Deformation-induced variations in microstructure evolution and mechanical properties of bi-modal Ti-55511 titanium alloy

Abstract

The deformation mechanisms of lamellar and equiaxed α phases of a Ti–5Al–5Mo–5V–1Cr–1Fe (Ti–55511) alloy with a bi-modal microstructure were investigated. Two types of bi-modal microstructures were introduced though different hot-rolling reductions. Accordingly, the tensile properties, fracture behavior, morphology evolution, and deformation twinning were comparatively analyzed. The results show that the sample with higher rolling reduction exhibits higher strength and better ductility, which are mainly owing to refined grain size and fragmentation of the grain boundary α phase. The deformation behaviors of the lamellar secondary α (αs) phase and equiaxed primary α (αp) phase have similarities as well as differences. During deformation, the {11¯01} twinning system is activated in both αp and αs phases. Two twin variants can be found in the lamellar phase, while three twin variants are formed in the equiaxed phase. With an increase in deformation, the twins in the equiaxed α phases extended, and consequently, the microhardness of the αp phases increased. Meanwhile, detailed TEM analyses demonstrated the presence of an α′ martensite phase in the equiaxed α phase, which intersected with the {11¯01} twin and is nearly perpendicular to the twin boundary. These findings provide insights into the deformation behaviors of bi-modal near-β Ti alloys, which can help in the optimization of deformation processes to achieve superior mechanical properties of bi-modal Ti alloys.