Designing microstructure of components made from Inconel 718 nickel based superalloy (IN718) with tailored mechanical properties for high temperature applications, require sequential thermo-mechanical processing. This often includes straining and annealing at solution annealing temperature (i.e. ≈980℃) followed by water quenching and subsequent aging heat treatments at lower temperatures. In addition to the microstructure development (i.e. precipitation) the aging heat treatment partially relieve the residual stresses generated at previous stages of forging and water quenching, however the stress field will not be completely relaxed. In this study, a series of experiments were conducted on round tensile specimens made from IN718 bar to investigate tensile stress relaxation behaviours at elevated temperatures used for aging heat treatments. The stress relaxation curves obtained can be described by a hyperbolic function with a non-zero asymptotic stress (σ∞), which seems to be proportional to the initially applied stress (σ0) for a fixed temperature. This behaviour is investigated at temperatures between 620℃ and 770℃ that is a temperature range used in industry to perform the aging heat treatments to obtain microstructures with tailored mechanical properties. It has been shown that the σ∞/ σ0 ratio has decreased rapidly with increasing temperature at this range. The relaxation behaviour has been assessed numerically and an empirical relationship has been defined for each temperature that can be used for modelling purposes
Hot deformation tests of superalloy 718 were carried out both in torsion and uniaxial compression in order to investigate the microstructural evolutions of this alloy. Post-deformation holding times were also performed. The obtained dynamic and metadynamic-most often partially-recrystallized microstructures were characterized using the EBSD technique. Particular attention was paid to the local crystallographic rotation of specific subgrains as it leads to the nucleation of most of the first new grains. Twin boundary generation, and consecutive disorientation, for producing the second and further generations of recrystallized grains, were also studied. Thanks to these data, an empirical model, based on Avrami formulation, was developed. Furthermore, as δ phase (Ni 3 Nb) precipitation can modify significantly the recrystallization and subsequent grain growth kinetics of alloy 718, a mechanism-based model was designed to take into account the coupling effects of recrystallization and δ phase precipitation.
In a polycrystalline γ-γ' nickel-based superalloy, γ' precipitates with a close-to-twin orientation relationship to their surrounding matrix (named T-type precipitates) are found in recrystallized grains located near specific unrecrystallized grains (named recovered grains). This type of γ/γ' interface has not been reported in literature before. Inherited from the ingot conversion process, recovered grains are characterized by a high density of micrometric and close-to-coherent γ' precipitates. Resulting from the interaction of the recrystallization front with the latter precipitates, the T-type precipitates appear to form onto the recrystallization front. The present paper details the crystallographic characteristics of the T-type precipitates. KeywordsGamma -Gamma Prime nickel-based superalloy, Interphase boundary, Twin boundary, Recrystallization.Polycrystalline γ-γ' nickel-based superalloys are commonly used to manufacture aircraft engine rotative parts due to their good mechanical behavior at high temperature (tensile, fatigue, creep resistance) [1]. Parts are usually derived from cast ingots through forging sequences [2,3]. They are made of a γ matrix in which γ' precipitates of various sizes are distributed. Both γ and γ' phases have a cubic structure but, while the γ phase is a Face Centered Cubic (FCC) non-ordered solid solution, the γ'-Ni 3 (Al,Ti) phase has a L1 2 ordered structure. Yet, the mechanical properties of the alloy do not only depend on the size and spatial distribution of the precipitates [4], but also on the γ/γ' interface characteristics [5,6]. Indeed, γ' precipitates are reported in literature as coherent, semi-coherent or incoherent to their surrounding matrix [7]. Coherency means that the γ and γ' crystal lattices perfectly coincide at the γ/γ' interface. This is made possible by a low lattice parameter mismatch between the γ and γ' phases [8]. Coherent γ/γ' interfaces have very low energies [9,10] and form during the cooling stages of the forging process for example. However, if some misfit dislocations pile up at the γ/γ' interface, the lattice matching is only partial and the interface is semi-coherent [7]. Semi-coherency occurs when external stresses (high temperature plastic deformation) or internal stresses (e.g. induced by long aging treatments leading to particle coarsening) generate dislocation loops which are incorporated into the γ/γ' interface as misfit dislocations [11]. Finally, when there is no crystal lattice matching, the γ/γ' interface is incoherent and has a high energy [9]. 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 recrystallizat...
Second phase particles (SPP) play an essential role in controlling grain size and properties of polycrystalline nickel base superalloys. The understanding of the behavior of these precipitates is of prime importance in predicting microstructure evolutions. The dissolution kinetics of the primary γ′ precipitates during subsolvus solution treatments were investigated for three nickel base superalloys (René 65, AD730 and N19). A temperature-time codependency equation was established to describe the evolution of primary γ′ precipitates of each material using experimental data, the Thermo-Calc software and the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The dissolution kinetics of precipitates was also simulated using the level-set (LS) method and the former phenomenological model. The precipitates are represented using an additional LS function and a numerical treatment around grain boundaries in the vicinity of the precipitates is applied to reproduce their pinning pressure correctly. Thus, considering the actual precipitate dissolution, these simulations aim to predict grain size evolution in the transient and stable states. Furthermore, it is illustrated how a population of Prior Particle Boundaries (PPB) particles can be considered in the numerical framework in order to reproduce the grain size evolution in the powder metallurgy N19 superalloy. The proposed full-field strategy is validated and the obtained results are in good agreement with experimental data regarding the precipitates and grain size.
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