Abstract
Iron–titanium alloys are promising candidates for hydrogen storage but are often affected by non-metallic impurities. Silicon is a common impurity present in FeTi from ilmenite and recycled steel, and its presence can promote the formation of intermetallic phases that reduce hydrogen storage capacity. A detailed understanding of the Fe–Ti–Si system is therefore essential for optimising alloy design. In this work, we elucidate the ternary phase equilibria in the Fe–Ti–Si system and quantify the effect of Si-containing secondary phases on the overall hydrogen storage capacity of the FeTi alloy. A Calphad thermodynamic model was developed, informed by density functional theory calculations and focusing on the equilibria of (FeTi)(1-𝓍)Si𝓍 alloys. Binary phase data were curated from the literature and extrapolated to the ternary system, with critical ternary intermetallic phases modelled explicitly. In particular, we applied the effective bond energy formalism to represent the Gibbs energy of the C14 Laves phase as a three-sublattice system, reflecting its crystallography. Using our new thermodynamic model, we predict that the addition of 6 wt% Si to the FeTi alloy already results in a reduction in hydrogen storage capacity of 50% caused by the stabilisation of phases that cannot store hydrogen reversibly. This work offers a comprehensive understanding of the Fe–Ti–Si system, which can direct the design of FeTi alloys with improved hydrogen storage properties and also provide guidance for extractive metallurgy.