Wrought Nickel-base Superalloy Research Articles

Microstructural Analysis on Grain Boundary of Boron– and Zirconium–Containing Wrought Nickel-Based Superalloys

Trace elements such as boron (B) and zirconium (Zr) can increase creep resistance in nickel-based superalloys. This study investigates the change of microstructures on the grain boundary (GB) in phase-controlled nickel-based superalloys through the addition of trace elements. The basis alloy without B and Zr has distributed micrometer-sized (Nb, Ti)C and Cr23C6 carbides at the GBs. Zr is detected alongside Nb and Ti within certain (Nb, Ti)C carbides and its addition increases the fraction of (Nb, Ti)C or (Nb, Ti, Zr)C carbides. B affects the formation of precipitates constructed by nanometer-sized precipitates, which are Cr23C6 carbides, Cr23(C, B)6 boro-carbides, and Cr-rich borides, surrounded by γ’ phases. This film structure, which includes nanometer-sized precipitates surrounded by γ’ phases, forms more continuously with the addition of B and Zr. It is constructed with precipitates of (Nb, Ti)C carbides and Cr23(C, B)6 boro-carbides surrounded by γ’ phases. Numerous nanometer-sized precipitates (i.e., (Nb, Ti)C and Cr23(C, B)6) are distributed alternately within the film structure. The effect of the addition of B and Zr is such that nucleation sites of each precipitate are formed simultaneously and alternately along the GBs. The experimental results were discussed by correlating them with the predicted fraction of stable phases depending on the temperatures of these alloys, using the JMatPro program.

Influence of cooling strategy on microstructure, deformation mechanism and mechanical properties of a nickel-based superalloy

In this study, the influence of cooling strategy after solution treatment on microstructure and mechanical properties of a novel cast and wrought nickel-based superalloy is studied. It was found that continuous oil cooling (CC) from the solution temperature led to formation of large primary γ' and ultrafine tertiary γ'. Interrupted cooling (IC), which involves furnace cooling from the solution treatment temperature (1080 °C) to a lower temperature (such as 1010 °C, 980 °C or 950 °C) first followed by an interrupted oil cooling, shows significant influence on microstructure. Owing to a short-period furnace cooling, secondary γ' started to form in these interrupted cooled samples and their volume fraction and size increased continuously with decreased IC temperature. In contrast, the volume fraction of tertiary γ' decreased with decreased IC temperature. The CC sample was found to deform mainly by dislocation glide/climb and extension of stacking faults at 650 °C/950 MPa. With increased volume fraction of large secondary γ' in the interrupted cooled samples, deformation became increasingly planar, i.e., the formation of stacking faults and nanotwins increasingly became the dominant deformation mechanisms. The cooling strategy shows no obvious influence on tensile properties and impact toughness but shows significant influence on stress rupture properties. With decreased IC temperature and coarsened microstructure, the stress rupture lifetime continues to decrease.

AUBERT & DUVAL AD730

Abstract Aubert & Duval AD730 (NiCr16Co9Mo3W3Ti3Al2 alloy) is a cast and wrought nickel-based superalloy that withstands high temperatures (up to 750 °C, or 1380 °F) while preserving strength, creep, and fatigue resistance at a competitive cost. The breakthroughmainly comes from its unique properties-versus-cost balance, due to its recycling and forging ability. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties as well as creep. It also includes information on high temperature performance and corrosion resistance as well as forming, heat treating, machining, and joining. Filing Code: Ni-782. Producer or source: Aubert & Duval S.A.

Evaluation of WEDM performance characteristics and prediction of machining speed during taper square profiling on Hastelloy-X

ABSTRACT Hastelloy-X is one of the wrought nickel-base superalloy comprising of oxidation resistance and high-temperature strength. Wire electric discharge machining process is the favoured technique for precise component fabrication of such materials. Apart from precision the most essential aspect with surface integrity and cutting velocity in manufacturing. This investigation deals with the effects of cutting speed override, corner dwell time, wire guide distance and wire offset on taper profile response characteristics. The variation of these parameters is studied by machining a simple square profile at 0°, 15° and 30° taper angles. It is observed that as the taper angle gets increased, both the machined errors at corners and machined roughness increased although the machining speed gets reduced. The recast layer and micro-hardness are explored for the taper square profile components. It is observed that the recast layer thickness was least for 30° taper profiles indicating least thermal degradation which was validated by micro-hardness. It was established that in a machined profile, that corners possessed lower hardness than the edges. The micro-hardness of inner profile corners was found to be lower than outer profile corners. Adaptive neuro-fuzzy interference system is better prediction computing method having least error ranging from 0–4.89%. In conclusion, the machining speed and taper angle also contribute to the output characteristics in wire electric discharge taper profiling.

Creep-Fatigue Behavior of a Newly Developed Ultra-Supercritical Steam Turbine Grade Nickel-Based Superalloy, HAYNES 282

Abstract Most engineering components used in gas/steam turbines are exposed to a range of complex loading conditions resulting from startup and shutdown procedures. These loading conditions involve superimposition of time-dependent creep on cyclic fatigue and can be simulated by properly designed high-temperature creep-fatigue tests. Creep-fatigue interaction is a function of duration and position of dwell in the loading waveform, and the material microstructure. The objective of this work is to investigate the creep-fatigue interaction response of a newly developed γ′-strengthened wrought nickel-based superalloy (HAYNES 282), which has a potential application in advanced ultra-supercritical steam turbines. Creep-fatigue tests are conducted at 760°C with strain dwell either at tensile peak or compressive peak or at both tensile and compressive peak positions for different dwell times of 100 and 1,000 s. The test results are analyzed with respect to evolutions of peak stress, stress amplitude, stress relaxation, hysteresis loop, inelastic strain energy density, and degree of softening. Degree of softening is found to increase with dwell position at tensile, compressive, and both peaks in that order. Tests with dwell at both tensile and compressive peak positions are found to be the most damaging, showing the least life. Between tensile dwell and compressive dwell tests, interestingly, those with compressive dwell show a significantly reduced life. Increasing dwell time aggravates the damaging effect manifold. The mechanism of fracture at the end of life is illustrated with fractographic characterization.

Influence of transition group elements on the stability of the γ″-phase in nickelbase alloys

Alloy 718-type wrought nickel-based superalloys are the materials of choice for many high temperature applications. They are strengthened by the γ″-phase that forms small precipitates with a large lattice distortion. However, at temperatures above 650 °C, this phase may transform to the thermodynamically stable δ-phase. It is therefore important to understand the influence of alloying elements on the stability of these phases. In this paper, density functional theory calculations at 0 K are performed to determine the effect of aluminium and of the transition group elements on the stability and the lattice parameters of the γ″-phase. It is shown that the substitution energy is mainly determined by two parameters, the charge transfer and the change in the lattice constant. Solution energies are compared to those in the δ-phase and some conclusions for the design of 718-type alloys are drawn.

Discontinuous precipitation leading to nano-rod intermetallic precipitates in an Al0.2Ti0.3Co1.5CrFeNi1.5 high entropy alloy results in an excellent strength-ductility combination

The phenomenon of discontinuous precipitation (DP) leading to the formation of nano-rod FCC (γ) + L12 (γ’) colonies, has been typically considered deleterious for mechanical properties. However, the present study shows clear evidence that substantially large fractions of FCC + nano-rod L12 microstructure within a thermo-mechanically processed high entropy alloys (HEA) or complex concentrated alloys (CCA) of composition Al0.2Ti0.3Co1.5CrFeNi1.5, formed via recrystallization coupled with discontinuous precipitation, can lead to an excellent combination of room temperature strength and ductility. The extent of thermomechanical processing can be engineered to modify the phase transformation pathway from homogenous L12 precipitation to discontinuous L12 precipitation in the same HEA. This predominantly FCC + nano-rod L12 microstructure exhibits a yield stress ~1.4 GPa, ultimate tensile strength ~1.6 GPa, and tensile ductility of ~14%, making it one of the best combinations of room temperature tensile properties for FCC-based HEAs, that have been reported to date, as well as better than current generation wrought nickel base superalloys. A high yield strength of the order of ~1 GPa is also retained to a temperature of 500 °C in this alloy. However, at higher temperatures (>550 °C), the DP microstructures exhibit a rapid decline in strength and become less competitive as compared to microstructures consisting of homogeneously precipitated L12 within the FCC matrix.

Nucleation mechanism of hetero-epitaxial recrystallization in wrought nickel-based superalloys

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050°C and cooled at 5°C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

Microstructure Evolution during Isothermal Aging for Wrought Nickel-Based Superalloy Udimet 520

Microstructure Evolution during Isothermal Aging for Wrought Nickel-Based Superalloy Udimet 520

High temperature deformation behavior and microstructure evolution of wrought nickel-based superalloy GH4037 in solid and semi-solid states

Abstract Cylindrical samples of Ni-based GH4037 alloy were compressed at solid temperatures (1200, 1250 and 1300 °C) and semi-solid temperatures (1340, 1350, 1360, 1370 and 1380 °C) with different strain rates of 0.01, 0.1 and 1 s−1. High temperature deformation behavior and microstructure evolution of GH4037 alloy were investigated. The results indicated that flow stress decreased rapidly at semi-solid temperatures compared to that at solid temperatures. Besides, the flow stress continued to increase after reaching the initial peak stress at semi-solid temperatures when the strain rate was 1 s−1. With increasing the deformation temperature, the size of initial solid grains and recrystallized grains increased. At semi-solid temperatures, the grains were equiaxed, and liquid phase existed at the grain boundaries and inside the grains. Discontinuous dynamic recrystallization (DDRX) characterized by grain boundary bulging was the main nucleation mechanism for GH4037 alloy.

Strain controlled isothermal low cycle fatigue life, deformation and fracture characteristics of Superni 263 superalloy

The total axial strain controlled low-cycle fatigue (LCF) behaviour of a wrought nickel-base superalloy Superni 263 has been evaluated at 298 K, 673 K, and 923 K employing the strain amplitudes in the range ± 0.25% to ± 0.80%. All the tests were performed using a triangular wave form at a constant strain rate of 10−3 s−1. The Superni 263 alloy was subjected to a solutionizing treatment at 1373 K/1.5 h followed by water quenching. An ageing treatment of 1073 K/8 h was given to the solutionized samples. The microstructure of the alloy is composed of γʹ in the intragranular regions and discrete M23C6 precipitates on the grain boundaries. Few undissolved (Ti, Mo)C and TiN precipitates were also noticed in the matrix. The strain amplitude and temperature played a significant role in the evolution of macroscopic cyclic stress response and the development of deformation substructure. The alloy displayed serrated flow in the plastic portions of stress-strain hysteresis loops in the tests conducted at 673 K and 923 K, predominantly at strain amplitudes higher than ±0.40%. Several manifestations depicting the occurrence of dynamic strain ageing (DSA) were derived by analysing both the macroscopic and micro-mechanistic information generated. The comparative evaluation of monotonic and cyclic stress-strain curves was conducted and the factors responsible for rapid cyclic hardening during DSA were identified. The fatigue life decreased drastically with increasing temperature, particularly at low strain amplitudes. The strain-life data could be represented well by using Basquin and Coffin-Manson relationships. The slope in Coffin-Manson relationship revealed more negative values under the influence of DSA. The crack initiation and propagation modes at all the test conditions remained transgranular without the indications of interference from creep and oxidation effects. The deformation substructure at elevated temperatures was characterized by the occurrence of co-planar slip, stacking faults, micro twinning and high dislocation density in between the slip bands.

Grindability of powder metallurgy nickel-base superalloy FGH96 and sensibility analysis of machined surface roughness

The grindability and sensibility analysis of surface roughness of powder metallurgy nickel-base superalloy FGH96 were studied in comparison to the wrought nickel-base superalloy GH4169. The effects of grinding parameters (such as workpiece infeed speed, depth of cut, and abrasive wheel speed) on grinding force, grinding temperature, specific grinding energy, abrasive wheel wear, and surface roughness were analyzed. The results show that the grinding force, grinding temperature, and specific grinding energy of GH4169 are usually higher than those of FGH96 under the given experimental conditions. However, the wear behavior of the brown corundum abrasive wheels when grinding these two kinds of nickel-base superalloy material is generally identical. The sensitivity of GH4169 workpiece surface roughness to depth of cut and workpiece infeed speed is higher than that of FGH96, but the sensitivity of GH4169 to abrasive wheel speed is less than that of FGH96. Finally, it is inferred that the grinding performance of FGH96 is slightly better than that of GH4169.

Solid-phase joining of the cast intermetallic Ni3Al-based alloys and wrought nickel-based superalloys via using superplastic deformation

The paper summarizes the results of solid-phase joining experiments performed for intermetallic single-crystal Ni3Al-based alloys (known as Russian VKNA-alloys) and wrought fine grained nickel-based superalloys using superplastic deformation of the wrought superalloys. For this, two single-crystal VKNA-alloys, VKNA and VKNA-25 and two Russian nickel-based superalloys, EK61 and EP975, were taken. It was shown that the formation of an ultrafine grained structure (d<1 µm) in the EK61 superalloy made it possible to obtain the solid-phase joint of EK61/VKNA under pressure welding at T = 800 °C using superplastic deformation of the nickel-based superalloy. Another nickel-based superalloy, EP975, was taken in a fine grained condition (d = 8 µm). High-quality solid-phase joints of EP975/VKNA and EP975/VKNA-25 were obtained under pressure welding at T = 1125 °C using the superplastic deformation of the EP975 superalloy. The obtained solid-state joints EP975/VKNA and EP975/VKNA-25 were subjected to heat treatment, which led to the formation of coarse γ grains in the EP975 superalloy. This also led to an increase in thickness of the γ phase interlayers up to 500 nm in both VKNA-alloys near the solid-phase joints

Effect of machining parameters on cutting force and surface integrity when high-speed turning AD 730\u2122 with PCBN tools

The novel wrought nickel-based superalloy, AD 730™, is a good candidate material for turbine disc applications at high temperatures beyond 650 °C. The present study focuses on the machining performance of this newly developed alloy under high-speed turning conditions with advanced PCBN tools. Meanwhile, the machined surface integrity as influenced by cutting speed and feed rate was also investigated. The surface integrity was thoroughly characterized in terms of surface roughness and morphology, machining-induced plastic deformation, white layer formation, and residual stresses. It has been found that the cutting speed and feed rate had a strong effect on the cutting forces and resultant surface integrity. The cutting forces required when machining the alloy were gradually reduced with increasing cutting speed, while at 250 m/min and above, the flank tool wear became stronger which led to increased thrust force and feed force. A higher feed rate, on the other hand, always resulted in higher cutting forces. Increasing the cutting speed and feed rate in general deteriorated the surface integrity. High cutting speeds within the range of 200–250 m/min and a low feed rate of 0.1 mm/rev are preferable in order to implement more cost-effective machining without largely reducing the surface quality achieved. The formation of tensile residual stresses on the machined AD 730™, however, could be of a concern where good fatigue resistance is critical.

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

ABSTRACTThe yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10−2, 1 × 10−3, 1 × 10−4 and 1 × 10−5 s−1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.

Comparative investigation on grindability of K4125 and Inconel718 nickel-based superalloys

Nickel-based superalloy is a typical hard-to-machining material in the aero-engine manufacturing industry. The grindability difference of two kinds of nickel-based superalloys, i.e., equiaxed cast nickel-based superalloy K4125 and wrought nickel-based superalloy Inconel718, are discussed in this article. The influence of grinding parameters (e.g., grinding speed, workpiece speed, and depth of cut) on the grinding force, grinding temperature, and ground surface quality are explored. The results illustrate that under the given grinding conditions, grinding K4125 generates higher forces than Inconel718. The temperature from the K4125 grinding process is beyond 400 °C, while the temperature from grinding on Inconel718 is below 200 °C. Moreover, because of the chip adhesion on the wheel surface when grinding K4125, not only the wheel wear is more severe but also the ground surface of K4125 is worse than that of Inconel718. Accordingly, it could be inferred that the grindability of K4125 is worse than that of Inconel718.

The effect of boron and zirconium on microstructure and tensile properties of the wrought nickel-based superalloy ATI 718Plus

In the present study, the effect of boron and zirconium on the microstructural characteristics, ductility and the tensile strength of the wrought nickel-based superalloy 718Plus were investigated. In this regard, five alloys with different amounts of boron 0.00–0.016wt%) and zirconium 0.0–0.1wt%) were cast via the vacuum induction melting and then purified via the vacuum arc remelting. Microstructural observations were carried out through optical and scanning electron microscopes and phase analysis was performed by x-ray diffraction analysis. The tensile test was carried out at room temperature and 704°C on the alloys. The results showed a slight effect of boron and zirconium on grain size while they had significant effect on the amount and the stability of the delta phase. These elements increase the lattice mismatch and the elastic strain between the γ and γ′ phases. Boron and zirconium increased the ductility at room temperature by 10% and 12% and at the 704°C by 8% and 5%, respectively. The effect of these elements on the strength at room temperature was negligible, while at 704°C, boron and zirconium increased the yield strength by 17% and 16% and the ultimate strength by 8% and 7%, respectively. The improved cohesion and strength of the grain boundaries and an increase in delta phase and the elastic strain between γ and γ′ phases are the most important causes for improving the ductility and strength of the alloy 718Plus in the presence of boron and zirconium.

Influence of transition group elements on the stability of the δ- and η-phase in nickelbase alloys

To improve the high-temperature capability of 718-type wrought nickel-base superalloys, the -phase () can be stabilized. However, this also reduces the size of the forging window because forging has to be done above the - and below the solvus temperature of the phase that is used to enable fine-grain forging, i.e. the δ-phase of type or the η-phase of -type. Understanding the influence of alloying elements on the formation of these phases is therefore important. In this paper, density functional theory calculations at 0 K are performed to determine the stabilizing effect of aluminium and of the transition group elements on the stability of the δ-phase and η-phase. Most of the transition group elements of 5th and 6th period stabilize the δ-phase, whereas the stabilizing effect on the η-phase is weaker. According to the calculations, Mo, Tc, W, Re, and Os may be expected to stabilize the δ-phase but not the η-phase, whereas Al and Zn strongly stabilize the η-phase. V, Zr, Ru, Rh, Pd, Ag, Cd, Hf, Ta, Ir, Pt, Au, and Hg stabilize both phases. For some elements (Cr, Mn, Fe, Co), magnetic effects in the δ and especially in the η-phase are shown to be significant at the concentrations studied here.

Electrochemical Dissolution Behavior of the Nickel-Based Cast Superalloy K423A in NaNO3 Solution

To date, many types of nickel-based superalloys have been widely used as key components in aircraft engines, and these can be categorized into wrought nickel-based superalloys and cast nickel-based superalloys according to the manufacturing process. However, regardless of the nature of the manufacturing process, nickel-based superalloys are difficult to machine using traditional methods Electrochemical machining (ECM) is an effective method for machining difficult-to-cut materials, especially nickel-based alloys, and there have been many reports concerning the electrochemical dissolution behavior of such alloys. However, the electrochemical dissolution behavior of cast superalloys has seldom been discussed.This study focuses on the electrochemical dissolution behavior of the cast superalloy K423A. The microstructure of the specimen was examined and numerous carbides were found to be distributed irregularly throughout the matrix, especially at the grain boundaries. The effects of the electrolyte and the electrolysis parameters on the electrochemical dissolution behavior were investigated by assessing the polarization curves and the open-circuit potentials. The results showed that a relatively stable and efficient dissolution process could be obtained using 10% NaNO3 solution at 40°C. Dissolution experiments were performed at various current densities and corrosion times. A highly efficient and erratic current efficiency curve was found. The corroded specimens were examined using a surface-finish measuring instrument, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The results showed that the microstructure of K423A, especially in terms of carbon and carbides, seriously interfered with the stability of the electrochemical dissolution process and led to a poor surface quality. A qualitative model has been proposed to illustrate the electrochemical dissolution behavior of K423A in NaNO3 solution.

Metallurgical Failure Investigation of Overheated Brackets Made of Nimonic Alloy 90

Abstract Fastening brackets for ceramic heat shields of gas turbine engine combustion chambers were found to be grossly overheated after only 1 100 operating hours. They were made from the wrought nickel-base superalloy Nimonic alloy 90. The affected parts were disassembled from the engine and replaced. One bracket was selected for an exemplary metallographic examination at the laboratory of the gas turbine engine manufacturer. The bracket exhibited severe deformation, oxidation, and creep damage. The alloy was selected properly for this application. The cause of the thermal overload is unknown to the investigating laboratory. It was recommended to find and remove the reason for the excess thermal exposure.