The effect of phase separation on subsequent crystallization in A18Gd6La2Ni4

To understand the crystallization (devitrification) kinetics of Al-rich AI-RE-TM (RE = rare earth, TM = transition metal) amorphous alloys, the devitrification of Al88Gd6La2Ni4 was studied by in situ electrical resistivity and transmission electron microscopy (TEM) investigations. This glass composition was chosen because it shows a well-defined glass transition temperature (T-g) and transforms to alpha -Al on partial devitrification. Surprisingly, we show that crystallization appears to be preceded by a phase separation into AI-rich and solute-rich amorphous regions, having a typical dimension of 40 nm. TEM studies reveal a preferential rapid nucleation of alpha -Al at the interface of the phase-separated regions. A Johnson-Mehl-Avrami analysis of crystallization-induced changes in the electrical resistivity shows limited agreement with the theory, with an Avrami exponent n close to unity. This is explained by a simple numerical model that is consistent with the microstructure; i.e., rapid nucleation at the phase boundary followed by diffusion-limited growth. (C) 2000 Acta Metallurgica inc. Published by Elsevier Science Ltd. All rights reserved.