The effect of Hf addition on the precipitation hardening and dynamic softening behavior of NiTi alloy during hot deformation

Nanoindentation test, high-temperature compression test, SEM, EBSD, and TEM observations are performed to investigate the effects of Hf additions on the microstructure and properties of NiTi alloy. The high-temperature deformation behavior of NiTi-8/12Hf alloys is discussed. The constitutive model and hot processing map are constructed, and the optimal processing interval of the two alloys is predicted. The results show that the content and size of precipitates increase with the addition of Hf. The nanoindentation hardness of the matrix with different Hf content increased from 3.36 ± 0.11 GPa to 5.47 ± 0.13 GPa. The main strengthening mechanism of the addition of the Hf to improve the strength is the solid solution strengthening effect and the load transfer effect. With the addition of 8 wt% and 12 wt% Hf, the instability areas are expanded. The optimal processing area of 8/12Hf alloys are 900 °C, 0.01s⁻¹. Under this condition, the microstructure of the alloy has a high DRX volume fraction and low average ρ_GND. As hot deformation progresses, the accumulated energy inside the alloy increases, leading to the activation of dislocations and the formation of more DDRX at grain boundaries. Dislocations within the grains aggregate to form LAGBs. The softening mechanism of 56Ni-Ti-8/12Hf alloy are DDRX and dynamic recover.