Cast Ni-base Superalloy Research Articles

Study of phase transformations in CMSX-6 and CMSX-8 superalloys

Nickel-based superalloys are extensively used mainly in the aircraft and aeronautic industry, particularly in the hottest parts of engines or turbo-reactors. The phase reactions occurring in these heat-resistant materials play a crucial role in many aspects of the processing and service of the highly alloyed materials. Cast Ni-based superalloys are obtained in a complex way and their structure is complicated. Differential scanning calorimetry (DSC) technique was applied for determination of temperature ranges of the phase transformations occurring in the CMSX-6 and CMSX-8 superalloys during heating/cooling processes. Thermophysical properties, including temperatures of the phase transformation, are the critical input parameters in mathematical models of solidification and casting of metallic materials. The literature data concerning phase transformations and performance of the heat treatment for CMSX-6 and CMSX-8 are incomplete and ambiguous. DSC results accompanied by scanning electron microscopy characterization of microstructure of CMSX-6 and CMSX-8 superalloy was applied. The present study will improve the understanding of the fundamental mechanisms of phase transformations of single-crystal nickel-based superalloys.

Analysis of the Effect of Cooling Rate on Microstructure in 17% Cr Ferritic Cast Steel

High Cr and Ni content steels are widely used in many manufacturing processes in the chemical and petrochemical industry. The automotive industry has also recognized the necessity of heat resistant alloys for a long time, for example, to apply them to exhaust systems to endure thermal loading and oxidation during the operation of engines. Various heat resistant alloys such as cast irons, stainless steels, and Ni-base super alloys have been considered as candidate materials of automotive exhaust systems. Among those candidates, ferritic stainless steels attracted a lot of attention due to their favorable low thermal expansion, sufficient mechanical strength at elevated temperature and excellent corrosion resistant properties [1]. Currently they are the leading engineering materials in several fields of applications that require resistance to wear, corrosion [2,3], creep or thermal fatigue [4]. The high corrosion resistance of these steels is due to alloying elements such as Cr, Ni and Mo. If the ferritic stainless steels are alloyed with strong carbide-forming elements, such as Mo, Ti, V and Nb, hard phases, MC carbides can be obtained in the soft ferrite phase [5,6]. The improvement of the properties of FeCrNi cast steels is directly related to the development of the microstructure, which mainly consists of a ferritic matrix and carbides and/or dispersed intermetallics [7,8]. The improvement is not always the hardening. The hardness is usually limited by the casting and the subsequent machining, so an annealing process is also inserted.

Investigation on the dissolution of η phase in a cast Ni-based superalloy

The dissolution behavior of η phase has been investigated in a cast Ni-based superalloy. The results showed that the platelets and blocks of η phase were formed within the interdendritic regions of the microstructure. Applying standard solution annealing at 1150–1160°C for a period of 4 h did not result in the complete dissolution of η phase. For the complete dissolution of η phase without residual incipient melting, a 2-step solution annealing has been recommended. After dissolution at high temperatures, the η phase transforms to two MC-type carbides: one is enriched in Ti, Nb, and Ta, and the other is of (Zr,Ti)C type.

Subcritical Crack Growth on Crystallographic Planes in a Ni-base Superalloy: Relevance to Orientations

Abstract Cast Ni-based superalloys, essential materials for gas turbines, possess large grains comparable to wall thickness of structural components: e.g., blades and vanes. Fatigue cracks propagating in these kinds of materials must be significantly influenced by crystallographic factors of materials: e.g. orientation of grains and grain boundaries. This paper covers such a basic problem. For the purpose, at first an effect of crystallographic orientation on the crack propagation rate is introduced, using a single crystal Ni-based superalloy CMSX-4 specimen with different primary and secondary orientations. Discussions are also made on what mechanics and mechanisms control the fatigue crack propagation behavior, on the basis of anisotropic fracture mechanics.

Primary α phase and its effect on the impact ductility of a high Cr content cast Ni-base superalloy

A high Cr content cast Ni-base superalloy, K4648, which contains 32–35wt.% chromium, was prepared by vacuum induction melting, and poured into testing bars or heavy section specimens. The as-cast, heat treated and fractured specimens were investigated by optical metallography, quantitative metallography, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The isothermal solidification followed quenching (ISQ) technique and differential scanning calorimeter (DSC) experiments were also carried out. The result indicated that the primary α phase precipitated from the residual liquid in interdendritic region at 1190°C near the solidus temperature. This phase can be represented as α-(Cr,Ni) in K4648 alloy because it solid solutionized by about 30at.% of element Ni. The Vickers microhardness value of primary α phase is 6.3GPa relative to 1.9GPa of the γ matrix at the load of 0.2N. This phase is brittle and tends to crack due to stress concentration during the solidification process. The primary α phase solutionized or transformed into M23C6 carbides during solid solution treatment in the range of 1180–1220°C. However, small amounts of α phase remained even at the obvious incipient melting temperature of 1200–1220°C. Therefore, primary α phase must be controlled during solidification process. The faster cooling during solidification process can decrease the amount of primary α phase in both as-cast and heat-treated K4648 alloys. Specimens cut from slow cooling heavy section castings with large amount of primary α phase possessed an impact ductility of 13.9J/cm2, which is much lower than 35.4J/cm2 of samples from faster cooling thin section testing bars. The extensive cracked α phase or transformed M23C6 carbides can be observed at the fracture surfaces and longitudinal sections of impact specimens. Large blocky primary α phase with a network-like distribution, forming as a result of slow cooling, has detrimental effects on the impact ductility of K4648 alloy and it is difficult to completely remove by heat treatment. Proper parameter to obtain faster cooling in the solidification of the alloy must be considered to avoid the detrimental primary α phase precipitation.

Long-term thermal exposure responses of the microstructure and properties of a cast Ni-base superalloy

Microstructural evolution and microstructure-property relationship during long-term thermal exposure in K446 alloy are investigated in detail. The coarsening rate of γ′ precipitates increases with the rise of exposure temperature, yet decreases with the increment of exposure time. Primary MC carbides present an excellent stability and decompose to a tiny degree by two reaction forms, which have an insignificant influence on the properties. TCP-μ phase precipitates from the supersaturated matrix in the forms of needles and granules and experiences a remarkable evolution with prolonged exposure; the μ phase near the grain boundaries (GBs) evolves slower than that within the grains due to the inhomogeneous distribution of elements. In addition, the precipitation of μ phase is determined to be more detrimental to the mechanical properties than the γ′ coarsening in the present alloy.

Advanced Ni base superalloys for small gas turbines

Ni base superalloy cast materials provide an outstanding balance of high temperature strength, fatigue resistance, oxidation resistance and coating performance, and can be produced to very tight tolerances in extremely complex configurations, such as axial and centrifugal integral cast turbine wheels. As a result, cast superalloys are used in the most demanding applications of aero and industrial gas turbine engines. Use of these materials is expanding to smaller microturbine, turbojet, turbocharger and missile engine applications due to this unique combination of desirable properties. This paper will present an overview of the application of investment cast Ni base superalloys and process capability to small turbine and missile engines.Les matériaux moulés de superalliage à base de Ni fournissent une balance exceptionnelle de résistance à haute température, de résistance à la fatigue, de résistance à l’oxydation et de performance du revêtement. On peut également les produire à des tolérances très serrées dans des configurations extrêmement complexes, comme des roues de turbine à coulée axiale et centrifuge d’une seule pièce. Conséquemment, les superalliages moulés sont utilisés dans les applications les plus exigeantes des turbines à gaz aéro- et industrielles. L’utilisation de ces matériaux s’étend à des applications de plus petite micro turbine, de turboréacteur, de turbocompresseur et de moteur de missile grâce à cette combinaison unique de propriétés désirables. Ce document présente une revue de l’application des superalliages à moulage de précision à base de Ni et de la capacité opérationnelle du processus pour les petites turbines et les moteurs de missile.

Key factors in quality control of a high Cr content cast Ni-base superalloy K4648

K4648 possesses the highest Cr content in applied cast Ni-base superalloys and has been applied to manufacture the integral castings of advanced gas turbines in China. However, the large blocky primary α phase in alloy had detrimental effect on the impact ductility, and the alloy severely reacted with the silica-based ceramic core during the solidification process. The investigation results indicate that the primary α phase precipitated in the late solidification at 1190 °C. The Vickers microhardness value of α phase is 6.3 GPa at the load of 0.2 N. The primary α phase transforms to M23C6 carbides after solid solution treatment at 1180 °C. The cracked α or transformed M23C6 can be observed at the fracture surface and longitudinal section of the impact specimens. Moreover, the severe reaction exists in the metal/ceramic interface. The reaction products consist mainly of the oxides which elements come from both alloy and silica-based ceramic core. For high Cr content superalloys, the solidified parameters should be controlled properly to inhibit primary α phase precipitation and more stable alumina-based core is recommended.

Solidification Behavior and Segregation of Re-containing Cast Ni-base Superalloy with Different Cr Content

The effect of chromium (Cr) on solidification and segregation behavior of Re-containing cast Ni-base superalloys was investigated by optical microscopy (OM), scanning electron microscopy (SEM) and electronic probe micro analysis (EPMA). The results show that Cr has significant effect on solidification and segregation behavior of Re-containing cast Ni-base superalloys. The liquidus and solidus of alloy decrease with increasing Cr in alloys. The segregation coefficient (K) of Mo increases and that of W and Re decreases gradually with increasing Cr element.

Combined Cycle Fatigue of Ni-Base Superalloy

Fatigue life of a cast Ni-base superalloy IN 713LC under combined cycle loading consisting of a superposition of low- and high-cycle fatigue at 800 °C was experimentally determined. No measurable effect of combined cycle was found for studied loading conditions. High scatter of fatigue life related to the initiation of cracks on casting defects was observed. Size of the largest defect in a specimen was predicted by the largest extreme value distribution method. The predicted size was compared with fractographic observation of defects resulting in final fatigue failure.

OS2502 温度勾配下の熱機械的疲労試験機の試作と二三の観察(OS25-1 高温強度・照射,OS-25 供用エネルギー機器の経年変化と健全性評価)

A new testing system has designed and developed to simulate an in-service condition of high temperature components with cooling holes under a thermal gradient condition. Using the new system, some experiments were carried out to assess the damage evolution, specifying a conventionally cast Ni-based superalloy specimen.

Advances in nickel-based cast superalloys

Nickel-based superalloys have served as the most competitive high temperature structural materials under highly stressed and aggressive operating conditions in a variety of applications for more than 60 years. The most demanding among all the applications has been the gas turbine aerofoil castings of modern aero-engines. These turbine parts operate in extremely aggressive environment of high velocity hot combustion gas-air mixture carrying highly corrosive ingredients at high pressure. Gas turbine aerofoil materials should therefore possess adequate resistance to creep, fatigue and aggressive environment. Materials design for such application therefore has been extremely challenging, particularly since the engine designers always aim at higher turbine entry temperature (TET) in order to achieve greater engine thrust and better fuel efficiency. In spite of enormous efforts made in the recent past towards developing ceramics and their composites, Ni-based superalloys continue to be most reliable blade and vane materials offering always the highest TET. This has been possible through better alloy design, improved blade cooling schemes, protective coatings and directional solidification (DS) of either columnar grains or single crystals (SC) along the most favorable 〈001〉 texture. During the last six decades, TET has gone up by about 500K. This article covers recent advances in cast Ni-based superalloys, including our own efforts in this direction. Extensive research at DMRL has led to the development of new generation Ni-based superalloys, designated as DMD-4 and DMS-4 for DS and SC processing, respectively. Simultaneously, expertise has been developed to cast DS and SC components for aero-engines. Technology has also been established for pilot scale production of these components.

Early growth of creep-fatigue small cracks in an advanced cast Ni-based superalloys

Naturally initiated creep-fatigue small crack growth behavior in a cast single crystal Ni-base superalloy, CMSX-2, was investigated at a temperature of 1223 K in vacuum and in air, by using the solid cylindrical smooth specimen of the CMSX-2 within 5 degree from 〈001〉 crystallographic orientation. This paper covers the following topics: (i) the naturally initiated creep-fatigue small crack propagation behavior relevant to the microstructure, compared with those of the physically long cracks; (ii) effect of loading frequency on the creep-fatigue small crack propagation; (iii) comparison of crack propagation between the single crystal alloy and a conventional cast polycrystalline one; and (iv) effect of environment on the creep-fatigue small crack propagation. The experimental results revealed the lack of similarity law in crack growth rates between the small and long cracks was significant: the former exhibited the growth rates remarkably higher than the latter at a given stress intensity factor range under the same test condition. It was also shown by the comparison of the crack propagation between in air and in vacuum that the role of creep, as well as that of environment, were essential in the creep-fatigue small crack propagation process.

Initiation and propagation of fatigue cracks in cast IN713LC superalloy

Abstract Fatigue life, initiation and propagation of cracks at 800 °C in a cast Ni-base superalloy IN 713LC were experimentally studied in high-cycle fatigue region. Load symmetrical cycling and cycling with high tensile mean load were applied. Both crystallographic crack initiation resulting in long Stage I crack growth and non-crystallographic Stage II propagation were observed. High scatter of fatigue life data was explained by: (i) variability in microstructural conditions for crystallographic crack initiation and propagation and by (ii) influence of casting defect size distribution. The fractographic observation supports the slip band decohesion mechanism of crack initiation and an important role of cyclic slip localization in persistent slip bands.

µ-Phase behavior in a cast Ni-base superalloy

The response of μ phase to applied stress and long-term thermal exposure has been investigated in the cast Ni-base superalloy K446. It is found that during stress rupture, the applied stress accelerates the precipitation and growth of μ phase. However, during thermal exposure the μ phase precipitating in the form of needles and granules experiences a complicated evolution. The needles, fiber- or sheet-shaped in three-dimension, align very regularly during exposure, either lying in three directions with an acute angle to one another or in two directions perpendicular to each other, the mechanisms of which are characterized in detail. In addition, it is concluded that an excessive precipitation of μ phase severely degrades the mechanical properties of the alloy, whereas its evolution behavior during thermal exposure is determined to have an insignificant influence on the properties.

High-cycle fatigue of Ni-base superalloy Inconel 713LC

Fatigue strength of three nominally identical batches of cast Ni-base superalloy Inconel 713LC under load symmetrical cycling and cycling with tensile mean load was experimentally determined in high-cycle fatigue region at a temperature of 800 °C in air. Examination of fracture surfaces, crack initiation sites, microstructure and casting defects has been performed. Statistical method of largest extreme value distribution has been applied to characterize the casting defect distribution. The aim of the study was to evaluate the mean stress effect on high-cycle fatigue life, to explain the scatter of fatigue life data and a scatter related to material produced under nominally identical production conditions.

Influence of Al-Si Layer on Structure and Properties of Cast Ni-Based Superalloys

High-temperature constructional parts of aircraft engines and energy units are exposed to high dynamic stress (fatigue processes and creep) and various temperatures in dioxide-corrosion condition (hot corrosion, oxidation and erosion). The improvement of aero-engine and turbine efficiency is possible through the increase of temperature in front of turbine. This requires the use of heat-resistant and creep-resistant materials, especially nickel-base superalloys which resist mentioned effects for a limited period of time. A deposition of protective layers should improve hot corrosion resistance. This paper is focused on microstructure of protective layers created by codeposition of Al and Si on nickel-base superalloys INCO 713 LC and INCO 738 LC after thermal and thermal-stress exposition and on microstructure of basic materials (substrates). The contribution also shows creep tests results for both superalloys with and without a protective layer.

Thermal stability of primary carbides and carbonitrides in two cast Ni-base superalloys

Thermal stability of primary MC carbide and M(CN) carbonitride in K452 and K446 alloys is investigated. In K452 alloy, primary MC is very unstable relative to M(CN) for the former contains a higher content of the weakeners (W and Mo) than the latter. The ratios of element contents (at.%) (W + Mo)/Cr, (W + Mo)/(Ti + Nb) and Nb/Ti are three important parameters, which together determine the thermal stability of primary MC. Also, primary MC degeneration plays an important role in the microstructural instability of K452 and K446 alloys.

Effects of Long-Term Thermal Exposure on the Microstructure and Properties of a Cast Ni-Base Superalloy

Microstructural stability and microstructure-property relationship during long-term thermal exposure in K452 alloy (a new Ni-base cast superalloy with ∼21 pct Cr, 11 pct Co, 3.5 pct W, 2.5 pct Al, 3.5 pct Ti, and others) are investigated. It is found that exposure temperature and time have significant effects on the microstructure and properties of the alloy. During exposure, the microstructure is degraded by γ′ coarsening, MC carbide (M mainly represents Ti, W, and Nb) degeneration, precipitation and evolution of grain interior (GI) M23C6 carbide, evolution of grain boundary (GB) microstructure, and precipitation of η phase. Among them, the γ′ coarsening is the leading reason for the decrease of strength of the alloy. The GI M23C6 and the η phase have negligible influence on the properties due to their relatively small populations. Blocky, closely spaced GB M23C6 particles engulfed in γ′ increase the stress-rupture life, whereas the formation of a continuous GB M23C6 chain has an opposite effect. A life peak occurs when the M23C6/γ′ structure at the GBs is in an optimal form. The degenerated MC is the preferred initiation site of microcracks. Its presence at the GBs promotes the onset of intergranular fracture, and leads to the decrease in mechanical properties.

Decomposition of primary MC carbide and its effects on the fracture behaviors of a cast Ni-base superalloy

Primary MC degeneration in long-term thermally exposed cast Ni-base superalloy K452 was investigated in detail. It is shown that the primary MC degeneration is a diffusion-controlled process where various transformation products are sequentially present within the reaction region during exposure. The primary MC degeneration produces a DM or SM microstructure on the MC/γ interface at the initiation of exposure, which hinders the diffusion of alloying elements via its ordered γ′ layers and leads to the presence of the η and α-(W, Mo) phases, respectively during the intermediate and late exposure. In addition, the primary MC deterioration is examined to be detrimental to the long-term thermally exposed alloy due to its contribution to the cracking incidence especially when the MC is present at the grain boundaries.