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-tosample variability in transformation temperature. The uncertainty in the austenite transformation temperature increases with decreasing size, reaching similar to,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 similar to 40 nm is required to develop multidomain martensite microstructures, and mechanical constraints reduce this critical size to similar to 17 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. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.