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

In this paper, very systematic experimental data on the orientation dependence of the twinning stress in Cu-8at.%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 tau(RSS)/tau(C) of the resolved shear stress to the critical stress of a twin system is greater than that of any other slip system; secondly, tau(RSS) is greater than a minimum stress necessary for twinning to occur; thirdly, tau(RSS) satisfies the sense of a twin shear. Other experimental results related to twinning stress in fee crystals are also discussed.