Is there a critical resolved shear stress for twinning in face-centred cubic crystals?

Abstract

In this paper, very systematic experimental data on the orientation dependence of the twinning stress in Cu–8 at.% Al single crystals are reported. A method, typical for ‘latent hardening experiments’, is employed to determine the critical stresses needed to activate twin systems in a dislocated crystal. Results show that activation of a particular twin system follows a critical resolved shear stress law, which is analogous to Schmid's law for slip. A criterion for the onset of twinning in fcc crystals, based on examination of a large body of experimental results, is also formulated. The criterion says that the onset of twinning occurs only when three necessary conditions are met simultaneously: firstly, the ratio τ_RSS /τ_C of the resolved shear stress to the critical stress of a twin system is greater than that of any other slip system; secondly, τ_RSS is greater than a minimum stress necessary for twinning to occur; thirdly, τ_RSS satisfies the sense of a twin shear. Other experimental results related to twinning stress in fcc crystals are also discussed.

Acknowledgement

This work is dedicated to the late Professor Zbigniew S. Basinski, a great scientist, a great man and a great Pole.