Influence of lamellar-α geometry characteristics on mechanical properties of bimodal microstructure in Ti-6Al-4V alloy

Abstract

From a new perspective with considering the combined effect of coordination deformation and heterogeneous effect, this work investigates the influence of the geometric characteristics of lamellar α on the mechanical properties of TC4 titanium alloy. To this end, four types (I-IV) of bimodal microstructures with similar equiaxed α (α_eq) phase content but differing geometric characteristics of lamellar α were designed. In Types I and II, the lamellar α exhibited a colony structure, with Type I having thinner lamella α and smaller α-colony size than Type II. In Types III and IV, the lamellar α exhibited a bi-lamellar structure consisting of basketwave lamella and dispersed α, where Type III had thinner basketwave-α and more dispersed α than Type IV. The tensile test and loading-unloading-reloading (LUR) test results showed that Types I-IV microstructures exhibited significant differences in tensile properties and hetero-deformation induced (HDI) stress. Specifically, in Types I and II, Type I exhibited a higher strength than Type Ⅱ without sacrificing plasticity due to the formation of more α-colony/colony interfaces and enhanced HDI strain hardening. In Types III and IV, Type III exhibited a significantly higher strength than Type IV while maintaining good plasticity due to enhanced dispersion strengthening and HDI strain hardening. Additionally, it can be found that Types III and IV are more advantageous than Types I and II for the combination of strength and plasticity. It could be attributed to that bi-lamellar structure in Types III and IV fabricated multiple strong heterogeneous interfaces, which could promote the formation and accumulation of geometrically necessary dislocations (GNDs) to enhance HDI stress and heterogeneous effect.