Accommodative {332}〈113〉 primary and secondary twinning in a slightly deformed β-type Ti-Mo titanium alloy

The {332}〈113〉 twinning microstructures in differently oriented grains were systemically examined by electron backscattered diffraction technique in a slightly deformed β-type Ti-15Mo polycrystalline alloy, and the selection of primary and secondary variants were quantitatively analyzed in terms of applied stress and strain accommodation. Primary twinning variants accompanied with their intersections and secondary twinning variants inside primary twins were easily activated in the grains. Presence of primary twinning variants in the grains with their tensile axes close to [1¯22] and [1¯11] directions exhibited the relatively higher contribution to macroscopic stain than those in the grains with their axes close to [011] and [001] directions. Selection of primary and secondary twinning variants not only obeyed the Schmid law with large Schmid factors from 0.3 to 0.5 but also exhibited the non-Schmid behaviors with low and negative values. The selection of twinning variants with non-Schmid behaviors was due to their high accommodative capacity to release the local internal stress concentration, which was evaluated by rotating the shear displacement gradient tensor expressed in the activated twinning variant reference frame into the accommodative twinning variant reference frame in neighboring grain or region. The occurrence of accommodative primary and secondary twinning was considered to further enhance an effectiveness of twinning deformation for the substantial strain hardening behavior of present alloy.