Crystallization of Amorphous Alloys
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INTRODUCTION
THE object of this article is to review the phenomena of crystallization of amorphous alloys in the context of solidification. Amorphous metallic alloys when they are made by rapid quenching of the liquid are glasses, preserving the structure of the liquid alloy but not in internal thermodynamic equilibrium, m At the elevated temperatures at which crystallization of the glass occurs, the glass undergoes structural relaxation and internal equilibrium is approached. In such a state, the amorphous alloy can be viewed as a highly undercooled (and therefore very viscous) liquid. Amorphous alloys can be made by routes other than rapid liquid quenching, but the properties of these materials, especially after some structural relaxation, are essentially indistinguishable from those of glasses.m Thus, for all types of amorphous metallic alloy, crystallization (sometimes termed devitrification) can be regarded as solidification occurring at an extreme of the temperature range. Not all amorphous solids fall into this picture, however. For example, amorphous silicon is covalent and tetrahedrally bonded, quite distinct from the denser metallic liquid. Crystallization of amorphous silicon is therefore a quite different transformation from crystallization of liquid silicon; this is illustrated, for example, by the different temperature dependences of the two processes. Izl Viewing the crystallization of amorphous metallic alloys as solidification, it is important to consider the thermodynamics of the process. It is expected that as the undercooling below the equilibrium liquidus is increased, the driving force for solidification (crystallization) will increase. No example is known of a stable amorphous alloy, and thus crystallization can always be expected if there is sufficient atomic mobility to make it kinetically possible. The crystalline phases which form may, however, be different at low and high undercooling. Indeed, there are (rare) cases in which the phase formed at low undercooling is less stable than the amorphous phase at high undercooling. I31 As for conventional solidification at low undercooling, crystallization of amorphous alloys involves the processes
A.L. GREER, Lecturer, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. This article is based on a presentation made at the "Analysis and Modeling of Solidification" symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2 ~ , 1994, under the auspices of the TMS Solidification Committee. METALLURGICALAND MATERIALSTRANSACTIONSA
of nucleation and growth. These will be considered separately in Section II, followed by a discussion of the relevance of amorphous alloys for grain refinement. Three aspects of crystallization studies will be stressed: interpretation in terms of solidification, contribution to understanding of conventional solidification, and potential for practical exploitation. II.
T E M P E R A T U R E D E P E N D E N C E OF N U C L E A T I
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