Study Of {332}<113> Twinning Behavior In β-type Ti-Mo Alloys

{332}<113> twinning,an unusual twinning mode in body-centered cubic(bcc)metals and alloys,has been demonstrated to be a fundamental plastic deformation mode in bcc metastable ? titanium alloys.a special feature of {332}<113> twins is that they easily undergo primary twinning,twin-twin intersection,and secondary twinning inside primary twins,which makes it as an important role in enhancing the strain hardening and thus improving the mechanical properties(twinning-induced plasticity(TWIP)effect).However,the relationship between twinning variant and strain hardening behavior is still not well understood because of a lack of study on the twinning behavior.Therefore,in this paper we focus on {332}<113> twinning behavior such as variant selection and twinning transfer in metastable ?-type Ti-Mo alloys.The {332}<113> twinning microstructures in differently oriented grains and twin-twin and twin-dislocation pairs at various grain boundaries were systemically examined by electron backscattered diffraction technique in Ti-15 Mo alloy and Ti-10Mo-5Zr alloy at strain of 5%.The selection of primary and secondary twinning variants,the contribution of twinning to plastic strain and the effect of grain boundary angle on twinning transfer behavior in neighboring grain were quantitatively analyzed in terms of Schmid factor,dodecahedron of {332} planes,strain accommodation mechanism,trace analysis and angle analysis.The results showed that primary twinning variants companied with their intersections and secondary twinning variants inside primary twins were easily activated in the grains.Presence of primary twins in the grains with their tensile axes close to [1?22] and [1?11] directions exhibited the relative higher contribution to macroscopic stain than those of 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 0.3-0.5 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.The twinning transfer occurred readily at the low angle grain boundaries without significant local stress concentration and formed a twin-twin pair dominated by applied stress,i.e.,macroscopic Schmid law.At the high angle grain boundaries,it often resulted in a twin-twin pair with non-Schmid factor twinning variant or a twin-dislocation pair with geometrically necessary dislocations at the boundary area dominated by the strain accommodation.The activation of non-Schmid factor twinning variant and slip system in neighboring grain was due to their high accommodative capacity while releasing the local internal stress concentration,which was caused by the twinning transfer at high angle grain boundaries.