omega phase acts as a switch between dislocation channeling and joint twinning- and transformation-induced plasticity in a metastable beta titanium alloy

We have investigated the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) as well as the influence of to phase on these two phenomena in a metastable beta-type Ti-25Nb - 0.7Ta-2Zr (at.%) alloy. We set off with two starting states: one is (Ji-free and the other one contains a high number density (3.20 +/- 0.78 x 10(24)m(-3)) of nanometer-sized (similar to 1.23 nm) omega particles. Deformation experiments demonstrate that the plastic deformation of the w-free alloy is mediated by stress-induced beta ->alpha martensitic transformation, (332) twinning and dislocation slip, where the former two induce joint TRIP and TWIP effects and the latter one carries the majority of the plastic strain. In the omega-enriched alloy, the to particles fully suppress the TWIP and TRIP effects and promote localization of dislocation plasticity into specific w-devoid channels. Atom probe tomography analysis reveals that the elemental partitioning between beta and omega to results in only subtle enrichment of solutes in the omega matrix, which cannot sufficiently stabilize the matrix to prevent martensitic transformation and twinning. A new mechanism based on the shear modulus difference between beta and to is proposed to explain the suppression of TRIP and TWIP effects by omega particles. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.