Icosahedral hydrides of the Ti₄₅Zr₃₈Ni_17-xCo_x nano-powders

In this communication we report the phase transitions associated with a hydrogen uptake in the quasicrystalline Ti₄₅Zr₃₈Ni_17-xCo_x alloys that were prepared by mechanical alloying. The crystal structure evolution of the alloys was characterized by means of the variety of diffraction techniques (X-ray, neutron, and electron) as well as extended X-ray absorption fine structure spectroscopy (EXAFS). That analysis was supported by the results of the thermodynamic measurements, namely differential scanning calorimetry (DSC) and thermal desorption spectroscopy (TDS). According to the neutron diffraction results, the transformation of the amorphous Ti₄₅Zr₃₈Ni_17-xCo_x phases into the quasicrystalline state starts at the temperature of 475°C. At 525°C the icosahedral phase (the i-phase) is well-developed. At higher temperatures the i-phase transforms into the approximant w-phase and eventually into the cubic phase (the c-phase). The investigated icosahedral nano-powders exhibit a high capacity for gaseous H₂ reaching 2.2 wt.% at elevated temperatures. After a hydrogen uptake, a complex phase constitution including the simple hydrides is evidenced. Although the i-phases form again after hydrogen desorption, they are accompanied by the additional Zr₃Co phase.