In-Situ Study on Tensile Deformation and Fracture Mechanisms of Metastable β Titanium Alloy with Equiaxed Microstructure

Understanding the mechanisms of deformation and fracture of metastable β titanium alloys is of great significance for improving formability and service life. By combining the in-situ tensile test, TEM characterization and EBSD analysis, the tensile deformation behavior, activation of slip systems, crack initiation, and propagation of a high strength metastable β titanium alloy (Ti-5Cr-4Al-4Zr-3Mo-2W-0.8Fe) with equiaxed microstructure are investigated. The equiaxed microstructure is composed of primary α (α(p)) phase, transformed β (β(t)) matrix phase, and secondary α (α(s)) phase. In contrast to the hexagonal α(p) grain with limited slip systems, the body-centered β(t) matrix has more slip systems, however the hindering effect of α(s) phases on dislocation slip leads to the different deformability of the α(p) phase and β(t) matrix. The equiaxed α(p) grains are more prone to deformation and rotation to coordinate the overall deformation. The shear band leads to the formation of sub-grain boundary and even the fragmentation of α(p) grains. As a result, the microvoids tend to nucleate at the grain boundary, phase interface, slip band, and shear band. The inhomogeneous deformation in the plastic deformation zone around the crack tip is the primary cause of damage. The crack propagation caused by microvoids coalescence advances along the grain boundaries and phase interfaces in the form of intergranular, and along the activated slip systems and shear bands in the form of transgranular. Pinpointing the situation in the equiaxed microstructure and combining that in other typical microstructures will help to summarize the universal deformation and fracture mechanisms of metastable β titanium alloy, and provide a basis for alloy design and microstructure tailoring.