1992 Volume 32 Issue 2 Pages 222-231
The effects of cold deformation on the morphology of α precipitates were examined for three β titanium alloys which were each mainly deformed by slip, twinning, and stress-induced martensitic transformation.
Although α precipitated preferentially at β 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 α precipitates aligned along the striations. In Ti-16V-10Sn, deformation twins were frequently accompanied by the internal twins, and the α phase preferentially precipitated at the twin boundaries, especially at the cross points with the internal twins, to finally form film-like α with occasional α precipitates within the twins. In Ti-16V-4Sn, stress-induced orthorhombic martensite (α'') 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 β phase during the early stages of aging, and the α phase precipitated preferentially at the interface between the β phases which were originally the α'' and the β matrix. During further aging, the α phase coalesced to form film-like α.
On the other hand, there was no significant difference reflected by the initial deformation modes in the α morphology after heavy deformation (60% rolling) among the three alloys, and the microstructures were considered to be of a microduplex structure.