Onset of plastic deformation and strain localization in relation to β phase in metastable β and dual phase Ti alloys

High strength metastable β titanium alloys are widely used in the aerospace industry for their superior mechanical properties, including an outstanding strength-to-weight ratio. Although prior studies have suggested a major influence of the elastic anisotropies of α and β phases on deformation, the understanding of the onset and the development of plasticity at the β grain scale is still limited. Strain localization was presently investigated in various Ti alloys with different microstructures. In-situ optical microscopy tensile tests were carried out in combination with digital image correlation and electron back-scattered diffraction in order to relate the local deformation behavior with the microstructure. The low directional modulus along the [001] direction of β phase was observed to generate high incompatibility stresses, which governs the onset of slip activity and strain localization. However, it was found to emerge differently depending on the presence of α phase. In α phase free microstructures, high incompatibility stresses across β grain boundaries triggers the onset of slip on the side exhibiting the highest stiffness. In α+β microstructures, β grains with a [001] direction aligned with the loading direction were found to experience an early onset of plastic deformation and subsequent strain localization. A mechanistic understanding was obtained through the simulation of micromechanical fields in α and β phases. The behavior of polycrystalline aggregates with explicit modeling of hierarchical dual phase microstructure inherent to β grains was studied within an anisotropic elasticity framework using fast-Fourier transforms-based simulations. An early onset of plastic deformation was found to occur in α phase embedded in [001] oriented β phase as compared to other β orientations due to stress partitioning driven by the stiffness difference. The effects of the Burgers orientation relationships, the β phase fraction and the selected set of single crystal elastic constants on stress heterogeneity were also examined and discussed.