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

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 alpha on the mechanical properties of TC4 titanium alloy. To this end, four types (I-IV) of bimodal microstructures with similar equiaxed alpha (alpha eq) phase content but differing geometric characteristics of lamellar alpha were designed. In Types I and II, the lamellar alpha exhibited a colony structure, with Type I having thinner lamella alpha and smaller alpha-colony size than Type II. In Types III and IV, the lamellar alpha exhibited a bi-lamellar structure consisting of basketwave lamella and dispersed alpha, where Type III had thinner basketwave-alpha and more dispersed alpha 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 alpha-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.