Influence of oxygen on the structure and devitrification pathways in Zr66.7Ni33.3 and Zr66.7Cu33.3 amorphous systems

Experimental and theoretical approaches were combined to investigate the role of O on the as-quenched structure and phase selection process during devitrification of melt-spun Zr66.7Ni33.3 and Zr66.7Cu33.3 metallic glasses. Oxygen was observed to degrade glass formation for both systems at relatively high contents approaching 15,000 pprn mass, but at lower amounts amorphous structures were obtained. In the case of Zr66.7Ni33.3 alloys, O contents ranging from below 250 ppm mass up to nominally 5000 ppm mass yielded qualitatively similar as-quenched amorphous structures, as determined from high-energy synchrotron X-ray diffraction. However, O did promote a dramatic change in the devitrification pathways of the Zr66.7Ni33.3 glasses. Whereas ribbons with no intentionally added O transformed directly to the c16 (Al2Cu-type) Zr2Ni structure, their counterparts with nominally 2000 and 5000 ppm mass O instead transformed from their amorphous structure to a metastable Ti2Ni-type "Big Cube" structure before ultimately forming the c16 structure with further heating. For the Zr66.7Cu33.3 glasses, the c11b (MoSi2-type) phase formed regardless of the O content up to the nominally 5000 pprn mass level probed in this study. First-principle calculations were performed to help understand these differences. It was found that the strong bonding between Zr's d-electrons and O's p-electrons contributes to increasing the stability of both systems as more O is added. In addition, the stronger bonding between Zr d-electrons and Ni d-electrons over that of Zr d-electrons and Cu d-electrons helps explain the easier formation of Zr64Ni32O16 BC phase observed in the experiments. (C) 2009 Published by Elsevier B.V.