Stress corrosion of Ni-based superalloys

Abstract

The development of gas turbines to increase fuel efficiency is resulting in progressively higher operating temperatures in the under platform regions of the blades. These regions have traditionally been considered low risk areas. However, higher metal temperatures combined with stresses and the deposition of contaminants from the cooling air system may result in complex degradation mechanisms. Static stress corrosion testing has been conducted on C-ring specimens at a range of stresses in a hot corrosion environment. Cracks were observed in C-rings after exposure times greater than 100 h. Scanning electron microscopy (SEM) systems were used to image cracks and characterise deposits to improve understanding of the mechanism. Finite element analysis (FEA) has been used to model the stress intensity under test conditions. CMSX-4 specimens subject to static stresses combined with hot corrosion demonstrated significant material degradation (crack initiation and propagation) suggesting a combined stress corrosion mechanism resulting in cracking.     

Microscale stray grains formation in single-crystal turbine blades of Ni-based superalloys

Microscale stray grains(MSGs),which are composed of misoriented and fragmented high-ordered den-drite arms,small-sized equiaxed grains,and columnar grains,are a novel grain defect that occurs within the shrouds of single-crystal(SC)blades made from Ni-based superalloy.State-of-the-art non-destructive testing methods are incapable of detecting this defect,resulting in a high-risk application of SC blades.This study aims to control this defect by systematically investigating directional solidification process-ing parameters,shroud dimensions,and rhenium(Re)content in the formation of MSG defects.The re-sults reveal that the defect forms exclusively at the overhanging extremities of the upper shrouds on the back side of the blades that face the heater.Increasing the withdrawal rate reduces the occurrence of MSG defects.As the shroud dimensions increase,MSG defects appear in the transition area between the downward suspended extremity of the blade shroud and the side of the blade body,as well as near the corner of the downward suspended extremity of the shroud.The occurrence of MSG defects increases with increasing shroud dimensions.Varying Re content sharply decreases the formation of MSG defects.A concentration-attached Rayleigh-Taylor instability(RTI)superheating(CARTISH)model,considering solid-ification shrinkage,was proposed to comprehend the formation of MSG defects.Simulation results based on this model are consistent with the experimentally observed distribution and degree of MSG defects under different conditions.Effective control of the CARTISH is critical to managing MSG defects.     

Microtwinning and other shearing mechanisms at intermediate temperatures in Ni-based superalloys

In Ni-based superalloys, microtwinning is observed as an important deformation mechanism at intermediate temperature and low stress and strain rate conditions. Current knowledge concerning this unusual deformation mode is comprehensively reviewed, and fundamental aspects of the process are further developed using state of the art experimental and modeling techniques. The nature of microtwins and the microtwinning dislocations at the atomic level have been determined using High Angle Annular Dark Field Scanning Transmission Electron Microscopy imaging. The results unambiguously confirm that the operative twinning dislocations are identical Shockley partials a/6〈1 1 2〉, and that they propagate through the γ′ precipitates in closely-separated pairs on consecutive {1 1 1} planes. The rate-limiting process of the microtwinning deformation mechanism is the diffusion-controlled reordering in γ′-phase. It is shown that reordering requires very simple, vacancy-mediated exchange between Al and Ni atoms. The energetic aspect of the vacancy-mediated exchanges is studied for the first time using ab initio calculations. The concept of reordering as a rate-limiting process is generalized and shown to be relevant for other, previously reported deformation mechanisms in superalloys such as a〈1 1 2〉 dislocation ribbons, and superlattice intrinsic and superlattice extrinsic stacking fault formation. Other diffusion phenomena associated with microtwinning, such as segregation of heavy elements, is also discussed and supported by experimental evidence. The influence of the γ/γ′ microstructure on microtwinning deformation mode is also discussed in light of observations and phase-field dislocation modeling results.     

Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys

In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elastic-plastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickel-base superalloys, IN100 and René 88DT. The material element that captures the fatigue crack-initiating features in René 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.     

Modelling of the thermo-physical and physical properties for solidification of Ni-based superalloys

The thermo-physical and physical properties of the liquid and solid phases are critical components in the modelling of casting simulations. Such properties include the fraction solid transformed, enthalpy release, thermal conductivity, volume and density all as a function of temperature. Due to the difficulty in experimentally determining such properties at solidification temperatures, little information exists for multi-component alloys. As part of the development of a new computer programme for modelling of materials properties (JMatPro), extensive work has been carried out on the development of sound, physically based models for these properties. Wide ranging results will presented for Ni-based alloys, which will include more detailed information concerning the phases formed during solidification and their composition and the density change of the liquid that intrinsically occurs during solidification due to its change in composition.     

Modelling the coefficient of thermal expansion in Ni-based superalloys and bond coatings

The coefficient of thermal expansion (CTE) of nickel-based superalloys and bond coat layers was modelled by considering contributions from their constituent phases. The equilibrium phase composition of the examined materials was determined using thermodynamic equilibrium software with an appropriate database for Ni-based alloys, whereas the CTE and elastic properties of the principal phases were modelled using published data. The CTEs of individual phases were combined using a number of approaches to determine the CTE of the phase aggregate. As part of this work, the expansion coefficients of the superalloy IN-738LC and bond coat Amdry-995 were measured as a function of temperature and compared with the model predictions. The predicted values were also validated with the published data for the single-crystal superalloy CMSX-4 and a number of other Ni-based alloy compositions at 1000 K. A very good agreement between experiment and model output was found, especially up to 800 \(^\circ \)C. The modelling approaches discussed in this paper have the potential to be an extremely useful tool for the industry and for the designers of new coating systems.     

Effect of strain rate on plastic deformation bonding behavior of Ni-based superalloys

Plastic deformation bonding (PDB) has emerged as a promising solid state bonding technique with limited risk of phase transformations and residual thermal stres...     

Effect of hafnium and tantalum on the microstructure of PM Ni-based superalloys

Journal of Materials Science - The effect of Hf and Ta on the microstructure of powder metallurgy Ni-based superalloys after heat treatment was investigated. Hf and Ta change the distribution and...     

Slurry aluminizing mechanisms of Ni-based superalloys incorporating an electrosynthesized ceria diffusion barrier

This study investigates the effect of an electrosynthesized ceria interlayer on the growth mechanisms of a full TBC system sintered from a slurry containing spherical Al micro-particles. A potential Selective Diffusion Barrier (SDB) ability of the ceria interlayer appears. It is demonstrated that ceria allows the Al enrichment of the substrate while limiting the Cr upward diffusion and segregation of Al_xCr_y. It is found that the ceria interlayer partially hinders the flow of solid and molten Al into the substrate at 700 °C from the spheres. Simultaneously, the flow of Al among the spheres results in the establishment of bridges and in the loss of sphericity all by maintaining a metallic core. In contrast, emptying of the particles and quick formation of δ-Ni₂Al₃ rapidly occur in the absence of ceria interlayer. Increasing the annealing temperature to 1100 °C for 2 h brings about the formation of a β-NiAl coating thicker than the one obtained without an interlayer and promotes thicker crusts and a greater sintering of the Al₂O₃ shell of the particles. This is proposed to result both from delayed Al inward diffusion and enhanced Ni outward diffusion promoted by the heat release accompanying the exothermic formation of the Ni_xAl_y phases.     

Nanoporous hard carbon membranes for medical applications

Current blood glucose sensors have proven to be inadequate for long term in vivo applications; membrane biofouling and inflammation play significant roles in sensor instability. An ideal biosensor membrane material must prevent protein adsorption and promote integration of the sensor with the surrounding tissue. Furthermore, biosensor membranes must be sufficiently thin and porous in order to allow the sensor to rapidly respond to fluctuations in analyte concentration. In this study, the use of diamondlike carbon-coated anodized aluminum oxide as a potential biosensor membrane is discussed. Diamondlike carbon films and diamondlike carbon-coated anodized aluminum oxide nanoporous membranes were examined using scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and platelet rich plasma testing. The diamondlike carbon-coated anodized aluminum oxide membranes remained free from protein adsorption during in vitro platelet rich plasma testing. We anticipate that this novel membrane could find use in immunoisolation devices, pacemakers, kidney dialysis membranes, microdialysis systems, and other devices facing biocompatibility issues that limit in vivo function.     

Interaction between Re and W on the microstructural stability of Ni-based single-crystal superalloys

The influences of Re and W as well as their interaction on γ′ and topologically close-packed (TCP) phases have been investigated in seven kinds of Ni-based single-crystal superalloys. The results show that after full heat treatment, the γ′ size is reduced with increasing Re, but does not change with increasing W. After thermal exposure at 1000°C for 1000?h, the TCP phase is dramatically increased with increasing Re, but increased slightly with increasing W. The TCP phase volume fraction in higher Re alloys is much more than that in lower Re alloys which have the same total content of Re and W. This indicates that W instead of Re could effectively improve the microstructural stability of Ni-based single-crystal superalloys.     

The precipitation and effect of topologically close-packed phases in Ni-based single crystal superalloys

The precipitation of topologically close-packed(TCP)phases is the result of microstructure instabilities of Ni-based superalloys.This review seeks to comprehensively collate all the available information on TCP phases in SX superalloys based on the latest findings.First,the thermodynamics and kinetics of the TCP phase precipitation are introduced.Meanwhile,the morphology,composition and orientation of TCP phases and their sequential transformation are summarized in detail.Further,the factors affecting the precipitation of these phases are sorted out.Besides,the proposed damage mechanisms of TCP phases are listed.Finally,several control and prediction methods of the TCP phase precipitation are reviewed,so the alloy designer can better balance the relationship between microstructure stabilities and properties of the superalloy.     

用多元线性回归法预测镍基高温合金的抗氧化性

用静态氧化法研究了3种铸造镍基高温合金K35,K44,K46在800℃及900℃的高温氧化动力学行为。结果表明,3种合金的氧化动力学曲线均符合抛物线规律,其中氧化速度常数为合金成分中每种元素作用的综合结果,用多元线性回归法计算了25种镍基高温合金在900℃氧化过程的作用系数,并给出了3种试验合金的氧化动力学回归方程,与3种合金的实验结果相比,相对误差小于13％。     

A hybrid replication technique for the analysis of precipitate-boundary interactions in Ni-based superalloys

A hybrid replication technique is described for complex alloy systems whereby the surface is etched to reveal the microstructure of the metallic phases prior to producing an extraction replica. By applying this approach to the Ni-based superalloy IN100, it has been shown that the inert carbide and boride particles can be extracted for analysis in the TEM whilst retaining information about their location with respect to the metallic phases in the alloy. This approach is particularly useful for studying the effects of inert particles on microstructural evolution, such as pinning effects during grain growth.     

Nanoporous silica membranes fabricated using multiwalled carbon nanotubes

Nanoporous silica membranes were fabricated using 3-aminopropyltriethoxysilane (APS) and acyl chloride-functionalized multiwalled carbon nanotubes (MWCNTs). The amine groups of silane reacted with the functional groups (e.g., acid chloride) that were attached to the sidewall of the MWCNTs. The APS that was grafted to the sidewall of the MWCNTs was polymerized in order to coat the MWCNTs wall through heating. The thickness of the silica layer on the surface of the MWCNTs was controlled by adjusting the growth time of the SiO2 layer. Approximately 20 nm-sized pores were formed through the removal of the MWCNTs using a simple thermal process, but some traces of the MWCNTs still remained. The porous properties of the nanoporous silica membrane were analyzed from the nitrogen adsorption-desorption isotherms that were obtained using a surface area and porosimetry analyzer. The structure and composition of the silane-modified MWCNTs were characterized using scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy.     

Nanoporous aluminum oxide membranes for filtration and biofunctionalization

Nanoporous aluminum oxide membranes with high open porosity are prepared by anodic oxidation. Conventional self-supporting as well as mechanically stabilized nanoporous membranes are produced from aluminum plates and microimprinted aluminum foils, respectively. The mechanically stabilized membranes are characterized by very thin membrane parts stabilized by surrounding thick bridges. The minimal thickness of these thin membranes with open pores on both sides is 1 microm, with a mean pore size of the parallel open pores of 185 nm. With these two kinds of membrane the flow rates for cross filtration can be tuned over a wide range. With the mechanically stabilized membranes, substantially higher flow rates are achieved and experiments that cannot be performed with thicker membranes become possible. The biofunctionalization of the pore walls with archaebacterial tetraether lipids is realized and proved using aminated semiconductor nanocrystals. The lipid layer deposited on the pore walls also changes the filtration properties.     

Nanoporous titania-coated alumina membranes: sol-gel synthesis and characterisation

In this work, nanoporous titania top layers were deposited by dip-coating process on microporous alpha-alumina substrates using the sol-gel process. The alumina substrates were synthesized by slip casting method using Taguchi optimising approach. The microporous substrate was then used to coat nanoporous titania layers by the sol-gel method. The thickness, pore size, structure and permeability of the membranes were characterised using SEM, XRD, STA and Hg-Porosimetry. The process conditions to achieve defect-free nanoporous titania layers with the average pore size of about 4 nm coated on the microporous alumina substrates with the average pore size of about 270 nm were determined.