Dislocations with virtually limitless ability for long range glide are the principal “carriers” of plasticity in crystalline solids. These, however, can also deform plastically to a much lesser extent by shear transformations such as by deformation twinning or martensitic shears.

    Amorphous solids, possessing no long range crystalline coherency lack the ability to deform plastically by dislocation motion, but instead, deform by recurring local shear transformations in fertile atom clusters possessing some “free volume”. In glassy metals having no directional bonding such clusters are relatively small with a size of c.a. 3-4 nm. In glassy polymers with long chain molecules with high back bone stiffness and little bond angle flexibility, but with relative ease for intermolecular rearrangements, the clusters undergoing shear transformations are considerably larger with dimensions of c.a. 10-15 nm. In space network glasses with strong covalent 3-D directional bonding the character of the basic unit plastic event had so far not been well understood.

    In this lecture the character of plastic behavior of amorphous silicon, as a representative of a covalently-bonded space network glass will be discussed, where the shear transformations occur preferentially in relatively large atom clusters possessing a high concentration of a “liquid-like” component. Unlike in metallic glasses where the preferentially transforming sites possess “free volume” the “liquid-like” component of silicon in a deforming atom cluster is of higher density and has a larger atomic coordination. Nevertheless, there is an interesting parallel between the steady state concentration of “liquid-like” component producing steady state plastic flow in amorphous silicon and the “mobile dislocation densities” in plastically flowing crystalline solids.