On the transformation pathways in TRIP/TWIP Ti–12Mo alloy

Diverse pathways of the deformation-induced transformations are investigated in the β-metastable Ti–12Mo alloy displaying combined TWIP ({332} β twinning) and TRIP (stress-induced α″) effects by means of Schmid factor analysis and in-situ microstructural characterizations. Two sets of Schmid factors, for {332} β twinning and stress-induced martensite, are calculated in grains of random orientation under uniaxial tension condition. Using in-situ EBSD characterization under tension, the correspondence is mapped between primary transformation products and the grain orientations. The correspondence results graphically in four grain-orientation domains in the inversed pole figure triangle, being twin-dominated, martensite-dominated, combination of both, and the domain unfavorable for both transformations. It is also found that the crystallographic partition of the domains, i.e. the selection of transformation pathways with respect to grain orientations, is related to the threshold of Schmid factor for the activation of each transformation and the transformation strain of the corresponding martensite variant. Guided by transformation partition mapping, grains at two special orientations, dominated by stress-induced martensite with-or-without primary {332} β twinning, are targeted in in-situ microstructural observations to clarify the transformation pathways and the underlying deformation mechanisms. Among the different pathways revealed, experimental evidences are highlighted on the unprecedented formation mechanisms of {332} β twins assisted by martensite {130} α″ deformation twinning via two different ways.