INFLUENCE OF ELEMENTAL B-ADDITION ON THE HEAT-TREATED CAST STRUCTURESOF TI-47AL-2CR-(2-4)NB ALLOYS

The influence of elemental B addition on the heat-treated cast structu re of Ti-47Al-2Cr-(2-4)Nb alloys has been investigated using x-ray dif fractometry, optical microscopy, scanning and transmission electron mi croscopy, and tensile testing. The phase sequence is beta --> beta + a lpha --> alpha --> alpha + gamma --> alpha + beta + gamma --> beta + g amma. The addition of (0-2 at. %) B does not change the phase sequence . It, however, tends to stabilize a phase by shifting the (alpha + bet a + gamma) three-phase region toward a higher (Cr + Nh) content. The B addition does not significantly alter the equilibrium composition wit hin (alpha + gamma) two-phase field. The B addition markedly accelerat es the lamellar formation kinetics and enhances the thickening rate of gamma plates, despite the fact that it increases both the misfit betw een alpha and gamma plates and the alpha/gamma interfacial energy. The acceleration of lamellar formation kinetics is thus believed to be pr imarily due to the enhancement of chemical diffusivity as a result of B addition. The B addition induces a significant refinement of heat-tr eated cast structure. This is primarily due to the role of boride to d isperse the interdendritic gamma regions to a fine network and to refi ne the dendrite cell size. Further refinement arises from the boride's role to act as the nucleation site for gamma grains and from the intr insic B effect to enhance the chemical diffusivity and the gamma thick ening rate. The addition of a small amount of B enhances both the stre ngth and tensile ductility of near gamma structure. The strengthening arises from grain size refinement and from the boride dispersion. The calculation of fracture strain suggests that an enhancement of ductili ty for small B addition (up to similar to 0.2 at. %) is mostly due to its effect to refine the gamma grains.