Characterization of a New Precipitate Phase in a Ni Rich Nitihf High Temperature Shape Memory Alloy

Aging the high temperature shape memory alloy 50.3Ni-29.7Ni-20Hf (at.%) at 600°C introduces a new precipitate phase, termed the “H-phase”. It plays an important role in improving shape memory properties such as strength, dimensional stability, work output and superelasticity behaviors. The precipitate phase was investigated by conventional electron diffraction, high angle annular dark field scanning transmission electron microscopy (HAADF STEM), and three dimensional atom probe tomography (3D APT). An atomic structural model is proposed based on the parent B2 phase, from which the H-phase is formed. This model was subsequently relaxed by ab initio calculations, using Vienna Ab Initio Simulation Package (VASP). As a result of the relaxation, atom shuffle displacements occur. HAADF STEM image simulations were also carried out on both the B2 based and the relaxed H-phase structure, using the multi-slice method and code by Kirkland. The simulated images on the relaxed structure yield much improved agreement with the experimental HAADF STEM images. The relaxed H-phase structure can also produce the observed electron diffraction patterns and is consistent with the composition analysis from 3D APT.  The relaxed H-phase structure has also been verified to be thermodymanically stable at 0 K.