Investigation of the omega-phase precipitation in the 0.506 at.% Ti-0.129 at.% Nb-0.365 at.% Al alloy by transmission electron microscopy and anomalous small-angle X-ray scattering

The evolution of omega particles after quench at different cooling rates fr om 1200 degrees C (beta-B2 + alpha)-phase field was studied by anomalous sm all-angle X-ray scattering (ASAXS) near the TiK absorption edge and transmi ssion electron microscopy (TEM). Two forms of the omega phase were observed : athermal for cooling rates R-c higher than 20 degrees Cs-1 and isothermal for lower R-c. ASAXS experiments confirmed this limiting cooling rate firs t determined by TEM. The experimental ASAXS curves were first analysed with the classical two-phase model of unmixing in order to obtain input data to introduce into the fitting with the procedure of Hennion et al. (1982, Act a metall., 30, 599). The size of the precipitates, the number per unit volu me, the diameter of the excluded spheres, and the volume fraction of the om ega phase were determined. The size of the w particles grows proportionally to the inverse of the cooling rate. ASAXS and TEM results are in good agre ement. The coarsening phenomenon involved in the B2 --> omega transition wa s observed in both athermal and isothermal omega phases.