Structural Relaxation, Crystallization Kinetics and Diffusion Study of Metallic Glasses

Abstract

<p> This study is on the thermodynamics, electrical and diffusion properties of five bulk metal glasses, new materials of great importance in technology, and on a longstanding problem of the residual entropy of the glassy state. It describes (i) an investigation of spontaneous structural relaxation, (ii) discovery of memory effect, and (iii) an investigation of crystallization kinetics both isothermally and on rate heating of bulk metallic glasses by measurements of their enthalpy change with time, temperature, and annealing conditions. Furthermore, it provides a real-time electrical resistivity study of structural relaxation effects, and an electron microscopy study of the interdiffusion kinetics of atoms across a junction interface, i.e., the so-called Kirkendall effect. </p> <p> It is shown that structural relaxation occurs according to a stretched exponential kinetics and distribution of relaxation times leads to memory effect for a glass sample of complex thermal history. This mechanism is confirmed by real time electrical resistivity measurements at different temperatures and explained in terms of the Ziman model. Crystallization of ultraviscous melts occurs in several steps but the first and major step follows the Kolmogorov-Johnson-Mehl-Avrami kinetics based upon the Poisson distribution of nucleation sites. Several other processes also occur including a possible spinodal decomposition with one phase remaining in the rigid glassy state. Thermally activated interdiffusion of atoms across a junction interface is inconsistent with the vacancy diffusion model. Finally, it is shown that contrary to the recent arguments based upon the Boltzmann equation, a glass has residual entropy. </p> <p> Seven papers based upon this study have been published in Journal of Non-Crystalline Solids, Journal of Chemical Physics, Journal of Physical Chemistry, Thermochimica Acta, and Philosophical Magazine. </p>