Effects of composition on transformation temperatures and microstructure of Ni-Ti-Hf shape memory alloys

Abstract High transformation temperature shape-memory alloys (HTSMA) usually present a martensitic transformation temperature (Ms) starting at 100 ºC. That is the case of high nickel Ni-Ti-Hf alloys. This article presents experimental results obtained from arc melting of Ni50Ti50-XHfX .at% (X = 8, 11, 14, 17 and 20 .at%) alloys. This process homogenized every composition with similar relative crystallinity. Results confirm that transformation temperatures (TT) increase with increasing the amount of Hf. A martensitic matrix is formed by two metastable phases: R and B19'. From all the alloys studied, the B19' phase presented the highest percent fraction. Gradually adding Hf3 .at% promoted a slow increase of crystalline fraction of R phase and a slow reduction of phase (Ti, Hf)2 Ni, located at grain boundaries. Coherent/semi-coherent interface between (Ti, Hf)2 Ni phase and the matrix may intensify the driving force for the formation of R phase, present on X-ray diffractograms.