Structural Imperfections in Amorphous Metals.
HARVARD UNIV CAMBRIDGE MA DIV OF APPLIED SCIENCES
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The experimental evidence of the existence and nature of imperfections in amorphous metals is reviewed. It derives from observations of microscopic homogeneity, density measurements, positron annihilation studies, inhomogeneous flow and measurements of the various atomic transport coefficients shear viscosity, diffusivity, structural relaxation rate and crystal growth rate. In contrast with crystal imperfections, which tend to be discrete and localized due to the constraints imposed by the long range periodicity, imperfections in amorphous metals tend to be more extended and diffuse. This is illustrated by the collapse and annihilation of a vacancy in a two-dimensional amorphous dynamic hard sphere model, and the observations are generalized for other defects in three dimensions. It is concluded that the most fruitful way of describing defects in amorphous metals is as sites with small local perturbations of the ideal short range order, which allows local configurational rearrangements. It is suggested that different types of rearrangement nearest neighbor changes, shear, volume annihilation are possible at different sites, and that the concentration and lifetime of each type of site determines the time constant of the corresponding atomic transport process. Author
- Properties of Metals and Alloys