Correlation between microstructures and tensile deformation behavior of a PM near α Ti–6Al–2Sn–4Zr–2Mo−0.1Si alloy

Abstract

A powder metallurgy (PM) near α Ti–6Al–2Sn–4Zr–2Mo−0.1Si (wt.%) alloy was fabricated by in-situ dehydrogenation and hot extrusion of a TiH2-based powder compact and heat treatments. Three types of microstructures were obtained in the as-consolidated state and after different heat treatments. They are a basketweave microstructure consisting of a network of inter-penetrating α plates filled with domains of ultrafine β transformed structure (βt) (Type I microstructure), an α/β lamellar microstructure with most β layers being thin and discontinuous plus α layers at prior β grain boundaries (Type II microstructure) and an α/βt lamellar microstructure (Type III microstructure). Although the three types of microstructures render the alloy with a similar yield strength (1085–1109 MPa) and ultimate tensile strength (1217–1239 MPa) due to the balanced effects of various strengthening mechanisms, the Type II microstructure exhibits a significantly lower tensile ductility than the other two types of microstructures (elongation to fracture: 5.5% vs. 13–14%). Examination of dislocations in the tensile deformed specimens reveals that dislocations cutting through the thin β layers in the Type II microstructure during deformation. Such interactions between moving dislocations and thin β layers in the Type II microstructure are expected to be the primary reason for the clearly lower strain hardening rate, premature fracture of grain boundary α layers and low tolerance to strain localization observed in the tensile tests, fracture surface examination and DIC analysis. This correlation between the microstructures and tensile deformation behavior strongly suggests that preventing moving dislocations cutting through thin β layers by turning them into thicker and ultrafine structure strengthened βt lamellae or blocks is critically important in ensuring high tensile ductility of high strength PM near α titanium alloys.