Advanced Superalloys Research Articles

Temperature/stress dependence of stress rupture behavior and deformation microstructure of an advanced superalloy for additive manufacturing

A self-developed crack-free advanced superalloy ZGH451 fabricated by direct energy deposition (DED) was applied to investigate the microstructure evolution, stress rupture behavior, and deformation mechanisms at moderate-high temperatures and high-low stress conditions. The high Ta/Al ratio induces large misfit lattice stress and low stacking fault energy of alloy, resulting in approximate cubic γ′ phases in dendrites and the formation of initial dislocation tangles. After the stress rupture test at 760 °C/780 MPa, high content cubic γ′ phases, small size of voids as well as preserved dislocation tangles are observed, showing stable structures with high-stress rupture resistance. High content and suitable size of cubic γ′ phases, initial dislocation tangles, and L-C locks hinder the dislocation motion, which decreases the minimum strain rate and prolongs life significantly, forming four stress rupture stages. Hence, the deformation mechanism is determined by dislocation piled-up on γ/γ′ interface, formation of stacking faults in γ′ phases, and dislocations shearing γ′ phases. However, the microstructure exhibits uneven structures composed of large sizes of rafted γ′ phases and voids at 980 °C/260 MPa. The rafted structure and high temperature provide continuous channels and enough energy for dislocation motion, resulting in the increase of minimum strain rate, decline of life, and typic three stress rupture stages, even though there are obstacles to dislocation movement caused by dislocation networks. The deformation mechanism transforms to form dislocation networks on γ/γ′ interface and dislocations shearing γ′ phases. Besides, the decomposition of carbides on GBs also depends on temperature, which decomposes into harmful chain-like M23C6 carbides at moderate temperatures and reinforced granular-shaped M6C carbides at high temperatures. The applied stress always decreases mechanical properties due to its degradation of microstructure induced by elongating the precipitates and defects.

A new approach to design advanced superalloys for additive manufacturing

A novel approach centered on a high Ta/Al ratio and a significant dependence on the boundary composition addressed the challenges in the manufacturability and material performance of additive-manufactured nickel (Ni)-based superalloys. This approach integrated alloy composition, defect management, and mechanical behavior considerations. This study investigated the impact of this approach on microstructural characterization, solidification behavior, elemental microsegregation, and precipitation, demonstrating its efficacy in crack resistance and performance enhancement. Ultimately, this approach facilitated the successful design of a crack-free Ni-based superalloy, ZGH451–1, featuring a high γ′-phase content. Extensive comparisons of mechanical properties, including tensile and creep tests, against reported AM-ed superalloys and partially heat-treated first-generation Ni-based single-crystal superalloys validated the superior mechanical properties and processability of the newly designed alloy. The rationality and feasibility of this approach for achieving collaborative optimization of properties and formability were demonstrated, paving the way for the design of new high-performance superalloys suitable for additive manufacturing.

Низкотемпературная сверхпластическая деформация никелевого сплава ЭК79 с ультрамелкозернистой структурой смешанного типа

One of the most effective ways to increase the processing plasticity of advanced superalloys (heat-resistant nickel-based alloys) is the formation of an ultrafine-grained (UFG) microstructure in bulk semi-finished products. Such a microstructure is a necessary condition for the manifestation of the structural superplasticity effect in the technological processes of manufacturing products from such superalloys. One of the most promising methods for producing UFG microstructures is thermomechanical treatment (TMT) according to the multiple isothermal forging scheme. It has been shown that the EK79 superalloy after TMT, with a gradual decrease in the processing temperature from 0.88 to 0.62 Ts (where Ts is the strengthening phase dissolution temperature) leads to the transformation of the initial microduplex fine-grained microstructure into a mixed UFG microstructure. Such a mixed UFG microstructure consists of: 1) relatively coarse (inherited from the fine-grain microstructure) particles – γ'-phase with a size of 3.0±0.8 μm; 2) γ-grains, and incoherent γ'-phase particles with a size of 0.3–0.5 μm; 3) strengthening coherent intragranular γ'-phase particles with a size of 0.05–0.1 μm, released upon cooling from the TMT temperature to room temperature. During uniaxial compression tests, the EK79 superalloy with such microstructure, demonstrates low-temperature superplasticity in the temperature range of 800–1000 °C. It has been found that an increase in the deformation temperature up to 1000 °C, leads to the increase of γ-phase grains to micron size. The maintenance of superplastic properties in the presence of relatively coarse incoherent particles in the microstructure of the second phase (γ'-phase) is apparently related to the fact that the deformation is localised in the UFG component.

Creep strengthening mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy at steady-state conditions under compressive stresses

The strengthening mechanism of steady-state creep is a critical factor in developing advanced superalloys. In this study, the steady-state creep mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy are investigated with the creep conditions of 750 °C/800 MPa, 900 °C/420 MPa and 1000 °C/137 MPa. Results suggest that the microstructures of steady-state creep samples are significantly different under the above three experimental conditions. A detailed TEM study shows that, Lomer-Cottrell locks and stacking faults interactions are responsible for the low creep rate at low temperature and high stress creep regime, dislocations cross-slip and the dislocation tangles contribute the main creep resistance at intermediate temperatures and moderate stress creep regime, interactions of stacking faults and dislocation networks are the strengthening mechanism for the high temperature and low stress creep regime. The steady-state creep rate of investigated alloy follows the classical power law equation, and the creep activation energy calculated by the power law equation is 378.46 kJ/mol. The results of this study provide insights into the effects of temperature and stress on steady-state creep mechanisms, and high activation energy makes the γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy an attractive candidate for further study as a high temperature structural material.

Homogenizing effects of Co on multi-scale elemental distribution in Ru-containing Ni-based single crystal superalloys

• Co alleviates large-scale heterogeneity of multiple microstructures of the alloys. • Co significantly reduces the partitioning coefficient of γ forming elements, which alleviates small-scale heterogeneity. • Co additions suppress TCP precipitation and increase microstructure stability. The effects of Co on multi-scale elemental distributions were studied in an advanced Ru-containing Ni-based single crystal superalloys. Two types scales of microstructural heterogeneities, the large-scale dendritic segregations and small-scale elemental partitions between γ/γ' phase were compared in as-cast, heat-treated and thermal-exposed alloys. The elemental homogeneities on both large and small scale were shown to be enhanced with Co additions, giving rise to the restricted precipitation of TCP phase. It is thus expected the potential role of Co to replace part of Ru in advanced superalloys.

Effect of pre-strain treatment on mechanical properties of a Ni-Co base disk superalloy

The effect of pre-strain treatment on mechanical properties of a Ni-Co base disk superalloy was studied. Results show that the pre-strain treatment, creep at 725 °C/630 MPa, improves the yield strength of the alloy at 23 °C and 760 °C. However, the creep life (at 760 °C/480 MPa) of the alloy was reduced by pre-strain treatment. The microstructure indicated that a lot of microtwins and Suzuki segregations were introduced by the pre-strain treatment, besides some unfavorable factors on mechanical properties, i.e., ripening of precipitates. The interactions of microtwin and Suzuki segregation with dislocations were discussed in this study, and it was deemed that the impediment of twin boundary and Suzuki segregation to the motion of dislocation may be responsible for the enhancement of the mechanical properties. Meanwhile, the microtwin and the Suzuki segregation have good stability at service temperature. If the problem of reducing the creep properties caused by pre-strain treatment would be solved, the introduction of microtwin and Suzuki segregation appropriately can be considered as a novel strengthening method in designing advanced superalloys.

Towards understanding the thermal history of microstructural surface deformation when cutting a next generation powder metallurgy nickel-base superalloy

Despite the ongoing progress in metallurgical characterisation of machined surfaces, knowledge of the thermal conditions under which they originate during the workpiece-flank interaction is still lacking. When cutting advanced superalloys, little is known about temperature evolution in the machined part volume, where workpiece material interacts with the tool flank. In this work, the characteristics of the thermal field and the resulting surface metallurgy induced in a next generation nickel-base superalloy have been studied for cutting scenarios involving different combinations of thermo-mechanical boundary conditions. Analysis of the thermal field evolution in the workpiece subsurface has allowed the heating and cooling rates induced by cutting to be revealed, allowing description of two distinct types of thermal cycle, with a Heating-Peaking-Cooling (H–P–C) and a Heating-Quasi-isothermal Deformation-Cooling (HQC) structure depending on the process aggressiveness. Subsurface thermal history has been found to relate with the severity of the cutting-induced deformation, as it combines information on thermal field magnitude and on the process rates. Furthermore, thermal balance equations have been applied to study the rate of the heat generation in the machined subsurface due to its own plastic deformation while interacting with the tool flank. This has revealed that the highest rate of heat generation induced by plastic deformation occurred in thin surface layers at the beginning of the workpiece-flank contact, which has been associated to the conditions under which white layers (WLs) are generated. Energy balance analysis has furthermore indicated the development of a less severe and less impulsive deformation process at higher subsurface depths, which has been linked to the formation mechanism of material drag (MD) layers. In this way, the thermal history of machined surfaces has been related to their resulting metallurgical integrity, allowing in-depth understanding of the physical conditions developing when cutting next-generation superalloys.

Synergistic strengthening mechanisms of rhenium in nickel-based single crystal superalloys

A loading-unloading-reloading strategy via atomistic simulations is proposed to reveal the strengthening mechanisms of rhenium in nickel-based superalloys at ambient temperature. Simulations reproduce the experimentally observed increased flow stress with addition of rhenium. Besides the usual solid solution strengthening, two novel mechanisms are uncovered. One is that rhenium facilitates the dislocation nucleation and, thus, increases the propensity of forest dislocation interactions. The other involves a critical role of rhenium in promoting accumulation of sessile stair-rod dislocations, served as obstacles for the glissile dislocations. The three mechanisms synergistically render a stronger alloy, which is heuristic to design advanced superalloys in extreme environments.

Experimental study of response parameters during machining of Inconel 718 with cryogenically treated ceramic round tool using cutting fluid

Highly advanced superalloys are being rapidly spreading throughout the globe. It's in need of the hour to produce similar materials which are being used in several industries similar as petrochemical, biomechanical, aerospace and marine industries. Inconel 718 is one similar superalloy which is being used due to its better characteristic features like high attrition resistance, high temperature burden conditions, thermal fatigue resistance, and cryogenic temperatures. Owing to the hardness conditions, tools indicate the low tool life and high wear characteristics. Ceramic insert is one such tool that is being used to machine Inconel 718 which is cryogenically treated to improve tool life. The use of emulsified cutting fluid reduces tool wear and improve durability of the tool, thereby improving the efficiency of the machining of Inconel 718. In this paper, experimental investigation has been carried to find the use of emulsified cutting fluid that improves the machinability of Inconel 718 based on parameters such as surface roughness and tool wear under the effect of cutting parameters which are cutting speed, feed rate and depth of cut.

Dual-processing by abrasive waterjet machining—A method for machining and surface modification of nickel-based superalloy

Abstract Abrasive waterjet (AWJ) is widely used for machining of advanced (e.g. nickel-based) superalloys as it offers high material removal rates and low cutting temperatures. However, the inadequate surface integrity, e.g. large number of scratches and embedded particles in the machined surface, which would induce severe deteriorations of the materials functional performance, has been one of the greatest issues of the AWJ machining technique. To solve this problem, this research proposed a dual-process abrasive waterjet machining method, whereby two different functions of abrasive waterjet were employed: materials removal (first process) and surface modification (secondary process), hence, to improve the workpiece surface integrity. Two types of entrained particles, i.e. with sharp cutting edges (e.g. garnet) and smooth surfaces (e.g. stainless steel ball), that depending on their kinetic energy density can either cut or modify the workpiece surface respectively, are employed for these the two constitutive processes of the proposed dual-waterjet machining method. A critical standoff distance and inclination angle of the waterjet nozzle has been defined for the surface modification process thus, to eliminate the embedded particles and scratches left by the first cutting process while also introducing a surface strengthening effect. To support this approach, a mathematical model has been proposed for determining the surface modification parameters (e.g. jet feed speed and abrasive flow rate). In-depth analysis of the microstructural and metallurgical alternations of the machined workpiece surface and superficial layer have also been conducted to reveal the mechanisms responsible for the surface damage elimination and surface strengthening. Moreover, a four point bending fatigue test has been conducted to validate the mechanical performance, whereby a significant improvement of the fatigue life on the machined workpiece was achieved when compared with the case that single AWJ cutting method (91 %) and conventional machining (34 %) are employed. This proves that the proposed dual-processing AWJ machining method is of high efficiency to improve the functional performance of components on a single machine tool platform.

The High Temperature Tensile and Creep Behaviors of High Entropy Superalloy

This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L12 ordered precipitates, so it is also named as “high entropy superalloy (HESA)”. The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors’ knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.

An investigation of diffusion-mediated cyclic coarsening and reversal coarsening in an advanced Ni-based superalloy

The anomalous cyclic coarsening behaviour of γ′ precipitates after ageing at 1073 K has been investigated for the low misfit commercial powder metallurgy (PM) Ni-based superalloy RR1000. Using scanning transmission electron microscope (STEM) imaging combined with absorption-corrected energy-dispersive X-ray (EDX) spectroscopy, the elemental segregation as a function of coarsening behaviour has been experimentally observed for secondary γ′ precipitates. Elemental EDX spectrum imaging has revealed nanoscale enrichment of Co and Cr and a depletion of Al and Ti within the γ matrix close to the γ-γʹ interface. Our experimental results, coupled with complementary modelling and synchrotron X-ray diffraction analysis, demonstrate the importance of elastic strain energy resulting from local compositional variations for influencing precipitate morphology. In particular, elemental inhomogeneities, as a result of complex diffusive interactions within both matrix and precipitates, play a crucial role in determining the rate of coarsening. Our findings provide important new evidence for understanding the microstructural evolution observed for advanced superalloys when they are exposed to different heat treatment regimes.

Resource recycling of superalloys and hydrometallurgical challenges

Superalloys are high melting temperature, excellent creep resistance, anti-corrosive, and oxidation resistant alloys; they are predominantly used in gas turbines of aircraft engines and power plants. Today, a series of superalloys is available according to their composition and applications. The tough nature of superalloys makes it difficult to recycle them after they are formed; this increases the demand and shortage of the energy-critical elements that are their raw materials and makes their processing one of the most demanding form of recycling today. A few processes are available, and some of these unique recycling systems are in use; however, a lack of data and/or information exists for two reasons: (i) scrap recycling is most frequently performed by superalloy manufacturers themselves (not always by the same unit) by recharging them into molten charges, and (ii) the recycler does not want to produce business for their rivals. This article presents an overview of the processes investigated to recycle superalloys, including the advantages and disadvantages of each. The available processes are not very technologically or economically feasible; therefore, further efforts are necessary to explore these processes and to make a value-added product by recycling superalloys rather than just re-melting them. Considering the continuously increasing number of alloyed elements in advanced superalloys, this paper also notes to the challenges associated with hydrometallurgical recycling of superalloys and the need for future studies on this topic.

Sulfidation Characteristics of an Advanced Superalloy and Comparison with Other Superalloys Intended for Gas Turbine Use

Sulfidation may occur even in an overall oxidizing environment beneath a corrosion product which assumes the role of a diffusion barrier allowing sulfur species transport at a faster rate when compared with that of oxygen species. The current paper presents sulfidation characteristics of an advanced single-crystal nickel-based superalloy (ANS) and compares performance with IN 792 and CMSX-4 superalloys. The results showed that all the superalloys were highly vulnerable to sulfidation and their lives were significantly reduced. Among them, the ANS was more susceptible to sulfidation and its life was reduced considerably. This is attributed to the changed chemistry of the advanced alloy. The results for ANS are compared with its oxidation data and the difference in its behavior is discussed. A degradation mechanism, which represents the deterioration of ANS under sulfidation conditions, is proposed based on the results obtained from different techniques. Finally, the necessity of protective coatings for shielding against high temperature sulfidation for potential application in enhanced efficiency of gas turbine engines is emphasized.

Advances in Superalloys

This topic concerns advances in superalloys, and is sponsored by the TMS High Temperature Alloys Committee. Near-term developments on nickel-base superalloys are presented, as well as longer-term considerations for new superalloys. The High Temperature Alloys Committee sponsored a very successful International Symposium on Superalloy 718 and Derivatives on October 10–13, 2010. A wide range of processing, properties, and applications of these alloys was considered in depth. The entire published proceedings are available at http://www.wiley.com/WileyCDA/WileyTitle/ productCd-0470943165.html. The High Temperature Alloys Committee also recently sponsored a symposium entitled ‘‘Advances in Processing’’ at the TMS 2011 Annual Meeting that was very well attended. Several interesting papers are presented from these two symposia. Superalloy 718 has a very versatile microstructure which can include c, c¢, c¢¢, and d phases. Rick Kearsey and coworkers describe how varied grain size, precipitate size, morphology, and phase fraction influence mechanical properties at high temperatures in one improved derivative alloy, ATI 718Plus . This alloy is shown to offer promising combinations of monotonic and cyclic mechanical properties in comparison with a superalloy employed in many turbine disk applications, Waspaloy. Some aerospace applications require small, complex-shaped components. A near-net shape metal injection molding process is investigated for use with Superalloy 718 by Eric Ott and coworkers. This process requires injection of a powder–binder blend into complex tooling, removal of tooling, debinding, densification, heat treatment, and final machining. With sufficient control of process and materials through appropriate industry specifications, this process is shown to have promising potential for aerospace components. Processing of superalloys for aerospace applications often includes shot peening after machining, to introduce compressive residual stresses near the surface that can prevent surface cracking. Verification of the stress magnitude is often required. R. Chandrasekar and co-authors describe the use of an advanced eddy current inspection technique to measure these stresses in Superalloy 718, and the effects of different microstructures on response. As the oil and gas industry has advanced to drill deeper wells with higher pressure, temperature, and corrosive media, conventional steel components have been selectively replaced with superalloys. John ‘‘Jack’’ deBarbadillo describes how Superalloy 718 and derivatives have been adapted to these demanding requirements. More advanced cast and wrought superalloys are being optimized to withstand extreme conditions in deep-water sour gas and oil drilling and production applications. Advanced ultrasupercritical power generation plants have the potential to attain significantly higher efficiency, reducing fuel consumption and gas emissions. Here also, many steel components will need to be replaced with superalloys for the associated increases in temperature. Paul Jablonski and coworkers describe the potential for cast versions of selected wrought superalloys to attain the temperature–stress requirements, in large cast components. Several candidates show promising results in screening tests at relevant conditions. Dispersion strengthening can be used to provide exceptional strength in superalloys at high temperatures. However, this can make the superalloys very difficult to fabricate, due to poor hot working and joining capability. Mike Fahrmann and co-author describe an innovative new Co-base superalloy that uses fine-scale stable nitrides for dispersion strengthening, introduced by a gas nitriding process after fabrication processes are completed. Thermodynamic and kinetic properties are shown to strongly influence the nitridation process. The mission of the TMS High Temperature Alloys Committee is to provide a means of communication among those interested in superalloys and other high-temperature alloys. Emphasis is placed on JOM, Vol. 64, No. 2, 2012

The Behaviour of Superalloys in Marine Gas Turbine Engine Conditions

This paper presents hot corrosion results carried out systematically on the selected nickel based superalloys such as IN 738 LC, GTM-SU-718 and GTM-SU-263 for marine gas turbine engines both at high and low temperatures that represent type I and type II hot corrosion respectively. The results were compared with advanced superalloy under similar conditions in order to understand the characteristics of the selected superalloys. It is observed that the selected superalloys are relatively more resistant to type I and type II hot corrosion when compared to advanced superalloy. In fact, the advanced superalloy is extremely vulnerable to both types of hot corrosion. Subsequently, the relevant reaction mechanisms that are responsible for slow and faster degradation of various superalloys under varied hot corrosion conditions were discussed. Based on the results obtained with different techniques, a degradation mechanism for all the selected superalloys as well as advanced superalloy under both types of hot corrosion conditions was explained. Finally, the necessity as well as developmental efforts with regard to smart corrosion resistant coatings for their effective protection under high temperature conditions was stressed for their enhanced efficiency.

Microscopic Observations on the Origin of Defects During Machining of Direct Aged (DA) Inconel 718 Superalloy

Surface quality of advanced superalloys after machining is one of the major issues in the aerospace industry because it directly affects service characteristics of the machined part. Tool life of cemented carbide inserts with the TiAlN coating during machining of direct aged DA 718 alloys under roughing and finishing conditions has been under study. The defect origin on the surface of the machined part was investigated. Metallographic observations of the DA 718 were made using optical metallography and SEM/EDS. To find out the origins of surface defect formation, the morphology of machined parts and cross sections of the machined surfaces have been investigated. Two major categories of defects were detected on the surface of the machined part: cracks and tears. The origin of the cracks on the machined surface is related to shearing of the primary complex TiC/NbC carbide revealed in a structure of DA 718 alloy. At the same time, Nb-rich regions of the primary complex carbide interact with the environment (oxygen from air) during machining with further formation of low strength oxide layer on the surface, forming tears.

Developments in superalloy castability and new applications for advanced superalloys

Increasing emphasis is being paid to optimising castability of cast superalloys to absolutely minimise defects which impact casting yield and alloy performance and to improve structural weldability of weld repairable alloys. The objective is to maximise casting yield and quality and minimise any rectification costs. To accomplish these aims the controls on deleterious elements such as [N], [O], S, P and Si at the ppm level, including accurate measurement, are critical. Conversely, the addition of highly reactive elements, La and Y at the ppm level, in conjunction with very low (1 ppm or less) sulphur content has dramatically improved bare oxidation resistance (for unshrouded blade tip life) and thermal barrier coating (TBC) adherence and life including prime reliant applications. This technology has been applied to the following new or improved cast superalloys: CM 939 Weldable®, CMSX-486® and CMSX-4® (SLS) [La+Y] (Cannon-Muskegon Corporation). In this paper, the role (both positive and negative) and control of critical elements at the ppm level as it applies to many advanced superalloys will be examined and discussed.

耐酸化性とクリープ強度に優れる第5世代Ni基単結晶超合金

The National Institute of Materials Science (NIMS) has utilized the in-house alloy design program to develop a 5th generation Ni-base single crystal superalloy, TMS-196 with superior high temperature creep and oxidation resistance by incorporating further ruthenium (Ru) and chromium (Cr) content over the compositions of 4th generation superalloys. With Ru additions in advanced superalloys to enhance phase stability, higher content of refractory elements can be accommodated to provide further strengthening; the associated oxidation resistance can be improved by the increase in Cr additions. In present article, TMS-196 has been subjected to cyclic/isothermal oxidation tests at 1100°C and creep at conditions between 800°C~1100°C/137 MPa~735 MPa. Preliminary studies indicate that the surface of oxidized TMS-196 can exhibit continuous alumina layer attributing good oxidation resistance similar to that of Rene'N5. Detailed TEM analyses have shown the alloy system of TMS-196 possessing strong resistance to γ′ deformation during creep deformation. Data regarding to oxidation and creep properties of some previous generations of superalloys are also included for comparisons and discussions in this research.

High-temperature strength of Ni-base superalloy coatings

Recently considerable attention has been paid to large utility gas turbines, because of their high efficiency and many other advantages. This accelerates the introduction of advanced superalloys and coatings. While these technologies are traditionally from aero turbines, many novel challenges have arisen in their application, e.g. long operating time, weight, cyclic duty, environment using cheaper fuel, and size. This paper describes the mechanical properties and failures of the superalloys and the coatings used at the hot sections of utility gas turbines. Special focus is put on thermo-mechanical fatigue failure, and the collaborative test results by the Subcommittee of “Superalloys and Coatings ” in The Society of Material Science, Japan, is introduced.