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The structural transformation of the CsCl-type compound TiNi has been investigated by X-ray-diffraction, calorimetric, and metallographic methods. The transition is a first-order one, with a latent heat of 370 ± 20 cal/g-atom. It occurs rapidly and with small hysteresis in bulk material, the transition temperature varying with composition within the range 48–52 at.-%nickel, and its martensitic character was confirmed by metallographic and electron-microscope observations. The martensite (low-temperature) structure is probably of low symmetry. There is some evidence that different transition products are formed on either side of the stoichiometry. Oxygen level significantly affects the transition behaviour.

It is shown that the structure homogeneity range of the B2 TiNi compound is very restricted below 500°C. A new phase, containing 58 at. pct Ni and structurally related to Ni3Ti, forms peritectoidally at 625° ± 20°C. The precipitation in nickel-rich TiNi is very slow, and a metastable, supersaturated B2 structure can be readily retained. At equilibrium the martensitic transformation takes place at ∼60°C, independent of the overall composition. Quenching from 600°C and above raises the transition temperature in titanium-rich, and lowers it markedly in nickel-rich compositions. The resulting martensite structure in the latter is also believed to change with increasing the prior anneal temperature. The observations reported previously on the transformation behavior must be considered as obtained on nonequilibrium, metastable materials, which may account for the discrepancies in the observed property variation. Close control of the processing variables is necessary to ensure reproducibility, particularly if nonequilibrium transformation behavior is desired.