Shear transformations metallic glasses

We follow the discussion of metallic glass alloys with a brief parallel consideration of the behavior of amorphous silicon that is based primarily on computer simulations that have not only introduced the corresponding behavior of space-network glasses but also permitted a much deeper mechanistic understanding both of structural relaxations, and, more importantly, of the nature of plastic shear relaxations by ubiquitous shear transformations in glassy solids of all types, particularly in glassy polymers. These are developed in detail later in Chapters 7 and 8. 

Pan, D., Inoue, A., Sakurai, T., and Chen, M. W. (2008) Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses, Proc. Natl. Acad. Sci., 105, 14769-14772. 

Figure 8.3 shows, for PC, that there is a substantial difference between the yield stress in compression and that in tension. As discussed in Chapter 7, for metallic glasses, and as outlined here in Section 8.6.1, this difference is a consequence of the quite considerable activation dilatation, e, kinematically coupled to the transformation shear strain. It arises from the transitory expansion of the transformation volume Qf of the ST due to the internal random rearrangement of molecular segments occuring during the relaxation event. 

Glasses, like metals, are formed by deformation. Liquid metals have a low viscosity (about the same as that of water), and transform discontinuously to a solid when they are cast and cooled. The viscosity of glasses falls slowly and continuously as they are heated. Viscosity is defined in the way shown in Fig. 19.7. If a shear stress is applied to the hot glass, it shears at a shear strain rate 7. Then the viscosity, ij, is defined by... 

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