FORMATION AND ELASTIC BEHAVIOR OF TiNi INTERMETALLIC SINTERED FROM A TiH₂–Ni POWDER MIXTURE

Abstract

The phase formation and consolidation processes in TiH₂–Ni powder mixtures to form TiNi intermetallic were studied. The use of titanium hydride as a precursor in optimal sintering conditions (900–1000 °С) allowed a material with optimum porosity to be produced and a liquid phase to be avoided during sintering. The phase formation processes were found to occur rapidly in the sintering of TiH₂–Ni mixtures. At sintering temperatures of 900–1000 °С, 70–82% TiNi formed. An additional phase that emerged in the material in all sintering conditions was Ті₂Ni intermetallic. The stability of this phase was explained by its affinity for oxygen and formation of complex stable oxides. The oxidation process was accelerated because the mixture was superfine and titanium resulting from the hydride decomposition was highly active. Moreover, interaction with oxygen began sooner than with nickel. The samples produced from the finest mixture were less homogeneous than the samples produced from a coarser mixture. High-speed interaction between titanium and nickel and rapid oxygen absorption were due significantly finer hydride powder. In these sintering conditions, the material was 12−15% porous and thus optimum for medical applications. Studies of the mechanical behavior of TiNi alloys revealed an abnormally low elastic modulus of 40 GPa. Experiments on cyclic loading–unloading showed that initial elastic strain was 1.1% and inverses transformation strain 0.7%. When strain reached 4%, the elastic modulus decreased to E ~ 32.7 GPa, total elastic strain increased to ε_el ~ 2.6%, and damping capability Q⁻¹ became equal to 0.036. The mechanical characteristics are close to those of human bones. The experimental results demonstrate that the sintered TiN materials possess the structure and mechanical properties that make them promising for the design of human bone implants.