Reactions of TiNi with oxygen

X-ray diffraction and metallography have been applied to studs, the formation conditions for the 17 phase in Ti4Ni2O in the reaction of TiNi with oxygen. The phase diagram for the Ti -Ni - D system indicates that the eta phase (Fe3W3C structure type) is a solid solution of oxygen in Ti2Ni. For TiNi made in an oxygen-bearing medium or from initial materials contaminated with oxygen, one gets the eta phase and Ni3Ti. Those phases are formed also in surface oxidation as products from the initial interaction of the alloy with oxygen. They, occur when there is a low oxygen partial pressure, as on annealing TiNi powder in a vacuum given by a rotary pump or in a lover tinder scale formed on a cast TiNi specimen on oxidation in air. The layer under the scale is formed because of the preferential loss of titanium from the TiNi and consists of Ti4Ni2O, Ni3Ti, and an Ni(Ti) solid solution, which in turn occurs because of preferential loss of titanium from Ni3Ti. In the subsequent oxidation stages, there is selective oxidation of Ti4Ni2O to lower titanium oxides. The decomposition of TiNi alloy containing oxygen when the composition is varied near the equiatomic one can be used for practical purposes. For example, the segregation of the hydride forming eta phase may improve the hydrogen uptake by the alloy, while the dispersed segregation of Ni3Ti and the lower oxides may favor hardening.