Influence of Melting Process on Hot and cold Rolling of NiTiHf Shape Memory Alloy

NiTiHf high-temperature shape memory alloy ingots with transformation temperatures above ۱۰۰ C were produced by vacuum induction melting (VIM) and vacuum arc melting (VAM). The effects of melting process was investigated on hot and cold rolling of cast samples. The amount of (Ti,Hf)۲Ni second phase which was formed during solidification and the (Ti,Hf)C formed due to graphite crucible using in VIM have significantly affected the microstructure of the cast sample due to poor coherency, which affected the rolling behavior. Optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were used to inspect the observed cracks in the microstructure after the hotrolling process. The results displayed that the formation of (Ti,Hf)C and the existence of (Ti,Hf)۲Ni second phase had harmful effects on the workability of the cast specimen due to the feeble coherency of (Ti,Hf)C and (Ti,Hf)۲ Ni with the matrix, which caused a failure in the hot-rolled specimen. The Ni۵۰Ti۴۰Hf۱۰ alloy produced by VAM shows better workability in hot and cold rolling due to lower amount of (Ti,Hf)۲Ni, (Ti,Hf)C phases.

Keywords:

workability , (Ti , Hf)۲Ni second phase , hot and cold rolling , shape memory alloy.