Effects of low-temperature aging on the microstructure and functional stability of a hot-rolled high-purity NiTi alloy

In this study, low-temperature aging treatments were conducted on a hot-rolled high-purity NiTi alloy. The effects of aging temperature (300 °C-500 °C) and aging time (1h–12h) on the microstructure, martensitic transformation behavior, and functional properties of the alloy were systematically investigated. The results reveal that, during aging at 300–450 °C, Ni₄Ti₃ precipitates disperse within the matrix, which suppress martensite nucleation and promote the intermediate R-phase transformation, causing the transformation path to change from a one-stage to two-stage or even multi-stage martensitic transformation. At 500 °C, the precipitates undergo re-dissolution into the matrix, and the transformation behavior reverts to a one-stage characteristic. Based on the room-temperature loading–unloading test results, it is found that different aging conditions affect the stress-induced martensitic transformation (SIM) process by altering precipitate morphology and transformation behavior, thereby modifying key parameters such as the critical transformation stress and plateau stress, and consequently regulating the functional stability of the alloy. With increasing aging temperature, the critical stress for SIM decreases from a maximum of 226.4 MPa at 300 °C to a minimum of 104.3 MPa at 450 °C, and then rebounds to 175.2 MPa at 500 °C. With prolonged aging time, the critical stress for SIM continuously decreases. The optimal functional stability of the hot-rolled high-purity NiTi alloy is achieved under the aging condition of 400 °C for 1 h. This study provides an in-depth analysis of the microstructure, martensitic transformation behavior, and functional properties of the high-purity NiTi alloy under various aging conditions, elucidating the “process - microstructure - transformation - performance” regulation mechanism. These findings offer a theoretical basis for optimizing the functional stability and promoting the widespread application of high-purity NiTi alloys.