Growth selection of deformation twins in hexagonal titanium

Abstract

A combination of experiments and molecular dynamics (MD) simulations was used to explore deformation twinning in hexagonal titanium. These were $\left\{10\overline{1}2\right\}$ tensile twins, and their relative area fractions were governed by crystallography. High-resolution microtexture measurements distinguished them as S-type and N-type. S-type involved ‘visible’ slip transfer. In particular, a near-boundary misorientation build-up in the twin-neighbor grain was observed. The N-type twins, in contrast, exhibited no such misorientation development. Collectively, N-type twins grew faster, but a few of them also had comparable growth rates as S-type twins. Mesoscopic differences in twin growth were supported microscopically. Only a few of the grain boundary microscopic deformation twins grew. Further, twin-matrix interfaces exhibited microscopic differences in morphologies and local residual strains. To expand the microstructural observations and to explore associated mechanism(s), focused MD-simulations were conducted. Deformation twins were simulated on bi-crystal models, incorporating differences in twin-neighbor slip transfer. S-type twins occurred at lower stresses and exhibited noticeable relative atomic displacements across twin-neighbor boundaries. This mimicked experimental misorientation build-up. Though nucleation and growth of all deformation twins introduced atomic shuffle, S-type twins were preceded by the formation of stacking faults and atomic shear. Further, the simulated growth of deformation twins was governed by stress-relaxation in the twin-embryo stage and by the accumulation of twin-matrix interfacial dislocations during subsequent twin growth. In summary, this study brought out a novel perspective on growth selection of deformation twinning in hexagonal titanium.