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Effect of aging treatment on heavily deformed microstructure of a 6061 aluminum alloy after equal channel angular pressing
Scripta Materialia 45 (2001) 901±907
www.elsevier.com/locate/scriptamat
Eect of aging treatment on heavily deformed microstructure of a 6061 aluminum alloy after equal channel angular pressing J.K. Kima, H.G. Jeongb, S.I. Hongc, Y.S. Kimd, and W.J. Kima* a
Department of Metallurgy and Materials Science, Hong-Ik University 72-1, Sangsu-dong, Mapo-ku, Seoul 121-791, South Korea b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan c Department of Metallurgical Engineering, Chungnam National University, Taedok Science Town, Taejeon 305-764, South Korea d School of Metallurgical and Materials Engineering, Kookmin University, Chongnung-dong, Songbuk-ku, Seoul 136-702, South Korea
Received 6 April 2001; accepted 27 June 2001
Abstract Pre-ECAP solid-solution treatment combined with post-ECAP aging treatment has been found to be more eective than pre-ECAP peak-aging treatment in enhancing the strength of a 6061 Al alloy. An increase of 40% in UTS and yield stress was obtained in the post-ECAP aged material compared to the T6 treated commercial 6061 Al alloy. Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Hot pressing; Transmission electron microscopy; Aluminum; Aging; Equal channel angular pressing
Introduction It has been well demonstrated that equal channel angular pressing (ECAP) is a method of producing very ®ne grain size (sub-micrometer or nano-meter) in ingotprocessed metallic alloys [1±6] without reduction in the cross-sectional dimensions of a sample. Fine grains are bene®cial in viewpoints of increased strength, toughness and fatigue life, and improved superplasticity. Alloy 6061 is one of many commercial aluminum alloys that can be signi®cantly hardened by a proper aging treatment. Ferrasse et al. [7] studied the eect of pre-ECAP heat treatment (i.e. peak-aging and over-aging) *
Corresponding author. Fax: +82-2-325-6116. E-mail address: [email protected] (W.J. Kim).
1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 1 ) 0 1 1 0 9 - 5
902
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
on development of sub-microstructure in the ECAP processed 6061 Al alloy. They found that the ECAP processed peak-aged material exhibited much higher strength than the ECAP processed over-aged material, although both materials had the similar subgrain sizes of 0.4 lm. The increase in strength after ECAP, however, was more pronounced in the overaged material (i.e. 180%) compared to the peak-aged material (i.e. 15%). They attributed this result to dierences in the primary obstacles to dislocation motion. In the peak-aged state, ®ne precipitate particles formed during aging treatment serve more eciently as obstacles to plastic ¯ow than new dislocations created during the ECAP process, while the new dislocations are more eective than the precipitate particles which are coarser and less dense in the over-aged state [7]. In this study, another heat-treatment method linked with ECAP process has been proposed to enhance the strength of a commercial 6061 Al alloy. Before ECAP processing, the alloy was solid-solution treated, quenched into water, and after the ECAP process the material was aged at relatively low temperature. The solid-solution treated alloy has an advantage over the peak-aged material in viewpoint of lowering load and processing temperature for ECAP. This is because the former has lower strength and larger ductility than the latter. The current results are compared with those obtained by Ferrasse et al. [7] and the dierences are discussed. Experimental procedure For sample preparation, a 6061 Al billet was extruded to a diameter of 18 mm at 450°C and cut to the length of 100 mm. The bar was solution treated at 530°C for 4 h. and then quenched into water. The grain size measured after the heat treatment was 40± 80 lm. ECA pressing was conducted using a solid die with an internal angle (U) of 90° between the vertical and horizontal channels at the pressing speed of 4 mm s 1 . For this die design, it has been shown that the eective strain accrued on a single pass through the die is 1 [8]. The bar sample was held at 125°C for 20 min and then pressed through the die pre-heated to 125°C. Repetitive pressings of the same sample were performed up to four passes, equivalent to strain of 4. All pressings were conducted by rotating each sample about the longitudinal axis by 90° in the same direction between consecutive passes (designated as route Bc [9]). After the ECA pressing, the bar was aged at 175°C or 100°C as a function of time. Micro-hardness and tensile tests were carried out to evaluate the strength and ductility of the ECAP processed materials. Vickers microhardness (Hv ) was measured on the y plain, parallel to the longitudinal axes, by imposing a load of 100 g for 15 s. Tensile specimens were machined parallel to the longitudinal axes with gauge lengths of 5 mm. Tensile test was conducted in air at room temperature, using a testing machine operated at a constant cross-head speed with an initial strain rate of 5 10 4 s 1 . The microstructure of each sample was examined using transmission electron microscopy (TEM). Samples for TEM were cut from the y plane and selected area electron diraction (SAED) patterns were taken from areas having a diameter of 10 lm.
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
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Fig. 1. TEM micrographs of the solid-solution treated 6061 Al alloys after ECAP process: (a) one pass and (b) four pass.
Results and discussion TEM micrographs of the as-ECAP processed samples are shown in Fig. 1, together with SAED patterns taken from regions within these areas. Fig. 1(a) is the microstructure after one pass, where parallel bands of elongated subgrains have formed. Further deformation (four pass) has broken elongated band structure, without reduction of the spacing between bands, to equiaxed subgrains by forming the boundaries perpendicular to the direction of the bands (Fig. 1(b)). The subgrain size was measured to be 0.3±0.4 lm, which is very similar to those measured in the ECAP processed peakaged and over-aged 6061 Al alloy studied by Ferrasse et al. [7]. This result implies that the size of subgrains formed during ECAP process is little aected by the density and size of precipitate particles. The SAED pattern shown in Fig. 1(b) exhibits diracted beams scattered more uniformly around rings compared to that for the one passed material, indicating the misorientation angle between boundaries increased. The eect of the number of pass on the hardness of the solid-solution treated materials is shown in Fig. 2. The hardness has increased by about 55% after one pass. After four pass, the increase in hardness was near 85%. This large increase in hardness can be directly attributed to the considerable substructure re®nement made through severe deformation (Fig. 1). The hardness data of the materials measured after aging treatment at 175°C for 2, 4 and 8 h are presented in Fig. 3(a). The aging treatment at this temperature after 8 h is the T6 peak-aging condition for a commercial 6061 Al alloy. In the unpressed material, there is a signi®cant increase in hardness (50% after 8 h). The pressed materials, on the other hand, exhibit the opposite trend: the hardness decreases with time. This result attests that in the pressed material, the eect of recovery or/and grain coarsening of heavily deformed substructure by annealing overwhelms the eect of precipitate hardening by aging. It was shown by Vinogradov et al. [10] and Valiev et al. [11] that grain growth occurs at as low as 150±200°C in severely deformed Cu. After the T6 treatment,
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J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
Fig. 2. Vickers hardness of the ECAP materials prior to aging.
Fig. 3. Vickers hardness of the ECAP materials after aging treatment at (a) 175°C and (b) 100°C, given as a function of time.
the pressed materials have a similar hardness and are harder only by 15% than the unpressed material. To reduce the softening eect due to annealing, a lower aging temperature of 100°C was applied. Fig. 3(b) shows the eect of aging at 100°C on the strength of the ECAP processed alloys as a function of time. An increase in strength with aging time can be seen in Fig. 2. All the pressed materials exhibit an increase in hardness with time (11± 12% after 48 h). The unpressed material also exhibits the same trend but the increment in hardness is much smaller than that observed in the unpressed material aged at 175°C (15% after 48 h at 100°C compared to 50% after 8 h at 175°C). The observed hardness increase in the pressed materials indicates that the age hardening eect is more dominant than the softening eect at 100°C. The optimum aging time at 100°C was determined to be 48 h Since the hardness started to fall after 48 h, probably due to substructure coarsening. The maximum hardness obtained after four passes was 160 Hv , which is a 40% increase in hardness compared to that of the T6 treated 6061 Al alloy
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
905
(115 Hv ). This increase is signi®cantly larger than that obtained from the pre-ECAP peak-aged 6061 Al alloy (i.e. 15%) [7]. Fig. 4 is a TEM micrograph showing spherical particles precipitated during aging at 100°C for 48 h. A high density of very ®ne precipitates of 20 nm exists in the matrix, especially around the dislocation substructure developed during severe deformation. This indicates that precipitates preferentially nucleate at the sites where dislocations are clustered. One interesting observation is that the spherical particles observed in the present study at 100°C are totally dierent in morphology from the precipitates commonly observed in commercial 6061 Al alloys [12±15]. 6061 Al alloys, spherical (or needle shaped) G.P. zones, needle shaped b00 , rod-shaped b0 and disc-shaped b particles have been observed [12±15]. One of the controversies concerning the precipitation reactions in 6061 Al alloys is whether the G.P. zones in this alloy system are spherical or needleshaped. Mondolfo [16] suggested that the precipitation sequence of G.P. zones begins with the formation of spherical zones that elongate along the cube matrix direction to assume a needle shape. The diameter of spherical zones were reported to be 5±8 nm. Note that the spherical particles observed in the present alloy are much larger than the size of the typical G.P. zones in 6061 Al alloys. The dierences in precipitate morphology and size may be associated with increased diusion and strong stress ®eld induced by the signi®cant increase of dislocation density and other defects, a detailed discussion of the structure and character of the spherical precipitates is beyond the scope of the present study. The engineering stress±strain curves obtained by tensile testing on the peakaged commercial 6061 Al alloy and four passed 6061 Al alloy with and without aging treatment at 100°C for 48 h are compared in Fig. 5. The combination of ECAP process with aging treatment at 100°C results in a signi®cant increase in tensile strength. There is an increase in yield stress and UTS by 40% compared to that of the peak-aged (T6)
Fig. 4. A TEM micrograph of the four passed material after aging treatment at 100°C for 48 h. Fig. 5. Comparison of engineering stress±engineering strain curves for the T6 treated commercial 6061 Al alloy and the ECAP processed 6061 Al alloy (four pass) with and without a low-temperature aging treatment.
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J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
commercial 6061 Al alloy, which is consistent with the hardness data in the present study. It should be noted here that even the ECAP-pressed solution-treated 6061 Al alloy (425 MPa) is stronger than that of the peak-aged 6061 Al alloy processed to the same strain by four passes (400 MPa) reported by Ferrasse et al. [7]. This result may be linked with the dierence in dislocation accumulation rate and stability of precipitates during ECAP. It was suggested by Hong et al. [14,17] that the hardening rates of the under-aged and solid-solution treated Al alloys are higher than those of peak-aged and over-aged Al alloys because dynamic recovery is more eectively suppressed by the high solute content in the matrix. Furthermore, needle-shaped h precipitates in Al±Cu alloys were found to be sheared into ®ne particles after a few passes by ECAP [18], which may weaken the strengthening eect of precipitates in the pre-ECAP aged material. A pre-ECAP solid solution plus post-ECAP low-temperature aging treatment is, therefore, more eective in improving the strength of 6061 Al alloy than the pre-ECAP peak-aging treatment.
Summary and conclusion In the present study, an eective heat-treatment method combined with ECAP processing has been proposed to enhance the strength of a commercial 6061 Al alloy. Before the ECAP process, the alloy was solid-solution treated and quenched into water and after ECAP processing, the material was aged at 100°C. An increase of 40% in UTS and yield stress was obtained in the post-ECAP aged material compared to the peak-aged (T6) commercial 6061 Al alloy. The improvement of the strength observed in the present study is better than that obtained in the pre-ECAP peak-aged material. The eective strengthening eect of the post-ECAP low temperature aging is linked to (1) higher dislocation accumulation rate in the solutionized matrix; (2) avoidance of fragmentation of precipitates in the pre-ECAP peak-aged Al alloys after a few passes of ECAP; (3) suppression of recovery by precipitation on the substructure after ECAP and reduction of the aging temperature; and (4) accelerated aging kinetics space at low temperatures, induced by a high dislocation density.
Acknowledgements This work was supported by Grant No. 2000-1-30100-013-3 from the Basic Research Program of the Korea Science and Engineering Foundation. References [1] Mishra, R. S., Valiev, R. Z., McFadden, S. X., & Mukherjee, A. K. (1998). Mater Sci Eng A 252, 174. [2] Valiev, R. Z., Kornikov, A. V., & Mulyokov, R. R. (1993). Mater Sci Eng A 168, 141. [3] Furukawa, M., Ma, Y., Horita, Z., Nemoto, M., Valiev, R. Z., & Langdon, T. G. (1998). Mater Sci Eng A 241, 122.
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[4] Berbon, P. B., Tsenev, N. K., Valiev, R. Z., Furukawa, M., Horita, Z., Nemoto, M., & Langdon, T. G. (1998). Metall Mater Trans A 29, 2237. [5] Iwahashi, Y., Horita, Z., Nemoto, M., & Langdon, T. G. (1998). Acta Mater 46, 3317. [6] Valiev, R. Z., Krasilnikov, N. A., & Tsenev, N. K. (1991). Mater Sci Eng A 137, 35. [7] Ferrasse, S., Segal, V. M., Hartwig, K. T., & Goforth, R. E. (1997). J Mater Res 12, 1253. [8] Iwahashi, Y., Wang, J., Horita, Z., Nemoto, M., & Langdon, T. G. (1996). Metall Mater Trans A 29, 2245. [9] Furukawa, M., Iwahashi, Y., Horita, Z., Nemoto, M., & Langdon, T. G. (1998). Mater Sci Eng A 257, 328. [10] Vinogradov, A., Mimaki, T., Hashimoto, S., & Valiev, R. (1999). Scripta Mater 41, 319. [11] Valiev, R. Z., Islamgaliev, R. K., & Alexandrov, I. V. (1999). Prog Mater Sci 74, 1. [12] Thomas, G. (1961±62). J Inst Met 90, 57. [13] Smith, W. F. (1973). Metall Trans 4, 2435. [14] Hong, S. I., Gray III, G. T., & Wang, Z. (1996). Mater Sci Eng A 221, 38. [15] Cordier, H., & Gruhl, W. (1965). Z Metallkde 56, 669. [16] Mondolfo, L. F. (1976). Aluminum Alloys: Structure and Properties (p. 566). London: Butter-Worths. [17] Hong, S. I., Gray III, G. T., & Lewandowski, J. J. (1993). Acta Metall Mater 41, 2337. [18] Murayama, M., Hono, K., & Horita, Z. (2000). In R. S. Mishra, S. L. Semiatin, C. Suryanarayana, N. N. Thadhani, & T. C. Lowe (Eds.). Ultra®ne Grained Materials (p. 145). Warrendale, PA: TMS.
www.elsevier.com/locate/scriptamat
Eect of aging treatment on heavily deformed microstructure of a 6061 aluminum alloy after equal channel angular pressing J.K. Kima, H.G. Jeongb, S.I. Hongc, Y.S. Kimd, and W.J. Kima* a
Department of Metallurgy and Materials Science, Hong-Ik University 72-1, Sangsu-dong, Mapo-ku, Seoul 121-791, South Korea b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan c Department of Metallurgical Engineering, Chungnam National University, Taedok Science Town, Taejeon 305-764, South Korea d School of Metallurgical and Materials Engineering, Kookmin University, Chongnung-dong, Songbuk-ku, Seoul 136-702, South Korea
Received 6 April 2001; accepted 27 June 2001
Abstract Pre-ECAP solid-solution treatment combined with post-ECAP aging treatment has been found to be more eective than pre-ECAP peak-aging treatment in enhancing the strength of a 6061 Al alloy. An increase of 40% in UTS and yield stress was obtained in the post-ECAP aged material compared to the T6 treated commercial 6061 Al alloy. Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Hot pressing; Transmission electron microscopy; Aluminum; Aging; Equal channel angular pressing
Introduction It has been well demonstrated that equal channel angular pressing (ECAP) is a method of producing very ®ne grain size (sub-micrometer or nano-meter) in ingotprocessed metallic alloys [1±6] without reduction in the cross-sectional dimensions of a sample. Fine grains are bene®cial in viewpoints of increased strength, toughness and fatigue life, and improved superplasticity. Alloy 6061 is one of many commercial aluminum alloys that can be signi®cantly hardened by a proper aging treatment. Ferrasse et al. [7] studied the eect of pre-ECAP heat treatment (i.e. peak-aging and over-aging) *
Corresponding author. Fax: +82-2-325-6116. E-mail address: [email protected] (W.J. Kim).
1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 1 ) 0 1 1 0 9 - 5
902
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
on development of sub-microstructure in the ECAP processed 6061 Al alloy. They found that the ECAP processed peak-aged material exhibited much higher strength than the ECAP processed over-aged material, although both materials had the similar subgrain sizes of 0.4 lm. The increase in strength after ECAP, however, was more pronounced in the overaged material (i.e. 180%) compared to the peak-aged material (i.e. 15%). They attributed this result to dierences in the primary obstacles to dislocation motion. In the peak-aged state, ®ne precipitate particles formed during aging treatment serve more eciently as obstacles to plastic ¯ow than new dislocations created during the ECAP process, while the new dislocations are more eective than the precipitate particles which are coarser and less dense in the over-aged state [7]. In this study, another heat-treatment method linked with ECAP process has been proposed to enhance the strength of a commercial 6061 Al alloy. Before ECAP processing, the alloy was solid-solution treated, quenched into water, and after the ECAP process the material was aged at relatively low temperature. The solid-solution treated alloy has an advantage over the peak-aged material in viewpoint of lowering load and processing temperature for ECAP. This is because the former has lower strength and larger ductility than the latter. The current results are compared with those obtained by Ferrasse et al. [7] and the dierences are discussed. Experimental procedure For sample preparation, a 6061 Al billet was extruded to a diameter of 18 mm at 450°C and cut to the length of 100 mm. The bar was solution treated at 530°C for 4 h. and then quenched into water. The grain size measured after the heat treatment was 40± 80 lm. ECA pressing was conducted using a solid die with an internal angle (U) of 90° between the vertical and horizontal channels at the pressing speed of 4 mm s 1 . For this die design, it has been shown that the eective strain accrued on a single pass through the die is 1 [8]. The bar sample was held at 125°C for 20 min and then pressed through the die pre-heated to 125°C. Repetitive pressings of the same sample were performed up to four passes, equivalent to strain of 4. All pressings were conducted by rotating each sample about the longitudinal axis by 90° in the same direction between consecutive passes (designated as route Bc [9]). After the ECA pressing, the bar was aged at 175°C or 100°C as a function of time. Micro-hardness and tensile tests were carried out to evaluate the strength and ductility of the ECAP processed materials. Vickers microhardness (Hv ) was measured on the y plain, parallel to the longitudinal axes, by imposing a load of 100 g for 15 s. Tensile specimens were machined parallel to the longitudinal axes with gauge lengths of 5 mm. Tensile test was conducted in air at room temperature, using a testing machine operated at a constant cross-head speed with an initial strain rate of 5 10 4 s 1 . The microstructure of each sample was examined using transmission electron microscopy (TEM). Samples for TEM were cut from the y plane and selected area electron diraction (SAED) patterns were taken from areas having a diameter of 10 lm.
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
903
Fig. 1. TEM micrographs of the solid-solution treated 6061 Al alloys after ECAP process: (a) one pass and (b) four pass.
Results and discussion TEM micrographs of the as-ECAP processed samples are shown in Fig. 1, together with SAED patterns taken from regions within these areas. Fig. 1(a) is the microstructure after one pass, where parallel bands of elongated subgrains have formed. Further deformation (four pass) has broken elongated band structure, without reduction of the spacing between bands, to equiaxed subgrains by forming the boundaries perpendicular to the direction of the bands (Fig. 1(b)). The subgrain size was measured to be 0.3±0.4 lm, which is very similar to those measured in the ECAP processed peakaged and over-aged 6061 Al alloy studied by Ferrasse et al. [7]. This result implies that the size of subgrains formed during ECAP process is little aected by the density and size of precipitate particles. The SAED pattern shown in Fig. 1(b) exhibits diracted beams scattered more uniformly around rings compared to that for the one passed material, indicating the misorientation angle between boundaries increased. The eect of the number of pass on the hardness of the solid-solution treated materials is shown in Fig. 2. The hardness has increased by about 55% after one pass. After four pass, the increase in hardness was near 85%. This large increase in hardness can be directly attributed to the considerable substructure re®nement made through severe deformation (Fig. 1). The hardness data of the materials measured after aging treatment at 175°C for 2, 4 and 8 h are presented in Fig. 3(a). The aging treatment at this temperature after 8 h is the T6 peak-aging condition for a commercial 6061 Al alloy. In the unpressed material, there is a signi®cant increase in hardness (50% after 8 h). The pressed materials, on the other hand, exhibit the opposite trend: the hardness decreases with time. This result attests that in the pressed material, the eect of recovery or/and grain coarsening of heavily deformed substructure by annealing overwhelms the eect of precipitate hardening by aging. It was shown by Vinogradov et al. [10] and Valiev et al. [11] that grain growth occurs at as low as 150±200°C in severely deformed Cu. After the T6 treatment,
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J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
Fig. 2. Vickers hardness of the ECAP materials prior to aging.
Fig. 3. Vickers hardness of the ECAP materials after aging treatment at (a) 175°C and (b) 100°C, given as a function of time.
the pressed materials have a similar hardness and are harder only by 15% than the unpressed material. To reduce the softening eect due to annealing, a lower aging temperature of 100°C was applied. Fig. 3(b) shows the eect of aging at 100°C on the strength of the ECAP processed alloys as a function of time. An increase in strength with aging time can be seen in Fig. 2. All the pressed materials exhibit an increase in hardness with time (11± 12% after 48 h). The unpressed material also exhibits the same trend but the increment in hardness is much smaller than that observed in the unpressed material aged at 175°C (15% after 48 h at 100°C compared to 50% after 8 h at 175°C). The observed hardness increase in the pressed materials indicates that the age hardening eect is more dominant than the softening eect at 100°C. The optimum aging time at 100°C was determined to be 48 h Since the hardness started to fall after 48 h, probably due to substructure coarsening. The maximum hardness obtained after four passes was 160 Hv , which is a 40% increase in hardness compared to that of the T6 treated 6061 Al alloy
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
905
(115 Hv ). This increase is signi®cantly larger than that obtained from the pre-ECAP peak-aged 6061 Al alloy (i.e. 15%) [7]. Fig. 4 is a TEM micrograph showing spherical particles precipitated during aging at 100°C for 48 h. A high density of very ®ne precipitates of 20 nm exists in the matrix, especially around the dislocation substructure developed during severe deformation. This indicates that precipitates preferentially nucleate at the sites where dislocations are clustered. One interesting observation is that the spherical particles observed in the present study at 100°C are totally dierent in morphology from the precipitates commonly observed in commercial 6061 Al alloys [12±15]. 6061 Al alloys, spherical (or needle shaped) G.P. zones, needle shaped b00 , rod-shaped b0 and disc-shaped b particles have been observed [12±15]. One of the controversies concerning the precipitation reactions in 6061 Al alloys is whether the G.P. zones in this alloy system are spherical or needleshaped. Mondolfo [16] suggested that the precipitation sequence of G.P. zones begins with the formation of spherical zones that elongate along the cube matrix direction to assume a needle shape. The diameter of spherical zones were reported to be 5±8 nm. Note that the spherical particles observed in the present alloy are much larger than the size of the typical G.P. zones in 6061 Al alloys. The dierences in precipitate morphology and size may be associated with increased diusion and strong stress ®eld induced by the signi®cant increase of dislocation density and other defects, a detailed discussion of the structure and character of the spherical precipitates is beyond the scope of the present study. The engineering stress±strain curves obtained by tensile testing on the peakaged commercial 6061 Al alloy and four passed 6061 Al alloy with and without aging treatment at 100°C for 48 h are compared in Fig. 5. The combination of ECAP process with aging treatment at 100°C results in a signi®cant increase in tensile strength. There is an increase in yield stress and UTS by 40% compared to that of the peak-aged (T6)
Fig. 4. A TEM micrograph of the four passed material after aging treatment at 100°C for 48 h. Fig. 5. Comparison of engineering stress±engineering strain curves for the T6 treated commercial 6061 Al alloy and the ECAP processed 6061 Al alloy (four pass) with and without a low-temperature aging treatment.
906
J.K. Kim et al./Scripta Materialia 45 (2001) 901±907
commercial 6061 Al alloy, which is consistent with the hardness data in the present study. It should be noted here that even the ECAP-pressed solution-treated 6061 Al alloy (425 MPa) is stronger than that of the peak-aged 6061 Al alloy processed to the same strain by four passes (400 MPa) reported by Ferrasse et al. [7]. This result may be linked with the dierence in dislocation accumulation rate and stability of precipitates during ECAP. It was suggested by Hong et al. [14,17] that the hardening rates of the under-aged and solid-solution treated Al alloys are higher than those of peak-aged and over-aged Al alloys because dynamic recovery is more eectively suppressed by the high solute content in the matrix. Furthermore, needle-shaped h precipitates in Al±Cu alloys were found to be sheared into ®ne particles after a few passes by ECAP [18], which may weaken the strengthening eect of precipitates in the pre-ECAP aged material. A pre-ECAP solid solution plus post-ECAP low-temperature aging treatment is, therefore, more eective in improving the strength of 6061 Al alloy than the pre-ECAP peak-aging treatment.
Summary and conclusion In the present study, an eective heat-treatment method combined with ECAP processing has been proposed to enhance the strength of a commercial 6061 Al alloy. Before the ECAP process, the alloy was solid-solution treated and quenched into water and after ECAP processing, the material was aged at 100°C. An increase of 40% in UTS and yield stress was obtained in the post-ECAP aged material compared to the peak-aged (T6) commercial 6061 Al alloy. The improvement of the strength observed in the present study is better than that obtained in the pre-ECAP peak-aged material. The eective strengthening eect of the post-ECAP low temperature aging is linked to (1) higher dislocation accumulation rate in the solutionized matrix; (2) avoidance of fragmentation of precipitates in the pre-ECAP peak-aged Al alloys after a few passes of ECAP; (3) suppression of recovery by precipitation on the substructure after ECAP and reduction of the aging temperature; and (4) accelerated aging kinetics space at low temperatures, induced by a high dislocation density.
Acknowledgements This work was supported by Grant No. 2000-1-30100-013-3 from the Basic Research Program of the Korea Science and Engineering Foundation. References [1] Mishra, R. S., Valiev, R. Z., McFadden, S. X., & Mukherjee, A. K. (1998). Mater Sci Eng A 252, 174. [2] Valiev, R. Z., Kornikov, A. V., & Mulyokov, R. R. (1993). Mater Sci Eng A 168, 141. [3] Furukawa, M., Ma, Y., Horita, Z., Nemoto, M., Valiev, R. Z., & Langdon, T. G. (1998). Mater Sci Eng A 241, 122.
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