Atomic-scale insights into the effect of Fe on phase transformation behavior of NiTi alloy

The addition of Fe into NiTi shape memory alloy (SMA) offers a viable method to tailor their mechanical properties and phase transformation temperatures to suit the requirements of aerospace applications. Nonetheless, the notably cryogenic martensitic transformation (MT) temperatures of NiTiFe alloys pose challenges in characterizing the deformation behavior and microstructure evolution associated with the MT. Molecular dynamics (MD) simulation emerges as a valuable tool for investigating the phase transformation phenomena of SMAs, effectively circumventing the obstacles encountered in experimental analyses. This research delves into the impact of Fe content on the MT in NiTi alloys induced by temperature and stress through MD simulations. A Ni-Ti-Fe ternary interatomic potential utilizing the second nearest-neighbour modified embedded-atom method (2NN MEAM) has been created and utilized. The study indicates that the addition of Fe in NiTi alloy inhibits the nucleation and growth of the martensitic phase, causing a reduction of the MT temperatures, and the increase of the critical stress needed for stress-induced MT in NiTi alloy. The free energy of the B2 phase and the B19′ phase, as well as the free energy difference between these two phases, decreased with increasing Fe content. Additionally, the T₀ temperature also exhibited a downward trend as the Fe content increased. The addition of Fe element in NiTi alloy elevates the elastic modulus of c’ and c₄₄, thereby enhancing the critical stress needed for stress-induced MT. To elucidate the mechanism behind the effect due to Fe addition, density-functional theory (DFT) analyses were employed to evaluate the charge density of NiTi and NiTiFe. The results indicated a higher charge density between Fe and Ti compared to that between Ni and Ti, signifying a stronger interaction force between Fe and Ti. This enhanced interaction leads to increased deformation resistance and decreased MT temperatures.