Physical metallurgy of a Ti-5%Ta-1.8%Nb alloy

Abstract

An alloy of Titanium with 5% Tantalum and 1.8% Niobium has been developed which possesses high corrosion resistance in highly oxidising environments. The microstructural basis that enabled design of optimum thermo-mechanical treatments has been established for this alloy. The classification of the alloy, transformation temperatures and different types of phase transformations were evaluated for the first time by experimental methods like metallography and calorimetry and empirical methods. Systematic microstructural modifications were introduced by thermo-mechanical treatments to improve corrosion resistance and mechanical properties. The Ti-5Ta-1.8Nb alloy exhibited interesting texturing behaviour. Deformation and transformation textures exhibited during unidirectional cold rolling and subsequent β→α+β transformation were studied using XRD and Electron Back Scatter Diffraction techniques. The cross section of a wire drawn specimen exhibited (1 0–1 0)_α texture while a cold rolled specimen showed (0 0 0 2)_α deformation texture along the length — width direction. The transformation texture by itself was found to be dependent on the type of deformation texture, (1 1 −2 0)_α in cold rolled and (1 1 −2 2)_α texture in the case of wire drawn alloy. A new method has been proposed to determine theoretical misorientation angle and axis between variants of hcp a product transforming from a parent bcc crystal, obeying Burgers Orientation Relationship. The role of variant selection mechanisms in the final texture of the alloy has been demonstrated by comparison of texture maps obtained by X-ray Diffraction with those computed.