Effect of post equal-channel-angular-pressing aging on the modified 7075 Al alloy containing Sc

Journal of Alloys and Compounds 450 (2008) 222–228

Effect of post equal-channel-angular-pressing aging on the modified 7075 Al alloy containing Sc W.J. Kim a,∗ , J.K. Kim a , H.K. Kim b , J.W. Park c , Y.H. Jeong c a

b

Department of Materials Science and Engineering, Hong-Ik University, 72-1, Sangsu-dong, Mapo-gu, Seoul 121-791, Republic of Korea Department of Automotive Engineering, Seoul National University of Technology, 172 Gongreung 2-dong, Nowon-gu, Seoul 139-743, Republic of Korea c Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Republic of Korea Received 25 July 2006; received in revised form 24 October 2006; accepted 25 October 2006 Available online 15 December 2006

Abstract The effect of post-ECAP low-temperature aging on mechanical properties of the 7075 Al aluminum alloy containing Sc (7 × 51) after a single pressing was examined. The best aging effect on strengthening was achieved at 373 K after 20–30 h. After the post-ECAP aging treatment, the yield stress and UTS of the 7 × 51 alloy have increased to 680 MPa and 730 MPa, respectively. This achievement is remarkable when compared with the 7075 Al alloy conventionally ECAP processed for three passes exhibiting the YS of 667 MPa and UTS of 677 MPa. The post-ECAP aging was also effective in improving the tensile ductility of the ECAPed alloy. The unECAPed and ECAPed 7 × 51 Al alloys before or after aging at 373 K showed a big difference in strain hardening ability. According to the model assessing the extent to which the hardening and softening mechanisms are active during the plastic deformation of the materials, the low strain hardening rate of the ECAPed alloy could be attributed to a significant contribution to softening by cross slip and a small contribution of precipitation to hardening. © 2006 Published by Elsevier B.V. Keywords: Metals and alloy; Precipitation; Microstructure; Metallography

1. Introduction Equal-channel angular pressing (ECAP) technique that does not require reduction of cross section of a workpiece during process is now well recognized as a quite effective method of enhancing strength of various metallic alloys through (sub) grain refinement through severe deformation [1–5]. Most of the ECAPed alloys show a common trend that yield stress considerably increases after the first ECAP followed by a slow increase with a further increase in plastic strain by ECAP, while the tensile ductility decreases by large extent after the first pressing and then less sensitively further increase in strain [1–5]. Recently, Kim et al. [6] studied the aging effect on strength of the ECAP processed 6061 Al alloys and found that pre-ECAP solution treatment combined with post-ECAP low-temperature aging was quite effective in enhancement of their strength. The yield stress (YS) of a single-passed 6061 Al after post-ECAP

∗

Corresponding author. Tel.: +82 2 320 1468; fax: +82 2 325 6116. E-mail address: [email protected] (W.J. Kim).

0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.10.151

aging (325 MPa) was higher by 55% than the same composition alloy ECAP-processed conventionally in fully annealed state (210 MPa) [5]. The YS value was even higher than that obtained after six passes in fully annealed state (290 MPa). The effect of post-ECAP aging treatment has been examined on another heat-treatable aluminum alloy system, 2024 Al [7]. A high yield stress of ∼630 MPa was obtained after a single pass. It was considerably higher by 110% than the 2024 alloy ECAPprocessed by one pass in fully annealed state (300 MPa) [7] and even higher in significance than that obtained after four passes (320 MPa). Above results demonstrate that the post-ECAP aging treatment is a very efficient method of improving strength of the age-hardenable alloys. In fact, it is so effective that a single ECAP pressing seems to be enough for desired strengthening. Reduction of ECAP pass number makes the ECAP process more economical and productive. Besides, it is also important in stopping the further decrease of elongation-to-failure followed by repetition of pressing, which is caused by decrease of strain hardening ability. The objective of the present study is to investigate the effect of post-ECAP low-temperature aging on mechanical proper-

W.J. Kim et al. / Journal of Alloys and Compounds 450 (2008) 222–228 Table 1 Chemical compositions of the 7 × 51 and 7075 Al alloys Alloys

Zn

Mg

Cu

Zr

Sc

Cr

Mn

7 × 51 7075

8.69 5.1–6.1

2.89 2.1–2.9

2.65 1.2–2.0

0.09 <0.25

0.09 –

<0.01 0.2–0.28

0.01 <0.3

ties of the modified 7075 Al aluminum alloy containing Sc (7 × 51). Aluminum containing scandium has been shown to have excellent mechanical properties at room temperature due to the presence of very fine and coherent Al3 Sc precipitates with high strength, which can be obtained at a high number density, thus effective as obstacles to mobile dislocations and stabilizing a fine-grain structure at high temperature [8]. Therefore, the ECAPed 7 × 51 alloy is expected to exhibit higher strength than the ECAPed 7075 Al alloy studied earlier [5]. The strain hardening behavior of the ECAP processed 7 × 51 Al alloys was com-

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pared with that of the 7 × 51 alloys before ECAP (unECAPed) and their strain hardening behaviors were analyzed based on a strain hardening model developed by Luk´acˇ and Bal´ık [9]. 2. Experimental procedures The chemical composition of the as-extruded 7 × 51 Al used in the present experiment is given in Table 1 with that of conventional 7075 Al alloy. Scandium content in the 7 × 51 alloy is 0.1%. The rods with a diameter of 14.5 mm and length of 100 mm were cut from the extruded material, solution treated at 743 K for 5 h, and then quenched into room-temperature water. The quenched samples were kept in a freezer for 2 h before being subject to ECAP. ECAP was conducted using a solid die made of SKD 61 with an internal angle of 90◦ between the vertical and horizontal channels and a curvature angle of 30◦ . For this die design, the effective strain accrued on a single pass through the die is ∼1 [10]. Molybdenum disulphide (MoS2 ) was used as lubricant. The solution treated workpieces were heated to 428 K or 453 K for 10 min in the preheated die and then pressed through the ECAP. After the first ECA pressing, the workpiece was immediately taken out of the die and kept in a freezer to prevent any unin-

Fig. 1. TEM micrographs of the unECAPed 7 × 51 Al alloy after solid solution treatment and water quenching showing (a) subgrains formed during extrusion, (b) two types of precipitates, (c) Al3 Sc particles associated with dislocations and (d) morphology and distribution of Al3 Sc particles.

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tended aging. The workpiece was later artificially aged as a function of time at three different temperatures: 373 K, 393 K and 413 K to determine the optimum post-ECAP aging condition. Micro-hardness and tensile tests were conducted to evaluate the strength and ductility of the ECAP processed 7 × 51 Al. Vickers micro-hardness (Hv ) was measured on the plain parallel to the longitudinal axes by imposing a load of 100 g for 15 s and taking 10 separate measurements at different randomly selected positions. Tensile testing was performed at room temperature and at an initial strain rate of 5 × 10−4 s−1 under constant crosshead speed condition. Samples for TEM were ground to a thickness of ∼100 ␮m. Thin discs were electropolished in an electrolyte consisting of 15% nitric acid, 85% perchloric acid and methanol at a temperature maintained between 233 K and 243 K.

3. Results 3.1. Microstructural observations Fig. 1(a) shows a TEM micrograph showing a typical microstructure of the unECAPed alloy (as-extruded) quenched into water after solid solution treatment. Subgrains of 2–5 ␮m in diameter formed during the extrusion process are observed. Fig. 1(b) shows a typical region where two types of particles coexist. Nearly spherical and very fine particles of Al–Sc–Zr are observed in matrix with oblong and coarser particles of CuZn2 . Their distribution is non-uniform. Many particles of Al–Sc–Zr are coupled with dislocations (Fig. 1(c)), indicating that dislocations offer preferred nucleation sites for precipitation. Fig. 1(d) is the dark field image showing the distribution of the Al–Sc–Zr compound particles. The volume fraction of the compound particles is relatively low since it is not aged yet. Fig. 2(a) is a TEM micrograph showing the microstructure after ECAP by a single pressing. Fine slip bands exist with presence of highly increased density of dislocations, which is a typical microstructure observed in many ECAPed alloys after one pass [4,6]. When the microstructure is compared with that from the quenched state before ECAP (in Fig. 1(a) and (c)), it is recognized that particles are present more densely in the matrix

possessing higher density of dislocations (Fig. 2(b)). The further precipitating might have taken place during the sample heating in the die before ECAP due to static aging or/and due to dynamic aging during ECAP. During dynamic aging, dislocation density being increased largely during ECAP should provide numerous sites for heterogonous nucleation for particle formation and fast path for diffusion for particle growth. The SADP analysis indicates that the Al–Sc–Zr compound has the similar crystal structure to that Al3 Zr with the zone axis of {0 1 3}. Thus, the observed Al–Sc–Zr compound is most likely Al3 (Zr, Sc) where some portion of Zr is substituted by Sc. Fig. 3(a) and (b) shows the TEM micrographs of the one passed alloy subject to low temperature aging at 373 K for 30 h. Little change in subgrain structure and particle density is observed, but the average particle size seems to have increased slightly after aging (Fig. 3(b)). 3.2. Hardness Fig. 4(a) shows the Hv hardness values of the unECAPed and ECAPed 7 × 51 Al alloys before aging, while Fig. 4(b) shows their Hv variation as a function of aging time at various temperatures in range between 373 K and 413 K. Significantly large increase in hardness was resulted by about 100% just after a single pass of ECAP. The alloy ECAPed at 428 K (Hv = 200) yields a higher hardness than that ECAPed at 453 K (Hv = 195), indicating that strain hardening was more prominent at a lower ECAP temperature. As an effort to increase the strength of the ECAP processed 7 × 51 further, post-ECAP aging was applied. Aging treatments were conducted at 373 K, 393 K and 423 K respectively. The best aging effect on strengthening was achieved at 373 K after peak aging time of 20 h in the alloy ECAPed at 428 K. At higher aging temperatures of 393 K and 413 K, peak hardness was obtained after shorter aging times (5 h and 2 h, respectively), indicating acceleration of aging process at higher temperatures. Aging effect on strengthening, however, is overall

Fig. 2. TEM micrographs of the 7 × 51 Al ECAP processed for one pass after water quenching showing (a) the microstucture containing high density of dislocations and (b) morphology and distribution of Al3 Sc particles.

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Fig. 3. TEM micrographs of the ECAPed 7 × 51 Al after aging at 373 K for 30 h showing (a) the microstructure not so different from that before aging and (b) morphology and distribution of Al3 Sc particles.

Fig. 4. (a) Vickers hardness of the unECAPed and ECAPed 7 × 51 Al alloys and (b) Vickers hardness of the ECAPed 7 × 51 Al as a function of aging time at various aging-temperatures in range 373–413 K.

small, since it is only 5% even at 373 K. This is in contrast to the case for the unECAPed 7 × 51 alloy where the aging effect is remarkable. For example, its hardness was increased by 70% after aging at 373 K for 10 h.

ening rates of the under-aged and solution treated Al alloys are higher than those of peak-aged and over-aged Al alloys, according to Hong et al.’s observation [11], because dynamic recovery is suppressed by high effective solute content in matrix. When

3.3. Tensile behavior The engineering stress–strain curves of the unECAPed alloy after water quenching and the alloys ECAP processed in supersaturated solid-solution state are shown in Fig. 5. Significant strengthening was achieved after a single pressing (increase by 120% in YS), agreeing with the hardness result in Fig. 4(a). The YS and UTS of the ECAPed alloy are as high as 650 MPa and 700 MPa, respectively. It is worthwhile to note that the 7075 Al alloy ECAP-processed in fully annealed state [5] exhibits much lower YS than the 7 × 51 Al alloy ECAPed in supersaturated solid-solution state (320 MPa versus 650 MPa), though the former was processed with multiple pressings (four passes) and its ECAP temperature was lower (293 K). The superior strengthening in the ECAPed 7 × 51 alloy is believed to be primarily related to high dislocation accumulation rate during ECAP. The hard-

Fig. 5. The engineering stress–strain curves of the unECAPed and ECAPed 7 × 51 alloys after water quenching.

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Fig. 6. (a) The engineering stress–strain curves of the ECAPed 7 × 51 alloys after post-ECAP aging treatment (a) at 373 K and (b) at 393 K and 413 K.

the post-ECAP aging treatment was applied, further strengthening (Fig. 6(a)) was resulted. The yield stress and UTS of the ECAPed alloy increased to 680 MPa and 730 MPa, respectively, after post-ECAP aging time of 30 h at 373 K. These values are higher by 5–7% than those of the ECAPed alloy before aging (YS = 650 MPa; UTS = 700 MPa). Tensile elongation decreased by almost half from 26% to 15% after ECAP process in supersaturated solid solution (Fig. 6(a)). This ductility decrease, agreeing with other investigators’ report on many ECAPed metallic alloys, is attributable to largely decreased strain hardening ability after large deformation. The aging treatment at 473 K, however, turns out to be effective in improving tensile ductility of the ECAPed 7 × 51 alloy, though its increment is small (from 15% to 17% after aging for 30 h). There have been efforts to improve the greatly reduced ductility of the ECAPed alloys by annealing them at elevated temperatures [12,13]. In most cases, however, this procedure accompanied some sacrifice of strength. Grain coarsening or/and relaxation of internal stresses during annealing is responsible for the strength reduction. In current case, increase in both strength and ductility could be achieved after heat treatment. A similar result was obtained in the ECAPed 6061 and 2024 Al alloys where the similar post-ECAP procedures had been applied [6,7]. When the aging treatment is carried out under the condition where the strain hardening effect by aging dominates the softening effects by recovery, it is thought that the ECAPed alloy can be stronger and more ductile after the aging treatment. Fig. 6(b) shows the engineering stress–engineering strain curves obtained when higher aging temperatures (393 K and 413 K) were used. Typical annealing effect on the ECAPed alloy mentioned above: decrease in strength with improved tensile ductility is observed. This result indicates that only a proper post-ECAP heat treatment allows the ECAPed alloy to have both strength and ductility improved. Recently, Zheng et al. [14] reported the large improvement of mechanical properties of the ECAPed 7075 Al alloys when the ECAP process similar to the current post-ECAP aging heat treatment was applied. The alloy was solution treated at 743 K for 0.5 h, ECA pressed at 393 K and then aged 393 K for 16 h. After the aging treatment on the one-passed alloy, the YS and UTS increased to 590 MPa and 610 MPa, respectively. For the three passed alloy after the same aging treatment, they increased to 667 MPa and 677 MPa, respectively. As the post-

Fig. 7. Comparison of the engineering stress–engineering strain curves between the unECAPed and ECAPed 7 × 51 Al alloys before and after aging at 373 K.

ECAP aged 7075 Al with three ECAP passes is still weaker than the the post-ECAP aged 7 × 51 after a single pressing, it can be concluded that the 7 × 51 alloy containing Sc is more advantageous in obtaining higher strength than the 7075 Al when ECAP is used for strengthening. Fig. 7 shows the comparison of engineering stress– engineering strain curves between the unECAPed and ECAPed 7 × 51 Al alloys before and after aging at 373 K. The aging effect is very significant in the unECAPed 7 × 51 Al alloy. The unECAPed alloy also exhibits a very large strain hardening ability. In terms of ductility, the unECAPed alloys show elongations of ∼15% similar to those of the ECAPed alloys but their uniform elongations are much larger (∼15% vs 2–3%). Necking at fracture shown in Fig. 7 supports their behaviors of uniform elongation. 4. Discussion Three strengthening mechanisms are considered to be involved in improvement of the mechanical properties of age-hardenable alloys. They are strengthening due to submicrometer-sized grains, strengthening due to the high dislocation density and strengthening due to the production of finer and more homogenous precipitates. Luk´acˇ and Bal´ık [9] developed a model to assess the extent to which the hardening and

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Fig. 8. The symbols represent the work-hardening rate measured from the flow curves in Fig. 7. The solid lines are the lines of best fit according to Eq. (1). Table 2 Parameters of best fit for Eq. (1), applied to the flow curves in Fig. 7

A B C

unECAPed in WQ

unECAPed + aging for 10 h

unECAPed + aging for 20 h

unECAPed + aging for 30 h

ECAPed in WQ (as-ECAPed)

ECAPed + aging for 10 h

ECAPed + aging for 20 h

ECAPed + aging for 30 h

2287.7 1726.2 1.5

27437.2 1963.4 4.0

26870.5 1964.5 4.9

28639.7 1761.1 4.3

2877.0 4930.7 50.12

2041.4 3006.8 44.9

2946.3 3817.6 53.4

3051.6 5657.8 56.0

softening mechanisms are active during plastic deformation of the materials. They assumed that the hardening occurs due to the multiplication of dislocations at both impenetrable obstacles and forest dislocations. On the other hand, annihilation of dislocations, due to cross slip and dislocation climb, was considered as the dominant softening process. The stress dependence of the work hardening rate for polycrystals was proposed in the following form: Θ=

A + B − C(σ − σy ) − D(σ − σy )3 (σ − σy )

(1)

Parameter A relates to the work hardening due to the interaction of dislocations with the non-dislocation obstacles. Parameter B relates to the work hardening due to the interaction with forest dislocations. Both parameters A and B do not depend on temperature [9]. The parameter C relates to recovery due to conservative slip and increases with increasing temperature, as can be expected from the thermally activated character of conservative slip. The parameter D connected with climb of dislocations also increases with increasing temperature as diffusivity is enhanced. The theoretical models were fitted to the experimental curves shown in Fig. 8 and the results are summarized in Table 2. In the fitting, it was assumed that D = 0 since dislocation climb rate in Al would be very low at room temperature. For the ECAPed alloy before aging, the parameter A is relatively low. Application of aging just slightly increases the A value, implying that contribution of post-ECAP aging at the low temperature of 473 Kto work hardening is not so significant. In contrast, the A values of the unECAPed alloys are significantly increased after aging. In addition, they are much larger than those of the ECAPed alloys after the same aging treatment. This result implies that interaction of dislocations with

precipitates is much stronger in the unECAPed alloys than in the ECAPed alloys. Degree of coherency mismatch between Al3 Sc and matrix may affect the interaction. Extremely high coherency mismatch is observed in a usual state of aged microstructure of Al–Sc alloys [8], resulting in significant lattice strain to block dislocation motion. In the ECAPed 7 × 51 alloys where dynamic aging during ECAP or post-ECAP aging takes place on tangled dislocations and severely elastically distorted lattice, the degree of coherency may be reduced. On the other hand, the values of B parameter in the ECAPed alloys are about two times larger than those of the unECAPed alloys. This may be related to that the forest dislocation density is higher in the ECAPed alloy, which agrees with a general observation that the ECAPed alloy has much higher dislocation density than the unECAPed alloy. The unECAPed and ECAPed alloys show a significant difference in value of C parameter. The value of C is much larger in the ECAPed alloys by more than 10 times. This suggests that cross slip makes a great contribution to the softening of the ECAPed alloys, which is reasonable since the ECAPed alloy has much higher dislocation density. As the distance between dislocations is shorter, a probability of dislocation–dislocation annihilation by cross slip becomes higher. 5. Conclusions The effect of post-ECAP low-temperature aging on mechanical properties of the 7075 Al aluminum alloy containing Sc (7 × 51) after a single pressing by ECAP was investigated. The best aging effect on strengthening was obtained at 373 K and the peak aging time at the corresponding temperature was 20–30 h. When the post-ECAP aging treatment was applied, the YS and UTS increased to 680 MPa and 730 MPa, respectively.

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The unECAPed and ECAPed 7 × 51 Al alloys before or after aging at 373 K show a big difference in strain hardening ability. According to the model of with Luk´acˇ and Bal´ık’s model assessing the extent to which the hardening and softening mechanisms are active during the plastic deformation of the materials, cross slip makes a significant contribution to the softening of the ECAPed alloys. However, contribution of precipitation to hardening is relatively small. In contrast, the unECAPed alloy shows a significant contribution of precipitation to hardening and low recovery rate by cross slip, leading to high hardening rate. Acknowledgement This work is supported by 21C Frontier R&D Program Ministry of Science and Technology (M105KO01000206K1501-00212). References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexadrov, Prog. Mater. Sci. 45 (2000) 103.

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