Strength-ductility relationship in solution treated and aged α plus βtype Ti-4.5%Al-3%V-2%Fe-2%Mo titanium alloy

The microstructural variation with solution treating or aging conditions in SP-700 alloy with Ti-4.5%Al3%V-2%Fe-2%Mo was investigated, and the effect of the microstructure on strength-ductility balance of this alloy was studied by tensile testing using both the smooth and notched specimens. Solution treating in the alpha+beta region formed a two-phase microstructure consisting of the primary a and transformed P phases, and the microstructure and hardness of the latter were widely varied by the cooling rate. The microstructures of the transformed beta obtained by water quenching were alpha" athermal omega and retained beta phases, while air cooling formed acicular alpha in beta matrix and its width became thicker with reduction of cooling rate. Waterquenched and aged alloy exhibited better strength-ductility balance compared with air-cooled and aged one, which appeared to be due to retardation of void formation during straining. Strength after aging in this alloy could be analyzed by a law of mixture, and micro-hardness and the volume fraction of the transformed beta phase were found primarily to control strength. Reduction of hardness difference between the.primary alpha and transformed beta phases by the increase of the oxygen content of this alloy improved strength-ductility balance. Notch tensile strength increased with strengthening due to age-hardening and also the increase of cooling rate after solution treating, and showed a peak value at tensile strength of around 1 300 MPa.