Fracture toughness and fracture mechanisms of a peak-aged cast Mg–Gd–Y–Zr alloy

This study addresses the fracture toughness and fracture mechanisms in a peak-aged cast Mg–6Gd–3Y–0.5Zr (wt.%) alloy using mode-I fracture toughness tests performed on precracked compact tension (CT) specimens. Pronounced stable crack extension is observed with arising P-Δ curves before rapid catastrophic failure. Some tests are interrupted within the stable extension stage, optical microscopy and electron backscattered (EBSD) analysis which are used to study the fracture mechanisms governing the crack extension behavior. Finite element analysis (FEA) is also conducted to simulate the stress–strain states experienced by the crack-tip microstructure. The results show that the crack extension process takes place in a discontinuous manner that resolves into two steps: First, grain-sized microcracks form in the basal planes of crack-tip grains; second, the microcracks coalesce with the main crack by local ductile fracture along grain boundaries and twin boundaries in between the cracks. The activity of deformation twinning is low at the crack tip. Analysis of resolved shear stress suggests that the formation of microcracks is associated with high basal slip activity poorly compensated by other deformation systems. It is argued the low fracture toughness of the current Mg–Gd–Y–Zr alloy lies in the material’s poor capacity of compatible deformation which requires mutual activation of slip and twinning.