Transformation Exclusion Zone and Stress-Induced Superelastic Response in Free-Standing NiTi nanoparticles: A Simulation Approach

Abstract

Abstract

Abstract This study uses classical molecular dynamics with the Ko–Grabowski–Neugebauer modified embedded-atom method potential to examine free-standing equiatomic Ni₅₀Ti₅₀ nanoparticles at two diameters only; the contrast between the two cases implies, but does not establish, a size-dependent behavior. Both diameters are at or below 10 nm, a regime difficult to access experimentally, so this is a limiting-case mechanistic study. B2 particles of 5 nm (4,745 atoms) and 10 nm (37,869 atoms) are cycled between 500 and 200 K at 0.35 K/ps and compressed axially between walls at 500 K (nominal strain 10%). No, thermally driven martensitic transformation (B2 → non-B2) is observed within 200–500 K, giving a lower-bound indication that rc > 5 nm (critical diameter > 10 nm) for this potential. Under mechanical loading, both particles exhibit superelastic-like recovery within the present single-cycle protocol, with the bulk shape recovered to within 0.5%. The 5 nm particle requires about twice the peak stress (~2.9 versus ~1.5 GPa, subject to volume-normalization uncertainty) to reach comparable core transformation, a qualitative contrast rather than a scaling law. RDF and bond-angle fingerprints of the non-B2 atoms indicate martensitic-like local order but no specific phase. At 10 nm, the surface retains ~40% B2 at peak compression while the core falls to ~17%, indicating a 1–2 nm transformation exclusion zone near the free surface. These are mechanistic observations from two single trajectories; scaling laws and uncertainty bounds require more sizes and replicas.