Crystallization Kinetics and Phase Transformation Mechanisms in Cu56zr44 Glassy Alloy

Abstract

The kinetics and phase selection mechanisms involved in the crystallization of an amorphous Cu-Zr alloy of eutectic composition (Cu56Zr44) were investigated using in situ high-energy X-ray diffraction (HEXRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) under isothermal and constant heating rate conditions. In situ HEXRD results for 10 K/min (10 A degrees C/min) heating indicate that the amorphous alloy devitrifies into CuZr2 and mainly Cu10Zr7 at the crystallization temperature of 725 K (452 A degrees C). The sequence continues with the precipitation of CuZr (B2) at 1004 K (731 A degrees C), where these three phases coexist until the decomposition of CuZr2 is observed at 1030 K (757 A degrees C). The two equilibrium phases Cu10Zr7 and CuZr (B2) remain present on further heating until melting at the eutectic temperature for the Cu56Zr44 alloy. TEM investigation of the isothermal [705 K (432 A degrees C)] crystallization sequence reveals primary nucleation and growth of the Cu10Zr7 phase, where growth of the Cu10Zr7 crystals is initially planar with a transition to a cellular morphology, associated with partitioning of Zr at the growth front. Related cellular structures and composition profiles are quantified. (C) The Minerals, Metals & Materials Society and ASM International 2015