Microstructural sensitivity and deformation micro-mechanisms of a bimodal metastable β titanium Ti–7Mo–3Nb–3Cr–3Al alloy

The bimodal microstructure of a metastable β Ti–7Mo–3Nb–3Cr–3Al (Ti-7333) alloy was successfully tuned through various thermomechanical processing steps, to achieve strength and ductility combinations, in the range between 903 and 1443 MPa and 2.9 % and 17 %, respectively. The influence of the microstructural variables on the deformation micro-mechanisms and fracture mechanisms were further elucidated by the assistance of TEM and fractographic analysis. The results indicate that the strength of bimodal Ti-7333 is mainly governed by secondary α_S precipitation through the introduction of a high density of α/β interfaces that effectively block dislocation transmission, whereas ductility is tuned through the control of the primary α_P, rod-like α_r and retained β phases. Moreover, ductility can be extremely deteriorated if the formation of a continuous grain boundary α_CGB phase is not prevented. Plastic deformation of bimodal Ti-7333 is dominated by dislocation slip and tangling, whereas the microstructural sensitivity that accelerated at high strength level is originated from the deformation incompatibility between the fine acicular α_S and parent β. In addition, the {10–11}<-1012>_α micro-twinning of equiaxed α_P phases also contributes to improved ductility and coordinates deformation to an extent. The fractographic analysis revealed either fracture by microvoid coalescence or a mixture of intergranular fracture and microvoid coalescence, depending on thermomechanical condition. The mixed fracture mode results in a relatively flat crack propagation path and reduced ductility; whereas a tortuous crack propagation is obtained when the alloy solely undergoes fracture by microvoid coalescence.