ω Precipitation in a Beta Metastable Titanium Alloy, Resistometric Study

Abstract

Beta-titanium alloys show an excellent capability for precipitation hardening. The microstructural evolution i.e. the nature, the morphology, and the degree of coherency of the phases formed during the transformations of the beta phase strongly determines the characteristics of the alloys. It is known that ω precipitates can be formed by: (i) A diffusionless martensitic mechanism during quenching; (ii) a diffusion controlled process during aging; (iii) A stress induced process. The formation of a large volume fraction of ω phase is generally recognised to lead to a drastic reduction in ductility; nevertheless ω crystals are considered as precursors for the α phase at the intermediate aging temperature, allowing in that way the mechanical properties of the (β+α) duplex microstructure to be improved. In the present paper, ω phase precipitation is studied in Ti–6.8Mo–4.5Fe–1.5Al (Low Cost Beta) using a four points resistometric method, as follows: 1-Precipitation ranges are studied during anisothermal aging, 2-Isothermal ω phase is studied during aging between 473 and 523 K and a growth model is proposed, 3-ω phase disappearance is studied and a mechanism linking ω_iso and α precipitation is discussed.