Multicomponent Zr-based bulk-glass forming alloys
Amorphous Zr-based alloys, e.g. Zr-Al-Cu-Ni, exhibit an interesting hydrogenation behaviour, which is mainly related with their short-range order atomic structure and composition of early and late transition metals. Absorbed hydrogen occupies tetrahedral interstitial site types (n) at different energy levels (En) which exist in the chemically random structure. Due to a high number of coordinative unsaturated surface atoms a high activity for electron transfer reactions is expected. But for spontaneously passive amorphous Zr-based alloys a large electrochemically active surface area is obtained after etching in HF solution. Cathodic charging in strongly alkaline electrolyte leads to a maximum H/M ratio of 1.65. A significant decrease of the thermal stability, i.e. a reduction of the supercooled liquid region and a change in the crystallisation sequence, occurs with increasing hydrogen content of the samples. The thermal hydrogen desorption has two distinct stages. At lower temperatures hydrogen desorbs from high interstitial-site energy levels. Zirconium hydrides are formed > 623K. At higher temperatures partial desorption of hydrogen occurs and transformation to hydride phases with lower hydrogen contents takes place. Hydrogen suppresses the oxygen-triggered formation of metastable phases, e.g. the quasicrystalline phase, and supports copper segregation processes. This effect can be utilized for surface reactivity enhancement. At high H/M ratios, a severe preferential Zr-hydride formation causes enrichment of Cu, Ni and Al in the residual amorphous phase resulting in the crystallisation of new stable intermetallic compounds.
Thermal desorption of the glassy Zr65Cu17.5Al7.5Ni10 alloy after cathodic hydrogen charging
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