A Brief Review of Spall Failure in Pure and Alloyed Magnesium

Abstract

The ratio of the yield strength to the density (specific strength) of magnesium and its alloys is relatively high, but cavitation-driven spall failure of these materials occurs at low threshold stresses and strains. The low failure threshold suggests a general susceptibility to catastrophic failure under tension at high rates of deformation. Higher material strength likely prevents such failure, but processing methods to achieve these strengths can introduce potential failure nucleation sites that can make failure more likely—the interplay between matrix strength and nucleation sites remains largely unknown. Here we examine the literature regarding experimental observation of spall failure of pure and alloyed magnesium with a particular emphasis on the trends between grain size and strain rate with respect to the spall failure strength. We then apply an analytical dynamic cavitation model to better understand the role of second phase particles in spall failure. We find that the conventional polycrystalline alloys with micron sized grains have microstructures containing second phase particles that act as failure nucleation sites, diminishing the potential to design against low failure threshold stresses. Additionally, nanocrystalline grained magnesium remains largely unexplored and may potentially offer an avenue for greater dynamic strength.