A study on crystal plasticity of face-centered cubic structures induced by deformation twinning

Abstract

The effect of deformation twinning on the accessible slip and twin shear systems of a dislocated face-centered cubic structure and their critical stresses is studied. In particular, the plastic shear modes of critical matrix structure – single glide oriented Cu-8at.%Al alloy crystals deformed by room temperature tension up to the onset of deformation twinning - and the plastic shear modes of bimodal twin/matrix layered structure – subsequently twinned Cu-8at.%Al crystals – were examined. It was found that all twin and slip systems of the matrix structure may operate independently and their critical stresses are dispersed by about fifteen percent around the mean value. However, not all of the accessible slip and twin systems of the twin/matrix structure may operate independently and their critical stresses may differ by a factor of well over four. This large discrepancy of the critical stresses is responsible for giant yield stress anisotropy of the twin/matrix layered structure; the maximal and minimal yield stresses may differ even by one order of a magnitude. The twin/matrix layered structure reveals also very strong asymmetry of the tension/compression yield stress. It is suggested, that the giant anisotropy and very strong asymmetry of the yield stress have the same physical origin, i.e. the internal stress field associated with extended configurations of cube dislocations located within a twin lamellae - the deformation twinning induced effect of cube dislocation stress. It is concluded, that the cube dislocation stress effect seems greatly responsible for plasticity and strengthening of mechanically twinned face-centered cubic structures.