA5052 after homogenization at 769 K for 4 h followed by 80% cold rolling and then heat-treatment at (a) 472 K, (b) 589 K, (c) 727 K and (d) 783 K for 4 h.
For these alloys, as the heat-treatment temperature increases from 589 K, the maximum magnitude of the serrations (Ds) [8,9] decreases in accordance with an increase in grain size. Figs. 6 /8 show the strain/stress drop relations for all three alloys heat-treated at different temperatures. For AA5182 heat-treated at 783 K, the stress drop has decreased to such a degree, that it is hard to measure it accurately. In addition, the magnitude of the stress drop does not increase significantly during the whole deformation process. All of these phenomena appear to support the theory that dynamic strain ageing occurs mainly at obstacles at
Fig. 4. The electrical resistivity of AA5052, AA5754 and AA5182 after homogenization at 769 K followed by 80% cold-rolling and then different heat-treatments.
solution. The large decrease in AA5182 indicates that a large amount of Mg atoms were precipitated out of solid solution since under such low temperature heat-treatment, the materials haven’t been recrystallized and there should be little change in resistivity if no Mg atoms are precipitated out. The precipitation of Mg atoms causes a decrease in the magnitude and also a delay of serrated yielding as will be seen later. 3.3. Tensile test and serrated yielding
Fig. 6. Stress drop as a function of strain for AA5052 annealed at different temperatures (/o˙/ /2.63 /10 4 s 1).
As is well known, Mg atoms are the main factor causing serrated yielding, also called the PLC effect, and its intensity increases with increasing Mg content. Serrated yielding is seen in all three alloys under the imposed testing temperature and strain rate range conditions. It also increases from AA5052 alloy to AA5754 alloy and then to AA5182 alloy under the same treatment and testing conditions. Fig. 51 shows the stress drop for all three alloys. 1 Plastic strain in this and some of the other figures refers to the plastic strain on the stress /strain curve.
Fig. 7. Stress drop as a function of strain for AA5754 annealed at different temperatures (/o˙/ /2.63 /10 4 s 1).
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3.4. Luders bands and acoustic emission
Fig. 8. Stress drop as a function of strain for AA5182 annealed at different temperatures (/o˙/ /2.63/10 4 s 1).
which the moving dislocations are temporarily held. The Mg atoms then diffuse to these dislocations and cause them to be locked. The grain boundaries are formed by large numbers of dislocations and they are strong enough to block the mobile dislocations. They are also high-energy areas that can attract large numbers of Mg atoms. When the mobile dislocations are held temporarily by the grain boundaries, Mg atoms can then diffuse to them and form atmospheres. The smaller the grains, the greater the number of grain boundaries along the line of the mobile dislocations and therefore the greater the number of dislocation tangles that are strong enough to hold the mobile dislocations long enough to let Mg atoms form atmospheres around them. See Fig. 9 for an illustration. Also this greater number of dislocation tangles attracts more Mg atoms. This double effect acts to make smaller grain samples possess larger stress drops. When there are only one or two grains within the test sample, the grain boundaries are rarely the barriers to the mobile dislocations. Mobile dislocations tend to move within one grain and there are no barriers that are as strong as the grain boundaries. Occasionally these mobile dislocations are held up by other obstacles, such as diffuse dislocation tangles or secondary precipitates, but they are not strong enough to hold the dislocations long enough to allow the Mg atoms to diffuse to them. In addition, Mg atoms tend to distribute more evenly within the grain and without other defects to assist their diffusion, only a very small number of these atoms move to the dislocations. Thus, the dislocations break away from the Mg atoms much more easily and therefore the stress drop is very weak.
Luders bands were observed when serrated yielding occurred on the stress /strain curve. They usually nucleated near the end of the samples and moved at a fast speed. During the subsequent deformation process, their thickness increased while their speed decreased. This indicates that initially the density of mobile dislocations was small and to accommodate the constant crosshead speed the dislocations had to move fast. However, in the later stages of deformation more dislocations were generated through multiplication and each one needed to make a much smaller movement in order to maintain the same crosshead speed. As more and more mobile dislocations were generated, the width of the moving Luders band increased. In the unrecrystalllized samples, i.e. samples with a 472 K heattreatment, the Luders bands started out with a larger width than those in the recrystallized samples. This in itself indicates that the initial dislocation density was smaller in the recrystallized samples than in the unrecrystalllized samples. Except for AA5182 alloy heattreated at 783 K, all other alloys exhibited Luders bands that were aligned about 50 /608 to the tensile axis. This observation can be explained from a consideration of the force that the moving dislocations encountered at the grain boundaries as they move along the gauge length. See Fig. 9(a) and Fig. 10 for an illustration. If the moving dislocations are aligned in other directions, such as illustrated in Fig. 11, the total boundary length they encounter would be longer than if they are aligned as in Fig. 9(a). Therefore the blocking force of the grain boundaries on the dislocations would be larger. To counter this force, the dislocations tend to align in a
Fig. 10. An illustration of the collective movement of a group of dislocations.
Fig. 9. An illustration of the collective movement of a group of dislocations along samples of different grain sizes.
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Fig. 11. An illustration of the collective movement of a group of dislocations oriented in two different directions along the gauge length of a single grain (the small dots around the dislocations are obstacles or Mg atoms).
direction of 50 /608 to the tensile axis. Thus, the mobile dislocations don’t have to overcome the blocking force of the grain boundaries all at once if they are aligned in the direction as depicted in Fig. 9(a). When some segments of the mobile dislocations were not confronted with grain boundaries, they advanced slightly and then were stopped by other grain boundaries. Other segments were first blocked, then they broke away and started moving. Therefore the moving dislocation group formed an angle with respect to the tensile axis. If the moving dislocations aligned the other way, such as indicated in Fig. 10, they had to overcome the obstacles at the same time, which required a much larger force. For AA5182 heat-treated at 783 K, the Luders bands aligned perpendicular to the tensile axis. In addition, these Luders bands moved at a much faster speed than those heat-treated under normal or lower temperature conditions using the same test conditions. As we have discussed before, AA5182 alloy after such a high temperature treatment has undergone severe abnormal grain growth, and its dynamic strain ageing effect is very weak. The Luders bands are mainly moving within one grain and don’t have to overcome such strong obstacles as grain boundaries. The alignment of the Luders bands were then related to this inner grain movement, thus the dislocations didn’t have to overcome the strong blocking force of the grain boundaries. By assuming the alignment in Fig. 9(a), the actual obstacles and Mg solute atoms that the mobile dislocations encountered would be less than if they were aligned as in Fig. 9(b). Since the mobile dislocations were not blocked by very strong obstacles, dislocation multiplication didn’t occur very often and when it did occur the number of newlygenerated mobile dislocations was much less. Therefore the width of the Luders bands didn’t change much during the whole deformation process and the bands continued to move at a high speed to accommodate the crosshead speed. Another phenomenon that was observed during the tensile test was acoustic emission. This mainly occurred in AA5182 alloy and only rarely in AA5052 and AA5754 alloys. It sounded like loud pops and was louder in AA5182 alloy than in the other two alloys. In AA5182 alloy it was easy to correlate the sound with the
characteristics of the stress /strain curve, since the sound was much louder and the stress drop was larger than in the other two alloys. The sound was always heard when the stress started to drop. Since the stress drop of one serration is commonly thought to be related to the breaking away of dislocations from the atmospheres, this sound was apparently caused by the large force that the dislocations had to overcome in order to break away from their atmospheres [10,11]. In addition, this acoustic emission only occurred during normal serrated yielding, that is, it did not occur when the PLC effect was very weak as in AA5182 alloy heat-treated at 783 K. Therefore the observed acoustic emission appears to be related to the actual number of Mg atoms taking part in serrated yielding. In AA5052 and AA5754 alloys, even though serrated yielding occurred, the critical number of Mg atoms was not large enough to cause significant acoustic emission. In AA5182 heat-treated at 783 K, even though the number of Mg atoms in the solid solution was much higher than in the other two alloys, the number of Mg atoms actually participating in dynamic strain ageing was small, therefore no acoustic emission was heard. 3.5. Critical strain For materials without heat-treatment and those with a 472 K heat-treatment after cold-rolling, there was a small amount of uniform deformation before the onset of serrated yielding. This small uniform strain is defined in the literature as the critical strain for the onset of serrated yielding [5,12]. This critical strain is thought to be necessary for the solute atoms to acquire a velocity large enough to diffuse to the mobile dislocations and form atmospheres. This event is accomplished with the aid of vacancies generated through plastic deformation. Our studies also show that the critical strain is larger for AA5182 alloy after a 472 K heat-treatment than without this heat-treatment. For the other two alloys, since the amount of critical strain was too small and the start of serrations was not easy to identify unequivocally, the measurement of their critical strain was not carried out. For the AA5182 alloy it can be seen there is a difference in the critical strain under the two condi-
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tions. Results show that there is a large drop in electrical resistivity for AA5182 alloy after the 472 K heattreatment and it is greater than it is after other higher temperature heat-treatments. It is commonly known that Mg atoms are precipitated out of solid solution in the temperature range 422 /477 K for alloys like AA5182, and that the resistivity decreases significantly due to this precipitation of Mg2Al3. The large decrease in resistivity can then be related to the large amount of Mg atoms precipitated out of solid solution in AA5182 alloy. In AA5052 and AA5754 alloys, there is a considerably smaller decrease of electrical resistivity, thus, not many Mg atoms are precipitated out since their original Mg concentration is low compared to AA5182 alloy. For AA5182 alloy annealed at 472 K, serrated yielding is expected to decrease when more Mg atoms are precipitated out of solid solution, since the Mg atoms in solid solution account for the serrated yielding phenomenon. This effect was observed in our studies since for the samples annealed at 472 K there was not only an increase in the critical strain for the onset of serrations but also a decrease in the intensity of serrations as compared to material without the 472 K anneal. See Fig. 12 for a comparison of the critical strain variation with respect to strain rate for the two different treatment conditions. Fig. 13 shows the stress /strain curve for AA5182 after the two different treatments. Indirectly, our results also discredit the dislocation / precipitate interaction model. As more solute atoms are precipitated out of solid solution, we did not see an increase in intensity of serrated yielding as is proposed in that model. On the contrary, the magnitude of serrated yielding clearly decreased.
Fig. 13. The stress /strain relation for AA5182 with two different treatment conditions, (/o˙1/ /2.63 /10 4 s 1) upper curve corresponds to the one without heat-treatment after cold rolling and the lower curve corresponds to the one with 472 K heat-treatment.
Fig. 14. Stress drop /strain relation for AA5052 heat-treated at 589 K and tested and two different strain rates: o˙1/ /1.05/10 3 s 1, and o˙2/ /2.63/10 4 s 1.
3.6. Strain rate effect Strain rate studies can give a good indication of the intensity of dynamic strain ageing. As can be seen in Figs. 14 /16, as the strain rate increases the magnitude of the stress drop decreases. This appears to be due to the fact that as the strain rate increases the velocity of mobile dislocations increases. Thus, the waiting time
Fig. 15. Stress drop /strain relation for AA5754 heat-treated at 589 K and tested and two different strain rate: o˙1/ /1.05/10 3 s 1, and o˙2/ / 2.63 /10 3 s 1.
Fig. 12. The critical strain as a function of strain rate for AA5182 under two different conditions, one without heat-treatment (room temperature) and the other one heat-treated at 472 K for 4 h.
Fig. 16. Stress drop /strain relation for AA5182 heat-treated at 589 K and tested and two different strain rate: o˙1/ /1.05/10 3 s 1, and o˙2/ / 2.63 /10 4 s 1.
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[13] that the mobile dislocations spend at obstacles, mainly grain boundaries, decreases, and therefore the number of Mg atoms able to diffuse to the dislocations to form atmospheres also decreases. The dislocations then break away from the diluted atmospheres. The force needed for the dislocations to break away thus decreases which is manifested as a decrease in the stress drop. 3.7. Summary (1) Serrated yielding occurs for all three aluminum alloys (AA5052, AA5754 and AA5182), with the intensity increasing as the Mg concentration increases. (2) The magnitude of serrations increases as strain increases before necking occurs and decreases as the grain size increases. (3) For AA5182 heat-treated at 783 K, which has undergone abnormal grain growth, the serrated yielding phenomenon becomes so weak that it is not easy to identify the stress drop. (4) Luders bands were observed in all samples tested and they usually increased in width and decreased in velocity as the plastic deformation proceeded. For AA5182 alloy heat-treated at 783 K, there was not much change in both width and velocity of the Luders bands. (5) Acoustic emission occurred in AA5182 alloy under normal treatment conditions and rarely in AA5052 and AA5754 alloys. It was not detected at all in AA5182 alloy heat-treated at 783 K. The acoustic emission appears to be related to the number of Mg atoms actually participating in the dynamic strain ageing process.
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(6) For AA5182 heat-treated at 472 K, there was a big decrease in electrical resistivity and correspondingly there was an increase in the critical strain for the onset of serrated yielding under this condition. All observations indicate that the number of Mg atoms in solid solution is the main factor associated with the intensity of serrated yielding and when Mg is precipitated out of solid solution, the effect decreases. (7) Our results are in support of the dislocation-solute atom interaction model for the explanation of serrated yielding and the existence of Luders bands. Our results tend to discredit the dislocation /precipitate interaction model used to explain both serrated yielding and the existence of Luders bands.
References [1] A.H. Cottrell, Phil. Mag. 44 (1953) 829. [2] R.K. Ham, D. Jaffrey, Phil. Mag. 15 (1967) 247. [3] P.J. Worthington, B.J. Brindley, Phil. Mag. 19 (1969) 1175. [4] A.W. Sleeswyk, Acta Metall. 6 (1958) 598. [5] P.G. McCormick, Acta Metall. 20 (1972) 351. [6] R. Ondera, H. Era, T. Ishibash, M. Shimizu, Acta Metall. 31 (1983) 1589. [7] D. Park, A Study of Serrated Flow, Strain Rate Sensitivity, and Formability in an Al /Mn /Mg Alloy, Ph. D. thesis, University of Kentucky, 1993, p. 93. [8] B.H. Tian, Mater. Sci. Eng. A347 (1998) 264. [9] P.R. Cetlin, Metall. Trans. 4 (1973) 514. [10] C. Chmelik, Acta Mater. 46 (1988) 4433. [11] E. Pink, Mater. Sci Eng. A280 (2000) 17. [12] D.M. Riley, P.G. McCormick, Acta Metall. 25 (1977) 181. [13] Z. Kovacs, Mater. Sci. Eng. A279 (2000) 179.
An investigation of serrated yielding in 5000 series aluminum alloys Wei Wen *, J.G. Morris Department of Chemical and Materials Engineering, University of Kentucky, 177 Anderson Hall, Lexington, KY 40506, USA Received 18 October 2002; received in revised form 20 December 2002
Abstract The effect of different thermal treatment temperatures (from 472 to 783 K) on the characteristics of serrated yielding of three commercial aluminum alloys, AA5052, AA5754 and AA5182, was investigated. In the high temperature treatment range, the stress drop (Ds ) decreases with increasing thermal treatment temperature. This is the result of an increase in grain size as the thermal treatment temperature increases and can be explained from the solute /dislocation interaction model. For these alloys without any thermal heat treatment after cold rolling and those with a 472 K heat treatment after cold rolling, there is a critical strain before the onset of serrated yielding. The critical strain is larger for those after 472K heat-treatment as compared to those without heattreatment. This result can be explained by the precipitation of Mg atoms from the solid solution when heat-treated at 472 K. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Aluminum alloys; Serrated yielding; Dynamic strain ageing; Critical strain
1. Introduction Serrated yielding, also known as the PLC effect, has been under intensive study for the past few decades. Till now, a few models have evolved to account for the different phenomena related to it. The two main branches of models are the dislocation-solute atom interaction models [1 /5] and dislocation-precipitate interaction models [6]. In the first models, researchers disagree at what point dynamic strain ageing occurs. Dynamic strain ageing is thought to be the underlying mechanism that causes the PLC effect. Solute atoms form atmospheres around the mobile dislocations and the dislocations then have to break away from the atmospheres and continue their motion. In one of the dislocation /solute atom interaction models [1 /3], dynamic strain ageing is thought to occur when the bulk diffusion of solute atoms has acquired a speed (through vacancies created during the deformation) high enough
* Corresponding author. Tel.: /1-859-257-4433; fax: /1-859-2321929. E-mail address: [email protected] (W. Wen).
to drag the moving dislocations. In the other model [4,5], the solute/dislocation interaction occurs mainly at the obstacles where dislocations are temporarily held. The solute atoms can then diffuse to these dislocations and cause atmospheres to form around them. To continue deformation, the dislocations have to break away from these atmospheres and multiply. This leads to serrated yielding. Our research results tend to favor the latter model. However, the results indicate that the obstacles arresting mobile dislocations are mainly grain boundaries since as the grain size increases, the grain boundaries */the main source of obstacles */decrease. Accordingly, the magnitude of serrations, the main measure of serrated yielding, decreases, which means dynamic strain ageing is not so intense. In addition, Mg atoms have long been thought to account for serrated yielding. This is also confirmed in our study. The three commercial materials have different Mg concentrations, and in accordance, the serration magnitude increases with increasing Mg content. At low test temperatures, when a large number of Mg atoms are precipitated out of solid solution, not only is the magnitude of the serrations decreased, but the serrated phenomenon is delayed.
0921-5093/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(03)00017-0
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2. Experimental Tests were carried out on three commercial Al /Mg alloys, i.e. AA5052, AA5754, and AA5182. The chemical compositions in weight percent are listed in Table 1. The materials were received in sheet form of 0.160 in. for AA5052 and AA5754, and 0.100 in. for AA5182. The as-received materials were then solution treated at 769 K for 4 h, cold rolled to 80% reduction and then heattreated at different temperatures ranging from 472 to 783 K for 4 h. Metallographic specimens were electropolished and anodized before examining on the optical microscope. The longitudinal-transverse sections were examined. Electrical resistivity tests were carried out for all the final conditions. For each sample, eight positions were measured and the average value was calculated. Tensile specimens with a gauge length of 31.7 mm and a width of 6 mm were cut from these samples. The cold rolled samples were also prepared for tensile tests for comparison with the other conditions. Tensile tests were conducted on an Instron tensile machine at room temperature with different initial strain rates ranging from 2.1 /103 to 3.29 /105 s1.
Fig. 1. The grain structure of AA5052 after homogenization at 769 K for 4 h followed by 80% cold rolling and then heat-treatment at (a) 472 K, (b) 589 K, (c) 727 K and (d) 783 K for 4 h.
3. Results and discussion 3.1. Microstructure Figs. 1 /3 show the grain structures for all three alloys under different heat-treatment conditions. With a 472 K heat-treatment, all three alloys possess a deformation structure with the grains elongated in the rolling direction. As temperature increases to 589 K, all three alloys are recrystallized. AA5182 alloy possesses an equiaxed grain structure while the other two alloys still possess partly elongated grains. With a further increase in temperature, the grains grow in all three alloys. Though there are some larger grains after the relatively low temperature heat-treatment, there are also a lot of newly nucleated grains which have a very small grain size. The average grain size is therefore smaller for the lower temperature heat-treated samples than for the higher temperature heat-treated samples. When the temperature increases to 783 K, AA5182 undergoes abnormal grain growth. Only one or two grains can be Table 1 Chemical compositions of the aluminum alloys used in this work (wt.%) Alloys
Mg
Si
Fe
Cu
Mn
Cr
AA5052 AA5754 AA5182
2.470 2.854 4.351
0.196 0.095 0.084
0.472 0.239 0.218
0.032 0.028 0.044
0.075 0.316 0.327
0.203 0.011 0.001
Fig. 2. The grain structure of AA5754 after homogenization at 769 K for 4 h followed by 80% cold rolling and then heat-treatment at (a) 472 K, (b) 589 K, (c) 727 K and (d) 783 K for 4 h.
observed in the metallographic specimens. This abnormal grain growth has a great effect on the serrated yielding as will be seen later in the discussion of the tensile results. 3.2. Electrical resistivity Fig. 4 shows the electrical resistivity change for the different heat-treatments. From room temperature to 472 K heat-treatment, there is a large decrease in electrical resistivity in AA5182 alloy, while there are only modest decreases in AA5052 and AA5754 alloys. According to earlier researchers’ results [7], Mg atoms in solid solution cause an average increase in resistivity of 0.54 mV cm per wt.%, while there is only a 0.22 mV cm per wt.% increase when the atoms are out of solid
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Fig. 5. Stress drop as a function of strain for all three alloys homogenized at 769 K for 4 h, then cold rolled to 80% reduction followed by annealing at 589 K for 4 h (/o˙/ /2.63/10 4 s 1).
Fig. 3. The grain structure of AA5052 after homogenization at 769 K for 4 h followed by 80% cold rolling and then heat-treatment at (a) 472 K, (b) 589 K, (c) 727 K and (d) 783 K for 4 h.
For these alloys, as the heat-treatment temperature increases from 589 K, the maximum magnitude of the serrations (Ds) [8,9] decreases in accordance with an increase in grain size. Figs. 6 /8 show the strain/stress drop relations for all three alloys heat-treated at different temperatures. For AA5182 heat-treated at 783 K, the stress drop has decreased to such a degree, that it is hard to measure it accurately. In addition, the magnitude of the stress drop does not increase significantly during the whole deformation process. All of these phenomena appear to support the theory that dynamic strain ageing occurs mainly at obstacles at
Fig. 4. The electrical resistivity of AA5052, AA5754 and AA5182 after homogenization at 769 K followed by 80% cold-rolling and then different heat-treatments.
solution. The large decrease in AA5182 indicates that a large amount of Mg atoms were precipitated out of solid solution since under such low temperature heat-treatment, the materials haven’t been recrystallized and there should be little change in resistivity if no Mg atoms are precipitated out. The precipitation of Mg atoms causes a decrease in the magnitude and also a delay of serrated yielding as will be seen later. 3.3. Tensile test and serrated yielding
Fig. 6. Stress drop as a function of strain for AA5052 annealed at different temperatures (/o˙/ /2.63 /10 4 s 1).
As is well known, Mg atoms are the main factor causing serrated yielding, also called the PLC effect, and its intensity increases with increasing Mg content. Serrated yielding is seen in all three alloys under the imposed testing temperature and strain rate range conditions. It also increases from AA5052 alloy to AA5754 alloy and then to AA5182 alloy under the same treatment and testing conditions. Fig. 51 shows the stress drop for all three alloys. 1 Plastic strain in this and some of the other figures refers to the plastic strain on the stress /strain curve.
Fig. 7. Stress drop as a function of strain for AA5754 annealed at different temperatures (/o˙/ /2.63 /10 4 s 1).
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3.4. Luders bands and acoustic emission
Fig. 8. Stress drop as a function of strain for AA5182 annealed at different temperatures (/o˙/ /2.63/10 4 s 1).
which the moving dislocations are temporarily held. The Mg atoms then diffuse to these dislocations and cause them to be locked. The grain boundaries are formed by large numbers of dislocations and they are strong enough to block the mobile dislocations. They are also high-energy areas that can attract large numbers of Mg atoms. When the mobile dislocations are held temporarily by the grain boundaries, Mg atoms can then diffuse to them and form atmospheres. The smaller the grains, the greater the number of grain boundaries along the line of the mobile dislocations and therefore the greater the number of dislocation tangles that are strong enough to hold the mobile dislocations long enough to let Mg atoms form atmospheres around them. See Fig. 9 for an illustration. Also this greater number of dislocation tangles attracts more Mg atoms. This double effect acts to make smaller grain samples possess larger stress drops. When there are only one or two grains within the test sample, the grain boundaries are rarely the barriers to the mobile dislocations. Mobile dislocations tend to move within one grain and there are no barriers that are as strong as the grain boundaries. Occasionally these mobile dislocations are held up by other obstacles, such as diffuse dislocation tangles or secondary precipitates, but they are not strong enough to hold the dislocations long enough to allow the Mg atoms to diffuse to them. In addition, Mg atoms tend to distribute more evenly within the grain and without other defects to assist their diffusion, only a very small number of these atoms move to the dislocations. Thus, the dislocations break away from the Mg atoms much more easily and therefore the stress drop is very weak.
Luders bands were observed when serrated yielding occurred on the stress /strain curve. They usually nucleated near the end of the samples and moved at a fast speed. During the subsequent deformation process, their thickness increased while their speed decreased. This indicates that initially the density of mobile dislocations was small and to accommodate the constant crosshead speed the dislocations had to move fast. However, in the later stages of deformation more dislocations were generated through multiplication and each one needed to make a much smaller movement in order to maintain the same crosshead speed. As more and more mobile dislocations were generated, the width of the moving Luders band increased. In the unrecrystalllized samples, i.e. samples with a 472 K heattreatment, the Luders bands started out with a larger width than those in the recrystallized samples. This in itself indicates that the initial dislocation density was smaller in the recrystallized samples than in the unrecrystalllized samples. Except for AA5182 alloy heattreated at 783 K, all other alloys exhibited Luders bands that were aligned about 50 /608 to the tensile axis. This observation can be explained from a consideration of the force that the moving dislocations encountered at the grain boundaries as they move along the gauge length. See Fig. 9(a) and Fig. 10 for an illustration. If the moving dislocations are aligned in other directions, such as illustrated in Fig. 11, the total boundary length they encounter would be longer than if they are aligned as in Fig. 9(a). Therefore the blocking force of the grain boundaries on the dislocations would be larger. To counter this force, the dislocations tend to align in a
Fig. 10. An illustration of the collective movement of a group of dislocations.
Fig. 9. An illustration of the collective movement of a group of dislocations along samples of different grain sizes.
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Fig. 11. An illustration of the collective movement of a group of dislocations oriented in two different directions along the gauge length of a single grain (the small dots around the dislocations are obstacles or Mg atoms).
direction of 50 /608 to the tensile axis. Thus, the mobile dislocations don’t have to overcome the blocking force of the grain boundaries all at once if they are aligned in the direction as depicted in Fig. 9(a). When some segments of the mobile dislocations were not confronted with grain boundaries, they advanced slightly and then were stopped by other grain boundaries. Other segments were first blocked, then they broke away and started moving. Therefore the moving dislocation group formed an angle with respect to the tensile axis. If the moving dislocations aligned the other way, such as indicated in Fig. 10, they had to overcome the obstacles at the same time, which required a much larger force. For AA5182 heat-treated at 783 K, the Luders bands aligned perpendicular to the tensile axis. In addition, these Luders bands moved at a much faster speed than those heat-treated under normal or lower temperature conditions using the same test conditions. As we have discussed before, AA5182 alloy after such a high temperature treatment has undergone severe abnormal grain growth, and its dynamic strain ageing effect is very weak. The Luders bands are mainly moving within one grain and don’t have to overcome such strong obstacles as grain boundaries. The alignment of the Luders bands were then related to this inner grain movement, thus the dislocations didn’t have to overcome the strong blocking force of the grain boundaries. By assuming the alignment in Fig. 9(a), the actual obstacles and Mg solute atoms that the mobile dislocations encountered would be less than if they were aligned as in Fig. 9(b). Since the mobile dislocations were not blocked by very strong obstacles, dislocation multiplication didn’t occur very often and when it did occur the number of newlygenerated mobile dislocations was much less. Therefore the width of the Luders bands didn’t change much during the whole deformation process and the bands continued to move at a high speed to accommodate the crosshead speed. Another phenomenon that was observed during the tensile test was acoustic emission. This mainly occurred in AA5182 alloy and only rarely in AA5052 and AA5754 alloys. It sounded like loud pops and was louder in AA5182 alloy than in the other two alloys. In AA5182 alloy it was easy to correlate the sound with the
characteristics of the stress /strain curve, since the sound was much louder and the stress drop was larger than in the other two alloys. The sound was always heard when the stress started to drop. Since the stress drop of one serration is commonly thought to be related to the breaking away of dislocations from the atmospheres, this sound was apparently caused by the large force that the dislocations had to overcome in order to break away from their atmospheres [10,11]. In addition, this acoustic emission only occurred during normal serrated yielding, that is, it did not occur when the PLC effect was very weak as in AA5182 alloy heat-treated at 783 K. Therefore the observed acoustic emission appears to be related to the actual number of Mg atoms taking part in serrated yielding. In AA5052 and AA5754 alloys, even though serrated yielding occurred, the critical number of Mg atoms was not large enough to cause significant acoustic emission. In AA5182 heat-treated at 783 K, even though the number of Mg atoms in the solid solution was much higher than in the other two alloys, the number of Mg atoms actually participating in dynamic strain ageing was small, therefore no acoustic emission was heard. 3.5. Critical strain For materials without heat-treatment and those with a 472 K heat-treatment after cold-rolling, there was a small amount of uniform deformation before the onset of serrated yielding. This small uniform strain is defined in the literature as the critical strain for the onset of serrated yielding [5,12]. This critical strain is thought to be necessary for the solute atoms to acquire a velocity large enough to diffuse to the mobile dislocations and form atmospheres. This event is accomplished with the aid of vacancies generated through plastic deformation. Our studies also show that the critical strain is larger for AA5182 alloy after a 472 K heat-treatment than without this heat-treatment. For the other two alloys, since the amount of critical strain was too small and the start of serrations was not easy to identify unequivocally, the measurement of their critical strain was not carried out. For the AA5182 alloy it can be seen there is a difference in the critical strain under the two condi-
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tions. Results show that there is a large drop in electrical resistivity for AA5182 alloy after the 472 K heattreatment and it is greater than it is after other higher temperature heat-treatments. It is commonly known that Mg atoms are precipitated out of solid solution in the temperature range 422 /477 K for alloys like AA5182, and that the resistivity decreases significantly due to this precipitation of Mg2Al3. The large decrease in resistivity can then be related to the large amount of Mg atoms precipitated out of solid solution in AA5182 alloy. In AA5052 and AA5754 alloys, there is a considerably smaller decrease of electrical resistivity, thus, not many Mg atoms are precipitated out since their original Mg concentration is low compared to AA5182 alloy. For AA5182 alloy annealed at 472 K, serrated yielding is expected to decrease when more Mg atoms are precipitated out of solid solution, since the Mg atoms in solid solution account for the serrated yielding phenomenon. This effect was observed in our studies since for the samples annealed at 472 K there was not only an increase in the critical strain for the onset of serrations but also a decrease in the intensity of serrations as compared to material without the 472 K anneal. See Fig. 12 for a comparison of the critical strain variation with respect to strain rate for the two different treatment conditions. Fig. 13 shows the stress /strain curve for AA5182 after the two different treatments. Indirectly, our results also discredit the dislocation / precipitate interaction model. As more solute atoms are precipitated out of solid solution, we did not see an increase in intensity of serrated yielding as is proposed in that model. On the contrary, the magnitude of serrated yielding clearly decreased.
Fig. 13. The stress /strain relation for AA5182 with two different treatment conditions, (/o˙1/ /2.63 /10 4 s 1) upper curve corresponds to the one without heat-treatment after cold rolling and the lower curve corresponds to the one with 472 K heat-treatment.
Fig. 14. Stress drop /strain relation for AA5052 heat-treated at 589 K and tested and two different strain rates: o˙1/ /1.05/10 3 s 1, and o˙2/ /2.63/10 4 s 1.
3.6. Strain rate effect Strain rate studies can give a good indication of the intensity of dynamic strain ageing. As can be seen in Figs. 14 /16, as the strain rate increases the magnitude of the stress drop decreases. This appears to be due to the fact that as the strain rate increases the velocity of mobile dislocations increases. Thus, the waiting time
Fig. 15. Stress drop /strain relation for AA5754 heat-treated at 589 K and tested and two different strain rate: o˙1/ /1.05/10 3 s 1, and o˙2/ / 2.63 /10 3 s 1.
Fig. 12. The critical strain as a function of strain rate for AA5182 under two different conditions, one without heat-treatment (room temperature) and the other one heat-treated at 472 K for 4 h.
Fig. 16. Stress drop /strain relation for AA5182 heat-treated at 589 K and tested and two different strain rate: o˙1/ /1.05/10 3 s 1, and o˙2/ / 2.63 /10 4 s 1.
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[13] that the mobile dislocations spend at obstacles, mainly grain boundaries, decreases, and therefore the number of Mg atoms able to diffuse to the dislocations to form atmospheres also decreases. The dislocations then break away from the diluted atmospheres. The force needed for the dislocations to break away thus decreases which is manifested as a decrease in the stress drop. 3.7. Summary (1) Serrated yielding occurs for all three aluminum alloys (AA5052, AA5754 and AA5182), with the intensity increasing as the Mg concentration increases. (2) The magnitude of serrations increases as strain increases before necking occurs and decreases as the grain size increases. (3) For AA5182 heat-treated at 783 K, which has undergone abnormal grain growth, the serrated yielding phenomenon becomes so weak that it is not easy to identify the stress drop. (4) Luders bands were observed in all samples tested and they usually increased in width and decreased in velocity as the plastic deformation proceeded. For AA5182 alloy heat-treated at 783 K, there was not much change in both width and velocity of the Luders bands. (5) Acoustic emission occurred in AA5182 alloy under normal treatment conditions and rarely in AA5052 and AA5754 alloys. It was not detected at all in AA5182 alloy heat-treated at 783 K. The acoustic emission appears to be related to the number of Mg atoms actually participating in the dynamic strain ageing process.
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(6) For AA5182 heat-treated at 472 K, there was a big decrease in electrical resistivity and correspondingly there was an increase in the critical strain for the onset of serrated yielding under this condition. All observations indicate that the number of Mg atoms in solid solution is the main factor associated with the intensity of serrated yielding and when Mg is precipitated out of solid solution, the effect decreases. (7) Our results are in support of the dislocation-solute atom interaction model for the explanation of serrated yielding and the existence of Luders bands. Our results tend to discredit the dislocation /precipitate interaction model used to explain both serrated yielding and the existence of Luders bands.
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