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We have developed a model which is able to describe the formation of high temperature microstructures in β metastable titanium alloys. It is based on the classical theory of nucleation, and on growth laws obtained by assuming that the processes are controlled by the diffusion of the alloying chemical species only. Two morphologies are described: grain boundary allotriomorphs, and Widmanstätten plates gathered in colonies growing from the allotriomorphs. Moreover, a statistical description of the nucleation conditions on β grain boundaries has been introduced in order to obtain the global kinetics of the transformation. The model predictions proved to be in good agreement with experimental results.

The influence of transformation temperature on microtexture development associated with a precipitation at b/b grain boundaries (GB) in the near-b Ti17 alloy was studied using electron backscatter diffraction and considering isothermal treatments. For the alloy studied and the temperature range considered, decreasing the transformation temperature decreased the local microtexture strength within each prior b grain because of a larger number of a WGB colonies (standing for a Widmanstätten GB) formed per b grain, each colony increasing by one the number of a orientations inside each prior b grain. This larger number of a WGB colonies was a consequence of faster formation along b/b GB of their precursors, the allotriomorphic a GB grains (standing for a-GB) at lower transformation temperatures, as evidenced by detailed examination of the first stages of a GB formation. a GB crystallographic orientations frequently followed a variant selection (VS) criterion based on the alignment of (0 1 1)b//(0 0 0 1)a GB //(0 1 1)b. From a statistically relevant number of observations, VS was found to be more frequent at a lower transformation duration and a lower temperature, but the effect was not significant enough to influence the final a microtexture, considered at the scale of one prior b grain. a GB grains that followed the VS criterion emitted two a WGB colonies on either side of the b/b GB more frequently than those with no particular orientation.

International audienceWe have investigated the microstructure evolutions in the Ti17 near Click to view the MathML source titanium alloy during heat treatments. The phase transformation has first been studied experimentally by combining X-ray diffraction analysis, electrical resistivity and microscopy observations. From a series of isothermal treatments, a IT diagram has been determined, which takes into account the different morphologies. Then, a Johnson–Mehl–Avrami–Kolmogorov (JMAK) model has been successfully used to describe the phase transformation kinetics during either isothermal or cooling treatments. Finally, the coupling of the JMAK model to the finite element software ZeBuLoN allowed us to investigate the evolution of the spatial distribution of the different morphologies during the cooling of an aircraft engine shaft disk after forging

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Overview Processing and Characterizing Titanium AlloysThe prediction of microstructure during processing needs to characterize the phase transformation occurring during the thermal treatments and their kinetics. In-situ high-energy synchrotron x-ray diffraction experiments performed during temperature variations allow the characterization of the phase evolution. For some transformation conditions, the continuous recording of diffraction diagrams evidences clearly intermediate phases. The quantitative analysis of the diffraction diagrams gives the transformation kinetics of each phase as well as their cell parameters. Transformation kinetics obtained by this method are

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 β-metastable phase. At low temperatures and low heating rates, the hexagonal ωisophase 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 α phase. For higher heating rates or for richer composition in β-stabilizing elements of the β-metastable phase, ω phase may not form and α’’ forms directly and again transforms into α phase. A direct transformation from β-metastable to a phase is observed for the highest heating rate. The formation of the metastable ωisoand α’’ phases clearly influences the final morphology of α.