A study on the mechanical behavior and microstructure of NiTiHf shape memory alloy under hot deformation

In the present work, the hot deformation behavior of Ni49Ti36Hf15 alloy was investigated. Compression tests were carried out at temperatures ranging from 800 to 1100 °C and at the strain rates of 0.001-1 s−1. The peak stress decreased with the increase in the deformation temperature and decrease in strain rate, which could be represented by the Zener-Hollomon parameter. It was realized that dynamic recrystallization (DRX) was responsible for flow softening. Most of the samples exhibited single peak stress followed by a gradual fall-down stress in stress-strain curves. Microstructure evolution showed that new recrystallized grains were formed in the vicinity of grain boundaries at 1100 °C. No new recrystallized grain was observed at 800 °C. However, DRX occurred at 900 and 1000 °C only at higher strain rates. The hyperbolic-sine-type constitutive model of Ni49Ti36Hf15 alloy was obtained to provide basic data for determining reasonable hot forming process. The high value of activation energy (410 KJ/mol) obtained for Ni49Ti36Hf15 can be attributed to the strengthening effect of (Ti,Hf)2Ni second phase. The study shows that Ni49Ti36Hf15 alloy was not suitable for hot working due to occurrence of flow localization at all temperatures and strain rates.