Effects of Defects on Stress-Induced Martensitic Transformation of Nanocrystalline NiTi Alloys: A Molecular Dynamics Study

The excellent functional properties of nanocrystalline NiTi alloys are associated with their unique material defects, including dislocations, vacancies and anti-site defects. However, the rules and mechanisms governing the effects of these defects on functional properties remain unclear. In this work, the effects of dislocation density (ρ = 0–1 × 1016 m−2), vacancy concentration (0–1%) and anti-site concentration (0–1%) on stress-induced martensite transformation of nanocrystalline NiTi alloy were investigated by molecular dynamics (MD) simulation, and the forward transformation stress (σtr), residual strain (ɛres) and transformation modulus (Htr) were analyzed quantitatively. Results show that σtr decreases with ρ due to the reduced martensite nucleation energy barrier (E). At the same defect concentration, vacancy models have higher E than that of anti-site models, resulting in a more significant increase in σtr for vacancy models. ɛres increases with ρ due to plastic deformation of disordered atom regions near dislocations and the accumulation of residual martensite. ɛres is insensitive to the concentrations of vacancy and anti-site defects. Disordered atoms near dislocations hinder growth of martensite, thus causing an increase in Htr with ρ. Models with vacancies, at the same defect concentration, have a higher fraction of disordered atoms within the interior of grains compared to models with anti-sites. Therefore, vacancy exhibits a more pronounced strengthening effect on Htr than anti-site defect.