2004 年 44 巻 11 号 p. 1911-1917
The microstructural variation with solution treating or aging conditions in SP-700 alloy with Ti-4.5%Al-3%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 α+β region formed a two-phase microstructure consisting of the primary α and transformed β phases, and the microstructure and hardness of the latter were widely varied by the cooling rate. The microstructures of the transformed β obtained by water quenching were α″, athermal ω and retained β phases, while air cooling formed acicular α in β matrix and its width became thicker with reduction of cooling rate. Water-quenched 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 β phase were found primarily to control strength. Reduction of hardness difference between the primary α and transformed β 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 1300 MPa.
