TRANSFORMATIONS IN A TI-24AL-15NB ALLOY .1. PHASE-EQUILIBRIA AND MICROSTRUCTURE

A variety of heat treatments have been employed to explore phase equilibria and the development of microstructure in a Ti-24Al-15Nb alloy. These include solution treatments both above and below the beta-transus, followed by controlled cooling and aging at temperatures high enough to preclude omega-phase formation. The phase fields beta, beta(0), alpha(2) + beta(0), alpha(2) + beta(0) + O, and beta(0) + O have been identified in the alloy, and schematic time-temperature-transformation (TTT) curves are proposed for continuous cooling transformations from the beta-phase. The composition of the alpha(2) and beta(0) phases in the alpha(2) + beta(0) region and of the alpha(2), beta(0) and O phases in the ternary phase field have been obtained by analytical electron microscopy.