準安定 β 型チタン合金 Ti-6.8Mo-4.5Fe-1.5Al の機械的性質に及ぼす遷移相および転位から核生成した α 相の効果

Abstract

Effects of α phase nucleating at a transient phase and at dislocations on mechanical properties in a metastable β titanium alloy, Ti-6.8Mo-4.5Fe-1.5Al(mass%), have been investigated by tensile test and optical, scanning and transmission electron microscopy. Two-step aging promotes the precipitation of fine α phase in grain interior because α phase nucleates at a transition phase of β′. Precipitate free zone appears at first, and finally disappears during aging. A specimen including precipitate free zone shows lower yield strength and larger elongation than that without precipitate free zone because dislocation can easily glide in the precipitate free zone, which can be verified from grain-boundary ductile fracture surface. The transition phase of β′ promotes the disappearance of precipitate free zone due to the promotion of α phase precipitation, resulting in enhancement of yield strength and in deterioration of elongation. Aging after tensile deformation produces α phase precipitation at dislocations. A specific variant of α phase is selected due to stress field around the dislocations included in a planar slip bands, resulting in the formation of a large colony of the specific α variant. Elongation is deteriorated because the large colony probably facilitates crack propagation.