Formation mechanism of Ni2Ti4Ox in NITI shape memory alloy

Abstract Ni 2 Ti 4 O x phase is known to affect the formability and mechanical properties of NiTi shape memory alloy (nitinol), however, the underlying mechanism of its formation remains unclear, although its presence has long affected the production of nitinol in industry. The present study aims to determine the formation mechanism of Ni 2 Ti 4 O x . A nitinol sample melted by vacuum induction melting technique was investigated. The as-cast sample possessed NiTi 2 segregation along the grain boundary, although solution heat-treatments at 1000 °C could eliminate NiTi 2 , Ni 2 Ti 4 O x could be observed, indicating that some NiTi 2 could transform into Ni 2 Ti 4 O x oxide. The correlation between the NiTi 2 segregation and the formation of Ni 2 Ti 4 O x has been elucidated by first principle calculation and near-edge x-ray absorption fine structure analysis. Results indicate that the formation enthalpy of Ni 2 Ti 4 O x is inversely proportional to the amount of oxygen pick-up. During the solution heat-treatment process, some NiTi 2 could be dissolved back to the NiTi matrix, while some NiTi 2 could transform into Ni 2 Ti 4 O x and be stabilized by absorbing more oxygen. Based on findings in this work, possible measures to minimize the formation of Ni 2 Ti 4 O x in nitinol are proposed.