Spall strength in alloyed magnesium: A compendium of research efforts from the CMEDE 10-year effort

Magnesium alloys are an attractive material system for protection applications due to their high specific strength and stiffness, but exhibit low ductility in these applications. The potential to address this shortcoming through materials-based-design has motivated the Center for Materials in Extreme Dynamic Environments (CMEDE) to focus on improving Mg systems over the past decade. The plastic anisotropy from the low-symmetry hexagonal-close-packed crystal structure of Mg, as well as defects in the microstructure such as voids and precipitates, may all play roles in spall (dynamic tensile failure at high strain rates), but experimental data assessing the effect of individual microstructure features on spall remains challenging to obtain. We begin the present study by reviewing spall investigations on pure and alloyed Mg from the literature, and then present a large number of spall experiments performed with a laser-driven micro-flyer apparatus on Mg-9Al (wt. %) thin foil specimens with various precipitate morphologies in order to address this shortcoming. The model Mg-9Al binary alloy is warm-rolled and processed in two conditions: (a) fully solutionized with no precipitates, and (b) peak-aged to generate high aspect-ratio precipitates (Mg(17)Al(12 )second phase particles/inclusions) with nm-scale thickness and um-scale length on the basal plane. The loading direction is varied between the normal-to and transverse-to rolling directions of the specimen in order to interrogate the effects of both plastic anisotropy of the matrix material and geometric anisotropy of the precipitates on the spall strength. Bayesian analysis of the results enables us to account for instrument uncertainty and microstructure variation in our study. We compare the experiments to numerical simulations using realistic precipitate geometries and spacings from electron microscopy observations, finding a significant decrease in spall strength in the Mg-9Al with precipitates despite the expected increase in quasi-static yield strength.