Combustion synthesis of TiNi intermetallic compounds

Abstract

Equiatomic titanium and nickel compacted powders were synthesized into a TiNi intermetallic compound using a thermal explosion mode of the self-propagating high-temperature synthesis (SHS) method in air. It was found that the combustion temperature (T _c) was lower than that produced in an argon-protected atmosphere but still higher than the melting point of the intermetallic. This allowed a cast product to be obtained. The ignition temperature (T _ig) decreased linearly with increasing heating rate. This was found to be dependent on the prior formation of TiO₂, the exothermic nature of which “triggered” the TiNi reaction once sufficient TiO₂ had been formed. The following formulation was proposed to estimate the amount of TiO₂, whose heat of formation was utilized in part to trigger the TiNi synthesis reaction, α ^r₂ , and that which was dissipated into the surroundings, α ^d₁

$$\begin{gathered} \alpha _2^r \Delta H_f (TiO_2 ,T_{ig} ) + (1 - \alpha _1^d - \alpha _2^r )\Delta H_f (TiNi,T_{ig} ) \hfill \\ = \int_{Tig(TiNi)}^{Tm(TiNi)} {C_{p_s } (TiNi)dT + \Delta H_m (TiNi) + \int_{Tm(TiNi)}^{Tc} {C_{p_1 } (TiNi)dT} } \hfill \\ \end{gathered} $$

The amounts of α ^r_s and α ^d₁ were also correlated with heating rates. Microstructural observations established that the formation of TiO₂ results in an outer layer of the product as TiO₂, a central area of TiNi and an intermediate outer layer of eutectic TiNi + TiNi₃. Some Ti₂Ni was also observed in the outer layer of the product.