EFFECTS OF COLD DEFORMATION ON THE MORPHOLOGY OF ALPHA-PRECIPITATES IN BETA-TITANIUM ALLOYS

The effects of cold deformation on the morphology of alpha precipitates were examined for three beta titanium alloys which were each mainly deformed by slip, twinning, and stress-induced martensitic transformation. Although alpha precipitated preferentially at beta grain boundaries in the undeformed specimens, slight deformation (5% rolling) added another preferential nucleation site peculiar to the deformation modes, which was clearly reflected by the microstructure after aging at 873 K. In Ti-15V-3Cr-3Sn-3Al, slip brought about a striated structure of planar slip bands caused by the extremely localized dislocations, and consequently led to directionally elongated alpha precipitates aligned along the striations. In Ti-16V-10Sn, deformation twins were frequently accompanied by the internal twins, and the alpha phase preferentially precipitated at the twin boundaries, especially at the cross points with the internal twins, to finally form film-like alpha with occasional alpha precipitates within the twins. In Ti-16V-4Sn, stress-induced orthorhombic martensite (alpha") tended to be produced in groups of martensite plates aligned in parallel by cold rolling, leading to a banded structure. This martensite substantially reverted to the beta phase during the early stages of aging, and the alpha phase precipitated preferentially at the interface between the beta phases which were originally the alpha" and the beta matrix. During further aging, the alpha phase coalesced to form film-like alpha. On the other hand, there was no significant difference reflected by the initial deformation modes in the alpha morphology after heavy deformation (60% rolling) among the three alloys, and the microstructures were considered to be of a microduplex structure.