The Grain Refinement And Thermal Stability Of A TA Containing And NB Containing Tial Based Alloy Subjected To A Long-Term Exposure At700℃in Air

The grain refinement of cast Ti-46Al-8Ta and Ti-46Al-2.5Nb alloys have been studied. Cast Ti-46Al-8Ta alloy with millimeter sized fully lamellar microstructure has been grain refined successfully through a specially designed heat treatment scheme, consisting of hot isostatic pressing, solid solution in single α phase region followed by air cooling, and annealing in an α+γ two-phase region. Convoluted lamellar colonies of～30micrometre in size have been produced throughout the cast alloy after securing a fully massive γ transformation. Compared to alloy Ti-46Al-8Ta, however, alloy Ti-46Al-2.5Nb was difficult to be grain refined. This is because the alloy containing less Nb would have the cooling rate nessissry for fully massive γ transformation much increased, i.e. a Jominy end quenching curve extended to the higher cooling rate. This actually makes the laboratory cooling practice, such as salt bath quenching, is not fast enough to satisfy the condition.The thermal stability of the grain refined alloy Ti-46Al-8Ta has been assessed through a serial exposure at700℃in air for up to5000hours. The changes in microstructure and phase structure were characterized using optical microscope, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The corresponding changes in mechanical properties were examined by tensile testing at room temperature. It was found that the refined grain structure was thermodynamically unstable. The fine convoluted structure (～30μm in grain size) was eventually coarsened to～100μm in size after5000h exposure at700℃. Along coarsening, great deal of B2(ω) phase (21.8vol.%), and equiaxed (13.3vol.%) formed by consuming the convoluted lamellar colonies, leading to a reduction both in strength and in ductility:16%reduction in0.2%proof stress and1/3reduction in elongation to failure.