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Microstructure and mechanical properties of ZK60 alloy processed by two-step equal channel angular pressing
Journal of Alloys and Compounds 492 (2010) 605–610
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and mechanical properties of ZK60 alloy processed by two-step equal channel angular pressing Yunbin He a,∗ , Qinglin Pan a , Yinjiang Qin a , Xiaoyan Liu a , Wenbin Li a , Yulung Chiu b , John J.J. Chen b a b
Department of Materials Science and Engineering, Central South University, Changsha 410083, PR China Department of Chemical and Materials Engineering, University of Auckland, Auckland 1142, New Zealand
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 26 November 2009 Accepted 28 November 2009 Available online 6 December 2009 Keywords: Equal channel angular pressing (ECAP) ZK60 alloy Microstructure Mechanical properties Dynamic recovery Recrystallization
a b s t r a c t A ZK60 alloy was processed by two-step equal channel angular pressing (ECAP) in order to refine the grain size and enhance the mechanical properties. The results show that fine grain size less than 1 m was obtained after the two-step ECAP process, 4 passes at 513 K, followed by another 4 passes at 453 K. The strength of ZK60 alloy decreased after the single-step ECAP process but recovered after two-step ECAP process. The strength decrease was related to the texture softening. Compared with the singlestep ECAP at 513 K, the two-step ECAP has significantly improved the yield strength and ultimate tensile strength of the alloy. The mechanism of the notable grain refinement of the ECAP process was mainly attributed to continuous dynamic recovery and recrystallization. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys have attracted significant interest in the last decades for their potential applications in automotive and electronics industry. However, the development of magnesium alloys is challenging, mainly because of their intrinsic poor deformability and limited ductility at low temperature, which is rooted in the hexagonal closed-packed crystal structure. It is well known that materials with fine grain size usually present enhanced yield strength and ductility at room temperature. Therefore grain refinement is expected to be an effective approach to improve the deformability of magnesium alloys. At present, a new processing method-equal channel angular pressing (ECAP) is available for the production of ultrafine grain fcc metals like Al and Cu [1–6]. The same success has also been achieved on some magnesium alloys [7–10]. Some fine-grained magnesium alloys produced by ECAP even show excellent superplasticity [9,11]. Basically, the grain refinement effectiveness of ECAP process not only depends on the configuration of the ECAP die such as intersection angle and out arc angle but also the ECAP temperature and processing route (designated as the rotation of
∗ Corresponding author at: Department of Materials Science and Engineering, Central South University, 154 Lushan South Road, Changsha, Hunan 410083, PR China. Tel.: +86 731 8830933; fax: +86 731 8830933. E-mail address: [email protected] (Y. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.192
the sample between each successive pass) [12]. It has been found that an increase in ECAP temperature results into larger grain size on aluminium alloys [13,14]. And the most probable reason was believed to be the acceleration of grain growth at higher temperature. For magnesium alloys, the ECAP temperature is usually above 473 K to avoid surface cracking [9,15–17]. However, high temperature leads to a rapid grain growth, which prevents to obtain an ultrafine grain structure. The ECAP temperature is sensitive to the initial microstructure of the starting materials. Generally, materials with fine grains usually show better deformability and thus can be deformed at lower temperature. Watanabe et al. [18] found that the ECAP temperature can be lowered to 433 K when the initial grain size was less than 5 m. Many researchers have developed several methods to obtain a fine-grained starting material for ECAP processing. Matsubara et al. [19] successfully developed a so-called EX-ECAP method where the billet was subjected to conventional extrusion prior to ECAP so as to obtain fine grain starting material. Jin et al. [20] developed a two-step ECAP method, where the sample was firstly ECAP processed at a higher temperature and then processed at lower temperature to achieve a final grain refinement to submicrometer. In comparison to the conventional extrusion, ECAP is a more rapid way to refine the grain size of the starting materials. After 4 passes at 498 K, the grain size of AZ31 alloy was successfully reduced down to ∼2 m [20], which prepared a fine-grained material for the subsequent ECAP at low temperature. Thus, the two-step ECAP was suggested to a more rapid way to refine the grain size of magnesium alloys.
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In this investigation, the two-step ECAP method was utilized to develop fine-grained ZK60 alloys. The grain structure and the mechanical properties after the two-step ECAP processing were studied. 2. Experimental materials and procedures The starting material used in the present study was a commercial magnesium alloy ZK60 (5.6 wt% Zn, 0.6 wt% Zr) extruded at temperature of 633–663 K. Rods of Ø20 mm and 100 mm long were machined out from the extruded bar and solution treated at 703 K for 10 h. These billets were pressed in an ECAP die with Ø20 mm channel, 90◦ intersection angle and a 20◦ curvature on the outer side of the channel intersection which imposes a strain of ∼1 for each pass. For single-step ECAP process experiments, samples were pressed through the die for 4 and 8 passes at 513 K. For two-step ECAP process experiments, samples were firstly pressed at 513 K for 4 passes and then pressed at 453 K for 2–4 passes. The pressing speed was controlled at about ∼20 mm/min. All the ECAP process experiments were carried out via route Bc where the sample was rotated 90◦ in the same sense along the longitude axis. A mixture of graphite powder of particle size less than 80 m and engine oil was used as the lubricant for the ECAP. After the ECAP processing, dog-bone shape tensile specimens with gauge dimensions of 2 mm × 3 mm × 5 mm were machined from the processed rod samples, with tensile axis lying along the longitudinal axes of the ECAP processed samples. Tensile
tests were carried out on an MTS universal testing machine at a nominal strain rate of 1.0 × 10−3 s−1 at room temperature. The microstructure of the ECAP processed alloy was characterized by the optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The specimens for optical microscopy examination were mechanically polished down to 1 m and then etched with a solution containing 1 g oxalic acid, 1 ml nitric acid, 1 ml acetic acid and 100 ml water. The specimens for TEM observation were firstly mechanically polished and then electro-polished using a Tenupol 5 jet polishing unit with a solution containing 11.4 g magnesium perchloric, 5.8 g lithium chloride, 500 ml methanol, and 100 ml butyloxy-ethanol at 228 K, 50 V. The TEM examination was carried out on a Tecnai G2 20 transmission electron microscope operating at 200 kV. The {0 0 0 2} pole figure was measured to evaluate the texture evolution during ECAP processes by performing X-ray texture analysis. The examining plane was parallel to the extrusion direction.
3. Results 3.1. Optical microstructure Fig. 1 shows the microstructure of as-received ZK60 alloy and that of the alloys after single-step as well as two-step ECAP pro-
Fig. 1. Grain structure of the ECAP processed ZK60 alloys observed by optical microscopy: (a) as-received, (b) 513 K-4 passes and (c) 513 K-8 passes; and scanning electron microscopy: (d) 513 K-4 passes + 453 K-2 passes and (e) 513 K-4 passes + 453 K-4 passes.
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cessing. It can be seen from Fig. 1a that the as-received ZK60 alloy exhibits a bi-modal grain structure, with coarse grains of ∼200 m and fine grains of ∼10 m. After ECAP for 4 passes at 513 K (Fig. 1b), the ZK60 alloy shows a fully recrystallized grain structure with significantly reduced grain size (the mean grain size ∼1.8 m). The sample after ECAP for 8 passes has a similar microstructure as that after 4 passes, with mean grain size of ∼1.6 m (Fig. 1c). It should be noted that some coarse grains with grain size of ∼10 m are still visible, indicating that grain growth has taken place during the ECAP process. The grain structure is similar with that reported by Lapovok et al. [21] on ZK60 alloy after ECAP passes at 473 K. No twining has been observed in the sample after ECAP at 513 K. After the two-step ECAP processing, the grain size becomes too small to be distinguished using optical microscope. SEM observation shows that after 2 passes at 453 K in the second step, the grain size was reduced to 1 m (Fig. 1d). Increasing the number of pass to 4 at 453 K, the grain size was further reduced to ∼0.8 m (Fig. 1e).
sibly caused by the deformation and recovery during ECAP. After 8 passes ECAP, heavy dislocation entanglement still retained in some area, while equiaxed dislocations-free grains can also be observed. TEM observation shows the existence of two types of Mg–Zn precipitates in the ECAP processed alloy. According to the study of Pan et al. [22], the large dark blocky particles as indicated by arrows in Fig. 2a and b might be MgZn2 , while the fine particles homogenously distributed within the alloy (Fig. 2a and c) might be MgZn precipitates. These precipitates are believed to block dislocation movement and grain boundary migration thus stabilising the fine grain structure. The TEM observation of the ZK60 alloys after two-step ECAP is shown in Fig. 3. Compared to the samples processed by single-step ECAP at 513 K, the two-step ECAP processed samples have finer grain structure. The grain size was reduced from 1.8 m after 4 passes ECAP at 513 K to 0.9 m and 0.7 m after 2 passes and 4 passes ECAP at 453 K, respectively.
3.2. TEM microstructure
3.3. Mechanical properties
Fig. 2a and b are the bright-field TEM images of the grain structure and corresponding selected area diffraction (SAD) patterns of the ZK60 alloys processed by single-step ECAP process at 513 K for 4 passes and 8 passes, respectively. In both samples, equiaxed grains of average size 1–2 m are observed. The result is accord with the observation of optical microscopy. Fig. 2c shows that dislocation arrays in the sample processed by ECAP at 513 K for 4 passes, plau-
The room temperature tensile stress–strain curves of the asreceived and ECAP processed ZK60 alloys are plotted in Fig. 4. The value of the yield tensile strength, the ultimate tensile strength and the elongation to failure are summarized in Table 1. After ECAP at 513 K, the ultimate tensile strength and yield strength are similar for both 4 passes and 8 passes alloy, while the latter shows higher value of elongation to failure. However, compared to the
Fig. 2. Representative bright-field TEM micrographs and corresponding SAD patterns showing the microstructure of the ZK60 alloys after ECAP at 513 K for 4 passes (a and c) and 8 passes (b and d).
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Fig. 3. Bright-field TEM micrographs of ZK60 alloys after two-step ECAP process: (a) 513 K-4 passes + 453 K-2 passes and (b) 513 K-4 passes + 453 K-4 passes.
3.4. Texture evolution The texture evolution during the ECAP process of ZK60 alloy is shown in Fig. 5. For the as-received alloy, the pole figure reveals a strong {0 0 0 2} basal texture indicating the basal plane in most grains is orientated perpendicular to the extrusion direction. After single-step ECAP process at 513 K for 4 passes, the texture remains basal with a slight spread and a new texture component locating at ∼45◦ from ED towards TD has formed. Increasing the pressing pass to 8, the basal texture has been totally replaced by the new formed texture component. The maximum intensity also increases from 4.77 to 7.05. The two-step ECAP process produces a similar texture as 8 passes ECAP at 513 K. After 4 passes at 453 K for the second step, the maximum texture intensity of {0 0 0 2} pole figure is highest among all the alloys, indicating that strong texture has been developed. The texture component retains when the pressing pass increases from 2 to 4 and maximum intensity increases from 7.1 to 12.3. Fig. 4. The stress–strain curve obtained from the tensile test of the ECAP processed ZK60 alloys.
as-received ZK60 alloy, the single-step ECAP process decreases the strength of the alloy though the grain size has been significantly reduced. It is believed that the strength decrease is related to the texture evolution during the ECAP processing according to Kim and Jeong [23]. After two-step ECAP process, the strength of the alloy was recovered compared with the as-received material. After 2 passes at 453 K, the yield strength was almost close to the as-received material. After 4 passes, the strength was higher than the as-received material. The strength was also significantly improved compared with the single-step ECAP process. The ultimate strength was increased by 15 MPa and 40 MPa after ECAP processing at 453 K for 2 passes and 4 passes and the yield strength was improved 45 MPa and 55 MPa, respectively.
4. Discussion The present work shows that ECAP can introduce a remarkable grain refinement to ZK60 alloy. The presence of equiaxed grains and serrated grain boundaries in Fig. 1 indicates the occurrence of recrystallization during ECAP process [24]. According to the results of Galiyev et al. [25] on compression experiments of ZK60 alloy, in range of 473–523 K, due to the activation of prismatic and pyramidal slip systems, the deformation of ZK60 magnesium alloy was mainly accommodated by continuous dynamic recrystallization associated with dislocation slip. The same mechanism was also employed by Su et al. [26,27] to explain the recrystallization process on AZ31 magnesium alloy. Since the ECAP process in the present study was carried out at 513 K, which is above the recrystallization temperature of ZK60 alloy (∼473 K), recrystallization is believed to play an important role during the ECAP process. It is evident from the TEM observation in Fig. 2 that after ECAP at 513 K for 4
Table 1 Mechanical properties of the ECAP processed ZK60 alloy. Samples
Ultimate tensile strength (MPa)
Yield strength (MPa)
Elongation to failure (%)
Mean grain size (m)
As-received 513 K-4p 513 K-8p 513 K-4p + 453 K-2p 513 K-4p + 453 K-4p
250 221 226 236 266
166 120 125 160 175
18.5 28.1 35.1 28.9 31.9
∼80 1.8 1.6 1.0 0.8
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Fig. 5. The {0 0 0 2} pole figures of the as-received material (a) and after ECAP process of 513 K-4 passes (b), 513 K-8 passes (c), 513 K-4 passes + 453 K-2 passes (d) and 513 K-4 passes + 453 K-4 passes (e).
and 8 passes, equiaxed grains with straight and well defined grain boundary are formed, indicating that recrystallization has taken place during the ECAP process. Further observation on TEM image in Fig. 6a shows that some dislocations are homogenously distributed within the grains at the initial stage of ECAP processing. The dislocation movement was blocked by the initial grain boundaries and resulted in a complicated network of dislocation entanglement. According to the low energy dislocation structure (LEDS) theory [28], the entangled dislocations are readily rearranged into dislocation boundaries and sub-grain boundaries, as shown in Fig. 6b. Upon further ECAP processing, the dislocation generation continues within the sub-grain interior. Mobile dislocations move across the sub-grains and are absorbed by the sub-grain boundaries, resulting in an increase in their misorientation. As a result, the low angle
boundaries (LABs) persistently evolved into high grain boundaries (HABs), which is a kind of continuous dynamic recovery and recrystallization. At high strain, dislocations generated by slip were mostly consumed by the transformation from LABs to HABs. Thus further straining is more effective in increasing the misorientation of the grain boundaries. However, the grain refinement is less effective. Another reason resulting in the less effectiveness of grain refinement at high strain might be grain boundary sliding (GBS). It has shown that magnesium alloys with fine grain size can be superplastically deformed around 473 K [18,29,30], where GBS was considered to be the main deformation mechanism. In present study, the deformation temperature was 513 K. After 4 passes ECAP processing, the grain size was reduced to 1.8 m and even smaller
Fig. 6. The dislocation configurations within the grain interior (a) and dislocation wall evolved from dislocation entanglement (b).
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after two-step ECAP processing. Therefore, during ECAP processing GBS was likely to operate in the ZK60 alloy with fine grains size having HABs. The occurrence of GBS can release the stress concentration resulting from dislocation slip and help to accommodate further deformation which leads to less accumulation of dislocation entanglement and eventually less effective grain refinement at high strain. According to the study of Su et al. [27], the recrystallization process can be considered as diffusion of atoms from one grain to the other by overcoming an activation energy through thermal activation. The velocity of the grain boundaries migration increases exponentially with temperature. Therefore low temperature is suggested to slow the transformation process of LABs to HABs, resulting in an increase in dislocation density. Therefore, more mobile dislocations are arranged into new sub-grain boundaries, which eventually result in a more effective grain refinement during ECAP process. In the present study, the two-step ECAP processing was carried out at low temperature of 453 K. At this temperature, the recovery process was restricted due to the low recovery rate. As a result, as shown in Fig. 1, the grain refinement is more effective than the single-step ECAP process at 513 K. The reduction in average grain size may also be resulted from the suppression of grain growth at low temperature. Generally, the room temperature properties of metals are greatly related to the grain size. According to the Hall–Petch equation, finer grain size usually has higher yield strength. However, texture also plays an important role on the deformation behaviour as texture evolution has great effect on the Schmid factor variation. Therefore, the tensile properties are determined both by the grain size and texture. Detailed effects of texture on the mechanical behaviour of ECAP magnesium alloys were well studied by Kim et al. [23,31], who reported that texture modification during ECAP has a great influence on the strength of magnesium alloys. The rotation of high fraction of basal plane to the favourable for slip can appreciably decrease the yield strength. In the present study, the texture for the as-received ZK60 alloy is mainly {0 0 0 2} basal texture, which has a high Schmid factor with the tensile direction. Therefore, the as-received alloy still exhibits high strength though having coarse grain size. After ECAP process, the basal texture gradually transformed to shear texture which is located at 45◦ from transverse direction towards extrusion direction. This kind of shear texture has low Schmid factor with tensile direction. Thus, though the grain size has been significantly reduced after ECAP process at 513 K, the yield strength of the ECAP processed alloy is still lower than that of the as-received alloy. However, the Hall–Petch equation is still valid for a constant texture. The two-step ECAP process produces similar texture with single-step ECAP but finer grain size. Therefore, the two-step ECAP also exhibits higher strength compared to the single-step ECAP process. 5. Conclusions The effect of two-step ECAP processing on the microstructure and mechanical properties of an as-extruded ZK60 alloy was investigated. The results obtained are summarized as follows:
(i) A two-step ECAP processing is more effective in refining grain size than single-step ECAP processing of as-extruded ZK60 alloy. After two-step ECAP of 513 K for 4 passes plus 453 K for 4 passes, a homogenous structure of fine grains, with grain size of 0.8 m, was obtained. The mechanism of the notable grain refinement of the ECAP process was mainly attributed to continuous dynamic recovery and recrystallization. (ii) The strength of ZK60 alloy was decreased after the single-step ECAP process but recovered after two-step ECAP process. The strength decrease was related to the texture softening. Compared with single-step ECAP at 513 K for 4 passes, the strength of the alloy was significantly improved after two-step ECAP at 513 K for 4 passes and 453 K for 4 passes. References [1] S.Z. Han, M. Goto, C. Lim, C.J. Kim, S. Kim, J. Alloys Compd. 434–435 (2007) 304–306. [2] S.Y. Chang, B.D. Ahn, S.K. Hong, S. Kamado, Y. Kojima, D.H. Shin, J. Alloys Compd. 386 (2005) 197–201. [3] V.V. Stolyarov, R. Lapovok, J. Alloys Compd. 378 (2004) 233–236. [4] Y. Amouyal, S.V. Divinski, L. Klinger, E. Rabkin, Acta Mater. 56 (2008) 5500–5513. [5] A.L. Etter, T. Baudin, C. Rey, R. Penelle, Mater. Charact. 56 (2006) 19–25. [6] J.M. García-Infanta, S. Swaminathan, C.M. Cepeda-Jiménez, T.R. McNelley, O.A. ˜ J. Alloys Compd. 478 (2009) 139–143. Ruano, F. Carreno, [7] M. Eddahbi, P. Pérez, M.A. Monge, G. Garcés, R. Pareja, P. Adeva, J. Alloys Compd. 473 (2009) 79–86. [8] G.B. Hamu, D. Eliezer, L. Wagner, J. Alloys Compd. 468 (2009) 222– 229. [9] W.N. Tang, R.S. Chen, E.H. Han, J. Alloys Compd. 477 (2009) 636–643. [10] W.M. Gan, M.Y. Zheng, H. Chang, X.J. Wang, X.G. Qiao, K. Wu, B. Schwebke, H.G. Brokmeier, J. Alloys Compd. 470 (2009) 256–262. [11] R. Lapovok, Y. Estrin, M.V. Popov, T.G. Langdon, Adv. Eng. Mater. 10 (2008) 429–433. [12] R.Z. Valiev, T.G. Langdon, Prog. Mater. Sci. 51 (2006) 881–981. [13] I. Gutierrez-Urrutia, M.A. Munoz-Morris, I. Puertas, C. Luis, D.G. Morris, Mater. Sci. Eng. A 475 (2008) 268–278. [14] P. Málek, M. Cieslar, R.K. Islamgaliev, J. Alloys Compd. 378 (2004) 237– 241. [15] E.J. Kwak, C.H. Bok, M.H. Seo, T.S. Kim, H.S. Kim, Mater. Trans. 49 (2008) 1006–1010. [16] B. Chen, D.L. Lin, X.Q. Zeng, C. Lu, J. Alloys Compd. 440 (2007) 94–100. [17] B. Chen, D.L. Lin, L. Jin, X.Q. Zeng, C. Lu, Mater. Sci. Eng. A 483–484 (2008) 113–116. [18] H. Watanabe, T. Mukai, K. Ishikawa, K. Higashi, Scripta Mater. 46 (2002) 851–856. [19] K. Matsubara, Y. Miyahara, Z. Horita, T.G. Langdon, Acta Mater. 51 (2003) 3073–3084. [20] L. Jin, D.L. Lin, D.L. Mao, X.Q. Zeng, W.J. Ding, Mater. Lett. 59 (2005) 2267– 2270. [21] R. Lapovok, R. Cottam, P. Thomson, Y. Estrin, J. Mater. Res. 20 (2005) 1375– 1378. [22] F.S. Pan, W.M. Wang, Y.L. Ma, R.L. Zou, A.T. Tang, J. Zhang, Mater. Sci. Forum 488–489 (2005) 181–184. [23] W.J. Kim, H.T. Jeong, Mater. Trans. 46 (2005) 251–258. [24] J.C. Tan, M.J. Tan, Mater. Sci. Eng. A 339 (2003) 124–132. [25] A. Galiyev, R. Kaibyshev, G. Gottstein, Acta Mater. 49 (2001) 1199–1207. [26] C.W. Su, L. Lu, M.O. Lai, Mater. Sci. Eng. A 434 (2006) 227–236. [27] C.W. Su, L. Lu, M.O. Lai, Philos. Mag. 88 (2008) 181–200. [28] D. Kuhlmann-Wilsdorf, Scripta Mater. 34 (1996) 641–650. [29] H.G. Jeong, Y.G. Jeong, W.J. Kim, J. Alloys Compd. 483 (2009) 279– 282. [30] V.N. Chuvil’deev, T.G. Nieh, M.Y. Gryaznov, V.I. Kopylov, A.N. Sysoev, J. Alloys Compd. 378 (2004) 253–257. [31] W.J. Kim, S.I. Hong, Y.S. Kim, S.H. Min, H.T. Jeong, J.D. Lee, Acta Mater. 51 (2003) 3293–3307.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Microstructure and mechanical properties of ZK60 alloy processed by two-step equal channel angular pressing Yunbin He a,∗ , Qinglin Pan a , Yinjiang Qin a , Xiaoyan Liu a , Wenbin Li a , Yulung Chiu b , John J.J. Chen b a b
Department of Materials Science and Engineering, Central South University, Changsha 410083, PR China Department of Chemical and Materials Engineering, University of Auckland, Auckland 1142, New Zealand
a r t i c l e
i n f o
Article history: Received 1 April 2009 Received in revised form 26 November 2009 Accepted 28 November 2009 Available online 6 December 2009 Keywords: Equal channel angular pressing (ECAP) ZK60 alloy Microstructure Mechanical properties Dynamic recovery Recrystallization
a b s t r a c t A ZK60 alloy was processed by two-step equal channel angular pressing (ECAP) in order to refine the grain size and enhance the mechanical properties. The results show that fine grain size less than 1 m was obtained after the two-step ECAP process, 4 passes at 513 K, followed by another 4 passes at 453 K. The strength of ZK60 alloy decreased after the single-step ECAP process but recovered after two-step ECAP process. The strength decrease was related to the texture softening. Compared with the singlestep ECAP at 513 K, the two-step ECAP has significantly improved the yield strength and ultimate tensile strength of the alloy. The mechanism of the notable grain refinement of the ECAP process was mainly attributed to continuous dynamic recovery and recrystallization. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys have attracted significant interest in the last decades for their potential applications in automotive and electronics industry. However, the development of magnesium alloys is challenging, mainly because of their intrinsic poor deformability and limited ductility at low temperature, which is rooted in the hexagonal closed-packed crystal structure. It is well known that materials with fine grain size usually present enhanced yield strength and ductility at room temperature. Therefore grain refinement is expected to be an effective approach to improve the deformability of magnesium alloys. At present, a new processing method-equal channel angular pressing (ECAP) is available for the production of ultrafine grain fcc metals like Al and Cu [1–6]. The same success has also been achieved on some magnesium alloys [7–10]. Some fine-grained magnesium alloys produced by ECAP even show excellent superplasticity [9,11]. Basically, the grain refinement effectiveness of ECAP process not only depends on the configuration of the ECAP die such as intersection angle and out arc angle but also the ECAP temperature and processing route (designated as the rotation of
∗ Corresponding author at: Department of Materials Science and Engineering, Central South University, 154 Lushan South Road, Changsha, Hunan 410083, PR China. Tel.: +86 731 8830933; fax: +86 731 8830933. E-mail address: [email protected] (Y. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.192
the sample between each successive pass) [12]. It has been found that an increase in ECAP temperature results into larger grain size on aluminium alloys [13,14]. And the most probable reason was believed to be the acceleration of grain growth at higher temperature. For magnesium alloys, the ECAP temperature is usually above 473 K to avoid surface cracking [9,15–17]. However, high temperature leads to a rapid grain growth, which prevents to obtain an ultrafine grain structure. The ECAP temperature is sensitive to the initial microstructure of the starting materials. Generally, materials with fine grains usually show better deformability and thus can be deformed at lower temperature. Watanabe et al. [18] found that the ECAP temperature can be lowered to 433 K when the initial grain size was less than 5 m. Many researchers have developed several methods to obtain a fine-grained starting material for ECAP processing. Matsubara et al. [19] successfully developed a so-called EX-ECAP method where the billet was subjected to conventional extrusion prior to ECAP so as to obtain fine grain starting material. Jin et al. [20] developed a two-step ECAP method, where the sample was firstly ECAP processed at a higher temperature and then processed at lower temperature to achieve a final grain refinement to submicrometer. In comparison to the conventional extrusion, ECAP is a more rapid way to refine the grain size of the starting materials. After 4 passes at 498 K, the grain size of AZ31 alloy was successfully reduced down to ∼2 m [20], which prepared a fine-grained material for the subsequent ECAP at low temperature. Thus, the two-step ECAP was suggested to a more rapid way to refine the grain size of magnesium alloys.
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In this investigation, the two-step ECAP method was utilized to develop fine-grained ZK60 alloys. The grain structure and the mechanical properties after the two-step ECAP processing were studied. 2. Experimental materials and procedures The starting material used in the present study was a commercial magnesium alloy ZK60 (5.6 wt% Zn, 0.6 wt% Zr) extruded at temperature of 633–663 K. Rods of Ø20 mm and 100 mm long were machined out from the extruded bar and solution treated at 703 K for 10 h. These billets were pressed in an ECAP die with Ø20 mm channel, 90◦ intersection angle and a 20◦ curvature on the outer side of the channel intersection which imposes a strain of ∼1 for each pass. For single-step ECAP process experiments, samples were pressed through the die for 4 and 8 passes at 513 K. For two-step ECAP process experiments, samples were firstly pressed at 513 K for 4 passes and then pressed at 453 K for 2–4 passes. The pressing speed was controlled at about ∼20 mm/min. All the ECAP process experiments were carried out via route Bc where the sample was rotated 90◦ in the same sense along the longitude axis. A mixture of graphite powder of particle size less than 80 m and engine oil was used as the lubricant for the ECAP. After the ECAP processing, dog-bone shape tensile specimens with gauge dimensions of 2 mm × 3 mm × 5 mm were machined from the processed rod samples, with tensile axis lying along the longitudinal axes of the ECAP processed samples. Tensile
tests were carried out on an MTS universal testing machine at a nominal strain rate of 1.0 × 10−3 s−1 at room temperature. The microstructure of the ECAP processed alloy was characterized by the optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The specimens for optical microscopy examination were mechanically polished down to 1 m and then etched with a solution containing 1 g oxalic acid, 1 ml nitric acid, 1 ml acetic acid and 100 ml water. The specimens for TEM observation were firstly mechanically polished and then electro-polished using a Tenupol 5 jet polishing unit with a solution containing 11.4 g magnesium perchloric, 5.8 g lithium chloride, 500 ml methanol, and 100 ml butyloxy-ethanol at 228 K, 50 V. The TEM examination was carried out on a Tecnai G2 20 transmission electron microscope operating at 200 kV. The {0 0 0 2} pole figure was measured to evaluate the texture evolution during ECAP processes by performing X-ray texture analysis. The examining plane was parallel to the extrusion direction.
3. Results 3.1. Optical microstructure Fig. 1 shows the microstructure of as-received ZK60 alloy and that of the alloys after single-step as well as two-step ECAP pro-
Fig. 1. Grain structure of the ECAP processed ZK60 alloys observed by optical microscopy: (a) as-received, (b) 513 K-4 passes and (c) 513 K-8 passes; and scanning electron microscopy: (d) 513 K-4 passes + 453 K-2 passes and (e) 513 K-4 passes + 453 K-4 passes.
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cessing. It can be seen from Fig. 1a that the as-received ZK60 alloy exhibits a bi-modal grain structure, with coarse grains of ∼200 m and fine grains of ∼10 m. After ECAP for 4 passes at 513 K (Fig. 1b), the ZK60 alloy shows a fully recrystallized grain structure with significantly reduced grain size (the mean grain size ∼1.8 m). The sample after ECAP for 8 passes has a similar microstructure as that after 4 passes, with mean grain size of ∼1.6 m (Fig. 1c). It should be noted that some coarse grains with grain size of ∼10 m are still visible, indicating that grain growth has taken place during the ECAP process. The grain structure is similar with that reported by Lapovok et al. [21] on ZK60 alloy after ECAP passes at 473 K. No twining has been observed in the sample after ECAP at 513 K. After the two-step ECAP processing, the grain size becomes too small to be distinguished using optical microscope. SEM observation shows that after 2 passes at 453 K in the second step, the grain size was reduced to 1 m (Fig. 1d). Increasing the number of pass to 4 at 453 K, the grain size was further reduced to ∼0.8 m (Fig. 1e).
sibly caused by the deformation and recovery during ECAP. After 8 passes ECAP, heavy dislocation entanglement still retained in some area, while equiaxed dislocations-free grains can also be observed. TEM observation shows the existence of two types of Mg–Zn precipitates in the ECAP processed alloy. According to the study of Pan et al. [22], the large dark blocky particles as indicated by arrows in Fig. 2a and b might be MgZn2 , while the fine particles homogenously distributed within the alloy (Fig. 2a and c) might be MgZn precipitates. These precipitates are believed to block dislocation movement and grain boundary migration thus stabilising the fine grain structure. The TEM observation of the ZK60 alloys after two-step ECAP is shown in Fig. 3. Compared to the samples processed by single-step ECAP at 513 K, the two-step ECAP processed samples have finer grain structure. The grain size was reduced from 1.8 m after 4 passes ECAP at 513 K to 0.9 m and 0.7 m after 2 passes and 4 passes ECAP at 453 K, respectively.
3.2. TEM microstructure
3.3. Mechanical properties
Fig. 2a and b are the bright-field TEM images of the grain structure and corresponding selected area diffraction (SAD) patterns of the ZK60 alloys processed by single-step ECAP process at 513 K for 4 passes and 8 passes, respectively. In both samples, equiaxed grains of average size 1–2 m are observed. The result is accord with the observation of optical microscopy. Fig. 2c shows that dislocation arrays in the sample processed by ECAP at 513 K for 4 passes, plau-
The room temperature tensile stress–strain curves of the asreceived and ECAP processed ZK60 alloys are plotted in Fig. 4. The value of the yield tensile strength, the ultimate tensile strength and the elongation to failure are summarized in Table 1. After ECAP at 513 K, the ultimate tensile strength and yield strength are similar for both 4 passes and 8 passes alloy, while the latter shows higher value of elongation to failure. However, compared to the
Fig. 2. Representative bright-field TEM micrographs and corresponding SAD patterns showing the microstructure of the ZK60 alloys after ECAP at 513 K for 4 passes (a and c) and 8 passes (b and d).
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Fig. 3. Bright-field TEM micrographs of ZK60 alloys after two-step ECAP process: (a) 513 K-4 passes + 453 K-2 passes and (b) 513 K-4 passes + 453 K-4 passes.
3.4. Texture evolution The texture evolution during the ECAP process of ZK60 alloy is shown in Fig. 5. For the as-received alloy, the pole figure reveals a strong {0 0 0 2} basal texture indicating the basal plane in most grains is orientated perpendicular to the extrusion direction. After single-step ECAP process at 513 K for 4 passes, the texture remains basal with a slight spread and a new texture component locating at ∼45◦ from ED towards TD has formed. Increasing the pressing pass to 8, the basal texture has been totally replaced by the new formed texture component. The maximum intensity also increases from 4.77 to 7.05. The two-step ECAP process produces a similar texture as 8 passes ECAP at 513 K. After 4 passes at 453 K for the second step, the maximum texture intensity of {0 0 0 2} pole figure is highest among all the alloys, indicating that strong texture has been developed. The texture component retains when the pressing pass increases from 2 to 4 and maximum intensity increases from 7.1 to 12.3. Fig. 4. The stress–strain curve obtained from the tensile test of the ECAP processed ZK60 alloys.
as-received ZK60 alloy, the single-step ECAP process decreases the strength of the alloy though the grain size has been significantly reduced. It is believed that the strength decrease is related to the texture evolution during the ECAP processing according to Kim and Jeong [23]. After two-step ECAP process, the strength of the alloy was recovered compared with the as-received material. After 2 passes at 453 K, the yield strength was almost close to the as-received material. After 4 passes, the strength was higher than the as-received material. The strength was also significantly improved compared with the single-step ECAP process. The ultimate strength was increased by 15 MPa and 40 MPa after ECAP processing at 453 K for 2 passes and 4 passes and the yield strength was improved 45 MPa and 55 MPa, respectively.
4. Discussion The present work shows that ECAP can introduce a remarkable grain refinement to ZK60 alloy. The presence of equiaxed grains and serrated grain boundaries in Fig. 1 indicates the occurrence of recrystallization during ECAP process [24]. According to the results of Galiyev et al. [25] on compression experiments of ZK60 alloy, in range of 473–523 K, due to the activation of prismatic and pyramidal slip systems, the deformation of ZK60 magnesium alloy was mainly accommodated by continuous dynamic recrystallization associated with dislocation slip. The same mechanism was also employed by Su et al. [26,27] to explain the recrystallization process on AZ31 magnesium alloy. Since the ECAP process in the present study was carried out at 513 K, which is above the recrystallization temperature of ZK60 alloy (∼473 K), recrystallization is believed to play an important role during the ECAP process. It is evident from the TEM observation in Fig. 2 that after ECAP at 513 K for 4
Table 1 Mechanical properties of the ECAP processed ZK60 alloy. Samples
Ultimate tensile strength (MPa)
Yield strength (MPa)
Elongation to failure (%)
Mean grain size (m)
As-received 513 K-4p 513 K-8p 513 K-4p + 453 K-2p 513 K-4p + 453 K-4p
250 221 226 236 266
166 120 125 160 175
18.5 28.1 35.1 28.9 31.9
∼80 1.8 1.6 1.0 0.8
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Fig. 5. The {0 0 0 2} pole figures of the as-received material (a) and after ECAP process of 513 K-4 passes (b), 513 K-8 passes (c), 513 K-4 passes + 453 K-2 passes (d) and 513 K-4 passes + 453 K-4 passes (e).
and 8 passes, equiaxed grains with straight and well defined grain boundary are formed, indicating that recrystallization has taken place during the ECAP process. Further observation on TEM image in Fig. 6a shows that some dislocations are homogenously distributed within the grains at the initial stage of ECAP processing. The dislocation movement was blocked by the initial grain boundaries and resulted in a complicated network of dislocation entanglement. According to the low energy dislocation structure (LEDS) theory [28], the entangled dislocations are readily rearranged into dislocation boundaries and sub-grain boundaries, as shown in Fig. 6b. Upon further ECAP processing, the dislocation generation continues within the sub-grain interior. Mobile dislocations move across the sub-grains and are absorbed by the sub-grain boundaries, resulting in an increase in their misorientation. As a result, the low angle
boundaries (LABs) persistently evolved into high grain boundaries (HABs), which is a kind of continuous dynamic recovery and recrystallization. At high strain, dislocations generated by slip were mostly consumed by the transformation from LABs to HABs. Thus further straining is more effective in increasing the misorientation of the grain boundaries. However, the grain refinement is less effective. Another reason resulting in the less effectiveness of grain refinement at high strain might be grain boundary sliding (GBS). It has shown that magnesium alloys with fine grain size can be superplastically deformed around 473 K [18,29,30], where GBS was considered to be the main deformation mechanism. In present study, the deformation temperature was 513 K. After 4 passes ECAP processing, the grain size was reduced to 1.8 m and even smaller
Fig. 6. The dislocation configurations within the grain interior (a) and dislocation wall evolved from dislocation entanglement (b).
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after two-step ECAP processing. Therefore, during ECAP processing GBS was likely to operate in the ZK60 alloy with fine grains size having HABs. The occurrence of GBS can release the stress concentration resulting from dislocation slip and help to accommodate further deformation which leads to less accumulation of dislocation entanglement and eventually less effective grain refinement at high strain. According to the study of Su et al. [27], the recrystallization process can be considered as diffusion of atoms from one grain to the other by overcoming an activation energy through thermal activation. The velocity of the grain boundaries migration increases exponentially with temperature. Therefore low temperature is suggested to slow the transformation process of LABs to HABs, resulting in an increase in dislocation density. Therefore, more mobile dislocations are arranged into new sub-grain boundaries, which eventually result in a more effective grain refinement during ECAP process. In the present study, the two-step ECAP processing was carried out at low temperature of 453 K. At this temperature, the recovery process was restricted due to the low recovery rate. As a result, as shown in Fig. 1, the grain refinement is more effective than the single-step ECAP process at 513 K. The reduction in average grain size may also be resulted from the suppression of grain growth at low temperature. Generally, the room temperature properties of metals are greatly related to the grain size. According to the Hall–Petch equation, finer grain size usually has higher yield strength. However, texture also plays an important role on the deformation behaviour as texture evolution has great effect on the Schmid factor variation. Therefore, the tensile properties are determined both by the grain size and texture. Detailed effects of texture on the mechanical behaviour of ECAP magnesium alloys were well studied by Kim et al. [23,31], who reported that texture modification during ECAP has a great influence on the strength of magnesium alloys. The rotation of high fraction of basal plane to the favourable for slip can appreciably decrease the yield strength. In the present study, the texture for the as-received ZK60 alloy is mainly {0 0 0 2} basal texture, which has a high Schmid factor with the tensile direction. Therefore, the as-received alloy still exhibits high strength though having coarse grain size. After ECAP process, the basal texture gradually transformed to shear texture which is located at 45◦ from transverse direction towards extrusion direction. This kind of shear texture has low Schmid factor with tensile direction. Thus, though the grain size has been significantly reduced after ECAP process at 513 K, the yield strength of the ECAP processed alloy is still lower than that of the as-received alloy. However, the Hall–Petch equation is still valid for a constant texture. The two-step ECAP process produces similar texture with single-step ECAP but finer grain size. Therefore, the two-step ECAP also exhibits higher strength compared to the single-step ECAP process. 5. Conclusions The effect of two-step ECAP processing on the microstructure and mechanical properties of an as-extruded ZK60 alloy was investigated. The results obtained are summarized as follows:
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