Precipitation in a near β titanium alloy on ageing: Influence of heating rate and chemical composition of the β-metastable phase

摘要

In the present study we focus on the precipitation processes during heating and ageing of β-metastable phase in the near β Ti-5553 alloy. Transformation processes have been studied using continuous high energy X-Ray Diffraction (XRD) and electrical resistivity for two different states of the β-metastable phase. Microstructures have been observed by electron microscopy. Different transformation sequences are highlighted depending on both heating rate and chemical composition of the P-metastable phase. At low temperatures and low heating rates, the hexagonal ω_(iso) phase is first formed as generally mentioned in the literature. Increasing the temperature, XRD evidences the formation of an orthorhombic phase (α"), which evolves toward the hexagonal pseudo compact a phase. For higher heating rates or for richer composition in β-stabilizing elements of the β-metastable phase, ω phase may not form and a" forms directly and again transforms into a phase. A direct transformation from β-metastable to a phase is observed for the highest heating rate. The formation of the metastable ω_(iso) and α" phases clearly influences the final morphology of α.