High strain rate deformation and spall damage in a Mg-Ag-Nd-Zr alloy: Effects of δ/γ precipitates

Plate impact experiments are conducted on aging-treated (AG) and solution-treated (ST) QE22 Mg alloy (Mg-2.5Ag-2.0Nd-0.7Zr) to investigate the effects of precipitates with different sizes on deformation and spall damage properties/mechanisms. The AG alloy contains the micron-scale intermetallic precipitates (δ) at grain boundaries and nano-scale precipitates (γ) in the matrix, while the ST alloy contains only the δ precipitates. Given the γ precipitate strengthening, the dynamic yield stress for the AG alloy (0.173 GPa) is about 40 % higher than that for the ST alloy (0.122 GPa). Upon shock loading, {1012}, {1011} and {1121} deformation twins are observed, and the γ precipitates significantly impede the growth of twins. The spall damage features of both samples are characterized and summarized via two dimensional scanning electron microscopy and three dimensional X-ray computed tomography. The voids/cracks are basically restricted within the δ phase at grain boundaries. In the AG alloy, the additional back-stress from matrix strengthened via γ precipitates suppresses void growth, leading to the less severe spall damage and 20 % higher spall strength (0.58–0.63 GPa) compared to the ST alloy (0.48–0.51 GPa). The present study provides insights into the roles of δ/γ precipitates in dynamic deformation and spallation of QE22 Mg alloy. Nano-scale γ precipitates improve both dynamic yield stress and spall strength, while micron-scale δ precipitates are conducive to the nucleation of spallation voids.