Effect of oxygen on phase formation and thermal stability of slowly cooled Zr65Al7.5Cu7.5Ni10 metallic glass

The effect of small amounts of oxygen (0.28-0.6 at.%) on the phase formation and the thermal stability of bulk samples of the Zr65Al7.5Cu17.5Ni10 alloy prepared by die casting into a copper mould has been studied by X-ray diffraction, microstructural characterization and differential scanning calorimetry. In contrast to rapidly quenched ribbons, the crystalline volume fraction in slowly cooled bulk samples increases with increasing oxygen content. Oxygen triggers the formation of a metastable f.c.c. NiZr2-type crystalline phase which also depends on the local cooling conditions during casting. The processes of the oxygen-induced crystallization are discussed in detail considering initial nucleation of metastable f.c.c. NiZr2-type nanocrystals which act as heterogeneous nucleation sites for subsequent formation of other crystalline phases. Furthermore, there is a strong influence of the oxygen content on the thermal stability during constant-rate heating to elevated temperatures. With increasing oxygen content a drastic reduction of the supercooled liquid region is observed, resulting mainly from a change of the crystallization sequence from a single- to a double-step process which is attributed to the oxygen-triggered formation and the subsequent transformation of the metastable f.c.c. phase. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.