Effect of Stoichiometry on a NiTiZr20 High Temperature Shape Memory Alloy

In recent years, Ni-rich NiTiHf high temperature shape memory alloys have been established as a premier option for high temperature actuation through research lead by the NASA Glenn Research Center. In particular, Ni_50.3Ti_29.7Hf₂₀ has a transformation temperature capability of >120ºC (M_F), high work output, and can be heat treated to form nanoscale coherent precipitates, which strengthen the matrix and enable near perfect dimensional stability. Even though this material is much cheaper than precious metals containing NiTiX alloys, its cost is still significant, due to the high level of Hf in the alloy. An even cheaper alternative to Hf is Zr – a chemical analog to Hf which can increase the transformation temperature of NiTi, though with a slightly weaker effect than Hf.

For this study, a range of alloy compositions were produced from ternary Ni_XTi_80-XZr₂₀ (X = 50 – 51 at% Ni). The melt weights of each element were precisely measured, and the samples arc melted in a copper hearth, with each button flipped multiple times to ensure complete mixing and an accurate composition. The materials were homogenized, solutionized, and then aged under various heat treat conditions to determine the effect of stoichiometry and heat treatment on the transformation temperatures, material properties, and precipitate size and morphology. The results of the mechanical testing, thermal testing, and microstructural analysis will be discussed along with a cost benefit analysis of the alloys compared to the baseline 20Hf alloy.