Simulation of structural phase transitions in NiTi

Abstract

By means of molecular-dynamics simulations, temperature driven diffusionless structural phase transitions in equi-and nearly equiatomic ordered nickel-titanium alloys were investigated. For this purpose, a model potential from the literature was adopted [W. S. Lai and B. X. Liu, J. Phys. Condens. Matter 12, L53 (2000)], which is based on the tight-binding model in second moment approximation. The model predicts a stable B19 ′ phase at low temperatures and a nearly cubic B2 phase at high temperatures. After an analysis of crystallography and energetics of the emerging structures, the experimentally known strong dependence of transition temperatures on composition is confirmed and related to lattice instability. Free energy calculations finally give insight into the driving forces of the phase transitions, and reveal free energy barriers inhibiting them below the transition temperatures.

Size effects in NiTi from density functional theory calculations

“…This finding is important because the B2-BCO-B2 phase transformation is not atomistically reversible and hence would not result in shape memory. These results were later confirmed in various ab initio studies and transformation paths between the above phases 25,26,24 and free energy barriers 27 were studied. However, the BCO phase has still not been confirmed experimentally and the B19' phase may be stabilized by internal stresses that developed during the martensitic transformation.…”

Simulation of the shape memory effect in a NiTi nano model system

“…values for Ni-Ni, Ti-Ti, and Ni-Ti interaction, have been determined by Lai and Liu [20]. The authors of the present paper have shown in a recent work [21], that the application of a cut-off function proposed by Baskes et al [22] with a width of δ = 0.2Å results in a stable monoclinic B19 ′ structure in a bulk system at low temperatures, with lattice parameters comparing well with experimental [23] and ab initio [24] values. In addition, an entropically stabilized nearly cubic B2 like structure is obtained upon heating.…”

“…The higher the Ni content, the smoother is the stress in this range, with less abrupt increases and releases. This behavior, as well as the lower stiffnesses at higher Ni contents can be explained by a growing lattice instability, what was figured out to be the reason for the strong decrease of transition temperatures with Ni content in a previous work [21]. Figs.…”

“…In that work, periodic boundary conditions were applied in order to simulate bulk behavior. The present paper deals with a nano structure, where angular distributions of the nearest neighbor lengths in B19 ′ are observed, which differ slightly from the one described in [21]. At free surfaces, these structures, denoted A and B, are energetically more favourable.…”

“…If the system is cooled out of this high-temperature phase, a martensitic ground state structure denoted B19 ′ emerges. This structure is closely related to B19 ′ , since it shows nearly the same peak structure in the radial distribution function, but the angular distribution of the nearest neighbor lengths is different, resulting in a ground state energy, which is about 0.04% lower than that of B19 ′ [21]. In that work, periodic boundary conditions were applied in order to simulate bulk behavior.…”