Study on hydrogen storage performance of as-milled Ti-V-Cr-Fe-Mn high entropy alloys

Abstract

This study aims to fabricate and optimize BCC-based high-entropy hydrogen storage alloys through mechanical alloying. The research systematically investigates the effects of processing parameters (including milling time, ball-to-powder ratio, rotation speed, and process control agent (PCA) addition), alloy composition, and heat treatment on the phase structure, thermodynamic stability, and hydrogen storage performance of the fabricated alloys. Furthermore, the study compares the microstructure and hydrogen storage properties of alloys fabricated by different methods (mechanical alloying and arc melting).

Firstly, the study optimizes the mechanical alloying parameters, revealing the critical roles of milling time, ball-to-powder ratio, and rotation speed in forming BCC structures and nanocrystalline grains in Ti-V-Cr-Mn-Fe alloys. The regulatory effects of PCA addition on powder yield and particle size are also analyzed. Subsequently, the impact of composition on hydrogen storage properties, including hydrogen absorption/desorption kinetics, thermodynamic behavior, and cycling stability, are explored by varying the Ti content and Mn/Cr ratios. It is found that increasing Ti content enhances the proportion of C14 Laves phases, while increasing Mn content effectively suppresses Laves phase formation, thereby increasing the BCC phase fraction and improving hydrogen storage kinetics.

Additionally, the role of heat treatment is examined. Microstructural evolution analysis reveals the phase transformation behavior among BCC, FCC, and Laves phases under different heat treatment conditions and their effects on hydrogen storage capacity. Specifically, as the temperature increases, the BCC structure first decomposes into a BCC + FCC dual-phase structure, followed by the precipitation of the Laves-2 phase within the FCC phase. After high-temperature treatment, the lattice constant of the BCC phase decreases, and the synergistic effect of the Laves and FCC phases results in a slight reduction in the hydrogen absorption and desorption capacity of the alloy. Finally, by comparing different fabricating process, the differences in microstructure and hydrogen storage performance of Ti25V35(CrMnFe)40 alloys prepared by these methods are investigated. The results suggested that Mechanical alloying significantly enhances initial the activation performance and hydrogen absorption kinetics of the as-milled alloys are improved compared to the counterparts of as-cast alloys.