Transformation strains and temperatures of a nickel-titanium-hafnium high temperature shape memory alloy

A combined experimental and theoretical investigation of the transformation temperature and transformation strain behaviors of a promising new Ni50.3Ti29.7Hf20 high-temperature shape memory alloy was conducted. Actuation behavior of single crystals with loading orientations near [0 0 1]B2, [1¯ 1 0]B2, and [1 1 1]B2, as well as polycrystalline material in aged and unaged conditions was studied, together with the superelastic, polycrystalline torsion response. These results were compared to analytic calculations of the ideal transformation strains for tension, compression, and torsion loading of single crystals as a function of single crystal orientation, and polycrystalline material of common processing textures. H-phase precipitates on the order of 10-30 nm were shown to increase transformation temperatures and also to narrow thermal hysteresis, compared to unaged material. The mechanical effects of increased residual stresses and numbers of transformation nucleation sites caused by the precipitates provide a plausible explanation for the observed transformation temperature trends. Grain boundaries were shown to have similar effects on transformation temperatures. The work output and recoverable strain exhibited by the alloy were shown to approach maximums at stresses of 500-800 MPa, suggesting these to be optimal working loads with respect to single cycle performance. The potential for transformation strain in single crystals of this material was calculated to be superior to binary NiTi in tension, compression, and torsion loading modes. However, the large volume fraction of precipitate phase, in part, prevents the material from realizing its full single crystal transformation strain potential in return for outstanding functional stability by inhibiting plastic strain accumulation during transformation. Finally, calculations showed that of the studied polycrystalline textures, [0 0 1]B2 fiber texture results in superior torsion performance, while [0 1 1]B2 fiber texture results in superior tensile behavior, and both [0 1 1]B2 and random textures will result in the best possible compression performance.