International audienceThe dynamic recrystallization mechanisms and kinetics of a recently developed γ-γ′ nickel based superalloy are studied at different subsolvus temperatures. Special care is paid to the evolution of γ′ particles and their influence on recrystallization phenomena. An original dynamic recrystallization mechanism is highlighted thanks to the combined use of Energy Dispersive Spectrometry and Electron BackScattered Diffraction mapping techniques. This mechanism leads to the formation of large heteroepitaxial pairs, made of a primary precipitate embedded in a recrystallized grain having the same crystallographic orientation. Recrystallization nuclei arise from inverse precipitation of γ phase at the rim of primary γ′ precipitates, prior to deformation. This heteroepitaxial recrystallization mechanism is reported here for the first time. It controls dynamic microstructure evolution at low strains and is then progressively replaced by the conventional discontinuous necklace recrystallization process at higher strains

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This paper demonstrates the existence of large γ' precipitates (several micrometres in diameter) that are coherent with their surrounding matrix grain in a commercial γ-γ' nickel-based superalloy. The use of combined energy dispersive X-ray spectrometry and electron backscattered diffraction (EBSD) analyses allowed for revealing that surprising feature, which was then confirmed by transmission electron microscopy (TEM). Coherency for such large second-phase particles is supported by a very low crystal lattice misfit between the two phases, which was confirmed thanks to X-ray diffractograms and TEM selected area electron diffraction patterns. Dynamic recrystallization of polycrystalline γ-γ' nickel-based superalloys has been extensively studied in terms of mechanisms and kinetics. As in many materials with low stacking fault energy, under forging conditions, the main softening mechanism is discontinuous dynamic recrystallization. This mechanism occurs with preferential nucleation on the grain boundaries of the deformed matrix. The latter is then being consumed by the growth of the newly formed grains of low energy and by nucleation that keeps generating new grains. In the case of sub-solvus forging, large γ' particles usually pin the migrating boundaries and thus limit grain growth to a size which is determined by the distribution of second-phase particles, in good agreement with the Smith-Zener model. Under particular circumstances, the driving force associated with the difference in stored energy between the growing grains and the matrix can be large enough that the pinning forces can be overcome, and some grains can then reach much larger grain sizes. In the latter exceptional case, some intragranular primary γ' particles can be observed, although they are almost exclusively located on grain boundaries and triple junctions otherwise. In both cases, primary precipitates have no special orientation relationship with the surrounding matrix grain(s). This paper demonstrates the existence of high fractions of large γ' precipitate (several micrometres in diameter) that are coherent with their surrounding matrix grain, in a commercial γ-γ' nickel-based superalloy. Such a configuration is very surprising, because there is apparently no reason for the coherency of such particles.

The creep-fatigue properties at 700°C of cast and wrought Udimet 720Li superalloy have been investigated. As expected, the introduction of a dwell time as low as 1s at maximum stress induces a creep-fatigue life debit compared to pure fatigue conditions. It is demonstrated that the mechanical behavior (plastic strain rate, ductility) is greatly affected by the grain size and precipitation distribution. Moreover, the creep-fatigue life seems to be hardly affected by the alloy microstructure, due to the development of localized damage. Whatever the environmental conditions (i.e. in air or under vacuum), failure has been identified to be intergranular.To get a better understanding of the contribution from grain boundaries to the first stage of plasticity and damage development, in-situ tensile tests were performed in a scanning electron microscope (SEM). In a first step to get a full understanding of the role of grain boundaries in controlling the mechanical properties and alloy durability, we first paid attention to the contribution of the local grain crystallography to the plasticity heterogeneity near grain boundaries. This was performed at room temperature where, compared to creepfatigue loading at 700°C, the role of viscoplasticity can be neglected. First evidences of slip activity were shown to depend only on the local Schmid factor and not on the grain size. In addition, these experiments were performed using samples with a homogeneous grain size and a homogeneous intragranular γ' particles distribution. By using the recently developed electron backscattered diffraction (EBSD) pattern cross-correlation technique, it has been shown that critical grain configurations exist, which lead to the activation of micro-volumes (~ 1-2 µm 3 at maximum) where intense plastic activity develop near grain boundaries. These volumes nucleate at grain boundaries separating two grains having a slip activity contrast: they develop in the grain less favorably oriented for slip, ahead of slip bands which had developed in the grain with easier slip. It is observed that the very first stages of development of intense plasticity micro-volumes are quite insensitive to a contrast in Schmid factors between neighboring grains. The local von Mises stress magnitude in these volumes can be as large as several GPa, sufficient enough to develop micro-cracking.