Role of atomic variability and mechanical constraints on the martensitic phase transformation of a model disordered shape memory alloy via molecular dynamics

We use molecular dynamics (MD) with an embedded atom model potential parameterized for NiAl to study martensitic phase transformations in a disordered shape memory alloy. We focus on the role of intrinsic atomic-level variability and mechanical constraints on the martensite and austenite transformation temperatures and on the martensite microstructure for specimens with varying size. We find that periodic system size has a weak effect on transformation temperature all the way to the nanometer scale, with the entropy-stabilized austenite phase slightly penalized with decreasing size. Atomic-level variability in these random alloys leads to significant sample-to-sample variability in transformation temperature. The uncertainty in the austenite transformation temperature increases with decreasing size, reaching ∼10% of the mean value for samples 10 nm on the side. Interestingly, the variability of the high-temperature martensite transition shows little size dependence. We find that a critical size of ∼40 nm is required to develop multidomain martensite microstructures, and mechanical constraints reduce this critical size to ∼7 nm, while significantly affecting the transformation temperatures. These results contribute to the understanding of martensitic transformation in nanocrystalline samples and of the fundamental limits of miniaturization of these alloys.