Synthesis of HEAs with Properties Potential of Hydrogen Storage

Hydrogen (H2) is considered the potential candidate to replace energy systems based on fossil fuels. The problem with its implementation is adequate storage. However, solid-state materials are one of the most promising options. Solid-state materials include metal hydrides [1]. Magnesium (M) hydrides and their alloys show the highest H2 storage capacity (7.6% by weight) [2]. However, they have a limited capacity, with a H/M ratio ≤ 2; although it is an acceptable storage capacity, they require using rare earth metals such as La to increase their capacity [3]. Therefore, exploring, evaluating and developing efficient alloys in H2 storage is necessary. It has been reported that alloys with a BCC-type structure have a higher capacity to store H2 [4] and that the structural deformation in the cell or interface of some metals can be favorable for forming hydrides [5, 6]. These two factors have resulted in a recent group of alloys known as Multicomponent Alloys (High Entropy Alloys-HEAs, Complex Concentrated Alloys-CCAs and Multi-Principal Element Alloys-MPEAs) being considered potential candidates as H2 storage materials [7-9]. Therefore, this research aims to design, develop and evaluate transition metal high entropy alloys (TM-HEAs) with a BCC-type structure, considering the MgAlTi(X)Ni system where X= Fe, Co and Cr. The TM-HEAs were elaborated by powder metallurgy from pure elements with purity ranging from 98% to 99.8% (Sigma Aldrich) in a High-Energy Ball Mill (SPEX- 8000 M) after 8 hours of milling. Hermetically sealed steel vials in an Ar atmosphere with steel balls and a ball-to-powder ratio of 5:1 were used. The microstructure of synthesized samples was analyzed by scanning electron microscopy (HITACHI SU3500 microscope) using a Secondary Electron (SE) and Energy-Dispersive X-ray Spectroscopy (EDS) detector. The structure of synthesized samples was analyzed by X-ray diffraction with an X’Pert PRO Panalytical device diffractometer using CuKα1 radiation; the 2θ interval explored was 10 – 90° with 0.017 step size. XRD patterns were indexed using the software TREOR, and Le Bail fits were performed using the program FULLPROF. Figure 1 shows the XRD pattern of the MgAlTiFeNi, MgAlTiCoNi and MgAlTiCrNi systems. The formation of characteristic peaks of the TiH2 phase is observed. In addition, the Indexed and Le Bail fits revealed that the reflections indicated forming a BCC-type structure in 3 systems with lattice parameter: a= 2.874, 2.932 and 2.884 Å, respectively. Figure 2 exhibits the SEM image and the corresponding EDS-mappings of a) MgAlTiFeNi, b) MgAlTiCoNi and c) MgAlTiCrNi systems. All transition metal elements (Mg, Al, Ti, Fe, Co, Cr and Ni) are homogeneously distributed in the MgAlTiFeNi, MgAlTiCoNi system. However, certain segregations of Cr are observed in the MgAlTiCrNi system. In summary, through powder metallurgy, it was possible to synthesize a BCC-type structure in MgAlTiFeNi, MgAlTiCoNi and MgAlTiCrNi systems with potential hydrogen storage properties. XRD pattern of equiatomic powder mix MgAlTi(X)Ni, where X= Fe, Cr, Co. SEM Micrographs and corresponding EDS-mapping of a) MgAlTiFeNi, b) MgAlTiCoNi and c) MgAlTiCrNi systems.