Microstructure et usinabilité d'alliages de titane béta-métastable

Titanium has become in recent years an interesting material in many applications that require a combination of high mechanical properties and low density. It has the peculiarity of undergoing an allotropic transformation at 882°C. Below this temperature, the alpha hexagonal phase is stable while above this temperature, the beta body-centered cubic is stable. beta-metastable titanium alloys are a new generation of titanium alloys that combines properties of both alpha and beta phases to optimize mechanical properties. Those alloys have a fully beta microstructure after quenching and the alpha phase can nucleate during an annealing. The size and homogeneity of the alpha phase depend on the nucleation of a prior omega phase at lower temperature. A double-steps heat treatment is often necessary to obtain such microstructure.This work is focused on the analysis of the microstructures and the related mechanical properties of three titanium alloys: Ti-15333, Ti-4733 and Ti-5553. The goal is to develop a thermomechanical treatment to optimize the mechanical properties of those alloys. It will also show the application of mechanical spectroscopy measurement for such optimization. Two peaks in internal friction have been discovered and are related with the nucleation of the phases omega (P1) and alpha (P2). Critical temperatures for those transformations were identified using internal friction. Transmission electron microscopy images and diffractions confirmed the nucleation of the omega and alpha phases. It was also showed that omega phase act as nucleation sites for the alpha phase and their presence lead to thinner and more homogeneous alpha precipitates. A recrystallization peak Pr was discovered in cold rolled samples. The recrystallization phenomenon was discovered to be dependent of the dissolution of the alpha phase at high temperature. It is proposed that the sudden increase of the dislocations' mobility after dissolution of alpha triggers the recrystallization. This result led to the development of a new grains refinement process for those alloys. Based on the internal friction results, a new thermomechanical treatment with four steps is proposed for those alloys: Plastic deformation, recrystallization, omega nucleation and alpha nucleation. The impact of beta grains refinement and alpha nucleation with optimal configuration was verified with hardness and traction tests. Many microstructures resulting from the different thermomechanical treatment were tested. Results show that beta grains refinement increases hardness, yield stress and ductility. In addition, a thin and homogeneous alpha phase is the most important parameter to achieve high hardness. Those results could lead to new methodology for analyzing phase transformation and mechanical properties optimization in beta-metastable alloys.