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

Abstract

Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature range from 300 °C to 750 °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 ω phase. This was followed by formation of the α phase, either in association with the ω phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly, isothermal decomposition of the metastable β phase resulted in the precipitation of ω phase exhibiting an ellipsoidal morphology. While precipitation of ω was immediate at 345 °C, an incubation period was observed upon aging at 390 °C. Isothermal aging above this temperature involved direct precipitation of the α phase, either homogeneously within the β matrix or heterogeneously at β grain boundaries. The extent of homogeneous vs heterogeneous α nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation and higher aging temperatures promote α heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization of the α phase.