Manipulating the strength and tensile ductility of a PM near α titanium alloy by adjusting the morphologies and volume fractions of α and βt domains

Abstract

Powder metallurgy (PM) near α Ti−6Al−2Sn−4Zr−2Mo−0.1Si−0.5Y (wt.%) alloy with a low oxygen content and fabricated by thermomechanical powder consolidation of an elemental powder blend with a trace amount of Y addition was subjected to two heat treatments to obtain different microstructures which were free of grain boundary α (αGB) layers. One heat treatment (960°C/1h/AC, AC = air cooling) led to the formation of an α/βt microstructure consisting of 75.3 vol% primary α (αp) domains with globular and platelike morphologies, 21.5 vol% β transformed structure (βt) domains comprising ultrafine secondary α (αs) lamellae in a β matrix and 3.2 vol% thin β strips. The other heat treatment (960°C/1h/WQ+580°C/6h/AC, WQ = water quenching) led to the formation of a microstructure consisting of 38.2 vol% isolated αp plates, slabs, and globules dispersed in a βt matrix comprising nanometer-sized αs lamellae and residual β strips. The first microstructure rendered the alloy with high tensile strength of 1128 MPa and excellent tensile ductility of 15.6%, thanks to the activation of abundant <a> and <c+a> slips in all α grains and tensile twinning in the “hard” <0001> micro-textured α grains with the c-axis parallel to the tensile force direction which mitigated the strain localization and enhanced strain hardening. On the other hand, the harder nanostructure of the βt matrix in the second microstructure resulted in a significantly increased tensile strength of 1351 MPa while maintaining an excellent tensile ductility of 10.5%. This can be attributed to the activation of abundant <a> and <c+a> slips in thin α plates and a limited degree of tensile twinning in the “hard” α domains, as well as a large number of statistically stored dislocations (SSDs) within the micron-scaled α slabs/globules. Both microstructures rendered the alloy with favorable ductile fracture.