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This study investigates the thermal and mechanical properties that arise from aging Ni-rich Ni–Ti (Nitinol) at temperatures below 250 °C, well below those commonly used to fabricate medical devices. We demonstrate that the Ni50.8–Ti49.2 composition decomposes at temperatures as low as room temperature and discuss the unusual changes in thermal and mechanical behaviors compared to common aging treatments, such as separation of Martensite and R-phase transformations. Using such aging treatments, superelasticity can be achieved without the presence of austenite at body temperature (Af > 45 °C, well above the body temperature). Furthermore, the influence of R-phase on mechanical response and the disparity between thermal and mechanical behavior is discussed in detail.

A Ni50.3Ti41.7Hf8 alloy was studied after two-step aging treatments consisting of 300 °C for 12 h followed by 550 °C for different times. An anomalous change in transformation temperatures was observed as the second aging time was increased from 7.5 to 13.5 h. Initially with increased aging time (0.5–7.5 h) at 550 °C, coherency strain fields about H-phase precipitates increased. The corresponding backstress favored martensite formation, hence an increase in transformation temperatures. However, a point was eventually reached where misfit dislocations relaxed those strain fields and the effect was reduced, resulting in a decrease in transformation temperatures.