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

A powder metallurgy (PM) near alpha 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 alpha plates filled with domains of ultrafine beta transformed structure (beta(t)) (Type I microstructure), an alpha/beta lamellar microstructure with most beta layers being thin and discontinuous plus alpha layers at prior beta grain boundaries (Type II microstructure) and an alpha/beta(t) 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 beta layers in the Type II microstructure during deformation. Such interactions between moving dislocations and thin beta 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 a 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 beta layers by turning them into thicker and ultrafine structure strengthened beta(t) lamellae or blocks is critically important in ensuring high tensile ductility of high strength PM near a titanium alloys.

Automated summary

A powder metallurgy near alpha titanium alloy was fabricated by in-situ dehydrogenation and hot extrusion, leading to three different microstructures. Despite similar strength, the Type II microstructure exhibited significantly lower ductility compared to the other types.