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

NiTiHf high-temperature shape memory alloy ingots with transformation temperatures above 100 °C were produced by vacuum induction melting (VIM) and vacuum arc melting (VAM). The effects of melting process and compositional changes were investigated on hot rolling of cast samples. The amount of (Ti,Hf)₂Ni 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 hot-rolling behavior. Optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were used to inspect the observed cracks in the microstructure after the hot-rolling process. The results displayed that the formation of (Ti,Hf)C and the existence of (Ti,Hf)₂Ni second phase had harmful effects on the workability of the cast specimen due to the feeble coherency of (Ti,Hf)C and (Ti,Hf)₂ Ni with the matrix, which caused a failure in the hot-rolled specimen. The Ni₅₀Ti₄₀Hf₁₀ alloy produced by VAM shows better workability in hot rolling due to lower amount of (Ti,Hf)₂Ni, (Ti,Hf)C phases.