Theoretical strength and prediction of structural defects in metallic glasses

Abstract

Theoretical strength plays a pivotal role in gauging the maximum stress and inferring structural defects in crystalline materials. However, its very existence and the expected prediction of possible defects in amorphous solids remain elusive. Here, by using a finite deformation theory, we obtain the theoretical strengths for several bulk metallic glasses under pure shear loading. The theoretical strengths obtained are only several times larger than the experimental yield stresses; in contrast, they are three or four orders of magnitude higher in crystalline materials. This striking closeness between the theoretical and experimental strengths suggests the absence of any extended structural defects that can substantially reduce the intrinsic strength in the amorphous metals. Instead, the atoms have to sever each individual bond to deform. Further investigation reveals that the deformation occurring at the theoretical stress proceeds with the mechanical instability with a vanishing shear modulus, or a mechanical spinodal. We deduce from these results that, different fundamentally from crystalline solids, there are no extended structural defects in the amorphous solids and the plastic deformation must be local and sensitive to sample conditions.