Ductilization of fracture of a high strength interwoven α/βt structured PM titanium alloy by in-situ formed submicron Y2O3 particles

Abstract

A method combining internal oxidation and thermomechanical consolidation of a composite powder with a trace amount of Y addition was used to fabricate powder metallurgy (PM) near-α Ti–6Al–2Sn–4Zr–2Mo−0.1Si−0.5Y (wt%) alloy, in which submicron Y2O3 particles were in-situ formed and distributed uniformly in the matrix to achieve a low oxygen content and an excellent combination of high tensile strength of 1242 MPa and excellent tensile ductility of 16.2%. The in-situ reaction of uniformly dispersed Y particles in the composite powders prepared by high-energy ball milling with the oxygen in the matrix to form submicron Y2O3 particles during powder consolidation can significantly scavenge oxygen in the matrix and eliminate the coarse grain boundary α layers. The Y2O3 particles also work as nucleants for heterogeneous nucleation of α grains in the prior β grains during cooling, thus promoting the formation of α plates with a larger number of orientation variations and a decrease of their aspect ratios in the interwoven α/βt microstructure consisting of fine α plates and β transformed structure (βt) domains, which is beneficial to suppress the initiation and rapid propagation of cracks during tensile deformation. It is demonstrated that with the high flow stress rendered by the ultrafine interwoven α/βt microstructure and oxygen scavenging effect of Y on the matrix, twinning across α/βt interfaces and decohesion between the submicron Y2O3 particles and matrix occur during tensile deformation. This further suppresses formation and propagation of microcracks often seen in the Y2O3 particle free PM near-α titanium alloy of the same composition in the same situation. It is believed that the twinning and decohesion of the interfaces between Y2O3 particles and matrix are responsible for the mitigation of strain localization needed for formation of microcracks, which is essential to maintain a good tensile ductility and allow the fracture to be totally ductilized.