Effect of trace boron addition on microstructures, textures, mechanical properties and fracture behaviors of near α titanium alloy

Abstract

B-free and B-containing near α titanium alloys were fabricated by induction skull melting method. Microstructures, textures, mechanical properties and fracture behaviors were investigated. Observations of as-casted microstructures revealed that prior β grains and α colonies were obviously refined after adding trace boron element, and widmannstatten structure of B-free alloy transformed to basket-weave structure of B-containing alloy. For as-forged alloys, prior β grains were broken and squashed, and they stretched along direction perpendicular to forging direction. TiB whiskers promoted dynamic recrystallization behavior, which led to the precipitation of more equiaxed α grains around them. Texture intensity of overall α phase obviously decreased after the addition of trace boron element, which manifested that TiB whiskers prohibited texture formation and improved microstructure homogeneity. B-containing alloy exhibited excellent combination of room temperature (RT) and high temperature (650 °C) mechanical properties. Fracture characteristics of B-free and B-containing alloys were dominant by abundant dimples and cleavage fracture of α lamellas during RT tensile processes. At 650 °C, the cleavage fracture of α lamellas was replaced by dimples, illustrating ductile fracture characteristics. Longitudinal and transverse fracture of TiB whiskers existed no matter at RT or at 650 °C for B-containing alloy. By observations of lateral surfaces, cracks and voids were easier to generate at boundaries between grains and colonies with large size for B-free alloy. For B-containing alloy, comparing with macro-zones in matrix, voids were inclined to generate around interfaces of TiB whiskers and matrix due to strain mismatch. It illustrated TiB whiskers accelerated the fracture of alloy. Voids, transverse fracture of TiB whiskers and lateral abhesion of TiB whiskers were detected at RT, while lateral abhesion of TiB whiskers was seldom discovered during 650 °C tensile process.