Polycrystalline Ni-based Superalloys Research Articles

On the Rate Dependence of Precipitate Formation and Dissolution in a Nickel-Base Superalloy

The temporal dependence of gamma ' dissolution in the polycrystalline Ni-base superalloy RR1000 has been studied with implications to thermo-mechanical processing. A resistivity-based method using an electro-thermal mechanical testing (ETMT), which overcomes the drawbacks associated with other approaches, such as calorimetry, dilatometry, and diffraction, has been used to explore the effect of transient and isothermal thermal cycles. This is supplemented by DICTRA numerical models that simulate the diffusion within the gamma phase up to the gamma /gamma ' interface. It is demonstrated that dissolution is affected by heating rate as well as the precipitate size. Below a threshold heating rate of sim 0.1 {^circ }C s^{-1}, the dissolution kinetics are marginally affected, however, is sensitive to microstructure. The role of precipitate size during dissolution is governed by diffusion flux in the gamma phase at the gamma /gamma ' interface, which is inversely proportional to size. It is argued that numerical simulations that predict constitutional liquation during rapid heating by altering the width of the computation domain to match the average precipitate size of the gamma ' population will yield inaccurate predictions. The influence of the heating rate on the nucleation undercooling, during subsequent cooling, has also been addressed. With increasing heating rates, the local gamma ' solvus temperature is shifted to progressively higher temperatures. Unless complete dissolution of gamma ' occurs prior to subsequent cooling, erroneous interpretations of nucleation undercooling can arise.

Twin boundary fatigue crack nucleation in a polycrystalline Nickel superalloy containing non-metallic inclusions

Non-metallic inclusions and twin boundaries are preferred fatigue crack nucleation locations in polycrystalline Ni-based superalloys. A Heaviside HR-DIC, EBSD and multi-scale modelling strategy (CPFE-DDP) was used to assess experimental observations of fatigue crack nucleation near agglomerated inclusions in RR1000 at elevated temperature. Inclusion fracture and decohesion were observed within the first cycle of loading. Fatigue crack nucleation at the non-metallic inclusion coincided with that in an adjacent coarse grain containing a twin boundary. DDP modelling of the twin boundary showed the establishment of slip activation parallel as well as oblique to the interface, as observed in DIC characterisation. The DDP results showed pile-up driven generation of local GND density at the interface, in turn driving high local stored energy density and fatigue crack nucleation. These conditions were shown to result from the anisotropic elastic constraint at the twin boundary. In addition, the complex stress field arising from the agglomerate drives plasticity near the twin boundary contributing to the necessary conditions for fatigue crack nucleation.

Effect of Loading and Unloading Strain Rates on Creep-Fatigue Damage of Polycrystalline Ni-Based Superalloy at Elevated Temperatures

Ni-based superalloys have been subjected to random creep-fatigue load in high-temperature environment to compensate the fluctuation of the output of renewable energies. However, under the creep-fatigue condition, intergranular cracking was found to be accelerated, and the fracture life of the alloy was significantly decreased. Therefore, it is essential to elucidate the mechanism of this acceleration of crack propagation rate of the creep-fatigue damage. In this study, intermittent creep-fatigue tests were applied to Alloy 617, one of the Ni-based superalloys, and the degradation process of its crystallinity was monitored by IQ (Image Quality) values obtained by EBSD (Electron Back-Scatter Diffraction) method. In particular, the effect of strain rate during loading and unloading processes were evaluated in detail. The results showed that under creep-fatigue loading, the faster the strain rate during unloading, the earlier the IQ value decreased and the shorter the time for the initiation of intergranular cracking. It was also found that intergranular cracking appeared when the IQ value reached a critical value regardless of the strain rate.

A unified dislocation density-based model for an aged polycrystalline Ni-based superalloy considering the coupled effects of complicate deformation mechanisms and initial δ phase

The flow characteristics and microstructure evolution of an aged nickel-based superalloy are researched by thermal compression experiments at different temperatures (1223–1283 K) and strain rates (0.001–0.1 s−1). Based on the theoretical analysis, the dynamic recrystallization kinetics and the evolution of grain size are quantitatively modeled. A unified dislocation density-based model is developed to characterize the flow features of the researched superalloy. In the developed model, the work hardening effect is taken into account by involving the coupled effects between the dislocations and substructures/grain boundaries/δ phases. Especially, the initial contents/morphologies of δ phase are reasonably considered. The dynamic softening mechanisms including dynamic recrystallization and dynamic recovery are considered. The comparisons between the predicted and experimental data indicate that the developed unified model can make accurate predictions for the flow stresses of the aged nickel-based superalloy at constant or variable strain rates. Also, it is found that the morphology of δ phase gradually transforms from spherical to short-rod-like or needle-like with the increased aging time. The dissolution and bend of δ phase induced by dislocation movement easily occur. Besides, the DRX behavior is accelerated, and the grains are refined as the aging time is increased.

Fatigue deformation behavior of revert Ni-based superalloys via electron beam technology at low and middle temperature

In this work, we investigated the low-cycle-fatigue (LCF) at room temperature (RT) and 750 °C of 100% revert commercial polycrystalline Ni-based superalloy (K417) blade re-melted by electron beam technology (EBT). The fatigue life of EBR-K417 reaches 2324, 6840, and 6925 cycles at 750 °C during the LCF, which is substantially improved compared with the as-prepared K417 (1050 cycles). Surface or subsurface defects became primary fatigue sources instead of inclusions inside alloy in purified revert K417, which is suggested to be the main reason for prolonging the fatigue life. In addition, the results and analysis show that the dislocation movement in K417 alloy at room temperature (RT) is mainly through strong-coupled-dislocation (SCD) shearing mechanism, and its contribution to yield stress is 203 MPa. Whereas the dislocation migration mechanism at 750 °C is mainly bypassing and cross slip, whose contribution to yield stress is 156 MPa. The theoretic critical resolved shearing stress (CRSS) for temperature at RT and 750 °C are calculated as 249 MPa and 198 MPa, which are very close numerically with the experimental CRSS values, 230.4 MPa and 204.6 MPa.

A Data-Driven Bayesian Model for Predicting Fatigue Crack Nucleation in Polycrystalline Ni-Based Superalloys

This paper develops a Bayesian, probabilistic crack nucleation model in the Ni-based superalloy Ren\'e 88DT for fatigue loading. A data-driven, machine learning approach is developed to identify the underlying mechanics driving crack nucleation. An experimental set of fatigue-loaded microstructures is characterized near crack nucleation sites using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) images to correlate grain morphology and crystallography to the spatial location of crack nucleation sites. A concurrent multiscale model that embeds polycrystalline microstructures, created from the EBSD images, in a self-consistent homogenized material is developed for low cycle fatigue simulations needed to create a database of state variables. The polycrystalline domain is modeled by a crystal plasticity finite element model (CPFEM), while a homogenized anisotropic plasticity model is used for the exterior domain. A Bayesian classification method is introduced to optimally select the most informative state variable predictors of crack nucleation and constructs a near-Pareto frontier of models with varying complexity. From this principal set of state variables, a simplified scalar crack nucleation indicator is formulated which encompasses all of the relevant components derived from the main discriminators. This Bayesian approach allows the micromechanical state variables responsible for causing crack nucleation events to come out naturally from existing data. The final result is a model that predicts the probability of nucleating a crack at a microstructural location, given the mechanical state of the material.

MULTIPLE REGRESSION MODELING THE CHEMICAL COMPOSITION EFFECTS ON THE TENSILE STRENGTH OF Ni-BASED SUPERALLOYS

Based on the previously obtained results of the Data Mining - based Classification and Regression Trees (C&RT) technique application to industrial Ni-based superalloys, multiple regression models were developed to describe quantitatively the chemical composition effects on the alloys tensile strength. The regression models obtained were verified for adequacy in three ways, contrary to the conventional approach requiring only several inadequacy variance values calculations. Namely, the following verification methods were applied: traditional R2 coefficient calculations; comparisons of computer and experimental frequency distributions of the quality indexes; Monte-Carlo simulated and experimental data based scatter plots comparing. It was emphasized that, according to the general statistic principle, close coincidence of the Monte-Carlo simulated and real frequency distributions of any measured characteristic values means full formal (statistical) identity of the real and simulated objects. Hence, in addition, coincidence of the corresponding scatter plots statistical parameters should be considered as a sign of the physical identity of real and modeled regression dependencies. All the applied verification methods show the excellent results that leads to conclusion about high workability of the obtained models in practical and theoretical applications. An additional general advantage of the models was noted: their physically interpretable structure, that provides the opportunity to determine and analyze the effects of individual chemical elements and their interactions under real industrial conditions. Most of the obtained results and the formulated conclusions, especially related to the chemical elements individual effects are in good accordance with the known experimental and theoretic data for polycrystalline Ni-based superalloys. Meantime, some unexpected features, particularly, for the interactions of Nb, Ti, Cu with each other and grain boundaries were revealed, showing necessity of further in dept investigations.

Deformation Mechanisms in Ni-Based Superalloys at Room and Elevated Temperatures Studied by In Situ Neutron Diffraction and Electron Microscopy

Polycrystalline Ni-based superalloys are one of the most frequently used materials for high temperature load-bearing applications due to their superior mechanical strength and chemical resistance. In this paper, we presented an in situ diffraction study on the tensile deformation behavior of the polycrystalline Ni-based superalloy VDM® Alloy 780 at temperatures up to 500 °C performed at the STRESS-SPEC neutron diffractometer at the Heinz Maier-Leibnitz Zentrum. A detailed microstructural investigation was carried out by electron microscopy before and after testing. The results of these studies allowed us to determine the deformation mechanism in the differently orientated grains. It is shown that the deformation behavior, which is mainly dislocation motion and shearing of the γ′-precipitates, does not change at this temperature range. The deformation is strongly anisotropic and depends on the grain orientation. The macroscopic hardening can mainly be attributed to plastic deformation in grains, where the (200) lattice planes were orientated perpendicular to the loading direction. Accordingly, a remaining lattice strain and high dislocation density were detected predominantly in these grains.

Characterization of oxidation mechanisms in a family of polycrystalline chromia-forming nickel-base superalloys

The oxidation properties of a family of novel polycrystalline Ni-based superalloys with varying Ti:Nb ratio have been studied, which has shown a correlation between increasing titanium content and accelerated oxidation kinetics. High-resolution characterization of microstructure and chemistry was carried out on the resultant oxide layers using SEM/EDX and Atom Probe Tomography to precisely locate Ti segregation within chromia scales, in order to identify a potential mechanism to explain this correlation. Despite some spread in the data, levels of Ti dissolved in the chromia oxide scales showed little correlation with the nominal Ti concentration or the oxidation properties of each alloy, indicating that oxidation rates are not reliant on dopant levels within bulk chromia. A number of grain boundaries within the oxide scale were targeted for APT analysis, as these are known to act as short-circuit diffusion paths. Segregation of Ti, Nb and Ta to oxide grain boundaries was observed, suggesting that ion transport rates may instead be mediated by dopants present at grain boundaries rather than dissolved within the bulk.

Fundamental Study on the Mechanism of Intergranular Cracking in Polycrystalline Ni-Based Superalloys by using Molecular Dynamics Analysis

Fundamental Study on the Mechanism of Intergranular Cracking in Polycrystalline Ni-Based Superalloys by using Molecular Dynamics Analysis

Enhanced creep performance in a polycrystalline superalloy driven by atomic-scale phase transformation along planar faults

Predicting the mechanical failure of parts in service requires understanding their deformation behavior, and associated dynamic microstructural evolution up to the near-atomic scale. Solutes are known to interact with defects generated by plastic deformation, thereby affecting their displacement throughout the microstructure and hence the material's mechanical response to solicitation. This effect is studied here in a polycrystalline Ni-based superalloy with two different Nb contents that lead to a significant change in their creep lifetime. Creep testing at 750°C and 600 MPa shows that the high-Nb alloy performs better in terms of creep strain rate. Considering the similar initial microstructures, the difference in mechanical behavior is attributed to a phase transformation that occurs along planar faults, controlled by the different types of stacking faults and alloy composition. Electron channeling contrast imaging reveals the presence of stacking faults in both alloys. Microtwinning is observed only in the low-Nb alloy, rationalizing in part the higher creep strain rate. In the high-Nb alloy, atom probe tomography evidences two different types of stacking faults based on their partitioning behavior. Superlattice intrinsic stacking faults were found enriched in Nb, Co, Cr and Mo while only Nb and Co was segregated at superlattice extrinsic stacking faults. Based on their composition, a local phase transformation occurring along the faults is suggested, resulting in slower creep strain rate in the high-Nb alloy. In comparison, mainly superlattice intrinsic stacking faults enriched in Co, Cr, Nb and Mo were found in the low-Nb alloy. Following the results presented here, and those available in the literature, an atomic-scale driven alloy design approach that controls and promotes local phase transformation along planar faults at 750°C is proposed, aiming to design superalloys with enhanced creep resistance.

Insight into microstructure-sensitive elastic strain concentrations from integrated computational modeling and digital image correlation

The microstructural origins of highly localized elastic strain concentrations in polycrystalline microstructures under monotonic loading are studied using grain-scale, in situ digital image correlation and crystal plasticity finite element method. It is shown that the locations of exceptionally high elastic strain concentrations in the microstructure depend on particular crystallographic and morphological orientations of grains and less so on crystalline details of their local neighborhood. Based on these results, we discuss how topological and crystallographic features of annealing twin boundaries can increase the likelihood of slip band initiation throughout the microstructure of polycrystalline Ni-base superalloys.

Nucleation mechanism of hetero-epitaxial recrystallization in wrought nickel-based superalloys

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050°C and cooled at 5°C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

Nucleation Mechanism of Hetero-Epitaxial Recrystallization in Wrought Nickel-Based Superalloys

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050 o C and cooled at 5 o C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

Mechanistic Modeling of Strain Hardening in Ni-Based Superalloys

This article presents a mechanistic approach for modeling the strain hardening response of polycrystalline Ni-based superalloys such as ME3, RR 1000, Alloy 720 Li, and IN 100. The mechanistic approach considers strain hardening in Ni-based superalloys in two stages: (a) self-hardening of individual {111} slip systems in the low plastic strain regime and (2) latent hardening of multiple {111} slip systems in the high plastic strain regime. Both strain hardening regimes have been modeled on the basis of interactions of superkinks with Kear–Wilsdorf locks and related to pertinent microstructural parameters such as the volume fractions of γ′ precipitates, grain orientation, and dislocation substructure. The mechanistic strain hardening model predicts that the strain hardening exponents in both the low plastic strain (n1) and the high plastic strain (n2) regimes increase with increasing values of the sum of the squares of the volume fractions of the primary and secondary γ′ precipitates, the number of {111} and {010} slip systems activated, and the critical height of the superkinks. A comparison of model predictions against experimental strain hardening exponents indicates good agreement between model predictions and experimental data. Implications of the operative strain hardening mechanisms during low-cycle fatigue and high-cycle fatigue are elucidated.

Strength prediction of Ni-base disc superalloys: Modified γ′ hardening models applicable to commercial alloys

The reported strength predictions of γ′ hardening so far for polycrystalline Ni-base superalloys tend to be underestimated. This is mostly due to the difficulty in isolating the γ′ hardening amidst the complicated multi-level strengthening mechanisms of the alloy and negligence of the contribution from γ/γ′ mixture strength. For these reasons, a series of single-crystal tie-line model alloys with specially designed γ′ size and volume fractions was adopted in this work, allowing a direct validation of classical γ′ particle hardening equations for the first time. Using the iso-work assumption, γ′ particle strengthening and γ/γ′ mixture strength at 650 °C were individually extracted from the compression test results. It was found that the classical strengthening models were unable to predict the expected strength increment with increasing γ′ volume fractions. Modified versions of the classical equations were proposed, which gave improved predictability across wider γ’ volume fractions and served as important basis for strength design application.

The effect of δ phase on the microplasticity evolution of a heat-treated nickel base superalloy

The micromechanical deformation behaviour of the commercial Ni-base polycrystalline Ni-base superalloy GH4169 has been investigated in terms of the volume fraction and grain boundary distribution of the δ (Ni3Nb) phase. The main objective of this work was to gain an understanding about the mechanisms of deformation of the superalloy at the grain scale. To that purpose, experimental and microstructural observations are relied upon to construct realistic models of the polycrystalline superalloy microstructure using crystal plasticity concepts. It was found that a volume fraction of the grain boundary δ phase in excess of approximately 3.0% results in a weakening of the room temperature strength, and the detrimental effect increases with increasing the δ phase volume fraction. This study also revealed that the grain boundary δ phase, being stronger than the surrounding grains, impedes the transmission of plastic slip through the grain boundaries, thus enhancing the localisation of plastic slip within the grains. However, the strengthening effect associated with such slip blocking mechanism by the δ phase is considerably lower than the weakening of the deformation resistance of the grains due to the depletion of the meta-stable γ″ precipitates within the grains resulting from the stable δ phase formation.

The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and γ′ phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of γ′-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.

Solving Recent Challenges for Wrought Ni-Base Superalloys

This paper reviews the status of technology in design and manufacture of new wrought polycrystalline Ni-base superalloys for critical engineering applications. There is a strong motivation to develop new alloys that are capable of operating at higher temperatures to realize improvements in thermal efficiency, which are necessary to achieve environmental targets for reduced emissions of harmful green-house gases. From the aerospace sector, the development of new powder metallurgy and ingot metallurgy alloys is discussed for disk rotor and static applications. New compositions for powder metallurgy contain about 50 to 55 pct of gamma prime (γ′) strengthening precipitates to ensure components operate successfully at temperatures up to 788 °C (1450 °F). In contrast, new compositions for ingot metallurgy aim to occupy a design space in temperature capability between Alloy 718 and current powder alloys that are in-service, and show levels of γ′ of about 30 to 44 pct. The focus in developing these alloys was design for manufacturability. To complement the aerospace developments, a review of work to understand the suitability of candidate alloys for multiple applications in Advanced-Ultra Supercritical (AUSC) power plants has been undertaken by Detrois, Jablonski, and Hawk from the National Energy Technology Laboratory. In these power plants, steam temperatures are required to reach 700 °C to 760 °C. The common thread is to develop alloys that demonstrate a combination of high-temperature properties, which are reliant on both the alloy composition and microstructure and can be produced readily at the right price. For the AUSC applications, the emphasis is on high-temperature strength, long-term creep life, phase stability, oxidation resistance, and robust welding for fabrications. Whereas for powder disk rotors in aircraft engines, the priority is enhanced resistance to time-dependent crack growth, phase stability, and resistance to environmental damage, while extending the current strength levels, which are shown by existing alloys, to higher temperatures.

A Study on Establishing a Microstructure-Related Hardness Model with Precipitate Segmentation Using Deep Learning Method.

This paper established a microstructure-related hardness model of a polycrystalline Ni-based superalloy GH4720Li, and the sizes and area fractions of γ’ precipitates were extracted from scanning electron microscope (SEM) images using a deep learning method. The common method used to obtain morphological parameters of γ’ precipitates is the thresholding method. However, this method is not suitable for distinguishing different generations of γ’ precipitates with similar gray values in SEM images, which needs many manual interventions. In this paper, we employ SEM with ATLAS (AuTomated Large Area Scanning) module to automatically and quickly detect a much wider range of microstructures. A deep learning method of U-Net is firstly applied to automatically and accurately segment different generations of γ’ precipitates and extract their parameters from the large-area SEM images. Then the obtained sizes and area fractions of γ’ precipitates are used to study the precipitate stability and microstructure-related hardness of GH4720Li alloy at long-term service temperatures. The experimental results show that primary and secondary γ’ precipitates show good stability under long-term service temperatures. Tertiary γ’ precipitates coarsen selectively, and their coarsening behavior can be predicted by the Lifshitz–Slyozov encounter modified (LSEM) model. The hardness decreases as a result of γ’ coarsening. A microstructure-related hardness model for correlating the hardness of the γ’/γ coherent structures and the microstructure is established, which can effectively predict the hardness of the alloy with different microstructures.