DEFORMATION TWINNING IN HCP METALS AND ALLOYS

The important role of deformation twinning in plastic deformation of h.c.p. metals and alloys is emphasized and the twin nucleation mechanisms are examined. A bifurcated homogeneous nucleation model is presented on the basis of the classical nucleation theory, the shape bifurcation theory and the elastic inclusion model. The activation energy for twin formation depends most sensitively on the twin-boundary energy. The structures and energies of (112BAR2) and (101BAR1) coherent twin boundaries are obtained by atomistic simulations using the Lennard-Jones potential for a model h.c.p. metal. The anisotropic coupling effect of normal stresses on twin dislocation mobility is determined over a wide temperature range (0-1200 K). The available experimental data on titanium and zirconium are consistent with the prediction from the proposed nucleation model and the temperature-dependent mobility of twin dislocations. Alloying effects on twinning and future research areas are discussed.