Shear transformations nucleation under stress

While the nucleation scenario under an applied shear stress that we are about to develop below closely resembles that of nucleation of a martensite embryo (Orowan 1954), it contains no corresponding interface energy, since in an amorphous structure the border of the shear transformation has no distinguishable feature that is different from the atomic packing in the surrounding amorphous structure. 

The Gibbs free energy of nucleation of the shear transformation under stress... 

We consider first the nucleation under an applied simple shear stress a without the presence of an accompanying mean normal stress We consider the nucleation as a reversible experiment in which the transformation strain in the volume element Qf can be built up by an imposed external shear strain y. As is clear, and as discussed by Orowan (1954), this results in a rise in the Helmholtz free energy AF(y) that initially is given by a quadratic function and for the whole process can be taken as being given by a cosine potential (Kocks et al. 1975) (see also Johnson and Samwer (2005)),... 

In the nucleation of the ST with transformation shear strain y and coupled dilatation e, the Gibbs free energy of activation under an applied shear stress a is still given by eq. (7.9) as... 

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