Experimental data were reported for the orientation dependence of the twinning stress in single crystals of Cu-8at%Al. A method which was typical of latent hardening experiments was used to determine the critical stresses which were needed to activate twin systems in a dislocated crystal. The results showed that the activation of a particular twin system obeyed a critical resolved shear stress law; analogous to Schmid’s law for slip. A criterion, for the onset of twinning in face-centered cubic crystals, was also formulated and was based upon the examination of a large body of experimental results,. The criterion stated that the onset of twinning occurred only when 3 necessary conditions were met simultaneously. Firstly, the ratio, τRSS/τC, of the resolved shear stress to the critical stress of a twin system had to be greater than that for any other slip system. Secondly, τRSS had to be greater than a minimum necessary stress for twinning to occur. Thirdly, τRSS had to have the sense of a twin shear.

Is There a Critical Resolved Shear Stress for Twinning in Face-Centred Cubic Crystals? M.S.Szczerba, T.Bajor, T.Tokarski: Philosophical Magazine, 2004, 84[3], 481-502