Study on grain size dependence of shape memory effect in nanocrystalline NiTi shape memory alloys with grain size below 20 nm based on molecular dynamics simulation

Behaviors of grain size (GS)-dependent shape memory effect in nanocrystalline NiTi shape memory alloys (SMAs) with GS below 20 nm were studied using molecular dynamics (MD) simulation and involved mechanisms were analyzed. Based on MD simulations, we hypothesize that the constraint of grain boundaries (GBs) may lead to increased stiffness near GBs compared with coarse-grained ones, forming rigid regions. Therefore, driving force aroused by the decrease of temperature is not sufficient to make intracrystalline austenite undergo martensitic transformation through shear deformation. With the decrease of GS, fraction of hypothesized rigid region increases, while fraction of hypothesized flexible region decreases, resulting in a decrease in the number of martensite nuclei and a reduction in martensite fraction. In the loading process, both martensite reorientation of accommodated variants and stress-induced martensite transformation of retained austenite occur in the alloy with larger GS, while only stress-induced martensite transformation occurs in alloys with GS below 10 nm, leading to an increased martensite fraction in all alloys. In addition, plateau stress rises with reducing GS and dramatic hardening effect occurs when the GS is below 10 nm. During the unloading process, reverse martensite transition occurs in partial stress-induced martensite formed during loading and fraction of reversed martensite rises with reducing GS. When heated, the fraction of martensite in unloaded nanocrystalline NiTi SMAs decreases sharply, resulting in a drastic drop in strain, and the shape memory effect strain decreases with decreasing GS because fraction of martensite decreases with the reduction of GS.