Evidence of multimicrometric coherent γ′ precipitates in a hot‐forged γ–γ′ nickel‐based superalloy

Abstract

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.

Accurate reconstruction of EBSD datasets by a multimodal data approach using an evolutionary algorithm

“…However, it can be computationally expensive and -this far-has not been able to differentiate γ and γ phases. Combined EDS-EBSD methods have been used in the last years but suffer from long indexing times (and so, increased beam drifting), post-processing times and limited resolution [12]. Using a completely different technique but a similar approach to dictionary indexing, with ion channeling imaging, the iCHORD method of Langlois et al [35] has been shown to enable the differentiation of phases [36].…”

Heteroepitaxial Recrystallization, a New Recrystallization Mechanism in Sub-Solvus Forged γ-γ' Nickel-Based Superalloys With Low Lattice Mismatch

“…Following this thermal treatment, cylindrical Udimet 720 samples (6 mm in diameter, 9 mm in height) were deformed by hot-compression up to a height reduction of 2:1 under the following conditions: 1000°C-0.1s -1 ; 1040°C-0.1s -1 and 1040°C-0.01s -1 . All samples were mechanically polished using diamond suspensions down to 1 µm in size and characterized using the coupled EDS-EBSD method described in [12] that allows for separating primary γ' precipitates from the γ matrix . Microstructure was characterized at the center of the deformed samples, where the strain level is the highest.…”

“…This mechanism is the dominant recrystallization mechanism at low strain levels (typically ε<0.6). Since a low lattice mismatch between γ and γ' phases is a necessary condition for the development of such large heteroepitaxial features [12], the possible occurrence of heteroepitaxial recrystallization in other γ-γ' superalloys with low lattice mismatch is investigated in the present article.…”

γ′ precipitates with a twin orientation relationship to their hosting grain in a γ-γ′ nickel-based superalloy

“…Secondary γ' precipitates which form during the cooling stages of the forming process, are coherent with a cube-cube orientation relationship: they evolve from spheres to {100} bounded cubes as their size and/or the lattice misfit increase [12]. On the other hand, primary γ' precipitates, derived from the as-cast microstructure and high temperature billet forging sequences, are usually incoherent with no special orientation relationship, excepted in case of heteroepitaxial recrystallization where primary precipitates are found coherent with an orientation very close to that of their surrounding matrix [13][14][15]. The present work points out another type of γ/γ' interface which, to the best knowledge of the authors, has not been reported in literature before: γ' precipitates with a close-to-twin orientation relationship to their surrounding γ matrix have been evidenced.…”

“…The first recrystallization mechanism is the Continuous Recrystallization (CRX), which means that cells resulting from the fragmentation of the grain under deformation recover and become new recrystallized grains once they are sufficiently misoriented with the surrounding matrix [22] (white arrow in Figure 1.d). The second mechanism is the HeteroEpitaxial Recrystallization (HERX) [13][14][15]: some γ' precipitates inside the recovered grain become surrounded by a coherent γ envelop of same crystalline orientation, this γ envelop can then grow in the hardened matrix as a new recrystallized grain (green arrow in Figure 1.d). For both recrystallization mechanisms, the newly recrystallized grains have orientations which are close to that of the nearby recovered grain, and consequently have all four <111> axes initially close to those of the recovered grain.…”