Precipitation Behavior of ω Phase and ω→α Transformation in Near β Ti-5Al-5Mo-5V-1Cr-1Fe Alloy during Aging Process

In this work, the precipitation behavior of the omega phase and omega ->alpha transformation in Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy was investigated during isothermal aging at 450 degrees C. The results show that the alpha precipitates increase with the increasing of aging time, resulting from the beta ->alpha and omega ->alpha transformations. The omega ->->alpha transformation involves the formation and evolution of the isothermal omega phase. The formation of the isothermal omega phase occurs after 30 min and ends at 120 min, which is caused by the embryonic omega phase to isothermal omega phase transformation. Small angle neutron scattering (SANS) results indicates that the evolution of the isothermal omega phase goes through the increasing average size and aspect ratio from 24.7 to 47.0 nm and from 2.1 to 2.7 respectively, and the morphology evolution of the omega particle from ellipsoid to spindle-like. Moreover, the isothermal omega phase assists the alpha phase to nucleate at the omega/beta interface, which involves the changes in elemental composition. The alpha phase is enriched in Al. Compared to the alpha phase, the element of Mo, V and Cr in the isothermal omega phase is lower. The Fe element is uniformly distributed in the isothermal omega phase and beta matrix but lean in the alpha phase.