Effects of ω phase formation on decomposition of α"/β duplex phase structure in a metastable β Ti alloy

The effects of omega phase precipitation on the decomposition processes of alpha "/beta duplex phase structure in a Ti-10V-2Fe-3Al alloy have been investigated by means of both X-ray diffraction and transmission electron microscopy. In the specimen quenched from beta phase region, thin alpha " martensite plates form within the beta phase matrix, the structure being consisted of both alpha " and beta phases. In the retained beta phase region, however, the diffuse scattering showing the pre-precipitation stage of omega phase particles can be recognised. By ageing at temperatures from 448 to 673 K, alpha " martensite plates transform gradually to beta and spherical omega phase precipitation follows. This omega formation stabilises beta phase and retards the beta-->alpha " transformation during the subsequent cooling. In the temperature range between 573 and 673 K where both omega and alpha phases form, omega phase particles precipitate first as well-defined and homogeneously dispersed spheres about 10 nm in diameter, and then very fine alpha phase platelets 100-200 nm long precipitate with replacing omega phase particles. At temperatures above 873 K, coarse monolithic alpha phase plates precipitate directly within beta matrix without omega phase precipitation. The interplanar spacing of the retained beta phase varies with the ageing condition due to the partition of alloying elements mainly between alpha and beta phases.