Phase transformations in Ti-6.8Mo-4.5Fe-1.5Al

Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature range from 300 degrees C to 750 degrees C utilizing hardness measurements, X-ray diffraction, optical microscopy, and electron microscopy. Artificial aging following solution treatment and water quenching initially involved growth of the athermal omega phase. This was followed by formation of the alpha phase, either in association with the omega phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly, isothermal decomposition of the metastable beta phase resulted in the precipitation of w phase exhibiting an ellipsoidal morphology. While precipitation of omega was immediate at 345 degrees C, an incubation period was observed upon aging at 390 degrees C. Isothermal aging above this temperature involved direct precipitation of the alpha phase, either homogeneously within the beta matrix or heterogeneously at beta grain boundaries. The extent of homogeneous vs heterogeneous alpha nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation and higher aging temperatures promote alpha heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization of the alpha phase.