Deformation mechanism of ω-enriched Ti-Nb-based gum metal: Dislocation channeling and deformation induced ω-β transformation

Gum metal, a class of multifunctional beta titanium alloys, has attracted much attention in the past decade due to its initially-proposed dislocation-free deformation mechanism based on giant faults, i.e., macroscopic planar defects carrying significant plastic strain. Special deformation features were observed in these alloys, such as plastic flow localization, pronounced surface steps, low work hardening, and large elongation. These were all proposed to arise from the special giant fault mechanism activated in the beta-Ti matrix, while the initial presence or mechanically-induced formation of other phases was debated in several follow-up studies. Here, we set off with Ti-Nb-based gum metal samples with confirmed presence of large amounts of nanometer-sized hexagonal omega particles. Deformation experiments demonstrate all the features observed in the original reports, mentioned above. However, careful characterization reveals that the deformation bands (similar to giant faults) where plastic flow localized are dislocation channels that are depleted of omega phase. These channels are proposed to form by a {1 1 2}< 1 1 1 > dislocation dissociation mechanism, promoting reverse transformation of the omega phase into the beta phase. The deformation induced omega-beta transformation and the associated dislocation channeling process can explain the presence of the aforementioned special deformation features in the current gum metal. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.