Conventional Superalloy Research Articles

Combinatorial development of multicomponent Invar alloys via rapid alloy prototyping

To develop novel Invar alloys in the practically infinite compositional space of multicomponent alloys, rapid alloy prototyping is used to investigate five multicomponent Invar alloys with 5 at.% addition of Al, Cr, Cu, Mn and Si to a super Invar alloy (Fe63Ni32Co5; at.%), respectively. All alloys show abnormally low thermal expansion coefficients below the Curie temperature and saturation magnetization deviating from the Slater-Pauling curve, revealing their Invar-behavior. The relationships among valence electron concentration, magnetic properties, and Invar behavior of the various multicomponent alloys are discussed. The Invar alloy with Cu addition is particularly promising, as it shows a 40 K larger temperature range (above room temperature) of low thermal expansion coefficient (<5.87×10−6 K) and slightly higher hardness compared to the conventional super Invar alloy. The work thus also successfully demonstrates the capability of using rapid alloy prototyping for developing multicomponent multi-functional alloys.

Effect of Pressure and Temperature on Densification in Electric Field-Assisted Sintering of Inconel 718 Superalloy.

Electric field-assisted sintering has ubiquitous merits over conventional sintering technology for the fabrication of difficult-to-deform materials. To investigate the effect of sintering pressure and temperature on the densification of Inconel 718 superalloy, a numerical simulation model was established based on the Fleck-Kuhn-McMeeking (FKM) and Gurson-Tvergaard-Needleman (GTN) models, which covers a wide range of porosity. At a sintering pressure below 50 MPa or a sintering temperature below 950 °C, the average porosity of the sintered superalloy is over 0.17 with low densification. Under a pressure above 110 MPa and a temperature above 1250 °C, the sintered superalloy quickly completes densification and enters the plastic yield stage, making it difficult to control the sintering process. When the pressure is above 70 MPa while the temperature exceeds 1150 °C, the average porosity is 0.11, with little fall when the pressure or temperature rises. The experimental results indicated that the relative density of the sintered superalloy under 70 MPa and 1150 °C is 94.46%, and the proportion of the grain size below 10 μm is 73%. In addition, the yield strength of the sintered sample is 512 MPa, the compressive strength comes to 1260 MPa when the strain is over 0.8, and the microhardness is 395 Hv, demonstrating a better mechanical property than the conventional superalloy.

Combinatorial Development of Multicomponent Invar Alloys Via Rapid Alloy Prototyping

To develop novel Invar alloys in the practically infinite compositional space of multicomponent alloys, rapid alloy prototyping is used to investigate five multicomponent Invar alloys with 5 at.% addition of Al, Cr, Cu, Mn and Si to a super Invar alloy (Fe 63 Ni 32 Co 5 ; at.%), respectively. All alloys show abnormally low thermal expansion coefficients below the Curie temperature and saturation magnetization deviating from the Slater-Pauling curve, revealing their Invar-behavior. The relationships among valence electron concentration, magnetic properties, and Invar behavior of the various multicomponent alloys are discussed. The Invar alloy with Cu addition is particularly promising, as it shows a 40 K larger temperature range (above room temperature) of low thermal expansion coefficient (<5.87×10 -6 K) and 2.8% higher hardness compared to the conventional super Invar alloy. The work thus also successfully demonstrates the capability of using rapid alloy prototyping for developing multicomponent multi-functional alloys.

An investigation on thermodynamics and kinetics of η phase formation in Nb-modified iron-nickel base A286 superalloy

In this study, precipitation of η phase (Ni3Ti) in conventional and Nb-modified (Nb-A286) A286 superalloys was evaluated at different aging times and temperatures. The TTP curve of the η phase formation was plotted using thermodynamic analyses, kinetics and microstructural studies. Depending on temperature and heat treatment, the η phase precipitated at the grain boundaries or twin sites, as a result of the γ′ phase or matrix austenite transformation. Heat treatment of conventional A286 superalloy and Nb-A286 was performed within a temperature range of 650 to 900 °C for 2 to 30 h. The η phase transformation was evaluated by scanning electron microscope (SEM) which is equipped to energy dispersive X-ray spectroscopy (EDS) and optical microscopy (OM). In the analyses based on thermodynamic calculations, the interaction of the Gibbs free energy of η phase formation and the diffusion activation energy of the elements, especially titanium and niobium, was considered. The microstructural studies showed that increasing the heat treatment time results in increasing the volume fraction of the η phase. By increasing the aging temperature to 840 and 860 °C for conventional A286 superalloy and Nb-A286 superalloy, respectively, the η phase volume fraction increased, however, further increase led to volume fraction decrease. The results of the thermodynamic analyses showed the tip of the TTP diagrams at temperatures of 860 and 820 °C for the A286 and Nb-A286 alloys, respectively. Investigation of kinetics calculations showed that η phase transformation depends on the diffusion of titanium, nickel, and niobium.

Performances of Ni3Al-based intermetallic IC10 in creep-feed grinding

Ni3Al-based intermetallic IC10 exhibits prominent high-temperature performance (e.g., high melting points, resistant to gas corrosion, and creep resistance); it acts as an ideal material employed to manufacture aero-engine turbine blades. Since the internal structure of the Ni3Al-based intermetallics differs from that of the conventional superalloy, its grindability significantly limits its application. Grinding parameters are vital factors affecting the surface quality of Ni3Al intermetallic IC10. In the present study, single-factor and orthogonal experiments were performed to ascertain the grinding temperature, grinding force, and grinding surface quality of Ni3Al-based intermetallic IC10. As revealed from the conclusion drawn in this study, surface quality and grinding force were considerably affected by cutting depth and feed rate, whereas they were less affected by wheel speed. The process of grinding with high feed rate or deep cutting depth would lead to the production of grooves, debris, delamination, and other defects on the test sample surface. The grinding temperature was deeply affected by the cutting depth. The empirical formulas of the relationships between process parameters and grinding temperature and force were calculated by orthogonal experiments, thereby verifying the correctness of single-factor experiments. Small cutting depth and low feed rate can improve surface integrity and reduce grinding surface defects.

Investigation on High Temperature Mechanical Behaviors of TiC-γ′ Reinforced Ni Composite

A duplex TiC and γ′-Ni3(Al,Ti) reinforced nickel composite was fabricated through hot-press sintering a 30 vol% Ti2AlC–Ni mixtures followed by thermal treatment method. Detailed microstructural detection was performed by adopting the route of high-resolution transmission electron microscopy. In situ generated submicron TiC particulate and nanosized L12-γ′ were homogeneously distributed in a fcc γ-Ni matrix. The high temperature tensile and compression behaviors of the TiC-γ′/Ni composites were evaluated in the temperature range of 25–800 °C. The results indicated that ultimate tensile and compression properties exhibited the dependence of temperatures. The tensile properties of the composite determined at 600 °C was 945 MPa, decreasing by 28.1% and 23.0% for elongation, compared to those of measured at room temperature (UTS = 1305 MPa, elongation = 5.35%). However, the yield strength exhibited the temperature independent until to 600 °C. The yield strength of the TiC-γ′/Ni composite are comparable with conventional superalloy under 600 °C, proving that TiC and γ′ particles constitute efficient reinforcements for Ni matrix according to thermal expansion dislocations strengthening, load-transfer and coherent strengthening mechanisms. The monotonic temperature dependence of yield strength at 600–800 °C indicates that plastic deformation of TiC-γ′/Ni composite follows the thermally activated mechanism. The TiC-γ′/Ni composite exhibited superior mechanical properties both at room and high temperatures, which can be ascribed to the synergistic strengthening effect in term of dislocation density, Orowan, coherent and atomic ordering strengthening mechanisms. The present work reveals the deformation behaviors of in situ TiC-γ′/Ni composite from room to high temperatures, which is critical to predict the failure and thereby optimize the microstructure to further enhance the high temperature properties of TiC-γ′/Ni composite.

Microstructure and microhardness studies of Inconel 718, manufactured by selective laser melting and subjected to severe plastic deformation and annealing

Microstructure investigations and microhardness measurements have been comparatively performed for the nickel-based superalloy Inconel 718 produced by selective laser melting (SLM) and conventional technology. The superalloy produced by SLM had a finer γ grain size and a developed dislocation substructure in contrast to a conventional wrought material. High pressure torsion of the superalloy led to a refinement in the microstructure and an increase in the microhardness in both superalloys. Annealing at 600 °C for 2 hours of the deformed superalloys caused an additional increase in microhardness. The microhardness of the SLM material was slightly higher than that of the conventional superalloy.

Comparative Assessment of the Surface Integrity of AD730® and IN718 Superalloys in High-Speed Turning with a CBN Tool

Nickel-based superalloys are typical materials used in components of aeroengines and gas turbine machinery. The strength properties of these alloys at high temperatures are crucial not only to the performance (e.g., power generation efficiency, energy consumption, and greenhouse gas emissions) of aeroengines and industrial gas turbines, but also to machinability during component manufacturing. This study comparatively evaluated the surface integrity of two superalloys, AD730® and Inconel 718 (IN718), during high-speed finishing turning using cubic boron nitride (CBN) tools. IN718 is a conventional superalloy used for the hot section components of aeroengines and industrial gas turbines, while AD730® is a novel superalloy with enhanced high-temperature mechanical properties and good potential as a next-generation superalloy for these components. High-speed turning tests of two superalloys were conducted using a CBN cutting tool and jet stream cooling. The achieved surface integrity of the AD730® and IN718 superalloys was characterized and analyzed to assess the comparability of these alloys.

Segregation and diffusion characterisation in two-stage homogenisation of conventional superalloy

The segregation behaviour of φ 406 mm IN718 ingot was investigated, and the experimental results indicated that Nb was the most segregated element. Two-stage homogenisation treatment was employed to relieve the extensive microsegregation in the as cast ingot. Historically, it was difficult to characterise the effects of the second stage homogenisation because of the uncertainty in locating the diffusion areas. To solve this problem, a 900°C/1 h treatment (‘Tag’ method) was conducted after homogenisation, which could form δ phase (Nb enriched) to trace the diffusion of Nb. The results revealed that the average composition of δ phase area, quantity of δ phase precipitates and sizes of δ phase precipitates change with the proceeding of homogenisation. Eventually, homogeneous distribution of elements could be achieved by means of two-stage homogenisation treatment (1140°C/60 h+1190°C/30 h).

Solidification process of conventional superalloy by confocal scanning laser microscope

The solidification process of a conventional superalloy, IN718, was investigated by confocal scanning laser microscope (CSLM). The liquid fraction during solidification was obtained as a function of real time and temperature in reference with the in-situ observation. The characteristics of L→γ transformation were analyzed and the γ growing rate of each stage was also calculated. Scheil equation was employed to predict the segregation behavior, and the predict results are in consistence with the experimental results. As a result, the confocal scanning laser microscope shows a great potential for solidification process research.

Cobalt-Based Alloys for High Temperature Applications

The isothermal oxidation behavior and thermal stability of a cobalt base alloy was investigated up to a period of 312 hr in air from 1000 to 1200°C. A comparison of oxidation behavior of this alloy with a conventional nickel-base superalloy (Inconel 713C) has been conducted in detail. This experimental alloy oxidizes by forming layers of Al2O3, Cr2O3, TiO2, CoO and traces of SiO2 with WO2 oxides on the surface of the specimen in contact with air. Scanning electron microscopy (SEM) was used to study the microstructure, morphology and compositions of oxides formed after the exposure. Thermal stability of the alloy after extended periods of exposures to air at 1000, 1100 and 1200°C was studied using transmission electron microscopy (TEM).

Low cycle fatigue behaviour of modified and conventional superalloy Inconel 718 at 650°C

The present investigation deals with the study and comparison of low cycle fatigue (LCF) behavior of modified IN 718 alloy with higher Al+Ti/Nb and Al/Ti ratios of 0.458 and 0.870, with that of conventional IN 718 alloy with corresponding ratios of 0.294 and 0.50 respectively. LCF tests were carried out in total strain control mode at total strain amplitudes (Δɛt/2) from ±0.625% to ±1.0%, in fully reversed loading (R= −1) at 650°C. The modified alloy was found to exhibit better LCF resistance than the conventional alloy.

Thermal Stability and Oxidation Resistance of Pt–10 Al–4Cr Alloy at Super-High Temperatures

The isothermal-oxidation behavior of refractory superalloy Pt–10Al–4Cr (in at.%) was investigated up to a period of 312 hr in air from 1200 to 1400°C. A comparison of the oxidation behavior of this alloy with a conventional Ni-base superalloy (Inconel 713C) shows an order of magnitude higher oxidation resistance. This experimental alloy oxidizes by forming Al2O3 and Cr2O3 (and perhaps trace amounts of PtO) with Al2O3 as the oxide layer in contact with air. Optical and scanning-electron microscopy (SEM) were used to study the microstructure, morphology, and composition of the scale formed after oxidation. The thermal stability of the alloy after extended periods at 1200, 1300, and 1400°C was studied using transmission-electron microscopy (TEM).

Effect of Ta on the Oxidation Resistance of a Cast Nickel-Base Die Material with Low Cr and High W Content

A conventional cast nickel-base superalloy with the chemical composition of Ni-16W-2Mo-1.5Cr-6Al wt % has been recently developed as isothermal forging die material operating at temperatures above 1000°C. The oxidation test results showed that the oxide scale spalled heavily after exposure in air at 1050°C for 100 h, which is unacceptable for the process of isothermal forging. To improve the high temperature oxidation resistance, 3 wt% Ta was added into the alloy. Specimens after oxidation test were analyzed by the scanning electron microscope (SEM) and the X-ray diffraction analysis (XRD). The results showed that the oxides were mainly NiO, NiWO4, and NiAl2O4 for all the samples. The oxidation resistance of the alloy with 3wt%Ta was fairly good at 1050°C with the mass gain rate of around 0.016g/m2•h and spallation of oxides less than 2g/m2 within 100h.The SEM results showed that a continuous protective layer of Al2O3 has been formed under the layer of NiO for the alloy containing Ta.

Oxidation Behavior of Rh–30Ni–18Nb Refractory Superalloy

The isothermal-oxidation behavior of Rh–30Ni–18Nb refractory alloy was investigated up to a period of 312 hours in air from 1000 to 1350°C. A comparison of the oxidation behavior of this alloy with a conventional Ni-base superalloy (Inconel 713 C) shows an order-of-magnitude higher oxidation resistance. This experimental alloy oxidizes by forming alternate layers of Nb2O5 and RhO while NiO is on the surface in contact with air. Optical and scanning-electron microscopy (SEM) was used to study the microstructure and morphology for the characterization of the oxides. Thermal stability of the alloy for extended periods of exposure to air at 1400°C was studied using transmission-electron microscopy (TEM).

The Effect of Heat Treatment on the Room Temperature and Elevated Temperature Fracture Toughness Response of Alloy 718

The effect of heat treatment on the JIC fracture toughness behavior of Alloy 718 was characterized at room temperature, 427°C and 538°C. Two different heat treatments were used: the conventional (ASTM A637) treatment, and a modified heat treatment designed to improve the toughness of Alloy 718 base metal and weldments. The elastic-plastic JIC fracture toughness response of the modified Alloy 718 was found to be superior to the JIC behavior exhibited by the conventional material over the entire test temperature range. Metallographic and fractographic examinations of Alloy 718 fracture surfaces revealed that the inferior fracture resistance of the conventional superalloy was attributed to the presence of coarse δ precipitates throughout the conventional matrix. The increased fracture toughness response of the modified Alloy 718 was related to the dissolution of coarse δ precipitates during the high temperature solution anneal employed in the modified treatment.

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides.