Effect of Heat Treatment on Microstructure and Mechanical Properties of Extruded PM Ti-6Al-4V Alloy With Deformation Flowlines

Two heat treatments, namely 955 degrees C/2h/AC (AC = air cooling) and 955 degrees C/4h/FC (FC = furnace cooling), were carried out on a powder metallurgy (PM) Ti-6Al-4V alloy fabricated by hot extrusion of a compact of Ti/Al60V40 powder blend. The as-extruded rod of the PM titanium alloy has a fully lamellar alpha/beta structure and deformation flow-lines in the longitudinal section. After the heat treatment of 955 degrees C/2h/AC, the softer deformation flow-lines became significantly clearer, and they had a higher V content (similar to 5.5 wt pct) than the nominal composition of the alloy. The deformation flow-lines encompassed a finer alpha/beta lamellar microstructure with a higher fraction of beta phase, while the matrix of the alloy under this heat treatment condition had an alpha/beta(t) lamellar microstructure, where beta(t) represents the beta transformed structure. On the other hand, after the heat treatment of 955 degrees C/4h/FC, the deformation flow-lines became discontinuous in the now coarser alpha/beta lamellar microstructure with a smaller fraction of beta phase. Both heat treatments cause a significant improvement in the tensile ductility of the alloy (8.2 vs. 12.1 and 17.2 pct) with only a slight decrease of the yield strength (891 vs. 863 and 852 MPa). Especially the second heat treatment leads to an excellent tensile strength-ductility combination with the product of tensile strength and elongation to fracture reaching 16.5 GPa pct. (C) The Minerals, Metals & Materials Society and ASM International 2022