Tailoring hydrogenation, thermodynamic properties and oxidation resistance of TiFe alloy by only regulating the stoichiometric ratio of Ti and Fe elements

Abstract

This study systematically investigates the effect of regulating the stoichiometric ratio of Ti and Fe elements on the microstructure, hydrogenation properties, thermodynamics and oxidation resistance of Ti_xFe_2−x (x = 1, 1.1, 1.2, 1.3) alloys. Microstructural analysis reveals that all as-cast alloys except x = 1 alloy consist of a TiFe main phase and Ti₂Fe secondary phase. Increasing Ti content leads to the expansion of the lattice parameters, unit cell volume and crystallite size of the TiFe phase, alongside an increase in the area percentage of a dark phase observed in the microstructure. Regarding hydrogenation, higher Ti content enhances first hydrogenation kinetics and increases the maximum hydrogen storage capacity from 1.91 wt% (x = 1.1) to 2.27 wt% (x = 1.3), which is attributed to the dark phase facilitating hydrogen dissociation and diffusion. However, the reversible hydrogen storage capacity decreases from 1.37 wt% (x = 1.1) to 1.10 wt% (x = 1.3), likely due to the formation of stable TiH_x hydrides. Thermodynamically, the activation energy of hydrogen absorption decreases with higher Ti content (from 18.62 kJ mol⁻¹ of x = 1.1 to 11.97 kJ mol⁻¹ of x = 1.3), indicating enhanced hydrogen diffusion. Conversely, the activation energy of hydride decomposition and the hydride stability increases with Ti content. Furthermore, the oxidation resistance of the alloys deteriorates as Ti content increases, ascribed to the higher oxidation sensitivity of Ti compared to Fe.