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 50.3 Ti 29.7 Hf 20 and Ni 50.3 Ti 29.7 Zr 20 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 50.3 Ti 29.7 Zr 20 have larger widths and span a higher number of precipitates compared with the Ni 50.3 Ti 29.7 Hf 20 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 50.3 Ti 29.7 Hf 20 are ~ 80–90 °C higher than those of Ni 50.3 Ti 29.7 Zr 20 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.