An investigation into the relationship between the hydrogen storage properties and the microstructure of mechanically alloyed mixtures of titanium, magnesium, and nickel

The hydrogen storage properties and the microstructures of titanium, magnesium, nickel mixtures mechanically alloyed using ball-to-powder mass ratios of 20:1, 40:1, and 70:1 have been studied. Materials have been produced that exhibit the ability to absorb moderate to significant amounts of hydrogen and show significant changes in the microstructure as a result of the ball-milling process.; The hydriding/dehydriding characteristics of samples of the Ti-Mg-Ni mixture were determined. The thermal analysis results obtained using differential scanning calorimetry, as well as the hydrogen uptake onset temperatures, weight percent hydrogen and hydrogen-to-metals ratios are presented. The microstructural characterization of mechanically alloyed Ti-Mg-Ni mixtures was performed using x-ray diffraction (XRD), scanning electron microscopy/x-ray dispersive spectrometry (SEM/XEDS), focus ionbeam milling (FIB), and Transmission electron microscopy/x-ray dispersive spectrometry.; Significant changes were observed in the hydrogen storage characteristics and the microstructures of Ti-Mg-Ni mixtures mechanically alloyed using the ball-to-powder mass ratios of 20:1 and 70:1. The hydrogen storage capacity of the material alloyed with a ball-to-powder mass ratio of 20:1 was 3.5 wt%. The Ti-Mg-Ni mixture mechanically alloyed using a ball-to-powder mass ratio of 70:1 exhibited an extraordinary hydrogen storage capacity. The resulting material was capable of absorbing 11 wt%. In addition, the onset temperature for hydriding decreased as the ball-to-powder mass ration increased.; XRD analysis shows that only the elemental planes of Ti, Mg, Ni are present in the ball-milled materials, where the suppression of the Mg reflections during ball-milling was observed. However, TEM results indicate that the complex arrangement of phases on the surface of the original Mg particle, indicating that XRD may not be a suitable technique for the study of these materials due to sampling limitations.; A convoluted microstructure with regions of heavy deformation was observed for the mechanically alloyed Ti-Mg-Ni mixture. Brittle fragments of Ti and Ni were incorporated into the Mg. The microstructure of the mechanically alloyed material using a ball-to-powder mass ratio of 70:1 exhibited a more refined microstructure than the material alloyed at a 20:1 ball-to-powder mass ratio.; The data suggest that the convoluted microstructures may play an important role in the ability of this mixture to absorb hydrogen. It appears that the convoluted microstructure may provide pathways for hydrogen diffusion in the Mg matrix. Incorporation of the Ti and Ni into the microstructure may provide catalytic sites within the matrix to aid in the diffusion of hydrogen in the Mg matrix, and/or may react with the hydrogen directly to form a metal hydride.