PHASE-TRANSFORMATION BEHAVIOR OF GAMMA-TITANIUM ALUMINIDE ALLOYS DURING SUPERTRANSUS HEAT-TREATMENT

Microstructure evolution in a wrought near-gamma titanium alloy, Ti-45 Al-2Cr-2Nb, was investigated by a series of heat treatments comprised of initial heating high in the alpha-plus-gamma phase field followed b y short-time heating in the single-phase alpha field. The initial heat ing step led to a dispersion of gamma particles which pinned the alpha grain boundaries. The kinetics of the gamma grain dissolution during subsequent heating in the single-phase field were interpreted in terms of models for both interface reaction-controlled and diffusion-contro lled processes. The model for diffusion-controlled dissolution yielded predictions comparable to the observed times, whereas the model for i nterface reaction-controlled behavior predicted dissolution kinetics o ver an order of magnitude slower than observed. The growth of the alph a grains, both before and after the dissolution of the gamma phase, wa s also modeled. Section size limitations to the ability to use supertr ansus heating to obtain uniform and moderately fine alpha grain sizes were examined using the transformation models and a simple heat transf er analysis approach. The results were validated through the heat trea tment of subscale and full-scale forgings.