ALTITUDE

Reducing polluting emissions is a major strategic challenge for the aviation industry. For that purpose, aerospace engine and airframe designers are constantly seeking lighter materials with high strength at higher service temperatures to improve fuel efficiency and to reduce aircraft weight. Thanks to their excellent specific strength, good damage tolerance and good corrosion resistance, titanium alloys are particularly attractive for aeronautical applications. However, their excellent properties deteriorate above 550°C: pushing this current upper limit by maintaining the desired properties above 650°C constitutes a strong scientific challenge. The present project aims at developing a novel titanium alloy able to meet this challenge by exhibiting good resistance to fatigue-creep interaction (dwell sensitive fatigue) as well as good mechanical properties up to at least 650 °C. At these temperatures, oxidation is an issue that will have to be taken into account. To reach this goal, the project will rely on some peculiar properties of several alloying elements: Mo for its beneficial effect on dwell fatigue resistance; C for its capability to improve the ability of Ti alloys to heat treatments at high temperatures; finally, Si combined with Ge, which are expected to improve the mechanical properties at the highest temperatures. Furthermore, assessing the possible effect of these elements on oxidation is an important point that will be undertaken. The project will be organised in two steps. At the first step, thermodynamic calculations together with characterization of the microstructures and properties of a dozen of alloys elaborated specifically for this step will permit to select promising compositions. We will investigate the influence of Mo, C, Si and Ge on (i) the formation and evolution of the microstructures, (ii) the associated mechanical properties (e.g. tensile strength at room and high temperatures), (iii) and the oxidation resistance, especially after ageing treatments. This first step will provide the 4 most promising alloys. At the second step, a deep investigation of the transformation kinetics, the resulting microstructure and their resistance to the environment will be carried out. We will pay a particular attention to the role played by Si, Ge and C in the mechanisms of the phase transformations, of oxidation, and of hot deformation. These new insights will be used in the modeling of some of these phenomena. Finally, the properties of the 4 alloys will be assessed in close-to-service conditions: in particular, creep fatigue (dwell effect), and fatigue crack propagation behaviour at high temperatures in connection with the oxidation behaviour. The set of results of the second step will permit to propose the best alloy capable of meeting the end-users requirements in terms of mechanical and environmental properties. The present project involves end-users (SAFRAN, AIRBUS), a titanium alloy producer (TIMET), an aeronautical research centre (ONERA) and academic partners specialized in metallurgy of titanium alloy (Institut Jean Lamour, Lorraine University) and oxidation mechanisms (Laboratory Interdisciplinaire Carnot de Bourgogne, Bourgogne-Franche Comté University). Hence, the composition of the consortium perfectly matches the final goal of the project.