Phase transformations and mechanical properties of the Ti-5553 beta-metastable titanium alloy

(eng) To compete with other structural materials despite their high cost, titanium alloys have to provide a significant performance advantage. A thorough understanding of the physical metallurgy of the alloys is therefore needed for an advanced optimization. The performance-driven approach taken by the titanium industry leads to the frequent development of new alloys, like the Ti-5553. The microstructure investigation carried out in this work deals with the formation of the alpha phase during isothermal heat- and thermomechanical treatments. Various kinds of defects are controlling the heterogeneous nucleation of alpha particles. Some treatments were specially designed to highlight a particular nucleation mechanism, like ageing on pre-bent specimens for the influence of deformation bands, or the use of fast quenching for the trapping of excess vacancies. However, the isothermal ageing treatments can only represent a part of the picture: As the heating and cooling rates cannot be infinite, they influence the phase transformations. In situ techniques are thus needed for monitoring the phase transformation during continuous heating and cooling. The free vibrational method, measuring the change of elastic modulus and the wave damping during heat treatment, is compared to other more conventional techniques like DSC, dilatometry, or post mortem metallography and hardness. The relationship between microstructure and mechanical properties was addressed at several levels. At the local scale, the nano-indentation technique was coupled with SPM imaging and EBSD to measure the hardness and elastic modulus of each phase, taking into account phase boundaries and crystal orientation. At the macro scale, tensile tests on notched specimens were used to link the microstructural features, the stress state and the fracture strain via a micromechanical model based on the growth of cavities nucleated on weaker points of the microstructure. The Ti-5553 alloy presents a very specific behavior compared to other titanium alloys. High temperature tensile tests were also performed in order to extract flow stresses, strain-rate sensitivity coefficients, activation energies for plastic deformation, for strain-rates ranging from creep to dynamic testing.