Phase Transformation of Zr65Cu27.5Al7.5 Metallic Glass

Abstract

The phase transformation by crystallization of a Zr₆₅Cu_27.5Al_7.5 metallic glass was examined as a function of annealing temperature, T_a. The crystallization temperature decreases gradually by annealing at the temperature over 20 K than glass transition temperature, T_g. At the initial stage of crystallization, Zr(Al, Cu) phase is observed in addition to a main phase of Zr₂Cu. Thus, the Zr₆₅Cu_27.5Al_7.5 metallic glass crystallizes through a eutectoid-like reaction of the two phases. The Zr(Al, Cu) phase is a metastable phase and disappears in the higher T_a range. The formation of metastable Zr(Al, Cu) may be attributed to the difficulty of diffusion of Al into Zr₂Cu and the strong chemical affinity between Al and Zr. The strong chemical affinity between Al and Zr seems to contribute to the retardation of nucleation and grain growth of the crystalline phases. The lattice spacing of Zr₂Cu increases with deconvolution of metastable Zr(Al, Cu) and hence Al atom is thought to be substituted with Cu atom in Zr₂Cu. The grain size of Zr₂Cu increases significantly accompanied by the disappearance of metastable Zr(Al, Cu). After the completion of crystallization, a part of supersaturated Al precipitates as the Zr₂Al phase from the Zr₂(Cu, Al) phase at annealing temperatures above approximately 860 K, accompanying a reduction of lattice spacing and a significant grain growth of Zr₂Cu. Thus, Al affects significantly the structure change through the crystallization. It is therefore concluded that the necessity of long-range redistribution of Al during the crystallization process plays a dominant role in the stabilization of supercooled liquid and the degradation of grain growth reaction.