H-Phase Precipitation and Martensitic Transformation in Ni-rich Ni–Ti–Hf and Ni–Ti-Zr High-Temperature Shape Memory Alloys

The distributions of H-phase precipitates in Ni_50.3Ti_29.7Hf₂₀ and Ni_50.3Ti_29.7Zr₂₀ alloys formed by aging treatments at 500 and 550 °C or slow furnace cooling and their effects on the thermal martensitic transformation have been investigated by TEM and calorimetry. The comparative study clearly reveals faster precipitate-coarsening kinetics in the NiTiZr alloy than in NiTiHf. For precipitates of a similar size of 10–20 nm in both alloys, the martensite plates in Ni_50.3Ti_29.7Zr₂₀ have larger widths and span a higher number of precipitates compared with the Ni_50.3Ti_29.7Hf₂₀ alloy. However, for large H-phase particles with hundreds of nm in length, no significant differences in the martensitic microstructures of both alloy systems have been observed. The martensitic transformation temperatures of Ni_50.3Ti_29.7Hf₂₀ are ~ 80–90 °C higher than those of Ni_50.3Ti_29.7Zr₂₀ in the precipitate-free state and in the presence of large particles of hundreds on nm in length, but this difference is reduced to only 10–20 °C in samples with small H-phase precipitates. The changes in the transformation temperatures are consistent with the differences in the precipitate distributions between the two alloy systems observed by TEM.