Investigation of the ω-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

Abstract

The evolution of ω particles after quench at different cooling rates from 1200°C (β-B2 + α)-phase field was studied by anomalous small-angle X-ray scattering (ASAXS) near the TiK absorption edge and transmission electron microscopy (TEM). Two forms of the ω phase were observed: athermal for cooling rates R_c higher than 20°Cs⁻¹ and isothermal for lower R_c. ASAXS experiments confirmed this limiting cooling rate first 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, Acta metall., 30. 599). The size of the precipitates, the number per unit volume, the diameter of the excluded spheres, and the volume fraction of the ω phase were determined. The size of the ω particles grows proportionally to the inverse of the cooling rate. ASAXS and TEM results are in good agreement. The coarsening phenomenon involved in the B2 → ω transition was observed in both athermal and isothermal ω phases.