Precipitation reactions in titanium-tantalum alloys

Abstract

Studies have been made of the tempering at 400° and 600°C of h.c.p. and orthorhombic martensitic phases in alloys with up to 32 w/o tantalum, and of the retained β phase in higher tantalum alloys. Precipitation of the equilibrium β phase as disc-shaped particles with the Burgers orientation relation and with a habit plane close to that expected for the reverse martensitic transformation was observed in martensitic specimens up to 32 w/o Ta, but at 36 w/o Ta the martensite first reverts to (non-equilibrium) β which then precipitates ω phase as an intermediate stage in the formation of the equilibrium β + α phases. The ω phase was also formed from retained β phase in Ti-53 w/o Ta, tempered at 400°c, and it was then difficult to produce α precipitation. Direct formation of α from β, however, was observed when this alloy was tempered at 600°c. Electron microscopy indicates ellipsoidal shapes for the ω particles, and this is consistent with the streaking observed in electron diffraction patterns; no evidence was found for the alignment of particles in rows.