Cast Ni-base Superalloy Research Articles

Effect of precipitates on high-temperature tensile strength of a high W-content cast Ni-based superalloy

Tensile tests for a high W-content (13–15.5 wt%) cast Ni-based superalloy were carried out at temperatures ranging from 20 °C to 1100 °C. Temperature was varied in increments of 100 °C after reaching 500 °C. The tensile strength of the superalloy used in this study was 150–200 MPa higher than that of other low W-content (<10 wt%) cast Ni-based superalloys at 1100 °C. Microstructural observations revealed that the tensile behavior of the alloy was controlled by the cross-slip mechanism which optionally operated in most of the γ′ precipitates and a large number of isolated stacking faults (with high densities) with two different orientations were formed as temperature reached 1000 °C. At temperatures higher than 1000 °C, the applied stress facilitated the strain-induced precipitation of tungsten-rich MC type carbide particles in the matrix and along the γ/γ′ interface. On the other hand, the low lattice misfit reduced the nucleation barrier, which resulted in the formation of nanosized γ′ phases in the matrix. The strong solid solution effect along with the formation of the tungsten-rich MC precipitates and nanosized γ′ phases improved the high-temperature (temperatures higher than 1000 °C) strength of the superalloy.

Effect of Aging Heat Treatment on the Microstructure and Creep Properties of the Cast Ni-Based Superalloy at Low Temperature

Effect of aging heat treatment on the grain boundary microstructure and creep properties of a cast Ni-based superalloy was investigated. With increasing aging temperature from 750 to 1000 °C, M23C6 carbides along the grain boundaries evolve from fine distributed block, continuous film into the coarse discrete block. Moreover, the M23C6 carbides are mainly enveloped within γ′ layers along grain boundaries during 1000 °C aging. Creep rupture lifetime and elongation at 760 °C and 645 MPa are improved with increasing the aging temperature. In particular, the creep rupture lifetime of the specimens aging at 1000 °C is one order of magnitude higher than that of the specimens aging at 750 °C. The enhancement of ductility induced by the γ′ envelopes plays a significant role in the improvement of creep rupture lifetime.

Properties of a HfC-Reinforced Nickel-Based Superalloy in Creep and Oxidation at 1100°C

A cast Ni-based superalloy strengthened by hafnium carbides was cast and characterized at 1100°C in creep and in hot oxidation. The as-cast microstructure formed by a dendritic matrix and eutectic scriptlike hafnium carbides (HfC) imbricated with the matrix in the interdendritic spaces is very favorable for its high mechanical properties at high temperature. This was confirmed by a three-point flexural creep test carried out at 1100°C under 20 MPa. The slow parabolic mass-gain kinetics showed that this alloy is very resistant against oxidation at 1100°C in air. The HfC carbides remained stable during these tests. The oxidation behavior was chromia-forming. This alloy is ready to be used in structural applications at a temperature reachable by only γ/γ′ Ni-based single-crystals or ODS superalloys.

Microstructure and stress-rupture life of high W-content cast Ni-based superalloy after 1000–1100 °C thermal exposures

High W-content cast Ni-based superalloy which can be served as isothermal forging die materials over 1050 °C was investigated in this paper. Based on the microstructure evolutions, stress-rupture life of the alloy were examined under 1100 °C /70 MPa, where the alloy was evaluated after thermally exposed at 1000, 1050, and 1100 °C for 100–1000 h, respectively. Results showed that increasing of exposure temperature can cause coarsening and coalescing of γ′ precipitates along with a decreasing in their volume fraction, especially at 1100 °C. The coarsening rate of the γ′ precipitates increased with the exposure temperature and followed by the modified Lifshitz-Slyozov-Wagner coarsening theory of Ostwald ripening. The coarsening rate coefficient (k) at 1000, 1050, and 1100 °C was calculated to be 5.72, 7.40, and 8.71 nm/s1/3, respectively. In addition, at those temperatures, it found the blocky M6C carbides in the interdendritic area to be precipitated and coarsened. Furthermore, the M6C carbides linked with each other like a chain wrapped around the grain boundary and along the eutectic γ′ phase. Because of the degradation of the microstructure, the stress-rupture life decreased after thermal exposure, from 94.8 h at 1000 °C to 49.3 h at 1100 °C for 1000 h. Finally, in view of the experimental results, a relationship between the microstructural evolution and the stress-rupture life of the alloy was discussed in details.

Microstructure evolution and mechanical properties of a new cast Ni-base superalloy with various Ti contents

Microstructure evolution of a new cast Ni-base superalloy with various Ti contents is studied by thermodynamic calculations and experimental observations. According to thermodynamic calculations, when Ti content increases from 2.0 wt% to 4.0 wt%, the γ′ weight fraction firstly increases, and then tends to be relatively stable because of the precipitation of η phase. Two alloys containing 2.4 wt% and 3.0 wt% Ti are prepared to verify the accuracy of calculation results. After full heat treatment, γ′ phase, MC carbide and M23C6 carbide are precipitated from two alloys, but no η phase is detected. The quantity of γ′ phase and MC carbide in 3.0Ti alloy is higher than that in 2.4Ti alloy because Ti acts as one of their main forming elements. The higher amounts of γ′ phase and MC carbide obviously improve the mechanical properties of 3.0Ti alloy. To verify whether there is η phase in two alloys or not, long-term aging treatments are carried out at 700 °C, 750 °C and 800 °C for 3000 h. After long-term aging, needle-like η phase is precipitated from 3.0Ti alloy but not from 2.4Ti alloy. The quantity of η phase in 3.0Ti alloy gradually increases with the increase of aging temperature and time. Especially, the precipitation of η phase preferentially takes place beside grain boundaries and near MC carbides. The growth of η phase consumes γ′ phase and causes the appearance of γ′ depleted zones.

Effect of Ta on microstructural evolution and mechanical properties of a solid-solution strengthening cast Ni-based alloy during long-term thermal exposure at 700 °C

Effect of Ta on microstructural evolution and mechanical properties of a solid-solution strengthening cast Ni-based superalloy, IN617B alloy, considered as a potential candidate for castings in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, has been studied in this paper. Detailed observation reveals the alloys with different contents of Ta have similar solution treated microstructure. During long-term thermal exposure at 700 °C, Ta addition refines γ′ particles and enhances the thermal stability of γ′ phase by decreasing the γ-γ′ lattice misfit. With increasing Ta content the solubility of Mo in matrix decreases, which promotes the transformation of film-like M23C6 at grain boundaries (GBs) into blocky M6C carbide with the reaction of M23C6→−Cr+Mo,TaM6C. Consequently, the morphology of GB carbide changes from continuous film structure to dispersed block structure. In addition, local concentration of metal elements leads to the precipitation of trace amount of σ phase which is not affected by Ta concentration. Nearly all the tensile properties at 700 °C of the alloy after long-term thermal exposure are improved noticeably with the increase of Ta content. But, the plasticity of the alloy with 2.0 wt. % Ta addition decreases remarkably due to the excessive Ta content leading to severe M6C coarsening at GBs. The alloy achieves optimal mechanical properties with about 1.0 wt. % Ta content.

Effect of N content on microsegregation, microstructure and mechanical property of cast Ni-base superalloy K417G

This study investigates the effects of nitrogen (N) content (0.0005–0.0045wt%) on the microsegregation, microstructures and high temperature mechanical properties of cast superalloy K417G. The results show that the segregation of Ti to interdendritic region becomes very serious with increased N content. In all three samples with different N contents, the alloys are typically dendritic morphology and γ′ phases show good cubic structure. With the N content increasing from 0.0005wt% to 0.0045wt%, the volume fraction of γ′ phase in the dendrite decreases, the sizes of γ/γ′ eutectic and MC carbide increase, and the volume fractions of γ/γ′ eutectic, MC carbide and micro-porosity firstly increase but then decrease. N addition changes the morphology of MC carbide from script-like shape to blocky shape, and Ti(C, N) is identified in the alloy with 0.0045wt% N. With the N content increasing, the ultimate and yield strength decrease at elevated temperature, and the ductile and stress rupture life firstly have a sharp decrease and then have a slight increase. The specimen with the minimal content of N shows the best comprehensive mechanical properties.

Numerical and experimental investigations on 342 nm femtosecond laser ablation of K24 superalloy

With the feature of ultrashort pulse width and ultrahigh peak power, femtosecond laser can realize real sense of ‘cold’ machining and it has been widely applied in micro and nano scale manufacturing. K24 is a new cast Ni-based superalloy with high strength, strong resistance to corrosion and excellent heat stability under high temperature. Micro drilling method of this kind of material has becomes a tough issue so far. In this paper, interaction process between 342nm femtosecond laser and K24 superalloy is theoretically described. Laser-dependent absorption coefficient and reflectivity of K24 superalloy are derived applying Planck equations while other physical properties like lattice heat capacity and electronic specific heat coefficient are calculated correlated with solid state physics utilizing linear superposition approximation. Based on these, finite difference method is employed to solve one-dimensional two-temperature model. Through corresponding verification experiments the accuracy of mathematical model is proved. The influence rules of machining parameters of 342nm femtosecond laser beam drilling, such as the feed distance of laser spot, scanning times, scanning velocity and laser average power, on the morphology of micro holes are deeply investigated through orthogonal experiments. Following theoretical analysis provide solid foundations for the future application of femtosecond laser drilling on superalloy.

Effect of cooling rate on microstructure, microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures, microsegregation and mechanical properties of cast superalloy K417G were investigated. Scheil-model was applied to calculate the temperature range of solidification. The casting mould with different casting runners was designed to obtain three different cooling rates. The microstructures were observed and the microsegregation was investigated. Also, high temperature tensile test was performed at 900°C and stress rupture test was performed at 950°C with the stress of 235MPa. The results showed that the secondary dendrite arm spacing, microsegregation, the size and volume fraction of γ′ phase and the size of γ/γ′ eutectic increased with decreasing cooling rate, but the volume fraction of γ/γ′ eutectic decreased. In the cooling rate range of 1.42°Cs−1–0.84°Cs−1, the cast micro-porosities and carbides varied little, while the volume fraction and size of γ′ phase and γ/γ′ eutectic played a decisive role on mechanical properties. The specimen with the slowest cooling rate of 0.84°Cs−1 showed the best comprehensive mechanical properties.

Metallography Evaluation of Cast and Wrought Ni-Base Superalloys

. The Ni-base superalloys have an interesting history and evolution since they start to be used in aero jet engines. Microstructures of superalloys have dramatically changed through the years as modern technology of its casting or forging becomes more sophisticated. The first superalloys have polyedric microstructure consist of gamma solid solution, some fraction of gamma prime and of course grain boundaries. As demands on higher performance of aero jet engine increases, the changes in superalloys microstructure become more significant. Further step in microstructure evolution was directionally solidified alloys with columnar gamma prime particles. The latest microstructures are mostly monocrystalline, oriented in [001] direction of FCC gamma matrix. What does not changed through the years is elementary FCC structure of matrix and fundamental group of alloying elements. All microstructure changes bring necessity of proper preparation and evaluation of microstructure. Except already mentioned structures have gamma double prime and various carbides form appear. These structural parameters have mainly positive influence on important mechanical properties of superalloys. However, some detrimental phases as Laves, σ-phase appears as well and have negative influence on heat resistance of superalloys. Paper deals with such microstructural evaluation of both groups of alloys – cast and wrought as well. Microstructure evaluation helps to describe mechanism at various loading and failure of progressive superalloys. Such example where microstructure evaluation is employed is fractography of failure surfaces after fatigue tests, which are as example of metallography evaluation described in this paper as secondary objective. Fatigue test done in this article were at high frequency with push-pull loading, so called high frequency fatigue loading (HFFL) and at low frequency three point flexure, so called low frequency fatigue loading (LFFL).

Initiation of fatigue cracks and lifetime of Ni-base superalloys at high mean stress and temperature

The effect of high-frequency cycling (120 Hz) superimposed on high tensile mean stress on a lifetime of cast Ni-base superalloys IN 713LC at 800 °C and MAR-M 247 at 900 °C was investigated. It has been found that both the alloys exhibit qualitatively similar behavior in creep/fatigue loading conditions. Superposition of small stress amplitudes on the tensile mean stress does not influence the time to fracture until the stress amplitude reaches a threshold value, which is dependent on the mean stress. The higher the mean stress the higher is the stress amplitude threshold value. The observed creep/fatigue behavior is governed by two mechanisms: (a) formation and growth of intergranular cracks by creep mechanism (b) initiation and growth of fatigue cracks from the intergranular creep cracks. The casting defects do not play any important role as regards the lifetime under investigated loading conditions.

Investigation of Solidification and Segregation Characteristics of Cast Ni-Base Superalloy K417G

Differential scanning calorimetry (DSC) analysis, isothermal solidification experiment and Thermo-Calc simulation were employed to investigate solidification characteristics of K417G Ni-base superalloy. Electron probe microanalysis (EPMA) was employed to analyze the segregation characteristics. Liquidus, solidus and the formation temperatures of main phases were measured. In the process of solidification, the volume fraction of liquid dropped dramatically in the initial stage, while the dropping rate became very low in the final stage due to severe segregation of positive segregation elements into the residual liquid. The solidification began with the formation of primary γ. Then with solidification proceeding, Ti and Mo were enriched in the liquid interdendrite, which resulted in the precipitation of MC carbides in the interdendrite. Al accumulated into liquid at the initial stage, but gathered to solid later due to the precipitation of γ/γ′ eutectic at the intermediate stage of solidification. However, Co tended to segregate toward the solid phase. In the case of K417G alloy, combining DSC analysis and isothermal solidification experiment is a good way to investigate the solidification characteristics. Thermo-Calc simulation can serve as reference to investigate K417G alloy.

Tensile Properties and Deformation Behavior of Several Cast Ni-Based Superalloys Fabricated by Different Solidification Ways

The tensile properties and deformation behavior of several cast Ni-based superalloys, respectively, in the equiaxed, columnar-crystal and single-crystal styles are comparatively studied. The effects of solidification way, heat treatment and strain rate on the tensile properties are discussed in detail. It is found that the reduction of grain boundaries by the feasible solidification ways offers cast Ni-based superalloys the potential capability of improving the mechanical properties, the ultimate achievement of which is also confirmed to lie on the appropriate modifications of chemical composition and heat treatment. The prolongation of solid solution facilitates the precipitation of fine secondary γ′ phase, whereas the extension of high-temperature aging leads to the coarsening of secondary γ′ phase. The combination of these two aspects has a crucial influence on the tensile properties. Under tensile applied stress, the surface grains of DZ-A alloy deform slightly, while the inner grains deform heavily. This deformation inhomogeneity is ascribed to the occurrence of cracks or oblique grains near the surface of specimens and the sliding or decohesion of grain boundaries between the surface and inner grains. Regardless of strain rate, the ILTDM (intermediate-low-temperature ductility minimum) phenomenon always happens in the temperature range from 400 to 600 °C in all the investigated alloys, the occurrence of which is closely related to the strong strain-hardening behavior in the deformation process. Finally, the interaction of slip bands which are the main deformation mode below 600 °C is established to be the essential reason for the strain hardening.

铸造镍基高温合金中初生 M C碳化物的退化过程和机理

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During longterm thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+g →M6C+g′, MC+g→M6C + M23C6+ g′ and MC+g→M6C + M23C6+h. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of g′ phase, coarsening of grain boundaries and precipitation of topologically closepacked (TCP) phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 °C for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field (HAADF) mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the g matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the g matrix, while Ti, W and Mo come from both primary MC and g matrix. The precipitation of h phase is determined by *国家自然科学基金项目51001101和国家能源局项目NY20150102资助 收到初稿日期: 2015-07-17,收到修改稿日期: 2015-11-24 作者简介:孙文,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00399 第455-462页 pp.455-462

The effects of solution treatment on the microstructure of the cast Ni-based IN100 superalloy

In this research, the effects of the partial, full and partial + full solution heat treatments followed by aging at 900 °C for 10 h, on the microstructure of cast Ni-based IN100 superalloy were assessed. It has been found that, the alloy in the partial + full solution treated condition had the optimal combination of γ’ morphology, volume fraction and size. In this condition, the alloy possesses a cubic primary γ’with an average size of 470±10nm and 45% volume fraction. Discrete M23C6 and M6C carbides were formed at the grain boundaries and the morphology of the cubic MC carbide was changed to the spherical shape. In addition, the volume fraction of γ’/γ eutectic phase dropped to half of its value, compared to the as-cast alloy. During partial solution treatment followed by aging, discrete carbides were formed at the grain boundaries. This treatment without full solutioning was not an effective method to provide an optimal volume fraction and arrangement of γ’ and MC carbides morphology. Full solutioning alone, changed the cubic morphology of the primary γ’ and the blocky MC carbides to the spherical shape.

Healing Process of Casting Pores in a Ni-based Superalloy by Hot Isostatic Pressing

Hot isostatic pressing (HIP) with a pressure of 180 MPa at a temperature of 1170 °C was introduced to an investment cast Ni-based superalloy (M91) turbocharger blade to explore the healing process of casting pores generated during investment casting. Optical micrograph and scanning electron microscopy (SEM) observations indicate that eutectic pores are the main cast defects in the as-cast blade before HIP. These pores normally locate at the solidification front of γ/γ′ eutectic with a size of a few micrometers to a few tens of micrometers. After HIP for 4 h, most of the pores were closed. Based on phase characteristics, these pores were healed by the formation of γ matrix with finer and irregular-shaped γ′ precipitates. Healing interface can be easily distinguished by SEM. Line scan by using energy dispersive X-ray spectroscopy (EDS) reveals a much higher Ti and Al concentration in the healing interface. It is proposed that solute diffusion toward the casting pores during HIP results in the formation of γ, and the much higher concentration of γ′-forming elements Al and Ti near the healing interface contributes to the precipitation of γ′ in the healed region in the succeeding cooling process after HIP.

Effect of microstructural characteristics on the low cycle fatigue behaviors of cast Ni-base superalloys

Low cycle fatigue behaviors of two cast Ni-base superalloys have been investigated. Effects of microstructure of a solution hardened alloy and a precipitation strengthened alloy on cyclic stress responses were studied at various temperatures and strain ranges. In the case of a solution hardened alloy, Hastelloy X, fatigue lives at lower temperatures are longer than those at higher temperatures regardless of total strain range. IN738LC which is strengthened by γ′ particles is founded to have longer fatigue life at low temperature than high temperature when total strain range is low. However, the alloy has longer fatigue life at high temperature than low temperature when the total strain range is high. Because γ′ particles are cut by coupled dislocation and slip band forms, deformation is inhomogeneous, and strain is concentrated to local region during low temperature fatigue test, while thermally assisted dislocation movements result in less stress concentration at high temperature. Therefore, fatigue life of the alloy at high temperature is longer than that of low temperature under high strain range condition. When strain range is low, oxidation plays important role in the fatigue behavior. Oxidation decreases fatigue life by means of affecting crack initiation and growth behavior at high temperature.

Microporosity Prediction and Validation for Ni-based Superalloy Castings

Microporosityin high performance aerospace castings can reduce mechanical properties and consequently degrade both component life and durability. Therefore, casting engineers must be able to both predict and reduce casting microporosity. A dimensionless Niyama model has been developed [1] that predicts local microporosity by accounting for local thermal conditions during casting as well as the properties and solidification characteristics of the cast alloy. Unlike the well-known Niyama criterion, application of the dimensionless Niyama model avoids the need to find a threshold Niyama criterion below which shrinkage porosity forms - a criterion which can be determined only via extensive alloy dependent experimentation. In the present study, the dimensionless Niyama model is integrated with a commercial finite element casting simulation software, which can now more accurately predict the location-specific shrinkage porosity volume fraction during solidification of superalloy castings. These microporosity predictions are validated by comparing modelled results against radiographically and metallographically measured porosity for several Ni-based superalloy equiaxed castings that vary in alloy chemistry with a focus on plates of changing draft angle and thickness. The simulation results agree well with experimental measurements. The simulation results also show that the dimensionless Niyama model can not only identify the location but also the average volume fraction of microporosity distribution in these equiaxed investment cast Ni-based superalloy experiments of relatively simple geometry.

Boron Depletion in a Nickel Base Superalloy Induced by High Temperature Oxidation

The conventionally cast Ni-base superalloy Rene 80 containing 90 wt ppm boron was oxidized in synthetic air at 850–1,050 °C for times up to 100 h. Extensive microstructural studies using GD-OES and SEM/WDX/EBSD revealed formation of a porous chromia scale containing TiO2 precipitates at its outer surface and semi-continuous layers of BCrO3 and CrTi2O5 at its base. The development of B-rich oxide in the scale is related to its high thermodynamic stability and the high alloy boron diffusivity. Its enrichment in the oxide scale resulted in B-depletion from the alloy matrix, which was extremely rapid at 1,050 °C, draining the boron content of a 2 mm thick alloy coupon in 100 h. After 100 h exposure at 950 °C, the B-content dropped below 40 wt ppm, but only minor depletion was found after 100 h at 850 °C. Predictions of B-depletion based on calculation of its diffusion from a thin, flat sheet were in good agreement with experiment at 1,050 °C. Precipitation of B-containing compounds (borides) within the alloy is shown to account qualitatively for slower B-outdiffusion at lower temperatures.

Thermal stability of primary MC carbide and its influence on the performance of cast Ni-base superalloys

Investigations into the thermal stability of primary MC carbide (TSMC) during long-term thermal exposure and its effects on the performance of nine cast Ni-based superalloys with different Nb/Ti and (W + Mo)/Cr ratios have been undertaken. The results show that the chemical composition of primary MC can be precisely modified by the Nb/Ti ratio of alloys while the distribution and morphology of primary MC carbides is not affected. During long-term thermal exposure, the TSMC is dependent not only on the composition of primary MC, i.e. the Nb/Ti and (Nb + Ti)/(W + Mo) ratios of primary MC, but also on the (W + Mo)/Cr ratios of the alloys, and the latter has a more significant effect than the formers. With the decrease of these parameters, the TSMC decreases and more degeneration reactions of primary MC occur during thermal exposure. Moreover, with the (W + Mo)/Cr ratio of the alloys decreasing from 0.55 to 0.37 and finally to 0.22, the transformed carbides from primary MC change from M6C to coexist of M6C and M23C6 and to M23C6. With the decrease of TSMC, the stress-rupture life of alloy at 800 °C/294 MPa is improved, because that more secondary carbides precipitate in the matrix which can retard or even suppress the topologically close-packed (TCP) phases.