IMPROVEMENT OF HYDROGENATION PROPERTIES FOR TI-BASED QUASICRUSTAL POWDERS PRODUCED BY MECHANICAL ALLOYING

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Besides the conventional solidification processes like rapid quenching, a combination of mechanical alloying (MA) and subsequent annealing in vacuum was shown to also produce the Ti-Zr-Ni icosahedral phase (i-phase) for a range of chemical composition. Depending upon the chemical compositions, minor phases such as the solid solution phase (α-Ti/Zr) and the Ti_2Ni-type phase (fcc structure) also formed after subsequent annealing The Ti-Zr-Ni i-phase powders could load hydrogen to a concentration greater than 60 at.%. The maximum concentration reached was 67 at% (H/M 【approximately equal】 2.02) which was obtained for the Ti_<45>Zr_<38>Ni_<17> i-phase powder during a high-pressure isotherm measurement. The substitution of Zr atoms for Ti ones in the Ti-Zr-Ni i-phase powders decreased the activation energy for hydrogen desorption and the hydrogen desorption temperatures. Besides the plateau-like pressure observed during the low-pressure isotherm measurement for the i-phase powder an additi … More onal plateau was also observed at about 0.5 MPa (desorption process) during a high-pressure isotherm measurement. The i-phase in the Ti-Zr-Ni powder remained stable even after hydrogenation, but the amorphous powders transformed into a fcc (Ti, Zr)H_2 hydride phase, whose activation energy for hydrogen desorption was generally higher than that measured for the i-phase.The i-phase could also be produced from Ti_<40>Hf_<40>Ni_<20> elemental powders by MA and subsequent annealing, coexisting with a Ti_2Ni-type crystal phase. The amount of the Ti_2Ni-type phase was much larger than obtained in the Ti-Zr-Ni powders. The hydrogen concentration in the Ti_<40>Hf_<40>Ni_<20> i-phase powder was about 55 at.% after loading from the gas phase at 573 K and an initial hydrogen pressure of 3.8 MPa. This was slightly lower than observed for the Ti-Zr-Ni i-phase powders. The activation energy for hydrogen desorption was also lower than for the Ti-Zr-Ni i-phases, suggesting that the substitution of Hf for Zr decreased the chemical interaction between hydrogen in the tetrahedral sites and the surrounding metal atoms. Less