Abstract
Ni2Ti4Ox 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 Ni2Ti4Ox. A nitinol sample melted by vacuum induction melting technique was investigated. The as-cast sample possessed NiTi2 segregation along the grain boundary, although solution heat-treatments at 1000 °C could eliminate NiTi2, Ni2Ti4Ox could be observed, indicating that some NiTi2 could transform into Ni2Ti4Ox oxide. The correlation between the NiTi2 segregation and the formation of Ni2Ti4Ox has been elucidated by first principle calculation and near-edge x-ray absorption fine structure analysis. Results indicate that the formation enthalpy of Ni2Ti4Ox is inversely proportional to the amount of oxygen pick-up. During the solution heat-treatment process, some NiTi2 could be dissolved back to the NiTi matrix, while some NiTi2 could transform into Ni2Ti4Ox and be stabilized by absorbing more oxygen. Based on findings in this work, possible measures to minimize the formation of Ni2Ti4Ox in nitinol are proposed.
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