The impact of dendrites and related compositional fluctuations on hydrogen absorption thermodynamics in bcc multi-principal element alloys

Abstract

Solid-state hydrogen storage is a key concept in the prospect of a sustainable hydrogen economy. Multi-principal element alloys (MPEAs) with a body-centred cubic (bcc) structure are promising hydride-forming materials, but often solidify with dendritic, compositionally segregated microstructures. This study examines how such compositional fluctuations affect hydride formation thermodynamics, using the Ti₃₀V₃₀Cr₂₄Nb₁₆ MPEA as an exemplar. Dendritic segregation was controlled by varying the solidification rate and eliminated through high-temperature solid-solution annealing. Rapid solidification by melt-spinning successfully suppressed dendrites but resulted in an alloy that did not absorb hydrogen, and <2 wt.% of a TiO-type oxide finely dispersed throughout the material. In contrast, the annealed alloy exhibited full hydrogen uptake (3.3 wt.%) and a flatter monohydride-dihydride transition plateau in the pressure-composition isotherms compared with the dendritic as-cast alloy. Despite these compositional fluctuations, the derived thermodynamic parameters (ΔH and ΔS) were indistinguishable within experimental uncertainty. Our experiments reveal that the compositional fluctuations caused by the dendrite formation influence the slope of the equilibrium plateau pressures, while the overall composition dominates the fundamental thermodynamic properties.