Investigation of Transformation Temperatures, Microstructure and Shape Memory Properties of NiTi, NiTiZr and NiTiHf Alloys

The influence of zirconium and hafnium additions in substitution to titanium, is investigated on TiNi shape memory alloy. Microstructural and thermomechanical studies are conducted on an arc-melted Ti38Ni50Hf12 alloy. Results are compared with those of an equiatomic TiNi. We confirm the increase of the phase transformation temperatures due to the additions of Zr and Hf. The presence of oxides in the cast alloy was detected. The lattice parameter is measured from a residue of electroextraction. It was found that a0=1,1547±0,0001nm. A stoichiometry of (Ti, Hf) 4Ni2 Ox (with x≤1) is suggested. It appears that this ternary alloy exhibit an improved shape recovery when it is pre-strained in austenite state (for Ms < T < As) . In this case it displays 100% of recovery for 4% of deformation