On the effects of reorientation and shear transfer during twin formation: Comparison between high resolution electron backscatter diffraction experiments and a crystal plasticity finite element model

The effects of reorientation and twin shear transfer on the load sharing between twin and parent pairs in hexagonal closed-pack (HCP) polycrystals have been examined by combined experimental and numerical methods. A highly textured Zircaloy-2 sample was uniaxially strained in a direction that favours twin formation and then unloaded to measure variations in residual elastic strains, lattice rotations, and stresses within twin and parent grains by the use of high resolution electron backscattered diffraction (HR-EBSD). The measured grain structures were imported into a finite element solver to study local stresses within each grain and their evolution as twins form. A crystal plasticity finite element code was modified to integrate the effect of twin shear strain into the constitutive equations. Results show that between reorientation and twin transformation strain, the later plays the more important role on determining the state of the stress in the parent, twin and the surrounding environment. The elastic energy of the parent grain was shown to reduce upon twin formation but then stay constant after the early stages of twin shear transfer. This can promote the formation of the next twin in preference to increasing the size of the current one. Comparison of the model with HR-EBSD measurements took into account that the residual stress variations were measured relative to the (unknown) stress state at the reference point within each grain.