Abstract : Industrialists of the aeronautic and energy fields are persistently trying to increase their turbomachine in-service temperatures in order to improve the engine efficiency and to decrease polluting emissions. This thesis deals with deformation mechanisms and damage processes that appears at 700°C under fatigue, creep and creep-fatigue conditions in two different metallurgical structures of the Udimet 720 alloy. Mechanical behaviour differences have been analyzed through their dislocation microstructures and the evolution of the mechanisms operating at this scale. The influence of test conditions (s, Dt) and of the different scales g' precipitation has been detailed especially concerning the transition between shearing and by-passing processes of the g' precipitates. Besides, cracks initiation and propagation processes have been characterized. If an inter-transgranular transition in cracks growth has been observed under particular test conditions, the damage of grain boundaries and twin boundaries dominates. Moreover, complex creep-fatigue-oxidation interactions have been highlighted by equivalent mechanical tests conducted under air and vacuum. At high temperature, the microstructure of this alloy at the precipitation scale is not stable. Ageing treatment effects have been quantified as well as their repercussions on the mechanical behaviour and the durability. The homogeneous and heterogeneous metallurgical structures have been systematically compared.