Effects of α Phase Nucleating at Transition Phase and Dislocation on Mechanical Properties in Metastable β Titanium Alloy Ti-6.8Mo-4.5Fe-1.5Al

Effects of a phase nucleating at a transition phase and dislocation on mechanical properties in a metastable beta titanium alloy, Ti-6.8Mo-4.5Fe-1.5Al(mass%), were investigated. Tensile test and optical, scanning, and transmission electron microscopy were used. Two-step aging promoted the precipitation of a fine a phase in grain interiors because the a phase nucleated at a transition phase of beta'. A precipitation-free zone appeared at first, but finally disappeared during aging. A specimen that included the precipitation-free zone showed lower yield strength and larger elongation than it did without the precipitation-free zone. This was because dislocation could easily glide in the precipitation-free zone, which was verified from examination of the intergranular ductile fracture surface. The transition phase of beta' promoted the disappearance of a precipitation-free zone, due to the promotion of a phase precipitation, resulting in enhancement of yield strength and deterioration of elongation. It is likely that the width of PFZ and the crystallographic orientation of beta grains determine fracture mode to be intergranular ductile, intergranular brittle or intragranular fractures. Aging after tensile deformation produced a phase precipitation at dislocation. A specific variant of a phase was selected due to a stress field around the dislocation that included a planar slip band. This resulted in the formation of a large colony of the specific a variant, while elongation deteriorated because the large colony probably facilitated crack propagation.