Microstructural evolution of the nickel-based superalloy AD730 during the industrial forging process ; Évolution de la microstructure du superalliage base nickel AD730 au cours des opérations de forgeage industrielles

Due to their excellent mechanical properties at temperatures up to 700°C, polycrystalline nickel-based superalloys are widely used in aero-engine turbine (or compressor) disk manufacturing. These alloys are usually processed following the conventional “cast-and-wrought” route. During this route, the cast ingot goes through a first series of forging operations which is named “conversion”. The goals of the conversion are to homogenize and refine the microstructure. It leads to a semi-finished product called billet. Then, the billet is forged again to obtain a draft of the final part. Yet, for the γ-γʹ nickel-based superalloys with high contents in alloying elements, it is common that the conversion process does not succeed in fully homogenizing the microstructure. Such is the case of the alloy AD730TM which has been recently developed by the Aubert&Duval Company. Indeed, AD730TM billets show both recrystallized equiaxed areas and characteristic recovered areas. The objective of the current PhD thesis is to understand how such microstructural heterogeneities can disappear during the last forging operations which lead to the final microstructure. First, the microstructural heterogeneities found in AD730TM billets have been characterized. Then, thermomechanical tests which aimed at simulating a forging process have been performed on billet samples in order to follow the evolutions of each local microstructure. The static (during thermal treatments) and dynamic (during deformation) evolutions of both equiaxed and recovered areas have been characterized using scanning electron microscopy, EBSD and EDS. A special attention has been paid to the influence of the γʹ precipitates on those evolutions. In particular, a specific interaction between a recrystallization front and γʹ precipitates have been studied in detail. This interaction, which has been weakly reported in literature so far, produces γʹ precipitates with either an imperfect twin orientation relationship to the matrix or an imperfect cube-cube orientation ...