• Teixeira, Julien

Abstract

The mechanical properties of the Ti17 alloy are strongly linked to the size, the morphology and the distribution of the α precipitates inside the β matrix. These microstructural parameters are dependent on the thermomechnical treatment applied to the part. Thus the control of the mechanical properties and the improvement of the processes goes through the knowledge of the final microstructure as well its evolution when applying these processes. For this purpose, two modeling approaches are developed, in the case of the thermal treatment following the high temperature forging:<br /> The first approach lies on the Johnson – Mehl – Avrami – Kolmogorov (JMAK) law, which allows the prediction of each α phase morphology amount, at a macroscopic scale. The metallurgical calculation needs the prior measurement of the isothermal transformation kinetics, in order to determine the JMAK law parameters. Controlled cooling treatments are simulated, the comparison with the experiment being conclusive. The case of large parts with temperature and microstructure gradients is then treated. The thermophysical data of the alloy and the transformation enthalpy are first measured, and coupled temperature and microstructure evolutions are predicted with the Finite Element calculation software ZeBuLoN, inside which we implemented our metallurgical model. The simulation results are compared to experimental results obtained for large cylinders equipped with thermocouples during the cooling. The obtained modelling tool is then applied to an industrial piece.<br /> The second approach lies on the modeling of the nucleation and growth of the α precipitates on the grain boundaries. This calculation at the microscopic scale of the precipitates allows to predict the influence of the plastic deformation in the high temperature β phase field. The model uses β microstructure parameters after the deformation (for instance the size of recristallized grain), in function of the deformation conditions. Transformation kinetics are predicted for several temperature controlled treatments, following either a simple solution treatment or a plastic deformation at given conditions. Concerning the influence of a prior plastic deformation, the comparison with experimental results in the literature is conclusive. Moreover, the model is useful for other alloys (knowledge of the equilibrium phases composition and of the diffusion coefficients) and it allows to accede to microstructural parameters as the precipitate size.

Topics

• impedance spectroscopy
• morphology
• polymer
• grain
• phase
• experiment
• simulation
• precipitate
• titanium
• titanium alloy
• forging