Notch strength and notch fracture mechanisms of a cast Mg-Gd-Y alloy

Abstract

This study aims to elucidate the mechanical properties and fracture mechanisms of cast Mg-6Gd-3Y-0.4Zr (wt.%) (GW63) alloy in the presence of geometrical discontinuities induced by notches. Notch strength is evaluated by room-temperature tensile tests using circumferentially notched bars of different notch acuity. Finite element calculations suggest that notch fracture is controlled by the von Mises stress and effective plastic strain at the notch tip. Through fractographic observations and EBSD analysis on samples subjected to interrupted tests, a cleavage micro-cracking behavior uncommon for conventional Mg alloys is found to dominate notch fracture. Grain-sized cleavage microcracks tend to occur along the basal plane of grains favorably oriented for basal slip, which is explained by crack nucleation at co-planar slip bands blocked by the grain boundary according to Smith and Barnby's theory. Failure is initiated by the accumulation and coalescence of such microcracks in the notch vicinity. The unexpected cleavage micro-cracking behavior is mainly ascribed to the RE solutes that suppress {10–12} twinning, and is responsible for the limited deformation capacity and notch strength of the cast-T6 GW63 alloy with coarse grains and significant strengthening due to precipitation.