Solid solution softening in dislocation-starved Mg-Al alloys

Here, we report molecular dynamics (MD) calculations of spall strength in pure magnesium and magnesium alloys with 1-8 at.% aluminum in solid solution. The Mg-Al solid solution alloys are found to have a weaker spall strength than pure magnesium, e.g. the spall strength of Mg-8 at.% Al is found to be 12% lower than pure magnesium. Moreover, we find that the spall strength of Mg-Al alloys monotonically decreases with increasing concentration of Al content. We term this phenomena solid solution softening (SSS) of spall strength, which is contrary to the conventional solid solution strengthening observed in most alloys (including Mg-Al alloys) deformed at quasi-static loading rates. We argue that this transition from solid solution strengthening at quasi-static rates to SSS at extreme loading rates is due to a transition from dislocation glide dominated failure at low rates to dislocation nucleation dominated failure at extreme rates. The distortion of the host lattice by solute atoms is found to retard dislocation glide, but aids dislocation nucleation. We thus conclude that SSS may manifest in any dislocation-starved system, regardless of loading rate. To this end, we propose a theoretical model for spall strength in dislocation-starved bulk and nanoporous alloys that exhibits remarkable agreement with our molecular dynamics calculations. Our theory bares some resemblance to the double kink nucleation enhancement theory for low-temperature SSS in some body centered cubic metals, e.g. refractory metals alloyed with group IV-VIII elements.