Synthesis, characterization and first hydrogen absorption/desorption of the Mg35Al15Ti25V10Zn15 high entropy alloy

Abstract

In this work, a novel Mg-containing high entropy alloy (HEA), namely Mg35Al15Ti25V10Zn15, was synthesized, characterized, and the phases formed after synthesis and after first hydrogen absorption/desorption were analyzed. The alloy composition was conceptualized aiming at increasing the atomic fraction of lightweight elements (35 at.% Mg and 15 at.% Al) to increase its gravimetric hydrogen storage capacity. Ti and V were selected to increase the hydrogen affinity of the alloy. Finally, Zn was selected as an alloying element because of its negative enthalpy of mixing with Mg, which could favor the formation of solid solution and avoid Mg segregation. The Mg35Al15Ti25V10Zn15 HEA was synthesized by high-energy ball milling (HEBM) in two different routes: under argon atmosphere (mechanical alloying – MA) and under hydrogen pressure (reactive milling – RM). The sample produced by MA was composed of a body-centered cubic (BCC) phase with an amount of unmixed Mg. When hydrogenated at 375 °C and under 4.0 MPa of H2, the MA sample absorbed about 2.5 wt% of H2 by forming a mixture of MgH2, a face-centered cubic (FCC) hydride, a body-centered tetragonal (BCT) phase and MgZn2. The desorption sequence upon heating was investigated and it was shown that the MgH2 is the first to desorb. The sequence of desorption is completed by the decomposition of the FCC hydride and BCT phase leading to the formation of the BCC phase. On the other hand, the alloy produced by RM was composed of a mixture of an FCC hydride and MgH2. During desorption upon heating, the MgH2 is also the first to desorb. At lower temperatures (below 350 °C), the FCC hydride decomposes partially forming the BCT phase. At higher temperatures, both FCC hydride and BCT phase decomposes forming the BCC phase. The desorption capacity of the RM sample was 2.75 wt% H2.