Synthesis of Ti–Fe Intermetallics from Element Powder Mixtures

Abstract—Fe2Ti and FeTi intermetallic compounds are of practical use as either hydrogen storages (FeTi) or magnetic materials (Fe2Ti). Owing to features of the equilibrium binary phase diagram, the preparation of the intermetallics by casting is difficult. Therefore, powder metallurgy methods along with preliminary mechanical activation of powder mixtures are widely used. The aim of the study is to investigate the possibility of preparation of single-phase compounds from titanium and iron powder mixtures having target compositions. Mechanically activated powder mixtures and products of combustion and subsequent annealing are studied by X-ray diffraction analysis, optical metallography, and scanning electron microscopy and energy dispersive spectroscopy used for the determination of the element composition. Powder mixtures are subjected to 20‑min mechanoactivation in an Activator 2S planetary mill at an intensity of 40 g; the ball-to-mixture ratio is 20. The mechanically activated mixtures are heated in a hermetically sealed reactor in an argon atmosphere at an average rate of 85 deg/min. Thermal curves, which are measured with thermocouples placed into a mechanoactivated mixture, demonstrate an abrupt rise (thermal explosion (TE)), which corresponds ~500°C and indicates the occurrence of an exothermic reaction in the mixture. The 2Fe + Ti composition is found to indicate the substantially higher rise as compared to that observed for the Fe + Ti composition. X-ray diffraction analysis shows that the Fe2Ti compound is the main reaction product for the both mixtures. The dominant formation of Fe2Ti and the high thermal effect of the 2Fe + Ti mixture as well are explained by the higher negative enthalpy of formation of Fe2Ti as compared to that of FeTi (–87.45 and –40.58 kcal/mol, respectively). High temperature homogenizing annealing of TE products results in the formation of a double-phase target product. After annealing, the contents of side phases and unreacted reagents slightly change. Based on the obtained data, it is inferred that the thermodynamic factor (enthalpy of formation of intermetallic) is the main factor determining the phase composition of the synthesis products in titanium and iron powder mixtures.