Influence of grain size and stacking-fault energy on deformation twinning in fcc metals

This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fee metals. Classical studies on fee single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystalline fee metals at low homologous temperatures has been proposed as (sigma(tw) - sigma(0))/G = C(d/b)(A), where sigma(tw) is the deformation twinning stress, oo is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger's vector, and C and A are constants determined to have values of 0.0004 and -0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.