Agradecimentos: Ricardo Floriano reports financial support was provided by State of Sao Paulo Research Foundation. Kaveh Edalati reports financial support was provided by MEXT. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Ver menos

Abstract: This study employs the mechanisms that rule the hydrogen storage properties, especially the influence of compositional and microstructural factors on thermodynamics and kinetics, to design a medium-entropy alloy derived from the Ti-ZrFe-Ni system. A novel non-equiatomic alloy (Ti21Zr21Fe41Ni17) was designed by applying semi-empirical models and computational thermodynamic calculations using the CALPHAD method. Four criteria were followed for design: valence electron concentration VEC = 6.2-6.5, atomic size mismatch d = 9.7%, atomic radius ratio of hydride-forming to non-hydride forming element rA/rB = 1.149-1.219, and the C14 Laves structure stability as the major phase. The designated material was synthesized by casting and structural analysis showed that the alloy crystallizes as a major C14 phase (92.8 wt%) and a minor BCC phase. Transmission electron microscopy showed the presence of nanograins with a strong tendency to form coherent boundaries between the C14 grains and the interphase of C14 and BCC grains. The alloy showed very fast kinetics after a simple thermal activation and reversibly absorbed 1.4 wt% of hydrogen with a relatively small hysteresis between the cycles. For the kinetics properties, it was suggested that the hydride phase nucleates preferably in coherent grain boundaries resulting in the fast hydrogenation of the alloy. For thermodynamic properties, chemical composition designed by the four mentioned criteria should be considered, while iron also plays a critical role. The high atomic percentage of iron (41%) – a non-hydride forming element – stabilizes the C14 phase because of the elevated absolute contribution of the interaction parameter (?ij) of the pair Fe-Zr, (?ij = -118.4 kJ/mol), which results in a negative enthalpy of the mixture, with the C14 structure Ver menos