The microstructural evolution and superplastic behavior at low temperatures of Mg-5.00Zn-0.92Y-0.16Zr (wt.%) alloys after hot extrusion and ECAP process

In this study, equal channel angular pressing (ECAP) was applied to a hot-extruded Mg-5.00Zn-0.92Y-0.16Zr (wt.%) alloy to produce an ultrafine-grained a-Mg structure of 0.6 mu m with uniformly distributed fine quasicrystal Mg3YZn6 particles. The basal planes in the as-ECAPed alloy were inclined approximately 45 degrees to the ECAP direction (EPD); thus, a high Schmid factor of 0.36 for (0 0 0 1)< 1 1 (2) over bar 0 > basal slip along EPD was obtained. Then, the superplastic behavior at low temperatures of 150-250 degrees C and initial strain rates of 1.67 x 10(-3) s(-1)-1.67 x 10(-1) s(-1) of the as-ECAPed alloy was investigated and compared with that of the as-extruded alloy. The microstructural development, texture evolution and cracking behavior during tensile tests were systemically investigated by electron backscattered diffraction (EBSD) analysis. During the tensile test along the EPD, the basal planes in the as-ECAPed alloy specimen were tilted approximately 15 degrees away from the original position; thus, the basal planes were still in a position favorable for the basal slip along the EPD. As a result, a maximum elongation of 865% was obtained at 200 degrees C and a strain rate of 1.67 x 10(-3) s(-1), as a result of this favored basal texture and the excellent thermal stability of the ultrafine-grained structure. This optimum superplastic temperature of 200 degrees C is much lower than that obtained for other Mg-Zn-Y-(Zr) alloys. In contrast, for the as-extruded alloy specimen, superplastic behavior did not occur until the critical condition of 250 degrees C and a strain rate of 1.67 x 10(-3) s(-1). Below this temperature and strain rate, the strong extrusion basal texture was maintained, and the cracks in the hot-worked coarse region (or un-DRXed region) were observed nucleating mainly associated with the formation of a 38 degrees {1 0 (1) over bar 1}-{1 0 (1) over bar 2} double twin. At this temperature and strain rate, the hot-worked coarse regions were significantly refined by the dynamic recovery and the following recrystallization process. In the case, the formation of a double twin was prevented and the facture was delayed. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.