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This study focuses on the single crystal （SC） superalloys CMSX-4 and DD5， designing and fabricating a SC test plate casting with five distinct stages. Castings with the same crystal orientation are selected to investigate their recrystallization （RX） behavior after solution heat treatment at 1300 ℃ for 2 h and 1310 ℃ for 4 h. The results indicate that CMSX-4 exhibits a stronger tendency for RX compared to DD5. Following a solution heat treatment at 1300 ℃， the CMSX-4 SC test plate displayed RX at the lower corners of 2-5 stage platforms， with the extent of RX expanding as the solution heat treatment temperature rose. Conversely， no RX was detected in the DD5 SC test plate post-treatment at 1300 ℃. Elevating the solution temperature to 1310 ℃ results in only a minor area of RX on the outer platform of the fourth stage in the DD5 SC test plate. The micro-shrinkage porosity and eutectic content of the as-cast and solution heat treated CMSX-4 alloy are both higher than those of the DD5 alloy. The higher content of eutectic and micro-shrinkage porosity provides more nucleation sites and quantities for RX in the CMSX-4 alloy， while the solution heat treatment temperatures above the γ' phase dissolution temperature weakens the pinning effect of coarse γ' phase on RX growth， and high melting point carbides and residual eutectic become important factors hindering RX growth. In addition， the high content of Co element reduces the stacking fault energy of the CMSX-4 alloy， making it with a high recrystallization tendency.

This study has investigated the effect of NaCl and different gaseous environments on the stress corrosion cracking susceptibility of CMSX-4 at 550 °C. The presence of SOx leads to the rapid dissociation of NaCl into Na2SO4 and the release Cl2 and HCl, which then trigger an active oxidation mechanism and stress corrosion cracking. The incubation time for crack initiation at 690 MPa and in the presence of a sulphur containing environment is 10 min. A working hypothesis is that stress corrosion cracking occurs due to the hydrogen released at the oxide/alloy interface when metal chlorides are formed; however, this hypothesis needs to be further explored. [ABSTRACT FROM AUTHOR]

In the present work, peripheral milling of CMSX-4 has been carried out at different machining conditions. Three levels of cutting velocity, feed rate and axial depth of cut have been selected to analyse the cutting forces and surface roughness during machining of CMSX-4. Full-factorial design (L27) has been considered for the machining. Furthermore, the analysis of variance has been carried out to find out the most influencing machining process parameters, which affect the cutting force and the surface roughness. The feed rate and depth of cut have been found to be the most influencing process conditions affecting the cutting force while cutting velocity and feed rate have been found to affect the surface roughness. A cross interaction between the machining process parameters has also been found to affect both cutting force and surface roughness during machining. The chip formation analysis has been carried out to analyse the effect of process parameters on the severity of localised shearing induced during the machining process. An increase in spindle speed beyond 3000 rpm has been found to be facilitating the formation of segmented chip. A segmented chip has also been formed at the combination of high feed rate and axial depth of cut. The low feed rate up to 0.04 mm/flute and axial depth of cut up to 0.1 mm accelerate the formation of continuous chip. [ABSTRACT FROM AUTHOR]

CMSX-4 nickel base superalloy is the second-generation alloy of this single crystal, which has improved its mechanical properties due to the lack of grain boundaries. According to the working conditions in using this superalloy, achieving less surface defects and lower surface roughness after the manufacturing process is very important. Therefore, the comparison of the surface of this superalloy after grinding, wire electro discharge machining (WEDM) and electrochemical machining (ECM) has been investigated by scanning electron microscope (SEM) and surface roughness. Surface roughness after WEDM, ECM and grinding are 3.337, 0.549, and 0.458 micro-meter, respectively. ECM and grinding processes are suitable from the point of view of surface roughness. On the other hand, in the SEM images after ECM, the defects caused by this process were not observed (compared to the other two processes). Besides, hardness after WEDM, ECM and grinding are 38.9, 39.7 and 40.1 HRC respectively. To conclude, the ECM process has desirable results and is a suitable alternative process for manufacturing parts with smooth surface and less surface defects. [ABSTRACT FROM AUTHOR]

Single crystal plate castings of superalloy CMSX-4 were prepared, providing simplified geometries of turbine blades featured with five stage platforms. The recrystallization behavior in the plate castings was investigated after solution heat treatments at different temperatures. The results show that after solution heat treatment, recrystallization occurs at the abrupt cross-section of every platform except the lowest one. The recrystallization area increases and then decreases as the platform's height increases. The outside platform of the casting is also more likely to result in recrystallization than the inside one. The nucleation and growth of the recrystallization grains are significantly influenced by the solution heat treatment temperature (TH). When TH is raised from 1303 ℃ to 1315 ℃, the recrystallization area increases by 30 times, from 9.7 mm2 to 293 mm2. Because of the abrupt section transition, the platform part of the casting will produce more plastic deformation and accumulate a large amount of deformation energy during solidification and cooling, and the abrupt change in cross-section will easily become the source of recrystallization during the subsequent heat treatment. Meanwhile, the difference in heat exchange conditions, cooling rate and deformation rate during solidification leads to uneven stress distribution. As a result, different recrystallization behaviors appear after solution heat treatment.Furthermore, increasing TH can effectively promote the dissolution of γ/γ' eutectics and coarse γ' phase, reduce the pinning effect of residual γ/γ' eutectics and coarse γ' phase in the recrystallization grain boundary migration process, and significantly increase the recrystallization defect area.

In this work the effect of crystal structure perfection on the creep resistance of single crystal superalloys. The CMSX-4 and CMSX-4®Plus with increased Re content is analyzed. Single crystal rods are made by directional solidification process at the withdrawal rates 3 and 5 mm/min. The evaluation of microstructure and crystal structure perfection are carried out by light microscopy and X-ray diffraction O-scan methods. Creep tests are performed according to ASTM E-139-11 standard at the temperature of 982 °C. Mathematical models for the creep resistance of the models are developed for both investigated superalloys. The influence of the crystal structure on the the investigated superalloys are built and the steady state creep rates are determined. Creep strength of both superalloys are compared. The results show that the CMSX-4®Plus nickel superalloy exhibits nearly two times higher creep resistance compared to the conventional CMSX-4 one. [ABSTRACT FROM AUTHOR]

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In the pursuit of more efficient gas turbine engines, components are required to operate for longer times at elevated temperatures. This increased time in service, together with a complex loading regime, can expose the material to environmental attack. This work has demonstrated that the interaction of stress, NaCl and a sulphur-containing environment is critical to cause crack initiation in the early stages of the exposure and accelerated corrosion rates in CMSX-4 at 550°C. The effect of having small concentrations of moisture in the gaseous environment or as water crystallisation in the salt is still to be investigated. A working hypothesis is that the interaction of alkali chlorides with a sulphur-containing atmosphere is the trigger to a self-sustaining cycle where metal chloride formation, vaporisation and oxidation lead to high amounts of hydrogen injection in a rapid manner and, therefore, hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

The pore size of nanoporous superalloy membranes produced by directional coarsening is directly related to the γ-channel width after creep deformation, since the γ-phase is removed subsequently by selective phase extraction. The continuous network of the γ′-phase thus remaining is based on complete crosslinking of the γ′-phase in the directionally coarsened state forming the subsequent membrane. In order to be able to achieve the smallest possible droplet size in the later application in premix membrane emulsification, a central aspect of this investigation is to minimize the γ-channel width. For this purpose, we use the 3w0-criterion as a starting point and gradually increase the creep duration at constant stress and temperature. Stepped specimens with three different stress levels are used as creep specimens. Subsequently, the relevant characteristic values of the directionally coarsened microstructure are determined and evaluated using the line intersection method. We show that the approximation of an optimal creep duration via the 3 w 0 -criterion is reasonable and that coarsening occurs at different rates in dendritic and interdendritic regions. The use of staged creep specimens shows significant material and time savings in determining the optimal microstructure. Optimization of the creep parameters results in a γ-channel width of 119 ± 43 nm in dendritic and 150 ± 66 nm in interdendritic regions while maintaining complete crosslinking. Furthermore, our investigations show that unfavorable stress and temperature combinations favor undirectional coarsening before the rafting process is completed. [ABSTRACT FROM AUTHOR]

Life prediction plays an important role in avoiding the catastrophic failure of a gas turbine blade subjected to simultaneous high thermal and mechanical load. The purpose of this paper is to predict the life of C3X blade of a nickel-based single crystal super alloy, CMSX-4 material under thermo-mechanical load at four different operating conditions. The blade was geometrically modelled and by using computational fluid dynamics approach with k-ε turbulence model the distribution of temperature, pressure and heat transfer coefficient on the surface of the blade was determined. Mechanical and thermal were determined by uncoupled heat transfer analysis and thermomechanical stresses by sequentially coupled thermo-mechanical stress analyses for a flight length of 5000 seconds. Finally, life of the blade was estimated considering fatigue-creep interaction by applying ductility exhaustion method and morrow mean stress correction. The results show that the critical damage occurred at trailing edge and fixed portion of the blade. In comparison to the mechanical or thermal load, there is a significant change in stress magnitude for the thermo-mechanical load. Also, the blade radial displacement is mainly ascribed to the thermal load rather than the mechanical load. [ABSTRACT FROM AUTHOR]

• Investigated the influence of sliver defects on the creep rupture life of nickel-based single crystal superalloys. • Revealed the damage mechanism of sliver defects on the creep life of single crystal superalloys through SEM and EBSD observations. • Developed creep constitutive equations for specimens containing sliver based on crystal plasticity theory, explaining the reasons for the decrease in sample life caused by sliver. • Predicted the damage tolerance of sliver defects with different orientations. Sliver defects are common casting imperfections in the preparation of single crystal blades, significantly impacting the high-temperature mechanical performance of blades and potentially leading to blade failure. A detailed study was conducted on the microstructure of sliver in the second-generation nickel-based single crystal superalloy CMSX-4, focusing on its influence on creep performance at 980 °C/250 MPa. Results indicate a substantial impact of sliver on the creep performance of single crystal superalloys, with a 26.37 % reduction in creep rupture time for samples with sliver compared to those without. Fracture analysis reveals that sliver primarily contributes to performance deterioration by promoting the extension of grain boundary microcracks through grain boundary sliding during the creep process, accelerating sample fracture. Utilizing crystal plasticity theory, a successful fit was achieved for the creep rupture life of samples with sliver, allowing the prediction of fracture life for samples with different orientations of slivers. Predictive results suggest that, for CMSX-4 alloy, the orientation of the sliver should be restricted to within 8°. [ABSTRACT FROM AUTHOR]

Superalloys are crucial in high-temperature applications, particularly in the gas turbine industries. With the development of high-temperature superalloy materials, efforts are still being made to improve the turbine entry temperature (TET), which will ultimately raise the efficiency of gas turbines. In this research, a second-generation single-crystal superalloy CMSX-4 based on nickel was prepared. The surface was coated by thermal barrier coating (TBC) with a bond coat of NiCoCrAlY with vacuum plasma spray (VPS) and a top coat with Yttria Stabilized Zirconia (YSZ) by Electron Beam Physical Vapor Deposition (EB-PVD) with a minimal current of 2.3 A with two sources compared to the conventional high-current that has been currently employed. These CMSX-4 specimens, both the as-cast and TBC coated, were used to test the effects of cyclic oxidation and hot corrosion at 1000 °C, simulating an aircraft engine. Also, a hot corrosion test using salt compositions (Wt%) of 60Na 2 SO 4 - 40NaCl was performed simulating an aviation engine environmental condition. A FE-SEM/EDS analysis was used to examine the surface and cross-section microstructures of oxidized, hot-corroded as-cast, and hot-corroded TBC-coated CMSX-4 and its corrosion products. Using the XRD technique, the phases that were found in the oxidized and hot-corroded samples were identified. The outcome demonstrated that the EB-PVD TBC-coating on CMSX-4 with beam switching technique and a lower electron beam current has adequate protection without any damage to the coatings at 1000 °C. • EBPVD processes with a beam-switching with continuous evaporation of two targets • Beam switching increased the coating window increasing productivity. • Successful coating with a 23 % lower electron beam current, was achieved. • Optimized scanning trajectory increased the aggressiveness of the electron beam. [ABSTRACT FROM AUTHOR]

The process to extract rhenium from a superalloy is an immense technical challenge due the complex chemistry involved. Being one of the rarest elements in the earth’s crust the scarcity and cost of rhenium makes it advantageous to recover the element from scrap superalloy. In this research the separation and monitoring of the different stages of the recycling process to extract rhenium from CMSX-4 superalloys using a distillation process were performed. This novel method combining distillation and use of exchange resins was used to separate rhenium from a complex mixture of metals in the CMSX-4 superalloy. The identification and quantitation of perrhenate and contaminants were performed by atomic absorption spectroscopy (AAS), Fourier transform infrared spectroscopy (FTIR), ion chromatography (IC) and Scanning Electron Microscopy- Energy Dispersive X-Ray (SEM-EDX). Perrhenate ions were extracted with purity close to 93%. The analytical characteristics for a novel infrared method to quickly identify perrhenate anions from CMSX-4 are presented. The main characteristics of the analytical validation were: LoD: 0.5% w/w; LoQ: 1.5% w/w; linear range 1.5-100% w/w; correlation coefficient R2 = 0.9905; precision (%RSD) for 10%w/w = 6.6 and 75%w/w = 4.1, respectively; accuracy (%) for 10% w/w 99.6% and 75% w/w=101.1, respectively.

Understanding the impact of thermo-solutal convection during alloy solidification is of central importance in the casting industry. Convective flow generated from density variations in the liquid is at the origin of many casting defects, such as chemical segregations and freckles, to which Ni-based superalloys used in jet engines are known to be highly sensitive. In this article, we present experimental results obtained during directional solidification of a CMSX-4 single crystal superalloy observed in-situ by means of synchrotron X-radiography at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The use of a high-temperature directional solidification furnace, an intense monochromatic beam and a high-speed X-ray sensitive camera allowed the direct observation of the development of solute flow to be performed during dendritic growth of a Ni-based alloy. The transient stage leading to the formation of a dendrite network and the interaction with the concomitantly developing convective pattern are studied. The impact of solute flow on the dendrite growth velocity is characterized for three cooling rates. Measurements of dendrite tip positions show that the convective flow above the solidification front induces a periodic oscillation of the dendrite growth velocity and that the oscillation amplitude decreases for higher cooling rates. The development of residual liquid areas is observed on the sides of the sample. Fragmentation of dendrite parts occurs during the solidification of the residual liquid areas and leads to the formation of spurious grains. The experimental results are analyzed by using a Rayleigh number-based model, highlighting the need for direct simulations in the future. Image, graphical abstract [ABSTRACT FROM AUTHOR]

To increase efficiency and reduce greenhouse gas emissions, the operating temperatures of combined power plants have increased. As a result, high-pressure turbine (HPT) blades are subjected to increasingly harsh operating conditions, which cause thermo-mechanical fatigue (TMF) damage and reduce the lifetimes of the gas turbine blades. Therefore, accurate analysis of the TMF characteristics caused by various strain and temperature conditions is needed to ensure the integrity and provide accurate lifetime assessments of the components of gas turbines. However, there have been few studies on the TMF characteristics of single crystal Ni-based superalloys, particularly CMSX-4. In this study, the TMF characteristics of a <001 > oriented single crystal Ni-based superalloy CMSX-4, which is used for HPT first-stage rotor blades, were investigated via strain-controlled TMF tests and the fatigue fractogra-phy was observed using scanning electron microscopy. Additionally, the relationship between the fatigue lifetimes of CMSX-4 and IN738LC was evaluated, and the tensile properties and microstructures at different temperature were investigated. Such TMF test results can be useful for analyzing the thermo-mechanical behavior and predicting the lifetimes of blades. [ABSTRACT FROM AUTHOR]

Coating application to superalloys provides oxidation and corrosion resistance. One of the key elements in gas turbine coatings is aluminium (Al) which forms a thin, dense alumina layer upon exposure to oxygen and impedes further oxygen diffusion into the alloy. However, Al in the coating depletes due to inward diffusion and oxide formation. The rate of Al depletion is a function of the coating composition/structure, substrate alloy composition/structure and service conditions. In this study, a PtAl diffusion coating, was applied onto polycrystalline IN738LC and single crystal CMSX-4. The coated systems were exposed to 1080°C for 1000 h. Both SEM and XRD analyses were carried out on the as-coated and tested samples. It was found that despite the much higher Al content in CMSX-4, Al depletion was greater. With the higher Cr in IN738LC, a diffusion barrier enriched in Cr/Mo was able to form and reduce the Al depletion. [ABSTRACT FROM AUTHOR]

Highlights • Optimised parameters for the extraction of rhenium from CMSX-4 superalloy reported. • Aqua-regia concentration to 100% (v/v) resulted in more than 30% gain in recovery. • Leaching kinetics fits into a chemical reaction controlled kinetics model in first 6 h. • Leaching kinetics fits a diffusion through the product layer model after 6 h. • Ultrasonic waves to the leaching process has proven useful to slightly increase yield. Abstract A study to leach and recycle rhenium from CMSX-4, a second generation ultrahigh-strength single crystal nickel-based superalloy, containing 3% rhenium is presented. Experimental factors involved in the leaching process of rhenium from CMSX-4 using aqua-regia solution have been investigated and reported. Experimental factors such as concentration of aqua-regia, the use of sonication, leaching period, solid-to-liquid ratio and stirring speed have been optimised and the leaching kinetics of rhenium and other component metals of superalloy CMSX-4 have been studied. Increasing aqua-regia concentration to 100% (v/v) resulted in more than 30% gain in recovery which can be attributed to the increased availability of leaching reagents. The leaching kinetics of rhenium from superalloy CMSX-4 in 100% aqua-regia solution fit into a chemical reaction controlled kinetics model for the first period of leaching. For the leaching period after 480 min, the leaching kinetics of rhenium fits a diffusion through the product layer model. Application of ultrasonic waves to the leaching process has proven useful to slightly increase yield. Given the small increase, however, sonication may not be economically feasible for recovery of rhenium on an industrial scale. Precipitation of other metals away from the rhenium rich solution has also been explored to prepare the solution for a downstream rhenium recovery process. It was found that that two unique precipitates can be recovered from the solution. The first precipitate, recovered at pH 5.05, has been determined to be mainly composed of the oxides of aluminium, chromium, molybdenum, and titanium. A green precipitate of mixed hydroxides of cobalt and nickel (MHP) which has commercial value can then be recovered at pH 7.0 leaving a rhenium enriched solution for further processing. [ABSTRACT FROM AUTHOR]

For nickel-based single-crystal superalloys, simultaneously reducing the segregation and optimising the microstructure is a primary challenge in the field of heat treatment. To further improve rupture life, the element partition, microstructure evolution, and creep behaviour of the second-generation nickel-based single-crystal superalloy CMSX-4 under five different solution processes were studied using electron microprobe, scanning electron microscopy, and transmission electron microscopy. Under the same conditions, both continuous heating and remelting heat treatment exhibited higher elements homogenisation efficiency. Moreover, it was found that the improvement of element segregation can make γ′ phase smaller and more uniform. These heat treatments reduced the incipient melting tendency or eliminated the incipient melting eutectic, thereby increasing the peak heat treatment temperature of the CMSX-4 alloy. However, remelting heat treatment results in a significant increase in the porosity of the alloy, leading to a deterioration in performance. Finally, based on the above study, a novel process combining stepwise heating and continuous heating was proposed, which not only increased the peak solid-solution temperature but also prevented the incipient melting and optimized the microstructure. After using the novel process, the durability of the alloy increased by 48 % from 240 to 356 h under the test conditions of 980 °C/250 MPa. The higher proportion of the γ′ phase, narrower matrix channel, and denser dislocation network were the main reasons for the improvement in the rupture life. [Display omitted] • A novel solid solution process was designed, resulting in a 48 % increase in durability (from 240 h to 356 h). • Discovered the potential risks and causes of performance deterioration in existing remelting heat treatment. • Established the influence of elemental partitioning behavior on the size, morphology and misfit relationship of γ′ phase. • Systematically studied the various heat treatments' effect on microstructural evolution and deformation mechanisms. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on the influences of laser trepanning drilling process parameters on the recast layer thickness and surface crack formation in CMSX-4 nickel-based superalloy angled holes. The effects of peak power, pulse frequency and the trepanning speed as input parameters were investigated in details by varying the laser drilling conditions using Taguchi orthogonal array-based design of experiment approach. Analysis of variance identifies the significant parameters affecting the output responses. It is found that the output responses are affected mainly by the peak power and trepanning speed. The experimental results reveal that the recast layer thickness increases with the increase of peak power and trepanning speed whereas the crack number density decreases with the increase of peak power only. Pulse frequency has no significant effect on both output responses within the range of values investigated. The knowledge gained in this parametric study could be used to improve the metallurgical characteristics of laser-drilled nickel-based acute angled holes. [ABSTRACT FROM AUTHOR]

The effect of laser trepanning speed and, as a result, recast layer thickness on the high temperature corrosion fatigue behaviour of CMSX-4 superalloy acute angled holes was investigated. The experimental test results show that an increasing laser drilling speed caused a reduction in corrosion fatigue life by 35–50% at 850 °C, under low cycle fatigue regime. This reduction was found to correlate directly with the recast layer thickness and surface anomalies within the recast layer produced during the laser drilling process. Corrosion had a smaller effect on the overall life of the laser drilled specimens under the conditions tested. The results presented show that laser trepanning speed is influential in limiting the life performance of laser drilled components in service. [ABSTRACT FROM AUTHOR]

Hot corrosion causes significant problems for both aerospace and power generation industries, where the combination of high temperature, corrosive gases, and contaminants severely limits component operating lifetimes in gas turbine hot gas streams. Multiple laboratory testing methodologies exist to study this hot corrosion, and these can be affected by a range of variables. This paper investigated the impact of varying deposit recoat cycle length when using the 'deposit recoat' testing method. CMSX-4 samples were exposed to simulated type II (pitting) hot corrosion conditions, with the same overall deposit load (averaged across the total exposure run), but different deposit recoat cycles. Post-exposure, samples underwent dimensional metrology analysis to compare metal loss resulting from different deposit recoat cycle lengths. Results for CMSX-4 suggest very small differences in corrosion losses, indicating CMSX-4 hot corrosion datasets obtained from deposit recoat experiments with different deposit recoat cycle lengths can be compared with confidence. [ABSTRACT FROM AUTHOR]

The freckle formation was investigated in directionally solidified CMSX-4 superalloy specimen with abrupt cross section variation to better understand that in the real hollow blade. Freckles were usually observed on the specimen surface within a range of 2–15 mm above the platform, and the freckles on both sides were slanted to respective edge. The freckling number was reduced with increasing withdrawal rate. Combining with the simulation, freckle formation was analysed. The deceleration of dendrites growth above the platform caused by the abrupt variation in cross section leads to an increase in the tendency of the fact that the interdendritic fluid-flow velocity is larger than the growth rate, and thus increasing the freckling tendency. The pressure difference in the horizontal direction induced by the concaved solidification interface forces the interdendritic segregated melt to flow toward the surfaces and edges, and slants the freckles. The freckle formation can be suppressed with increasing withdrawal rate for the refined dendrite microstructure. [ABSTRACT FROM AUTHOR]

Selective electron beam melting (SEBM) is a powder-bed-based additive manufacturing process that was used to produce cylindrical and columnar-grained parts made of Ni-base superalloy CMSX-4 from pre-alloyed and atomized powder. SEBM is characterized by high temperature gradients during solidification, which results in a very fine microstructure that is several orders of magnitude smaller than in conventionally cast material. This opens up new perspectives regarding time-consuming solution heat treatment. Nevertheless, microstructural heterogeneities, such as segregation and porosity, are still present on a much smaller scale, and also the high susceptibility to cracking of this alloy class during welding has to be taken into account. Since the latest generation of hot isostatic presses (HIP) are able to simultaneously heat-treat and eliminate porosity owing to their quenching capability, such a HIP is used in this work. The influence of different HIPing-heat-treatment-strategies with variation of temperature and time at a constant pressure on the SEBM-microstructure was investigated with emphasis primarily on segregation and porosity. The results demonstrate that only a few minutes of solutioning are sufficient to dissolve segregations and to close pores. The initial degree of homogeneity of the SEBM-material is responsible for the short solutioning-time. [ABSTRACT FROM AUTHOR]

A large test sample of single-crystal superalloy (CMSX-4) deposited with a MCrAlY coating of γ and β phases was subjected to a thermal-cycling fatigue (TCF) test at 1100 °C for about 300 cycles. Near the coating-substrate interface where an inner β-depletion zone often develops in the coating for other superalloy-MCrAlY systems, a continuous β-layer was formed instead for this particular system after the TCF test. The formation of the β-layer was related to the accumulation of Al at the interface. Simulations using an oxidation-diffusion model were carried out on this and a number of other superalloy-MCrAlY systems. It was derived that the formation of the β-layer at the interface was likely due to the high Al activity of CMSX-4 which resisted the inward diffusion of Al from the coating. The simulation results also indicated a positive effect provided by the formation of the β-layer on the coating's oxidation resistance. [ABSTRACT FROM AUTHOR]

The solidification microstructure development of single crystal CMSX-4 superalloy produced by the Bridgman method was investigated through experimental measurements and numerical modelling. The numerical investigation was performed using the commercial software ProCAST/CAFE and included the following variables: nucleation potential, heat transfer coefficients and emissivity. The predicted solidification microstructure, nucleation and growth, single grain selection and texture to the growth direction < 001> were validated by SEM microstructural and X-ray diffraction investigations. Modelling predictions of the temperature gradient and of the solidification velocity for different heat extraction conditions were used to calculate the primary and secondary arm spacing evolution with the distance from the cooling plate. The results were calibrated with experimental measurements, and fitted equations were established as a function of temperature gradient and solidification rates. It was found that appropriate selection of the numerical modelling input parameters allows prediction of microstructural features that match reasonably well the experimentally measured ones. [ABSTRACT FROM PUBLISHER]

The effects of different thermal barrier coating (TBC) top coat structures and substrate alloys on the isothermal oxidation behaviors of TBC systems were investigated at 1080 °C in lab air. The tested TBC systems consisted of two nickel-based superalloy substrates (CMSX-4 and IN738LC), a platinum aluminide bond coat and two 8YSZ top coats (vertical cracked and columnar structured). Samples with IN738LC substrate demonstrated longer isothermal oxidation lives than the counterparts with CMSX-4 substrate. Outward refractory elemental diffusion in coating systems with CMSX-4 substrate and void formation at the interface between thermally grown oxide and bond coat was found to be responsible for the early failure of TBCs. Columnar structured YSZ top coat seemed to provide better protection of the bond coating and substrate, marginally delaying the failure of the both coating systems with IN738LC and CMSX-4. [ABSTRACT FROM AUTHOR]

First principles calculations are used to investigate the segregation behaviour of Co, Cr, Re, Mo and W to intrinsic and extrinsic stacking faults in γ′-Ni 3 Al. It is shown that the change in stacking fault energy depends on local alloying concentration and is related to subtle changes in the electronic structure of the alloying elements and adjacent nickel atoms. The results are consistent with observed stacking fault segregation in commercial superalloys and in particular the behaviour of Co and Cr. [ABSTRACT FROM AUTHOR]

Small punch testing is now a widely recognised approach for obtaining useful mechanical property information of critical structural components, particularly in the nuclear industry. However, to date the utilisation of this method has been limited to isotropic materials such as aluminium alloys and steels. This paper will look to utilise the small punch (SP) test to assess the creep response of 〈001〉-orientated CMSX-4 1 1 CMSX-4 is a registered trademark of Cannon-Muskegon Corporation. at temperatures above 950 °C. An orthogonal rafting regime of the γ′ structure is observed in the post-test microstructure due to the biaxial tension state typically produced in a SP test. Interpretation of the SP results to correlate with uniaxial creep data is carried out by employing the k sp approach in order to provide a platform for future material assessment. [ABSTRACT FROM AUTHOR]

The effect of temperature on the tensile behavior and deformation mechanisms in a single crystal superalloys CMSX-4 is addressed and deduced by transmission electron microscopy in the temperature range from room temperature to 1100 °C. It is found that the tensile yield strength reaches a peak at 800 °C. And then, the yield strength decreases with increasing temperature. At room temperature, anti-phase boundary shearing dominates the plastic deformation. From 800 °C to 850 °C, the plastic deformation mechanism is mainly controlled by stacking fault shearing. The Kear-Wilsdorf locks have also appeared. When the temperature reaches at 950 °C, dislocation loops with anti-phase boundary shearing of the γ ′ precipitates are presented. Above 950 °C, the plastic deformation mechanism is processed by the rafted structure of the γ ′ precipitates by-passing, i.e., Orowan by-passing and dislocation climb. Finally, according to the experimental results, the variety of stacking faults with temperatures and the relationship between the yield strength and plastic deformation mechanism are discussed. • The tensile deformation mechanism of CMSX-4 is dependent on the temperatures. • The Kear-Wilsdorf locks are responsible for higher tensile yield strength at 800 and 850 °C. • The temperature for SSFs formation under tensile tests is lower than that at compression tests. • The deformation mechanism at 950 °C is controlled by dislocation loops with APB cutting of γ ′ precipitates. • Above 950 °C, the deformation mechanism is addressed by rafted γ ′ phase by-passing. [ABSTRACT FROM AUTHOR]

The effects of different substrates (CMSX-4 and IN738LC) on cyclic oxidation behavior of two thermal barrier coatings (TBCs) were investigated in this study. Cyclic oxidation test was conducted at a peak temperature of 1080 °C with 30 min hold time. The TBC systems tested consisted of two nickel-base superalloy substrates (CMSX-4 and IN738LC), a platinum aluminide bond coat and two types of 8YSZ top coat (vertical cracks (VC) and columnar structure). Samples with CMSX-4 substrate showed greater cyclic oxidation lifetimes than that with IN738LC, and VC YSZ on CMSX-4 offered longer cyclic life than those with columnar structure. After the oxidation test, the samples were analyzed using a scanning electron microscope (SEM) and an X-Ray diffractometer. The failure location of all TBC systems examined was found to be in the vicinity of TGO, consistent with past research of similar coating systems. Alumina and spinels (CS) were detected on all spalled surfaces although the amount of each phase differs from sample to sample. Ta-rich oxide was found on spalled coating surfaces with CMSX-4 substrate while islands of YSZ on the exposed TGO surface of samples with IN738LC substrate. It is believed that alloy composition, TGO rumpling and stresses induced by TGO growth and coefficient of thermal expansion (CTE) mismatch between different layers all contributed to the reduced cycles to failure of TBC systems with IN738LC substrate. [ABSTRACT FROM AUTHOR]

The solid-state diffusion in binary and ternary systems is overall treated and modeled using the bi-velocity method. The mathematical formulation is given, and the boundary conditions discussed. The model allows the calculation of concentration profiles, spatial distribution of the entropy production rate, and diffusion path. The last two can be used to identify the phases present in the diffusion zone of the ternary system, and determine the thicknesses of the respective layers. The model is applied for the first time to simulate diffusion during aluminization of nickel and its super-alloys, MAR-M200+Hf and CMSX-4, and the predictions are compared with the results of the experiments performed by the authors. The results give a new quantitative explanation of the high-active and low-active regimes of aluminization. It is, in particular, confirmed that the high-active and low-active regimes of nickel aluminization can be carried out by a variation of the HCl/H2 flow ratio. The most important, and a brand new, result concerns the entropy production rate. It is shown that its maxima coincidence with the two-phase zone boundaries in real space, which gives new insight into the modeling of the diffusion in ternary systems. [ABSTRACT FROM AUTHOR]

The paper presents the analysis of numerical simulation of the Bridgman directional solidification process performed on CMSX-4 rods. The numerical simulation was studied applying the ProCAST software. The constitutive law parameters of the normal Gaussian distribution were used to describe the nucleation process. The coefficients of the equation were determined and used to calculate the growth rate of dendrite tip. The analysis of the as-cast microstructure was carried out with the use of Aphelion software in order to determine the average area of grains and their quantity. The experimental verification of both nucleation and grain growth coefficients used for the simulation of the directional solidification process confirmed that the model was correct and described well the investigated process of directional solidification using the Bridgman method. [ABSTRACT FROM AUTHOR]

Combined cycle power systems offer increased efficiency of electricity generation and lower environmental emissions of CO2, SO x and NO2, as well as being adaptable to most fossil/biofuels. Industrial gas turbines are at the heart of such power stations and are being developed to perform at higher firing temperatures and pressures to achieve even greater efficiencies, with lower emissions. Fuel gases derived from renewable fuels, such as biogases from digestors or syngases from solid fuel gasification, may contain contaminants that are extremely corrosive to the gas turbine components (e.g. blades and vanes) located in the hot combusted gas path. Such damage can result in a gradual loss of turbine efficiency and reliability. Therefore, it is of paramount importance that the materials used for gas turbine components that operate in these environments provide acceptable and predictable in-service life times. Single crystal superalloys (e.g. CMSX-4) were developed to have improved mechanical properties (creep and fatigue) at increasing component operating temperatures, especially in relatively clean aero-engine operating environments. This paper describes work carried out to investigate the development of hot corrosion processes on CMSX-4 (uncoated and Pt-Al coated) in a range of potential environments for blade materials in industrial gas turbines fired on biomass derived fuel gases. A series of laboratory tests has been carried out using the 'deposit recoat' technique, with exposure conditions covering: deposits of 80/20 and 50/50 (Na/K)2SO4, with additions of lead, a gas composition of 100 vpm SO x, 100 vpm HCl in simulated combustion gases, deposition flux of 15 µg/cm2/h, temperature of 700 °C, for periods up to 1000 h. During their exposure the materials were monitored using traditional mass change methods. However, quantitative damage data in terms of metal loss was obtained using dimensional metrology, pre-exposure contact measurements combined with post-exposure measurements of damage observed by optical microscopy on polished cross-sections. These measurement methods allowed the distribution of damage to be determined and the material sensitivity to such hot corrosion processes to be quantified. [ABSTRACT FROM AUTHOR]

CMSX-4 and IN738LC, industrial gas turbine (IGT) aerofoil alloys with low (6.5 wt%) and high (16 wt%) chromium contents respectively, were exposed to type II hot corrosion conditions (700 °C, SO X in air, alkali sulphate/chloride deposits) for up to 1000 h. By comparing detailed pre- and post-exposure dimensional metrology from the samples, the fraction of each sample's surface undergoing the incubation stage (lower levels of sample damage due to the presence of a protective scale) or the propagation stage (higher levels of sample damage due to the direct attack of the base alloy) of type II hot corrosion damage was determined. Corrosion pit development under type II conditions may be described as an extreme event. As such, the transition from incubation to propagation may be modelled using Weibull statistics, which were found to give a good fit to the spread of incubation lifetimes in the exposed IN738LC samples. Much shorter incubation lifetimes were found for the lower chromium content alloy (CMSX-4). Propagation rates for CMSX-4 were found not to be constant over time. Instead, when exposed to the higher deposition fluxes, the propagation rates fell after long exposure times as the resultant deposit/corrosion product inhibited the transportation of reactive species. This improved, quantitative understanding of the transition from incubation to propagation damage under type II hot corrosion conditions will assist in the development of quantitative hot corrosion damage models, enhancing IGT component lifetime prediction. [ABSTRACT FROM AUTHOR]

Nickel-based superalloys are predominantly used as structural materials in high-temperature applications due to their exceptional high-temperature strength resulting from precipitation hardening of the coherent γ'-phase. The self-assembly of the γ'-cubes to interconnected networks can be utilized by electrochemical leaching to produce nanoporous superalloy membranes with open porosity and, therefore, functional properties. So far, only γ'-membranes consisting of the intermetallic compound Ni3(Al, Ti, Ta) have been manufactured and analyzed. In this work, the production of the mirrored γ-membranes, consisting of the nickel solid solution, is described. [ABSTRACT FROM AUTHOR]

Abstract: The reduction of wall thickness of turbine blades brings coating systems into focus with a high oxidation resistance and mechanical strength at high temperatures. Therefore a multi-layer coating with a nominal composition of 77Pt–12Al–6Cr–5Ni (at.%) was applied to single crystal Ni-based superalloy CMSX-4 substrates by electron beam physical vapor deposition. The multi-layer structures were homogenized at 800–1000°C under continuous argon/2%-hydrogen gas flow for various times. Microstructures were investigated using scanning electron microscopy and transmission electron microscopy. Phase compositions were determined using energy dispersive X-ray spectroscopy in scanning and transmission electron microscopy. The coatings transformed into two phase microstructure with a fcc γ-phase and a L12 ordered γ′-phase during annealing. [Copyright &y& Elsevier]

CMSX-4 samples were exposed to type II hot corrosion conditions (700 °C, 300 vpm SO X in air) for between 12.5 and 500 h. Two deposits (4/1 molar ratio Na 2SO 4/K 2SO 4 and 49/1 molar ratio Na 2SO 4/NaCl) were tested with fluxes between 0 and 10 μg cm −2 h −1. Dimensional metrology studies showed similar metal losses for samples exposed with the same flux but to different salts. Duplicate samples exposed under identical conditions gave repeatable metal loss data. Samples exposed to low deposition fluxes for short exposure periods maintained surfaces in incubation despite the low chromium content of CMSX-4. For higher fluxes, incubation times and propagation rates were determined. [ABSTRACT FROM AUTHOR]