Polycrystalline Ni-based Superalloys Research Articles

Advanced Polycrystalline γ′-Strengthened CoNiCr-Based Superalloys

Novel compositionally complex CoNiCr-based superalloys with excellent mechanical properties have been developed, which combine the multiprincipal element nature of high-entropy alloys with the precipitation strengthening in superalloys. A series of advanced polycrystalline γ′-strengthened CoNiCr-based superalloys, called CoWAlloys, with varying contents of Al, W, Ti, Ta, Mo, and Nb are investigated in terms of microstructure, thermophysical properties, yield, and creep strength. The microstructure of all CoWAlloys consists of an fcc solid solution matrix phase (approximate γ composition in at. pct: 50Co–20Ni–20Cr–10X (X = other alloying elements)), which is strengthened by a multicomponent γ′ (Ni,Co)3(Al,Ti,Ta,W,Nb)-based precipitate phase with a very high-volume fraction of around 60 vol pct (approximate γ′ composition in at. pct: 45Ni–30Co–25X). These alloys have high solidus temperatures above 1300 °C and moderate γ′ solvus temperature between 985 °C and 1080 °C leading to a large processing window. The increasing content of γ′-forming elements Ti, Ta, W, and Nb decreases this window, but increases the γ/γ′ lattice misfit and the anti-phase boundary energy, which contribute to a significantly higher yield and creep strength. Their properties are discussed in comparison with conventional polycrystalline Ni-base superalloys and so-called L12-strengthened high-entropy alloys, revealing that the creep strengths of the CoWAlloys are significantly higher. This is due to the reduced strain rate sensitivity of the CoWAlloys due to different underlying deformation mechanisms: By increasing the anti-phase boundary energy, a transition to stacking fault shearing and microtwinning occurs, which leads to the enhanced creep strength. Based on these results, guidelines and strategies for the design of next-generation advanced high-temperature polycrystalline superalloys are proposed.Graphical

Strain localization in the Alloy 718 Ni-based superalloy: From room temperature to 650 °C

Irreversible deformation in relation to the microstructure was investigated for a polycrystalline Ni-based superalloy (Alloy 718) from room temperature to 650 °C using high-resolution digital image correlation (HR-DIC) techniques. Interrupted tensile tests were performed under a protective atmosphere to ensure the stability of the speckle pattern to track kinematics fields from surface analyses. In-plane strain localization was captured using HR-DIC on scanning electron microscopy (SEM) images. A statistical analysis of different strain localization events in relation to the microstructural features was conducted, i.e., intragranular slip localization, slip localization parallel to and near Σ3-twin boundaries (Σ3-TB), and grain boundary sliding (GBS). Alloy 718 exhibited slip localization at room temperature and 350 °C. Intense strain localization develops parallel and in the vicinity of Σ3-TB from the onset of the microplasticity. Few intense slip stimulated-grain boundary sliding events were found due to slip localization on both grains adjacent to the grain boundary. At 650 °C, Alloy 718 experienced grain boundary sliding at the onset of the yield without particular slip localization in adjacent grains. At lower temperatures, strain localization parallel to and near Σ3-TB was intense, and intragranular slip localization intensified with increasing macroscopic deformation. Particular microstructural configurations were found at 650 °C leading to premature damage: (i) sub-surface cavitation at grain boundaries, and (ii) grain boundary cracking due to intense shearing near a Σ3-TB.

The Effect of Nb, Ta, and Ti on the Oxidation of a New Polycrystalline Ni-Based Superalloy

The effect of variations in Nb, Ta, and Ti concentrations in exchange for Al on the oxidation resistance of a new polycrystalline Ni-based superalloy (C19) was studied in air at 800 °C for up to 1000 h. An external scale of Ti-doped Cr2O3 and a sub-scale of discontinuous Al2O3 intrusions formed on the surface of all the studied alloys. Contrary to other reports, increasing the Nb concentration improved the oxidation performance and may have promoted the formation of a CrTaO4 layer, thereby reducing oxygen ingress. The addition of Ta also significantly improved oxidation resistance and reduced the depth of the Al2O3 intrusions. Increasing the Ti concentration did not significantly affect the oxidation performance, potentially due to the relatively low Ti concentrations investigated. Several of the studied alloys with modified Ta and Ti concentrations showed regions of continuous Al2O3 scale formation, suggesting that the compositions are in a transition regime between Cr2O3-forming and Al2O3-forming behaviour. The findings suggested that part of the Ti content in C19 could potentially be replaced with Nb, Ta and/or other elements to further enhance oxidation resistance and other desirable properties. Overall, the insights gained could serve as a guide to optimise the composition of C19 and similar alloys for enhanced oxidation resistance.

Delayed parabolic oxidation via transient thermal exposures on a polycrystalline nickel-based superalloy

Multiple oxides were observed during the transient oxidation stage of the polycrystalline Ni-based superalloy, RR1000, before a protective Cr2O3 scale formed. Thermogravimetric analysis, synchrotron grazing incidence X-ray diffraction, and electron microscopy were performed on samples subjected to isothermal exposures at 800 °C for up to 100 h. Transient effects governed the early stages up to 40 h. NiO, spinels (NiCr2O4, (Ni,Co)(Cr,Co)2O4), Cr2O3/(Cr0.88Ti0.12)2O3, NiTiO3, and CrTaO4 formed during the initial stage with pseudo-linear kinetics. At the onset of parabolic kinetics, extensive Cr2O3 and TiO2 growth dominated scale formation with the former emerging as the major passivating oxide.

Microstructural Stability and Evolution in a New Polycrystalline Ni-Base Superalloy

Polycrystalline Ni-base superalloys that offer equivalent or superior performance to current commercial alloys at lower overall cost are of widespread industrial interest. In this work, a new polycrystalline Ni-base superalloy with low elemental cost has been characterized and compared to current commercially available alternatives. Through a combination of scanning electron microscopy, thermal analysis, synchrotron X-ray diffraction, and hardness testing, a broad preliminary investigation of fundamental alloy properties has been performed, identifying the key areas for further alloy development opportunities.

Competitive role of primary [formula omitted] precipitates and annealing twins on the heterogeneous deformation of a polycrystalline ni-based superalloy: Crystal plasticity modeling and experiments

A crystal plasticity modeling framework is developed and used in combination with experiments for studying the deformation behavior of a polycrystalline Ni-based superalloy. In particular, the monotonic stress-strain response across a range of temperatures and the misorientation development at room temperature are predicted. The model accounts for individual deformation of the matrix and primary γ′ phases, with consideration for the various strengthening mechanisms observed in these alloys. The as-received microstructure has been characterized using Energy Dispersive X-Ray Spectroscopy (EDS) and the concentration of the solute atoms present in the γ and primary γ′ phases obtained from these measurements are used to inform the solute solution strengthening model. Further, the Electron Back Scatter Diffraction (EBSD) microstructures are directly used as inputs for the simulations. The model is first calibrated to the experimental stress-strain response at 293 K and 923 K and then validated by predicting the mechanical response at 523 K and 723 K. Interrupted tensile testing has been performed using a combination of Scanning Electron Microscopy (SEM) and EBSD to study the misorientation developments during deformation. These deformed microstructures are correlated with the simulated EBSD microstructures, in terms of the competitive misorientation developments at grain boundaries, annealing twin boundaries and phase interfaces. Higher misorientation developments were observed at the twin boundaries as compared to the grain boundaries and phase interfaces, indicating a higher propensity of heterogeneous deformation.

A model for dislocation creep in polycrystalline Ni-base superalloys at intermediate temperatures

A model for creep at intermediate temperatures in polycrystalline Ni-based superalloys is presented. The model is based on describing stacking fault nucleation, propagation and subsequent shear within the γ matrix and γ′ precipitates. The critical energy for stacking fault nucleation is obtained by minimising the energy to form a stacking fault from dislocation partials, which is promoted by local stress concentrations. The extent of stacking fault shear at a γ′ precipitate is estimated using a force balance at the γ/γ′ interface to determine the critical shear distance The model results are validated against creep experimental data in several polycrystalline superalloys showing good agreement. Individual contributions to creep from key microstructural features, including grain size and γ′ distribution, are studied to identify which ones are more significant. Similarly, it is shown that one of the main factors controlling the creep rate is the stacking fault energy in the γ as it dictates the stacking fault nucleation and shear rates. Parameter analysis on alloying additions typically used in commercial superalloys demonstrates which elements have the strongest effect on creep, highlighting how the present model can be used as tool for alloy and microstructure design against dislocation creep.

γ′-size effect on the deformation microstructure and texture in a Ni-based superalloy using in-situ neutron diffraction

The γ′-size effect on the evolution of microstructure and grain rotation during tensile deformation in a polycrystalline Ni-based superalloy was investigated by in-situ neutron diffraction technique. Diffraction profile analysis was employed to evaluate the dislocation density and deformed subgrain size distribution, revealing that alloys with fine and coarse γ′ possess dislocation shearing and Orowan dislocation bypassing, respectively. The high strain hardening in the coarse γ′ specimen is attributed to the more pronounced accelerated accumulation of dislocations compared to the fine γ′ specimen. Moreover, the specimen with coarse γ′ precipitate exhibits a lower orientation rotation rate, resulting in weaker <111> and <100> fiber texture components upon tensile straining. The use of neutron diffraction combined with electron microscopy provided multiscale insights into the deformation microstructure and changes in the lattice orientation.

Evaluating Wagner Oxidation Criteria for Protective Al2O3 Scale Formation in Ni-Based Superalloys

An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al2O3 scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al2O3 scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.

Temperature Range of Heating Rate Dependent Reactions Leading to Spinel Formation on a Ni-Based Superalloy

Improved oxidation kinetics for a polycrystalline Ni-based superalloy used in turbine disc applications has been shown to be possible by controlling the heating rate of the first thermal exposure to 5 °C min−1. The beneficial effect arises from the formation of a protective layer of NiCr2O4, instead of the more usually formed doped Cr2O3. This study shows that it was possible to form the NiCr2O4 at temperatures up to 725 °C, within the operational conditions for this alloy, and that at higher temperatures Cr2O3 formed. The improvements in alloy performance extended to the internal oxidation processes where reduced depths of degradation were observed. It is demonstrated here that Al2O3 formation is less thermodynamically stable when the highly protective NiCr2O4 oxide is present at the alloy surface compared to the doped Cr2O3. Synchrotron XRD was performed on samples removed during the heating stage and provided evidence of the oxidation sequence occurring, enabling refinement in the thermodynamic calculations and suggesting an additional route to the formation of the NiCr2O4.

Microstructure and mechanical degradation of K403 Ni-based superalloy from ultra-long-term serviced turbine blade

The effective characterization of the mechanical property degradation of turbine blades in service for an ultra-long-term had a significant impact on the service safety and maintenance decisions of turbine blades. In this study, the microstructure observation and mechanical property study of the K403 polycrystalline Ni-based superalloy turbine blade under four service times were carried out, using microstructure characterization methods to quantify the microstructure evolution of the superalloy at different parts of the turbine blade with increasing service time. The effects of the microstructure degradation of the superalloy under ultra-long-term service on the mechanical properties of the turbine blade were analyzed. The results show that the microstructural degradation characteristics of the superalloy turbine blades under ultra-long-term service were strongly correlated with the spatial structural characteristics of the turbine blades after service, and it was found that the carbon elements and other elements inside and at the boundary of the carbide showed regular migration behavior with the increase in service time. The regular influence of microstructural degradation of superalloy turbine blades under ultra-long-term service on their mechanical properties was demonstrated. These results can provide empirical support for the study of the predicted residual properties of nickel-based superalloy turbine blades under ultra-long-term service.

Oxidation Behavior of the Polycrystalline Ni-Base Superalloy VDM® Alloy 780

VDM® Alloy 780 is a newly developed polycrystalline Ni-base superalloy with high contents of Co, Cr, and Al intended for operating temperatures up to 750 °C. The alloy is precipitation strengthened by the γ′ phase, which is analyzed by atom probe tomography. Additionally, δ and η phases are utilized for grain boundary pinning. It is shown that the δ and η phases precipitate either plate like or in a fine lamellar structure inside each other. VDM® Alloy 780 shows superior oxidation resistance in comparison with Udimet 720Li and A718Plus, as seen by a lower mass gain and thinner oxide layers at 800 °C and 900 °C. This superior behavior is analyzed in detail by TEM and STEM investigations of the oxide scales from which it is concluded that the Al/Ti ratio in these alloys plays an important role on the oxidation behavior.

Deformation Mechanisms Rationalisation to Design for Creep Resistance in Polycrystalline Ni-Based Superalloys

Creep strength in polycrystalline Ni-based superalloys is influenced by the formation of a rich variety of planar faults forming within the strengthening gamma ' phase. The lengthening and thickening rate of these faults – and therefore the creep rate – depends on an intriguing combination of dislocation interactions at the gamma -gamma ' interface and diffusional processes of the alloying elements at the core of the fault tip. The effect of alloy composition on this process is not fully understood. In this work we use correlative high resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy to study the deformation faults in two different Ni-based superalloys with carefully designed ratios of disordering-to-ordering-promoting elements (Co-Cr against Nb-Ta-Ti). The results show that the additions of ordering-promoting elements reduce the diffusional processes required for the faults to lengthen and thicken thus reducing the creep rates found for the higher Nb-Ta-Ti alloy. These insights provide a path to follow in the design of improved grades of creep-resistant polycrystalline alloys beyond 700 ^circ C.

Oxidation-induced recrystallization and damage mechanism of a Ni-based single-crystal superalloy during creep

The creep tests for Ni-based single-crystal superalloy are carried out under 980 °C, and the microstructure at the crack tip is characterized at micro- and nano- scales. Numerous γ’ phase dissolution, nucleation and growth of fine grains are observed near the crack. In addition, γ’ phase did not effectively hinder the nucleation and growth of recrystallization grains at the oxidation-affected zone. Instead, recrystallization promoted the degradation of the γ’ phase to dissolve along the recrystallized grain boundaries. Combining the sub-grain boundary growth model proposed by Winning et al. and the discontinuous recrystallization model suggested by D.G. Cram, a recrystallization and oxidative damage model for Ni-based single-crystal superalloy is proposed. • Local polycrystallization or recrystallization will greatly damage the high-temperature properties of the alloy. From the point of view of Ni-based single-crystal superalloy design, polycrystallization during medium-high temperature creep is very rare and should be strictly avoided. • Many researches have paid attention to the recrystallization of polycrystalline Ni-based superalloys. However, there is no report about the appearance of polycrystallization region in Ni-based single crystal superalloys during creep. Additionally, systematic research and explanation on the mechanism of discontinuous recrystallization of Ni-based SC superalloy are also very lacking. • In this paper, it is found for the first time that in the process of medium-high temperature (980 °C) creep under low stress level, polycrystallization region and a large amount of γ’ phase dissolve appeared at the crack tip, which leads to a sharp decline in the properties of the alloy. Researchers believed that the γ’ phase is the main factor that hinder the recrystallization of Ni-based single-crystal alloy. Nevertheless, this paper propose that the local strain and depletion of solute atoms makes the sub-grain boundaries tend to become the nucleation of recrystallization.

Early Stages of Fatigue Crack Initiation in Relation With Twin Boundaries in Polycrystalline Ni-Based Superalloy AD730™

Early Stages of Fatigue Crack Initiation in Relation With Twin Boundaries in Polycrystalline Ni-Based Superalloy AD730™

The Isothermal Oxidation of a New Polycrystalline Turbine Disk Ni-Based Superalloy at 800 °C and Its Modification with Pre-oxidation

Ni-based superalloys with enhanced oxidation resistance at high temperatures are crucial for next-generation gas turbine engines. A new polycrystalline Ni-based superalloy (C19) that combines improved microstructural stability with environmental resistance has been developed. Its oxidation resistance has been determined through measurements of the specific mass change and morphological evolution of the formed oxides following furnace exposures at 800 °C in air for up to 1000 hours; the results of which were benchmarked against Nimonic 105. C19 showed hybrid Type II/Type III behavior as a marginal Al2O3 former and performed similarly to established superalloys at 750 °C. The Wagner model for the transition from internal to external oxide formation predicted that C19 should form a continuous Al2O3 scale at higher temperatures. A pre-oxidation treatment at 1100 °C for 1 hour was, therefore, selected and shown to dramatically improve the oxidation resistance during subsequent exposure at 800 °C. Oxide cross-sectional analysis showed that C19 formed a continuous and protective Al2O3 scale after the pre-oxidation treatment, whereas Nimonic 105 retained discontinuous Al2O3 finger-like intrusions beneath a Cr2O3 overscale.

Creep resistance improvement of a polycrystalline Ni-based superalloy via TiC particles reinforcement

In order to develop highly efficient aircraft engines, the use of alloys for turbine blades that combine high creep resistance and lower density brings substantial benefits. That may be achieved by the introduction of a reinforcing phase, such as a discontinuous reinforcement. So far, most researched particle-reinforced Ni-based superalloys made use of sub-micrometer particles, such as oxides and borides, often produced by in-situ reactions. These are proposed solely as dislocation immobilizers, in a role comparable to that carried out by γ’ precipitates, while metal matrix composites (MMCs) containing micro-scale particles offer a load-transfer potential. To combine both approaches, a recently designed polycrystalline Ni-based γ’-strengthened superalloy reinforced with TiC particles was produced by low energy mixing of powders, followed by pressure sintering. The composite, along with a non-reinforced reference, was submitted to compression creep tests at 973 K (700 °C), with stresses varying from 280 to 500 MPa. Selected specimens had their microstructure evaluated with techniques such as EBSD and XRD. When simulating a working turbine, the material density influences directly the centripetal forces acting on a section of a turbine blade, and consequently the creep rates. In the proposed composite, not only an increase in the threshold stress by a load-transfer component was obtained, but also the density reduction affected remarkably the creep rates. Moreover, the presented MMC not only presented a superior creep resistance in relation to the non-reinforced alloy, but also exhibited higher stability of primary and secondary γ’ volumes after 500 h in creep tests.

Temperature and ΔK dependence of grain boundary effect on fatigue crack propagation in a two-dimensional polycrystalline Ni-base superalloy

Effects of grain boundaries (GBs) on fatigue crack propagation (FCP) in a cast Ni-base superalloy were experimentally investigated using “two-dimensional” polycrystalline specimens extracted from a directionally solidified material. FCP tests were conducted at room temperature (RT), 700 °C and 900 °C under the stress intensity factor range, ΔK, increasing conditions, and distinctive FCP behavior both in grain interiors and around the GBs was observed in-situ. At RT, the crack propagated in the shearing mode along slip planes. At 700 °C, the cracking mode transitioned from the opening mode to the shearing mode with the increasing ΔK. At 900 °C, only the opening mode cracking occurred. The shearing cracks at RT and in the high ΔK region at 700 °C were retarded near the GBs, whereas the opening mode crack in the low ΔK region at 700 °C was not affected by the GBs. The retardation was attributed to a transition from the shearing mode to the opening mode induced by the slip plane incompatibility at the GBs, and the degree of the retardation showed a good correlation with the size of the shearing crack plane. At 900 °C, the crack propagated along high angle GBs with higher FCP rates than those in grain interiors. The acceleration was greater at the GBs with larger misorientation, which was attributed to higher GB energy. In high ΔK regions, secondary cracks were nucleated along high angle GBs ahead of main crack tips possibly because of external stress normal to the GBs and/or the strain incompatibility at the GBs.

Evolution of creep rupture mechanism in advanced powder metallurgy superalloys with tantalum addition

To further understand the effect of Ta additive on the creep damage for polycrystalline Ni-base superalloys, creep rupture tests on powder metallurgy (PM) superalloys with various Ta were carried out under different conditions in this study. The results indicated that the multiple crack propagation and damage mechanisms could occur at the identical creep condition. These different fracture mode areas had distinct boundaries on the fracture surface, its size could be altered by Ta addition. However, the effects of Ta were radically changed by the creep conditions. The creep rupture mechanism transformed significantly with temperatures and applied stresses, and even subversive laws appeared in the creep life for high-Ta alloys. The substructure observation after creep rupture demonstrated that the annealing coherent twin boundaries gradually lose coherence with the matrix, which could become sites for cavities nucleation or crack propagation that followed. Furthermore, the creep-induced microtwins were critical for the creep damage and rupture. The electron back-scattered diffraction (EBSD), high-resolution transmission electron microscope (TEM) imaging, and quantitative elemental analysis assisted in comprehending these alterable creep rupture mechanisms.

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.