Molecular dynamics investigation on mechanical behaviour and phase transition of nanocrystalline NiTi shape memory alloy containing amorphous surface

Abstract

It is well known that surface severe plastic deformation will cause the surface amorphization of nickel-titanium shape memory alloy (NiTi SMA), so it is of significance to investigate the influence of amorphous phase on mechanical behaviour and phase transition of NiTi SMA. Nanocrystalline polycrystal (PC) models with the average grain size of 9, 12 and 15 nm, respectively, are established by molecular dynamics (MD) software. On the upper and lower surfaces of the aforementioned three PC models, the corresponding amorphous layers are formed to obtain the amorphous-nanocrystalline-amorphous (ANA) counterparts. MD simulation is used to investigate mechanical behaviour and phase transition of nanocrystalline NiTi SMA with amorphous surface. The phase transition stress, the maximum tensile stress and the irrecoverable strain increase with decreasing grain size. Furthermore, grain orientation has a great impact on phase transition of NiTi SMA. The existence of amorphous phase leads to superelastic degradation due to its plastic deformation, but it can greatly improve the tensile stress of NiTi SMA. The irrecoverable strain is reduced from 1.3% to 0.9% when the thickness of amorphous layer is decreased from 10 to 5 nm. The existence of amorphous phase results in reduction of martensite variants in the selected grains.