STRUCTURAL INTERPRETATION OF THE NUCLEATION AND GROWTH OF DEFORMATION TWINS IN ZR AND TI .1. APPLICATION OF THE COINCIDENCE SITE LATTICE (CSL) THEORY TO TWINNING PROBLEMS IN HCP STRUCTURES

A step-wise nucleation and growth mechanism based on the coincidence site lattice (CSL) theory is proposed for deformation twinning in h.c.p. structures. Lattice transformation during twinning is accomplished by a coordinated movement of a large number of atoms between two lattice match planes from the matrix to twin positions rather than a layer by layer movement through twinning dislocations as proposed for twinning dislocation theories. The sidewise propagation and thickening of a twin lamella proceeds in a step-wise manner with lattice match planes being the coherent boundaries between the matrix and twins. This model predicts that twinning in h.c.p. lattices can occur at a high velocity close to the sound speed, which is impossible according to twinning dislocation theories. The proposed mechanism is also consistent with other observations such as lack of critical-resolved-shear-stress for twinning, emissary dislocations, and insensitivity to temperature. Dislocation reactions may be involved in twinning although they are, at high stresses, not required.