The effect of microstructure on the plastic strain localization and fatigue crack initiation in cast Mg–8Gd–3Y–0.5Zr alloy

Abstract

Strain-controlled fatigue tests were performed on a cast Mg–8Gd–3Y–0.5Zr (wt. %, GW83K) alloy to investigate the effect of microstructure on strain localization and fatigue crack initiation. The results show that, in T4 alloys, a large number of basal slips in individual grains and most grains in the bulk material are involved in plastic deformation and fatigue damage because persistent slip bands (PSBs) are dense and uniform. The interaction between dense PSBs and grain boundaries (GBs) occurs continuously along a wide range of GBs, resulting in uniform distribution of residual strains along GBs. The grain-level strain localizations within and among grains are effectively alleviated. In T6 alloys, however, due to the sparse and inhomogeneous PSBs, only a few basal slips in individual grains and only a few grains in the bulk material participate in plastic deformation and fatigue damage. The interaction sites between sparse PSBs and GBs is significantly limited, which leads to highly concentrated deformation at a few specific areas of the GBs. It results in high trans-granular strain localization in long range stretching across both grain interiors and GBs in short cycles. The strain localizations within and among grains are relatively aggravated. Moreover, the saturated strains of the T4 alloys are much larger than those of the T6 alloys. Therefore, under the same loading condition, it takes more cycles for T4 alloys, while fewer cycles for T6 alloys, to reach the saturation strains and initiate micro-cracks. In other words, the crack initiation life of T4 alloys is much longer than that of T6 alloys.