Effect of alpha Phase Nucleating at Transition Phase and Dislocation on Mechanical Properties in Metastable beta Titanium Alloy Ti-6.8Mo-4.5Fe-1.5Al

Effects of alpha phase nucleating at a transient phase and at dislocations on mechanical properties in a metastable beta 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 alpha phase in grain interior because alpha phase nucleates at a transition phase of beta'. 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 beta' promotes the disappearance of precipitate free zone due to the promotion of a phase precipitation, resulting in enhancement of yield strength and in deterioration of elongation. Aging after tensile deformation produces alpha phase precipitation at dislocations. A specific variant of a 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 a variant. Elongation is deteriorated because the large colony probably facilitates crack propagation.