Cyclic shear behavior of conventional and harmonic structure-designed Ti-25Nb-25Zr β-titanium alloy: Back-stress hardening and twinning inhibition

Spark plasma sintering was used to fabricate two Ti-25Zr-25Nb beta-titanium alloys having a homogeneous microstructure and a so-called harmonic structure. Mechanical properties were evaluated by simple shear cyclic tests. The sample with homogeneous microstructure shows a large cumulative strain similar to 400% and a two-stage work hardening associated with intense {332} < 11.3 > mechanical twinning activity. Conversely, the harmonic-designed microstructure exhibits lesser cumulative strain similar to 200% but a higher cyclic strength. A complete twinning inhibition is observed. Indeed, the ultrafine-grained shell of the harmonic structure dispersed stress concentration. It accommodates plastic incompatibilities due to grain size difference by accumulating geometrically necessary dislocations at the shell/core vicinity. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.