Compression along the Easy-Glide Orientation of Ultrafine and Fine-Grained Mg-3Al-1Zn Alloy
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er the last few years, the demand of light weight materials for structural application has increased; therefore, the mechanical properties of magnesium alloys are of great interest. To improve the strength of magnesium alloys, reducing grain size is an effective and relatively simple way. In the literature, there have been many attempts to develop fine-grained Mg alloys that have had a certain degree of success.[1–9] For example, Ding et al.[8] reported a very high 0.2 pct yield stress of 372 MPa of ultrafine-grained (UFG) AZ31 Mg alloy, which has a grain size of 0.37 lm. Unfortunately, like other UFG materials, the improvement of strength is accompanied by the reduction of ductility at room temperature. For Mg alloys, poor workability limits their applications; therefore, it is essential that ductility is improved together with strength. The low ductility of Mg alloys at room temperature is due to the limited number of slip systems. Despite so, in several articles,[10–12] it was reported that the ductility of Mg alloys was significantly increased when the basal plane was oriented in favor of basal slip. The aim of the present research is to study the deformation behavior of textured UFG and FG Mg alloys, which were compressed along the easyglide orientation, an orientation that let the basal plane have the largest resolved shear stress, so that basal slip dominates the deformation process. W.T. LEE, Y.W. CHOU, and C.I. HSIAO, Research Students, C.P. CHANG and P.W. KAO, Professors, and L. CHANG, Associate Professor, are with the Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted July 15, 2010. Article published online October 28, 2010 3282—VOLUME 41A, DECEMBER 2010
Equal channel angular pressing (ECAP) was used to produce UFG AZ31 Mg alloy. Billets for ECAP were cut from hot-rolled ½-in. AZ31 plate. ECAP was carried out in a die with the die angle F = 120 deg and outer die angle W = 0 deg. Three axes, X, Y, and Z, are used to define the coordinate system of the channels and the specimen after extrusion, as shown in Figure 1. A multitemperature pressing procedure was used, which consists of three sequential steps: (1) four passes at 473 K (200 C) with 10 cm/min pressing speed, (2) four passes at 423 K (150 C) with 3 cm/min pressing speed, and (3) two passes at 398 K (125 C) with 1 cm/min pressing speed. Between each pressing step, the billet was rotated 180 deg about the exit axis. After ECAP, a grain size of 0.46 lm was obtained, and specimens were annealed between 448 K and 498 K (175 C and 225 C) with different annealing times in order to generate different grain sizes ranging from 0.62 to 3.22 lm. Equivalent circle diameter (ECD) was used to measure the grain size, D. ECD was determined by measuring a grain area A, where A = pECD2/4. Grain size of 0.46 lm was measured by transmission electron microscopy (TEM), and other grain sizes were measured by scanning electron microscop
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