Characteristics of slowly cooled Zr-Al-Cu-Ni bulk samples with different oxygen content

Bulk samples of the glass-forming Zr65Al7.5Cu17.5Ni10 and Zr55Al10Cu30Ni5 alloys with 3 mm diameter were prepared by die casting into a copper mould. The oxygen content of the samples was varied between 0.26 at.% and 0.73 at.% by adjusting the oxygen partial pressure in the argon atmosphere upon casting. Characterization of the microstructure of as-cast samples and of specimens continuously heated to 873 K was carried out by x-ray diffraction (XRD), optical microscopy (OM) and transmission electron microscopy (TEM). Thermal stability was investigated by constant-rate differential scanning calorimetry (DSC). The phase formation and the thermal stability of the slowly cooled zirconium-based bulk samples are essentially influenced by the oxygen content of the material. Furthermore, the sensitivity to oxygen depends on the composition of the alloy. In bulk Zr65Al7.5Cu17.5Ni10 samples only small oxygen traces induce nucleation and crystal growth during slow cooling whereas Zr55Al10Cu30Ni5 samples are completely amorphous for all oxygen contents investigated. The processes of the oxygen-induced phase formation are discussed in detail also with respect to the results obtained for the heat treated samples. With increasing oxygen content the thermal stability deteriorates, as it is obvious from a diminution of the supercooled liquid region (Delta T-x = T-x - T-g) which is mainly due to a reduction of the crystallization temperature T-x. Furthermore, the thermal behaviour of Zr65Al7.5Cu17.5Ni10 and Zr55Al10Cu30Ni5 reveals significant differences.