Effect of composition on the amount of second phase and transformation temperatures of NixTi90−xHf10 shape memory alloys

The influence of Ni and Ti content of NixTi90−xHf10 shape memory alloys on the presence of a second phase [Ni(Ti+Hf)2] and transformation temperatures (TTs) is determined. The Ni content of the alloys in this investigation ranges from 49.0 to 50.2 at.% in increments of 0.2 at.%. The presence of second phase is investigated by metallographic examination and image analysis of homogenized alloys (900°C for 360 ks), and their TTs are determined by differential scanning calorimetry. The amount of second phase decreases with increasing Ni content and is least with a Ni content of 50.2 at.%. The martensite to austenite and austenite to martensite TTs remain constant as the Ni content increases from 49.0 to 50.0 at.%. A rapid decrease in the TTs is observed when the Ni content of the NixTi90−xHf10 alloy exceeds 50.0 at.%.