Nickel-base Superalloy FGH96 Research Articles

A novel creep life prediction model for variable loads considering the sequence effect

Residual creep life prediction under variable creep loading is an important issue in structural design. The loading sequence has a significant effect on the rupture life, which refers to the sequence effect, including the temperature sequence at constant stress and the stress sequence at constant temperature. This paper investigates the creep life prediction model taking into account the sequence effect. First, the experimental results from the literature were collected to summarize the sequence effect they revealed. Then various creep life prediction models were reviewed and their main limitations were pointed out. A novel creep life prediction model was proposed based on the time fraction model. Finally, two-level tests were carried out on nickel-based superalloy FGH96 to validate the model. In addition, two-level and multi-level creep test results from public literature were used to validate. The results show that the proposed model effectively consider the sequence effect under various loading conditions.

Effect of sustained load on fatigue crack growth behavior of FGH96 at elevated temperature

Accurate prediction of fatigue crack growth behavior is crucial to damage tolerance assessment of turbine disk. Powder nickel-based superalloy is a kind of preferred material of turbine disk. For the purpose of better understanding the characteristics of its crack growth behavior under dwell fatigue conditions at elevated temperature, dwell fatigue crack growth tests of powder nickel-based superalloy FGH96 were conducted with compliance method at three temperatures (600℃, 650℃, 700℃) and multiple dwell time conditions (from 0 s to 60 s). The results show that sustained load can accelerate fatigue crack growth rate of FGH96 significantly at high temperature, and it is more pronounced with higher temperature and prolonged dwell time. At higher ΔK level, the acceleration effect brought by sustained load seems to diminish, this is ascribed to rapid crack growth rate which leads to insufficient time for time-dependent damage occurring at crack tip zone. The effect of temperature on dwell fatigue crack growth rate is more remarkable from 650℃ to 700℃, compared with that from 600℃ to 650℃. The fatigue crack growth resistance of FGH96 is sensitive to sustained load at 700℃, just 1 s dwell loading can produce remarkable acceleration effect on fatigue crack growth rate. The time-dependent crack extension in a loading cycle increases with dwell time in a decelerated manner. Finally, A phenomenological-based crack growth model considering time-dependent damage was proposed, the comparation between the proposed model and the experiment data confirms that it works well for a wide range of temperature and dwell time.

Influence of Deformation Rate on Nucleation and Growth of Recrystallized Grains during Hot Deformation of Nickel-Based Powder Superalloys

High temperature uniaxial compression deformation test was carried out on nickel-based powder superalloy FGH96 alloy by thermal processing simulation test machine, and the relationship between grain boundary characteristics and nucleation mechanism of recrystallized grains, growth characteristics of recrystallized cores and related dynamic softening mechanisms was studied. According to the correlation and evolution law of grains, combined with the change trend of grain orientation during the deformation process, the recrystallization behavior characteristics of the grains with special orientation generated during the deformation process were analyzed. When the deformation temperature is 1070 °C, the evolution rules of the substructure grain boundaries with an orientation difference of less than 10° are different under different deformation rate conditions, which indicates that the dynamic softening mechanism of FGH96 alloy is slightly different with the increase of deformation rate. There are different. With the increase of the deformation rate, the nucleation mechanism induced by Σ3 twinning gradually replaced the nucleation mechanism of continuous subgrain rotation and coarsening, and the grain growth rate gradually slowed down.

The microstructure effect on fatigue and dwell-fatigue in a nickel-based superalloy

In this study, the fatigue behaviors of a nickel-based superalloy FGH96 with different microstructural morphologies generated from different heat-treatment processes were investigated. The fatigue and dwell-fatigue tests with the same maximum stress of 1200 MPa were conducted at 650 °C. The relationship between the γ' morphology and the fatigue properties was established. The results indicate that the critical shearing stress of dislocation movement played a key role in affecting fatigue life. No apparent difference can be observed from the fracture morphology at the macroscopic scale for normal fatigue and dwell-fatigue tests. However, in the crack initiation region, the crack was observed to grow along the grain boundary for the dwell-fatigue condition, resulting in the intergranular fracture, similar to creep failure. The introduction of dwell time in the fatigue loading brings larger cyclic strain deformation originates from massive dislocation slip. The dwell debit for the considered loading condition is about 100 times.

Flow Stress Evolution of P/M Nickel-base Superalloy During High-temperature Plastic Deformation

In this work, the use of scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction technique and experimental study of the nickel-base superalloy FGH96 recrystallization behavior of alloy. Through experimental study found that the thermal deformation, FGH96 alloys under different plastic deformation at high temperature deformation process parameters, mainly dynamic softening and strain hardening in two stages.

Prediction on grinding force during grinding powder metallurgy nickel-based superalloy FGH96 with electroplated CBN abrasive wheel

In this article, a grinding force model, which is on the basis of cutting process of single abrasive grains combined with the method of theoretical derivation and empirical formula by analyzing the formation mechanism of grinding force, was established. Three key factors have been taken into accounts in this model, such as the contact friction force between abrasive grains and materials, the plastic deformation of material in the process of abrasive plowing, and the shear strain effect of material during the process of cutting chips formation. The model was finally validated by the orthogonal grinding experiment of powder metallurgy nickel-based superalloy FGH96 by using the electroplated CBN abrasive wheel. Grinding force values of prediction and experiment were in good consistency. The errors of tangential grinding force and normal grinding force were 9.8% and 13.6%, respectively. The contributions of sliding force, plowing force and chip formation force were also analyzed. In addition, the tangential forces of sliding, plowing and chip formation are 14%, 19% and 11% of the normal forces on average, respectively. The pro-posed grinding force model is not only in favor of optimizing the grinding parameters and improving grinding efficiency, but also contributes to study some other grinding subjects (e.g. abrasive wheel wear, grinding heat, residual stress).

An investigation on machined surface quality and tool wear during creep feed grinding of powder metallurgy nickel-based superalloy FGH96 with alumina abrasive wheels

In this study, the machined surface quality of powder metallurgy nickel-based superalloy FGH96 (similar to Rene88DT) and the grinding characteristics of brown alumina (BA) and microcrystalline alumina (MA) abrasive wheels were comparatively analyzed during creep feed grinding. The influences of the grinding parameters (abrasive wheel speed, workpiece infeed speed, and depth of cut) on the grinding force, grinding temperature, surface roughness, surface morphology, tool wear, and grinding ratio were analyzed comprehensively. The experimental results showed that there was no significant difference in terms of the machined surface quality and grinding characteristics of FGH96 during grinding with the two types of abrasive wheels. This was mainly because the grinding advantages of the MA wheel were weakened for the difficult-to-cut FGH96 material. Moreover, both the BA and MA abrasive wheels exhibited severe tool wear in the form of wheel clogging and workpiece material adhesion. Finally, an analytical model for prediction of the grinding ratio was established by combining the tool wear volume, grinding force, and grinding length. The acceptable errors between the predicted and experimental grinding ratios (ranging from 0.6 to 1.8) were 7.56% and 6.31% for the BA and MA abrasive wheels, respectively. This model can be used to evaluate quantitatively the grinding performance of an alumina abrasive wheel, and is therefore helpful for optimizing the grinding parameters in the creep feed grinding process.

Evaluation of the influence of surface crack-like defects on fatigue life for a P/M nickel-based superalloy FGH96

To study the influence of defects, a crack-like artificial defect was introduced at the surface of smooth specimens of a powder metallurgy nickel-based superalloy FGH96. Two kinds of size were set and fatigue tests were conducted at both room temperature (RT) and elevated temperature (ET) with several different stress levels. Results first show that the existence of surface defects significantly reduces fatigue life and the negative effect of temperature is also evident. Meanwhile, specimens with identical conditions exhibit little scatter in life which provides the possibility to evaluate fatigue lives with a relatively deterministic method. To further reveal the failure process, both macro and micro fractography were carefully examined and specimens at RT and ET show evident differences in crack initiation and propagation. Finally, a life prediction method based on the fracture mechanic was proposed. Five stress intensity calculation methods with different strategies for the surface crack in round specimens were applied and their influence on life prediction was further discussed. Given the relative high loading condition, the plasticity modification for different method is also included. Besides, this calculation method was checked for application on other loading conditions. Finally, the final stress intensity value and the parameter sensitivity analysis were also discussed.

Effect of cooling rate during quenching on the microstructure and creep property of nickel-based superalloy FGH96

The effect of cooling rate during quenching on the microstructure and creep property of nickel-based superalloy FGH96 was investigated. Three groups of samples were quenched continuously with three fixed cooling rates, respectively, then subjected to a creep test under a constant load of 690 MPa at 700°C. Clear differences in size of secondary γ′ precipitates, creep properties and substructure of creep-tested samples were observed. The quantitative relationship among cooling rate, the size of secondary γ′ precipitates, and steady creep rate was constructed. It was found that with increasing cooling rate, the size of secondary γ′ precipitates decreases gradually, showing that the relationship between the size of secondary γ′ precipitates and the cooling rate obeys a power law, with an exponent of about −0.6, and the creep rate of steady state follows a good parabola relationship with cooling γ′ precipitate size. For 235°C/min, FGH96 alloy exhibited very small steady creep rate. The density of dislocation was low, and the isolated stacking fault was the dominant deformation mechanism. With decreasing cooling rates, the density of dislocation increased remarkably, and deformation microtwinning was the dominant deformation process. Detailed mechanisms for different cooling rate were discussed.

Grindability of powder metallurgy nickel-base superalloy FGH96 and sensibility analysis of machined surface roughness

The grindability and sensibility analysis of surface roughness of powder metallurgy nickel-base superalloy FGH96 were studied in comparison to the wrought nickel-base superalloy GH4169. The effects of grinding parameters (such as workpiece infeed speed, depth of cut, and abrasive wheel speed) on grinding force, grinding temperature, specific grinding energy, abrasive wheel wear, and surface roughness were analyzed. The results show that the grinding force, grinding temperature, and specific grinding energy of GH4169 are usually higher than those of FGH96 under the given experimental conditions. However, the wear behavior of the brown corundum abrasive wheels when grinding these two kinds of nickel-base superalloy material is generally identical. The sensitivity of GH4169 workpiece surface roughness to depth of cut and workpiece infeed speed is higher than that of FGH96, but the sensitivity of GH4169 to abrasive wheel speed is less than that of FGH96. Finally, it is inferred that the grinding performance of FGH96 is slightly better than that of GH4169.

Effect of inclusions on low cycle fatigue lifetime in a powder metallurgy nickel-based superalloy FGH96

Powder metallurgy (P/M) nickel-based superalloy FGH96 is widely used for turbine discs in aero-engines. With an increasing trend towards powder metallurgy routes for turbine discs, undesired non-metallic inclusions comparable to the grain size are unavoidably introduced during the manufacturing process. In this study, a series of strain controlled low cycle fatigue (LCF) tests were conducted on the specimens cut from a P/M FGH96 turbine disc at elevated temperature. The microscopic mechanisms of the effects of the size and inclusion location on LCF lifetime were investigated by using scanning electronic microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) for the fractographic analysis of specimens. It is revealed that the P/M FGH96 superalloy is sensitive to surface defects, such as surface scratches and surface inclusion; while the specimen for which the crack nucleates from internal inclusion usually has a longer fatigue lifetime due to smaller stress concentration at the crack tip. Moreover, the inclusion involving its location and size has a significant influence on the P/M FGH96 superalloy’s LCF lifetime. Then, a modified model was proposed to quantify the inclusion effect on the LCF lifetime. Finally, a probabilistic model based on Bayesian approach was formulated to describe the scattering in LCF lifetimes induced by random inclusions.

Effect of Solution Treatment Temperature on the Grain Growth Behavior of Fine Grained FGH96 Superalloy

The effect of solution treatment temperature on the grain growth behavior of fine grained nickel-base superalloy FGH96 has been investigated. The results showed that with the increase of solution treatment temperature, the grain growth of fine grained FGH96 alloy (<5μm) experienced 3 stages, the slow growth stage below 1120°C, the steady grain size stage from 1120°Cto 1250°C, which revealed the fine grained FGH96 alloy with wide solution treatment window compared to coarse grain, and the exponential growth stage. At the same time, the primary γ′ and MC carbides on impeding grain boundary migration to limit grain growth was demonstrated. The grain growth model has been established, and the activation energy of grain growth was calculated to be 288 KJ/mol through linear fitting.

Microstructure Characterization of Inertial Friction Welding Zone for Nickel-Based Superalloy FGH96

The inertial friction welding of similar PM FGH96 superalloy was investigated. The inertial friction welding introduced steep thermal gradients of welding zone for PM FGH96 superalloy, the whole welding process only needed several seconds, therefore, it’s a fast heating and cooling completely recrystallization process. Dramatic changes in the microstructure were observed over a narrow weld zone, which across the weld interface was measured to be about 1.0-1.2mm. Significant changes in the secondary and tertiary γ′ distribution can be observed over the first 2mm from welding line, while very fine tertiary γ′ particles precipitated in a unimodal size distribution and in a high density at welding line. The fine secondary re-precipitated γ′ under fast cooling was spherical in shape, which gradually transformed into elliptical, cube with the distance from welding line. No significant texture or grain distortion was observed in the extensively plastically deformed region due to recrystallization.

Effects of microstructure and γ′ distribution on the hot deformation behavior for a powder metallurgy superalloy FGH96

Abstract

Study on the Recrystallization Behavior of an as-Extruded PM Superalloy

The powder metallurgy (PM) nickel-base superalloy FGH96 was extruded at temperature from 1050°C to 1180°C,and the as-extruded samples in the extrusion temperature 1050°C were heat-treated at from 1000°C to 1100°C during 10min, 30min, 60min and 90min apart. The microstructure of recrystallization, γ' phase and growth behavior of grain were discussed. The results indicate that the extrusion temperature have a severely influence on the dynamic recrystallization and γ' phase behavior. The content and pattern of γ' phase is influenced by extrusion temperature and the big size of γ' phase in the grain boundary hinder effectively the growth of dynamic recrystallized grain. The degree of dynamic recrystallization is completed when the extrusion temperature increases. When the extrusion temperature reach 1100°C, the big size of γ' phase lie the grain boundary dissolves gradually and the grain grows obviously. As the heat temperature of as-extruded samples increases, the small size of γ' phase dissolves early and the degree of recrystallization is completed. When heat temperature reach 1080°C, the small size of γ' phase begin to dissolve and the recrystallization grain begins to grow, for the strain energy is given off and existence of interface energy. When heat temperature reach 1100°C, the recrystallization grain become uniform, for the pinning of big size of γ' phase. As the time of heat treatment elongate, the recrystallization grain size increase and the content of γ' phase decrease. When the heat temperature is 1080°C, the final microstructure is fine and uniform grain due to the pinning of big size of γ' phase. When the heat temperature is 1100°C, large of big size of γ' phase dissolve and the largest grain size reach 8μm. The nucleation and growth of recrystallization grain is influenced by small size of γ' phase and the fast growth of recrystallization grain is due to the big size of γ' phase in the grain boundary.

Surface Segregation and Oxidation Behavior of Superalloy Powders Fabricated by Argon Atomization

Particle surface component and valence state of nickel-based superalloy FGH96 powders fabricated by argon gas atomization have been measured by X-ray phoelectron spectrometer (XPS) to recognize the effect of powder size, surface segregation and heat treating on the oxidation behavior of powders. Type and phase structure of prior particle boundaries (PPBs) precipitated in HIPed superalloy have been studied also. The results showed that prior particle surface segregation and oxidation happened during the powder collection and storage with the C, O, Ti elements enrichment and Ti, Cr, Al, Zr elements oxidation in the surface respectively. During the heat treating of 1150/2h, normal segregation element Ti and Zr enriched more apparently in the particle surface, on the contrary, negative segregation elements Ni, Al and Co diffused into the particle inner. Furthermore, the higher oxygen level on particle surface enhanced to form much more oxides such as ZrO2, TiO2 and Cr2O3. During HIP process of 1150/2h/150MPa, the oxide ZrO2 as the nuclei accelerated the preferential precipitation of MC-type carbides which forms the continuous harmful PPBs. Definitely, PPB precipitation depended on the prior particle surface segregation and oxidation, surface elements Ti, Cr and Nb enrichment and oxidation reaction enhancement in high temperature such as 1150. Small powders always had little segregation in the surface and thin oxidation layer, but had more oxygen content per weight for its higher specific surface. So using some powders with optimum particle size and lower oxygen content can help to reduce the PPB precipitation.

Effect of heat treatment on the micro-indentation behavior of powder metallurgy nickel based superalloy FGH96

The influences of solution treatment temperature on microstructure and micromechanics of P/M nickel-base superalloy FGH96 were investigated by micro-indentation methods. The alloy was heat-treated at the temperatures of 1050°C, 1150°C, 1220°C and 1310°C, respectively. The micro-indentation tests were conducted in the indenter load range from 500mN to 4500mN and the loading rate range from 5.19mN/s to 103.71mN/s at room temperature by using a sharp Berkovich indenter. The influence of solution treatment temperature on microstructure was analyzed based on microstructural observations using both optical and scanning electron microscope. The micro-hardness, Young’s modulus and yield stress were obtained by means of Oliver–Pharr method and reverse analysis algorithms, respectively. The results show that both of micro-hardness and Young’s modulus are significantly affected by indentation depth and solution treatment temperature. Based on microstructure analysis, these effects were attributed to the changes of precipitate properties, e.g., size, distribution and morphology, and the relationship between microstructure and micromechanics was established. Then, the deformation mechanism was explained on the basis of dislocation–dislocation and dislocation–precipitate interactions. In this paper, the descending Young’s modulus was related to localized stress concentration and microcrack propagation. The results reveal that the damage variable is high for P/M nickel-base superalloy FGH96 after high temperature solution treatments.

Fatigue Deformation Behavior of FGH96 Superalloy under Low Cycle Fatigue Loading

Low cycle fatigue failure tests of the powder metallurgical nickel based superalloy FGH96 at 550°C and 720°C were carried out under total strain-controlled mode (R=-1). The fatigue failure behaviors were investigated by analyzing cyclic stress response and observing microstructure after fatigue through scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that FGH96 superalloy exhibits cyclic stability at 550°C, and cyclic softening afterwards at 720°C with Δε/2=0.4% , and of cyclic softening at 720°C with Δε/2=0.7%. After high temperature low cycle fatigue, the γ′ precipitates are cuboidal for all samples. No coarsening of γ′ precipitates was detected at 550°C and at 720°C with Δε/2=0.4%, but small γ′ precipitates get together to be larger precipitates at 720°C with Δε/2=0.7%. EBSD shows that the continuous small angle grain boundaries are produced during the fatigue. The small angle grain boundaries have a significant increase at 720°C, especially that case at Δε/2=0.7%.

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The effects of indentation size and loading rate on micro-hardness of the P/M nickel-base superalloy FGH96 were investigated by the micro-indentation tests performed in the indenter load rang from 100 mN and 450 mN and the loading rate range from 1.94 mN/s to 19.37 mN/s at room temperature. The results show that the micro-hardness obviously decreased with the increase of indentation depth, while the influence of indentation depth on elastic modulus was slight. The average value of elastic modulus was 193.97 GPa. Moreover, the rate-dependence phenomenon was prominent during indentation tests as well. It was found that the micro-hardness increased with the increasing loading rate. By means of Nix and Gao model, the size effect during indentation tests was studied. The discipline between H2 and 1/h exhibited a better linear relationship. Further, the density of statistically stored dislocations (SSDs) obtained from the model exhibited a decreasing tendency with the increase of loading rate. In contrast, the density of geometrically necessary dislocations (GNDs) showed an inverse relationship. According to the dynamic material model, the equivalent strain rates during indentation tests were obtained. It was found that the equivalent strain rate decreased with the increase of indentation depth. This phenomenon was considered to be related with the plastic power density (PPD), which represented the kinetic energy of GNDs. By means of Zener–Hollomon parameter, the relationship between activation energy and kinetic energy was established. Furthermore, the dislocation-power theory was developed and applied to interpret the deformation mechanism and the influence of indentation size and loading rate during indentation tests. It should be pointed that the decreasing kinetic energy caused by the decrease of PPD was closely related to the geometry changes, and the decrease of equivalent strain rate would lead to the increase of activation energy which exhibited ascending deformation resistance. In this paper, the influences of indentation size on micro-hardness could be attributed to the variation of equivalent strain rate.

Investigations of inhomogeneous mechanical properties and plastic deformations in HIPed P/M nickel-base superalloy FGH96 by using micro-indentation methods

The purpose of this paper is to investigate the influence of non-uniform precipitate distribution on local plastic deformation in HIPed P/M nickel-base superalloy FGH96 by using micro-indentation methods. The micro-indentation tests were conducted in the indenter load range from 100mN to 450mN and the loading rate range from 1.94mN/s to 19.37mN/s at the room temperature. Then, the load–displacement data of precipitate concentration areas (PCA) and precipitate sparsity areas (PSA) were obtained, respectively. The results show that both PCA and PSA exhibited decreasing micro-hardness during indentations, which proved that the size effect was significant in P/M nickel-base superalloy FGH96 as well. The high micro-hardness of PCA indicates that γ′ precipitate could effectively improve the resistance to deformation. In contrast, the influences of indenter load and loading rate on Young’s modulus were slight. Through plotting test data, it was revealed that a better linear relationship existed between H2 and 1/h. Moreover, the densities of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) were obtained by data fitting. Like other alloys, the GND density fast decreased with the increase of indentation depth during indentations. The SSD density of PCA was higher than that of PSA when the loading rate was lower than 12.91mN/s, indicating that the γ′ precipitates could block the dislocation movements and induce strong interactions. However, this mechanism was inconspicuous at high loading rates due to precipitation shearing mechanism. In addition, the FEM method and forward analysis algorithms were used to predict the micro-indentation parameters. Comparing with FEM method, the prediction results were more accurate for forward analysis algorithms. The constitutive equations of PCA and PSA were established by means of reverse analysis algorithms. The microstructural simulation reflected that the interactions between PCA and PSA were significant during uniaxial compression. The high stress concentration existing in PCA between two adjacent PSAs may cause microcracks and void dislocations at the large deformations.