Progress in Materials Science Vol. 30, pp. 81 to 134, 1986 0079-6425/86 $0.00+ .50 Printed in Great Britain. All rights reserved Copyright Lf?'1986 Pergamon Journals Ltd THERMODYNAMIC AND KINETIC ASPECTS OF THE CRYSTAL TO GLASS TRANSFORMATION IN METALLIC MATERIALS William L. Johnson Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, California 91125, U.S.A. CONTENTS 1. INTRODUCTION 8 1 2. KINETICS AND THERMODYNAMICS--GENERAL BACKGROUND 83 2.1. Free Energy Diagrams and Metastable Phases 83 2.2. Kinetic Constraints and the Approach to Equilibrium 87 2.3. Unstable Phases, Gibbs Criterion and Other Approaches 89 3. COMPOSITION-INDUCED DESTABILIZATION AND VITRIFICATION OF CRYSTALLINE ALLOYS UNDER ISOTHERMAL CONDITIONS 94 3.1. Glass Formation and Growth in Diffusion Couples--Experimental Results 95 3.2. Diffusion Couples--Free Energy Diagrams--Polymorphous Diagrams, and Instability 100 3.3. Diffusion Couples--Kinetic Aspects 103 3.4. Hydrogen in Metallic Alloys 109 3.5. Heterogeneous Nucleation Sites for Glass'--Competition with Compounds [12 3.6. Powder Metallurgy, Lamellar and Filamentary Composites, and Bulk Metallic Glasses 114 4. OTHER EXAMPLES OF INDUCED VITRIFICATION--EXTERNALLY DRIVEN SYSTEMS I 16 4. I. Radiation-induced Atomic Mixing in Diffusion Couples--Driven Composition Profiles I 17 4.2. Irradiation of Metals and Intermetallic Compounds--Defects and Atomic Site Disorder--Driven Disordering 119 4.3. Mechanical Alloying--Mechanically Driven Systems 125 4.4. Pressure-induced Vitrification 127 5. FINAL REMARKS AND SUMMARY 129 REFERENCES | 31 1. INTRODUCTION The study of amorphous metallic alloys dates back to the investigations of Buckei and Hilsch ~I) during the 1950s. By quenching the vapors of certain elemental metals or a binary mixture of two metal vapors onto a cryogenically cooled substrate, they were able to produce amorphous metallic alloy films. Using an in situ electron diffraction technique, they characterized the atomic scale structure of these films to establish their amorphous nature. The crystallization of the amorphous phase upon subsequent heating of the films above cryogenic temperatures was observed as well. Nearly all of these "vapor quenched" films were found to crystallize well below room temperature. A major breakthrough in this field occurred in 1959 when Pol Duwez and his colleagues ~2) found that amorphous alloys could also be produced by the rapid cooling of an alloy melt to ambient temperature. Using quench techniques capable of achieving cooling rates of the order of 106 K/s, Duwez's group showed that the process of nucleation and growth of crystalline phases could be kinetically bypassed in certain alloy melts to yield a configuration frozen liquid, a metallic glass. TurnbulP was quick to point out that, in analogy with conventional melts (e.g. fused silica), metallic melts should exhibit a glass transition. Experiments by Chen and Turnbull confirmed this 81 JPMS 30 2 A