This paper discusses the structural and compositional changes at the nanometer scale associated with the nucleation and growth of alpha precipitates in the beta titanium alloy Ti-5553 (Ti-5Al-5Mo-5 V-3Cr-O.5Fe) with omega precipitates acting as heterogeneous nucleation sites. The microstructural evolution in this alloy, during beta-solutionizing, quenching and aging type heat-treatments, lias been investigated by combining results from scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy, high-resolution TEM and three-dimensional atom probe (3DAP) tomography. Athermal omega precipitates form in this alloy on quenching from above the beta transits temperature. On isothermal annealing at low temperatures, these omega precipitates coarsen to form chemically ordered omega precipitates, accompanied by the nucleation of the stable alpha phase. Annealing at higher temperatures leads to dissolution of omega and further growth of alpha precipitates accompanied by clustering of different a variants in self-accommodating morphologies. 3DAP results indicate that annealing at lower temperatures (approx 350 deg C) leads to initial nucleation of a precipitates with a non-equilibrium composition, nearly identical to that of the 3 matrix. Subsequent aging at higher temperatures (approx 600 deg C) leads to more pronounced partitioning of alloying elements between the two phases. These results indicate that the structural body-centered cubic to hexagonal close-packed transformation and the compositional partitioning of alloying elements occur in sequential steps, resulting in a mixed-mode dispiacive-diffusional transformation, similar to the bainite transformation in steels.
展开▼
