β rich α-β 型チタン合金 Ti-6Al-2Sn-4Zr-6Mo の組織と機械的性質

Abstract

The effect of microstructures in mechanical properties of β rich α-β titanium alloy Ti-6Al-2Sn-4Zr-6Mo was investigated. The β phase formed at the solution temperature above 900°C transformed into the α''(orthorhombic martensite) phase with retained β phase by quenching treatment, but transformed into the acicular α phase with the retained β phase after slow cooling. Solution treatment in the temperature range between 825°C and β transus temperature (952°C) and aging (STA) improved the tensile properties. But STA materials showed the lower fracture toughness than 80 kgf/mm3/2 on account of the presence of the fine acicular α grains between primary α grains. Double solution treatment and aging (STSTA) improved the fracture toughness. The volume fraction of primary a grains was optimized by first solution treatment and second solution treatment controlled the shape of the secondary acicular α grains. The higher fracture toughness in the STSTA materials appeared to be dependent upon the morphology of the secondary acicular α grains. The materials which were double solution treated at 940°C for 1 h oil quenching (OQ) and at 830°C for 2 h air cooling (AC) and aged at 590°C for 6 h AC, possessed the following properties.Tensile strength : 121 kgf/mm2Tensile elongation : 14.4%Fracture toughness : 113 kgf/mm3/2