On the nucleation and growth of {112¯2} twin in commercial purity titanium: In situ investigation of the local stress field and dislocation density distribution

Guo, Y.; Schwiedrzik, J.; Michler, J.; Maeder, X.

Acta Materialia 120 (2016)

Compression testing was conducted on a micro-pillar consisting of a soft-hard grain pair. A {112¯2}<112¯3¯> contraction twin variant nucleated at the edge of the hard grain and elongated progressively into the pillar towards the grain boundary. High angular resolution EBSD scans were performed in situ during the compression testing in order to characterize the elastic strain field and dislocation density distribution at the twin-parent interface during the twin growth. The local/global shear stress ratio on the active twin variant increased from ∼1.6, before macroscopic yielding, to ∼4.2 at peak load, and the rate of increase is inversely proportional to the macroscopic strain level. While the active twin variant does not have the highest global shear stress, it does experience a higher local shear compared to other twin variants, which may contribute to its nucleation. The elongation of the active twin variant involves a competition between local shear stress and dislocation density in front of the twin tip. This could lead to a discontinuous elongation process and stop twin from reaching grain boundary—a phenomenon frequently observed from large grain materials. After removal of the external loading, twinning shear partially reversed sign along the twin-parent interface, indicating a tendency to de-twin.