Heterogeneous multiphase microstructure governing crack-path evolution and impact toughness in Mg-7Gd-2Y-1Zn-0.5Zr (wt.%) alloy

In this study, the roles of heterogeneous multiphase microstructure in governing crack-path evolution and impact toughness were systematically investigated in as-extruded and peak-aged Mg-7Gd-2Y-1Zn-0.5Zr alloy. Compared to the peak-aged alloy, the as-extruded alloy exhibits pronounced tensile twinning and a higher density of geometrically necessary dislocations around the V-notch before the occurrence of crack initiation under the same loading impact condition, leading to an enhanced impact toughness. In contrast, due to the formation of dense β´ nano-precipitates within the α-Mg matrix, the peak-aged alloy shows a higher strength but a reduced impact toughness, which is ascribed to the limited twin activity and suppressed plastic deformation, resulting in an inferior impact toughness dominated by cleavage and intergranular failure. Meanwhile, the formation of delamination microcracks aligned parallel to the lamellar-shaped γ´ phase can be detected, in which the γ′ phase acts as preferential paths for crack advancement. Furthermore, the crack deflection and crack bridging induced by non-dynamic recrystallized grains in the as-extruded alloy can enhance the energy dissipation to achieve a ductile fracture mode. Load-displacement curve analysis indicates that the superior fracture resistance of the as-extruded alloy is predominantly derived from both crack initiation and stable crack propagation, leading to remarkable rising dynamic R-curve behavior on the basis of J-based measurements of nonlinear-elastic fracture mechanics. Atomic-scale TEM observations reveal that the as-extruded alloy can accommodate the plastic deformation through the concurrent activation of non-basal <a> and pyramidal <c + a> dislocations, whereas only limited non-basal <a> slip occurs in the peak-aged alloy. Besides, the accumulation of non-basal <a> and <c + a> dislocations near the γ′ phase induces pronounced lattice distortion and local stress concentration, thereby promoting interfacial delamination and enhancing the impact toughness. It is anticipated that this work can provide fundamental insights into the microstructure-toughness relationship in Mg-RE alloys for the microstructural design of high-performance Mg alloys with improved impact resistance.