Microstructure and property relationship controllable by thermomechanical processing of Mg-Al-Zn-Y-Ca alloy sheets


Sheet processing of conventional Mg alloys, such as Mg-3Al-1Zn (in wt.%), typically leads to the formation of so called basal-type texture in which the crystallographic -axis mostly align parallel to the sheet normal direction (ND). Such strong basal-type texture causes a limited formability under loading in the sheet normal and planar directions, due to the low Schmid factor for the basal -slip. By altering the strong basal-type texture, obtained by alloying addition of rare earth elements (REE) or Ca, the sheet formability can be considerably improved. The texture weakening and formability improvement have been reported in Mg-Zn-REE or Mg-Zn-Ca alloy systems. Regarding further properties such as corrosion resistance and non-flammability, it is important to provide a way of achieving weak texture and fine grained microstructure simultaneously in various alloy systems, especially Mg-Al alloys. The present study examined the influence of thermomechanical processing condition on the microstructure and texture evolution of a modified AZ31 alloy by Ca and Y addition. The rolling of AZXW3100 (Mg-3Al-1Zn-0.5Ca-0.5Y) alloy was conducted by using various parameters; 2 rolling temperatures of 450°C and 500°C, and the different pass reduction degrees (φ) of 0.1 ~ 0.3. The sheet rolled at 450°C and φ = 0.1 shows a relatively strong texture with a basal pole split into the sheet rolling direction (RD). On the contrary, the sheet rolled at 450°C with a larger step reduction degree, φ = 0.2 and φ = 0.3, shows the formation of significantly weak texture with basal pole split into the RD and the sheet transverse direction (TD). The rolling at 500°C results in a further distinct texture type in which the basal poles are largely tilted into the RD. The rolling condition influences also on the grain sizes of the recrystallized sheets, varying within a range of 6 µm and 21 µm after annealing at 400°C for 60 min of the differently rolled sheets. The Erichsen index (IE) of the sheets varies in accordance to the texture sharpness, i.e. the weaker texture the higher IE. The sheet having the basal pole split into the TD shows an excellent formability with IE of 8.5, which is about 3-fold higher value than that obtained in a conventional Mg alloy sheet. The experimental results indicate a clear relationship among the microstructure, texture and sheet formability which are controllable by thermomechanical treatments of the newly developed Mg alloy.
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