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

This article investigates the microstructural variables influencing the str ess required to produce deformation twins in polycrystalline fee metals. Cl assical studies on fee single crystals have concluded that the deformation- twinning stress has a parabolic dependence on the stacking-fault energy (SF E) of the metal. In this article, new data are presented, indicating that t he 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 str ess in the polycrystal. A new criterion for the initiation of deformation t winning in polycrystalline fee metals at low homologous temperatures has be en 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 sh ear modulus, d is the average homogeneous slip length, b is the magnitude o f 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 c ritical in building the necessary dislocation density while maintaining rel atively large homogeneous slip lengths.