High entropy alloys containing immiscible Mg and refractory elements: Synthesis, structure, and hydrogen storage properties

Abstract

In this work, the influence of Mg incorporation on the structural properties of the Mgx(TiVCrNb)100−x (x = 0, 10, and 20) hydrides was investigated and correlated with the hydrogen sorption properties. For material’s synthesis, the TiVCrNb pre-alloy was prepared by arc melting and subsequently, Mg was introduced via reactive high-energy ball milling (RBM) under H2 pressure. The hydrogen uptake of as-prepared hydrides strongly depends on the Mg content, as revealed by thermogravimetric analysis: 1.05, 1.28, and 1.62 H/M for the samples with x = 0, 10, and 20, respectively. The desorbed materials reabsorb hydrogen at 25 °C with fast kinetics, achieving capacities of 0.98, 1.23, and 1.49 H/M for x = 0, 10 and 20, respectively. The Pressure-Composition-Isotherm (PCI) curves present a flattening of the plateau pressure with increasing Mg content, which could explain better uptake for Mg-rich materials. Remarkably, Mg content significantly impacts the overall crystallinity of the RBM hydrides. Synchrotron XRD and related pair distribution function analysis (PDF) demonstrate that increasing Mg amount prevents the loss of long-range ordering during the ball milling process. Positron annihilation spectroscopy (PAS) shows that the as-prepared hydrides have a significant concentration of vacancies and dislocations usually introduced by ball milling. Coincidence Doppler broadening spectroscopy (CDB) hints that Mg atoms are preferentially located in the vicinity of vacancies, which may help relaxing the structure. In summary, this study highlights the potential benefit of adding Mg to refractory high entropy alloys produced by RBM, as Mg enhances crystallinity and subsequently improves the hydrogen storage properties.