Effects of ω Phase Formation on Decomposition of α″⁄β Duplex Phase Structure in a Metastable β Ti Alloy

Abstract

The effects of ω phase precipitation on the decomposition processes of α/β 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 β phase region, thin α martensite plates form within the β phase matrix, the structure being consisted of both α and β phases. In the retained β phase region, however, the diffuse scattering showing the pre-precipitation stage of ω phase particles can be recognised. By ageing at temperatures from 448 to 673 K, α martensite plates transform gradually to β and spherical ω phase precipitation follows. This ω formation stabilises β phase and retards the β→α transformation during the subsequent cooling. In the temperature range between 573 and 673 K where both ω and α phases form, ω phase particles precipitate first as well-defined and homogeneously dispersed spheres about 10 nm in diameter, and then very fine α phase platelets 100-200 nm long precipitate with replacing ω phase particles. At temperatures above 873 K, coarse monolithic α phase plates precipitate directly within β matrix without ω phase precipitation. The interplanar spacing of the retained β phase varies with the ageing condition due to the partition of alloying elements mainly between α and β phases.