Engineering the Martensitic Transformation Hysteresis of Ni-Rich NiTi Alloys

The shape memory behavior in NiTi alloys can be exploited for a wide variety of applications that require active materials. The application dictates the transformation temperatures and hysteresis of the alloy. NiTi alloys with high Ni contents can be precipitation heat treated to produce large changes in the transformation temperatures, as well as increases in strength and dimensional stability. The effect of aging on the Ms temperature has been previously studied in the literature; however, few studies have investigated long duration aging, and little attention has been paid to the effect of precipitate formation on the thermal hysteresis.

In the current work, a systematic study of heat treatments was performed to study the effect of aging time, aging temperature, and initial Ni composition on the transformation temperatures and the thermal hysteresis under zero stress conditions using differential scanning calorimetry. Heat treatments and NiTi compositions were chosen in order to ensure that only Ni4Ti3 type precipitates formed during the aging process.

The results showed aging led to three different transformation paths; single step B2-B19, B2-R-B19, and multiple step transformation. At low aging temperatures, the transformation temperatures were initially suppressed but increased after sufficient aging durations. At higher aging temperatures the transformation temperatures only increased with aging time. In high Ni content materials the transformation temperatures were suppressed more than in the low Ni content materials.

The thermal hysteresis was highest in materials aged at short times at low temperatures, when the average spacing between adjacent precipitates was small. As the materials continued to aged, the thermal hysteresis decreased with time. As the aging temperature increased, the thermal hysteresis decreased.