Nickel-base Superalloy IN738LC Research Articles

Gamma prime formation in nickel-based superalloy IN738LC manufactured by laser powder bed fusion

Complex components for high-temperature gas turbine applications require materials that offer a combination of excellent high-temperature strength and oxidation resistance. Nickel-based superalloys with high gamma prime (γ´) volume fractions are particularly suited for these applications, especially combined with additive manufacturing for intricate geometries. Despite the complex thermal history that these materials experience during laser powder bed fusion (LPBF) processing, γ´formation is suppressed when manufacturing IN738LC, which has a medium equilibrium γ´content of about 40–50 vol%. This study follows γ´formation in LPBF IN738LC during subsequent annealing treatments at temperatures ranging from 745 °C to 865 °C, creating an experimentally determined TTT (temperature-time-transformation) diagram. This diagram is largely based on scanning electron microscopy (SEM) imaging supported by Vickers hardness measurements and scanning transmission electron microscopy (STEM) bright field imaging. Atom probe tomography (APT) of the as-built material indicated nm-sized regions depleted in Cr and enriched in Ni, Al, and Ti, but show no characteristic superlattice patterns in TEM diffraction. APT and TEM diffraction analysis of material annealed at 850 °C for 3 min confirmed the presence of the γ´phase but indicated that γ´had formed through spinodal decomposition instead of precipitation.

High-Temperature Low Cycle Fatigue of Nickel-Based Superalloy IN738LC

Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L12 γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

High-cycle fatigue behavior of IN 738LC superalloy at high temperatures

The presented study is focused on experimental results of high-cycle fatigue life of a cast polycrystalline nickel-based superalloy IN 738LC at temperatures of 800, 900, and 950°C. The absence of hot isostatic pressing (HIP) led to the occurrence of casting defects in the microstructure. The fatigue tests were performed in symmetrical loading in laboratory air. The fracture surfaces of specimens were studied by scanning electron microscopy to describe the influence of temperature on the fatigue damage mechanism and fatigue crack initiation. Change in the primary crack propagation mechanism from crystalline (stage I regime) to non-crystalline (stage II regime) mechanism was observed with increasing temperature. However, the effect of changing mechanism caused by temperature dependence was unfavorably influenced by casting defects.

Towards creep property improvement of selective laser melted Ni-based superalloy IN738LC

• SLM IN738LC superalloy was treated with unconventional two-step heating schemes. • The 950 °C first step was found the most effective in redistributing the carbides. • Carbide redistribution significantly reduced grain boundary pinning and promoted grain growth. • Larger grain size led to 56% to 339% increments in creep life depending on creep strength. Nickel-based superalloy IN738LC produced by selective laser melting (SLM) exhibits inferior high-temperature creep properties than its cast counterparts due to relatively smaller grain size, particularly for the plane normal to the building direction. This work studied effects of post heating strategy on the microstructure and especially the grain size to improve the high temperature creep resistance. The as-built microstructure exhibited a fine grain size and large quantities of M C carbides that could effectively hinder grain growth. It was found that unconventional two-step heat treatments could lead to substantial grain growth, and the effect is particularly prominent at a specific temperature. The ease of grain growth was explained after classifying the microstructural evolution (boundary carbide transformation) during each heating step and related to the reduced grain boundary pinning force from M C carbides. Creep tests validated the effect of the new heat treatment scheme on the SLM-processed IN738LC at 850 °C. An extended creep fracture life (1.5 to 4 times improvement) and lower secondary creep rates were achieved with samples subjected to the newly optimized two-step heat treatment. The complete creep curves are also firstly presented for SLM-IN738LC, confirming the effectiveness of grain growth and highlighting the importance of dedicated heat treatment for SLM superalloys.

Single-track investigation of IN738LC superalloy fabricated by laser powder bed fusion: Track morphology, bead characteristics and part quality

• Single-track laser powder bed fusion experiments were performed on IN738LC. • Bead dimension varied significantly with respect to used processing parameters. • Balling occurred when solidification time was less than spread time of droplets. • Conduction mode transferred to keyhole as normalized enthalpy was larger than ∼ 50. • High porosity appeared in bulk samples at both low and high energy input densities. Single-track experiments were performed to investigate how the scan speed and the laser power affected the behavior of the single tracks during laser powder bed fusion, including the track stability, the melt pool dimension and the bead mode, using the nickel-based superalloy IN738LC. The processing parameters had an evident effect on the track morphology and the bead features in the experimental conditions investigated. The balling phenomenon was studied by a competitive model between the spread and the solidification of droplets. The analysis and experimental results clearly demonstrated that the track lost its stability when the droplets solidified before they spread out on the substrate with a contact angle greater than 90°. The keyhole mode was investigated by establishing the relationship between the normalized enthalpy and the normalized bead depth. The conduction mode would convert to the keyhole mode as normalized enthalpy was greater than ∼ 50. Finally, bulk samples were built with the same parameters as the single-track testing. It can be seen that the parts with high porosity appeared at both low and high energy input densities due to the un-melted powders and the keyhole pores, respectively.

Hot corrosion behavior of aluminized and Si-modified aluminized coated IN-738LC produced by a novel hot-dip process

Hot-dip aluminum and Si-modified aluminum diffusion coatings were applied on the substrate of nickel-base superalloy IN-738LC to enhance its hot corrosion behavior for high-temperature applications. Aluminizing salt bath consisted of KCl, NaCl, NaF, Na3AlF6, and Al powder, while the Si-modified salt bath contained NaCl, NaF, KCl, Na2SiF6, and Na3AlF6. A temperature of 750 °C and an operating time of 30 min were applied for coating. At this condition, the thicknesses of about 48 and 35 µm were achieved for aluminum and Si-modified aluminum coatings, respectively. All coated samples were immersed in the molten salt of Na2SO4–25 wt%NaCl composition for 60 and 140 h for the hot corrosion test. It was found that the Si-modified coated samples had better corrosion resistance. The high hot corrosion resistance was believed to be due to the formation of the protective and continuous scale of Al2O3 and SiO2 and no sign of sulfidation was detected at 750 °C.

3D orientation data – A comparison of diffraction contrast tomography and serial sectioning electron backscatter diffraction for the nickel-base superalloy IN738LC

For specimen characterization, X-ray microscopy offers a variety of advantages like non-destructive investigations and reduced preparation effort for the samples. Orientation measurements are possible using diffraction contrast tomography (DCT). However, lateral resolution is lower as compared to scanning electron microscopy combined with electron backscatter diffraction (EBSD). A sample of a nickel-based superalloy IN738LC was investigated three-dimensionally for direct comparison of EBSD and DCT, which clearly reveals the advantages, disadvantages and limits of both methods. Images obtained by EBSD showed more microstructural details, but also artefacts resulting from the previous sample preparation. DCT seemed more suitable for 3D-imaging, but was not able to resolve some of the smaller grains.

Fine Precipitates in Nickel Base Superalloys

Presence of fine, secondary/tertiary precipitates in superalloys improves especially the creep-fatigue properties of these alloys. It is conveniently accepted that the fine precipitates form non-isothermally, for example, during cooling from an aging temperature or isothermally during a secondary, lower temperature aging. In the current study, several single-aging treatments were conducted to assess the formation of the fine precipitates in the polycrystalline, nickel-base superalloy IN738LC. The agings were carried out stress-free at 950oC, 1050oC, 1120oC, and 1140oC for various times. Stressed agings at 950oC and 1050oC were also conducted. A time-dependent isothermal formation of the fine precipitates was observed. The formation time decreased as the aging temperature increased. It is suggested that dissolution of some coarse precipitates, evolution of the precipitate-matrix interface toward a fully faceted one, and increased matrix channel width saturate the channels and control the formation of the fine precipitates.

Metastable carbides and their impact on recrystallisation in IN738LC processed by selective laser melting

Selective laser melting of nickel based superalloys opens up new possibilities for gas turbine engine manufacturers; namely, efficient production of low component volumes, decreased component cost through reduced post-machining procedures and access to new component geometries that cannot be fabricated by conventional processing methods. However, processing high performance nickel-based superalloys components via additive manufacturing, without the occurrence of defects, is challenging and requires a better understanding of the resulting microstructure, especially as different compositions can lead to significant changes in the microstructure obtained. In addition, carefully selected post-processing heat-treatments are required to alter the microstructure and attain the required mechanical properties. In this study, the microstructure of the nickel base superalloy IN738LC has been characterised in the as-deposited and stress relieved heat-treated states, as well as following high temperature heat treatments. The data acquired highlights the influence of the stress relief step on the recrystallisation temperature and its relation with the distribution of carbide particles present in the alloy's microstructure.

Oxidation Kinetics and Oxide Scale Characterization of Nickel-Based Superalloy IN738LC at 900 °C

The high-temperature isothermal oxidation behavior of the polycrystalline nickel-based superalloy IN738LC was investigated at 900 °C in air for up to 1000 h. The results from the tests suggest that the alloy showed single-stage parabolic oxidation behavior during isothermal oxidation. The oxidized samples were characterized using SEM and SEM/EDS, and the results show that the alloy is comprised of an outer dense chromia scale with titania proving Type II oxidation behavior. In addition, the formation of a spinel composition adjacent to the external layer and a discontinuous needle-shaped alumina scale in the alloy subsurface zone were also observed. The depletion of gamma prime (γ′) phase leads to a precipitate-free zone formation in the subscale zone. A JMatPro thermodynamic analysis showed that an increase in titanium content from 1 to 3.44 wt.% increased the chromium activity by 50%. Therefore, the results suggest that the presence of high amounts of titanium (~3.44 wt.%) in IN738LC increased the oxidation kinetics by increasing the chromium scale growth rate and resulting in an oxidation rate constant of 2.79 × 10−6 mg2 cm−4 s−1.

Melt pool simulation for the evaluation of process parameters in selective laser melting

With increasing industrial interest and significance of the selective laser melting the importance for profound process knowledge increases so that new materials can be qualified faster. Also it is the basis for an educated evaluation of possible process innovations. Therefore a 3D numerical model for the selective laser melting process is presented that allows a detailed look into the process dynamics at comparably low calculation effort. It combines a finite difference method with a combined level set volume of fluid method for the simulation of the process and starts with a homogenized powder bed in its initial configuration. The model uses a comprehensive representation of various physical effects like dynamic laser power absorption, buoyancy effect, Marangoni effect, capillary effect, evaporation, recoil pressure and temperature dependent material properties. It is validated for different process parameters using cubic samples of stainless steel 316L and nickel-based superalloy IN738LC. The results show the significance of evaporation and its related recoil pressure for a feasible prediction of the melt pool dynamics. Furthermore a possible way to reduce the times and costs for material qualification by using the simulation model to predict possible process parameters and therefore to reduce the necessary experimental effort for material qualification to a minimum is shown.

Mechanism of Al-Si codeposition on In738LC through single-step diffusion process

The out-of-pack cementation method was applied to coat the Nickel-based super alloy IN738LC with Si-modified aluminide. Different mixtures of the pack containing pure Al, Si, alumina powders as an inert filler, and NH4Cl as a halide salt activator were used for co-deposition at 950 and 1,050°C. The results showed although the amount of subhalide vapour pressures of Al and Si were equal at 1,050°C, Al atoms diffused only after Si atoms had been deposited. Before heat treatment, the coating thickness formed by the pack containing 2.3 wt.% Al was 6 µm more than that containing 3.5 wt.‰ Al. After heat treatment, thickening coatings formed within the pack containing 2.3 wt.% Al was less than that obtained in the other pack containing 3.5 wt.% Al. This study suggests that increasing the depositing temperature will increase the thickness at a constant composition of the pack powder.

Mechanism of Al-Si codeposition on In738LC through single-step diffusion process

The out-of-pack cementation method was applied to coat the Nickel-based super alloy IN738LC with Si-modified aluminide. Different mixtures of the pack containing pure Al, Si, alumina powders as an inert filler, and NH4Cl as a halide salt activator were used for co-deposition at 950 and 1,050°C. The results showed although the amount of subhalide vapour pressures of Al and Si were equal at 1,050°C, Al atoms diffused only after Si atoms had been deposited. Before heat treatment, the coating thickness formed by the pack containing 2.3 wt.% Al was 6 µm more than that containing 3.5 wt.‰ Al. After heat treatment, thickening coatings formed within the pack containing 2.3 wt.% Al was less than that obtained in the other pack containing 3.5 wt.% Al. This study suggests that increasing the depositing temperature will increase the thickness at a constant composition of the pack powder.

Microdamage-coupled inelastic deformation analysis of ceramic thermal barrier coatings subjected to tensile loading

Abstract To protect the hot parts of a gas turbine (GT) from high-temperature combustion gas, thermal barrier coatings (TBCs) are deposited on the surface of the blades and the inner surface of the combustor. Thermal stress is cyclically generated in TBC systems by the start-steady-stop operation of the GTs, resulting in TBC spallation. Recently, it has become necessary for electricity generation companies to adopt TBC life assessment technology to determine the appropriate interval to maintain the TBC. Therefore, we decided to develop a numerical code for damage-coupled inelastic deformation analysis of GT components coated with TBCs. In this study, a finite element (FE) code was developed based on the commercial FE software MARC by implementing both the complete deformation model of the inelastic constitutive equation for TBCs, which was developed by us, and the Chaboche-type viscoplastic constitutive equation for nickel-based superalloy IN738LC. The simulation of the damage process, including crack initiation and propagation in TBC systems, was conducted under tensile loading using this FE code. It was shown that the crack propagation feature simulated using the FE code completely matched the continuous observation results obtained in the high-temperature tensile test.

Investigation of isothermal and thermo-mechanical fatigue behavior of the nickel-base superalloy IN738LC using standardized and advanced test methods

The uniaxial and biaxial-planar isothermal fatigue behavior as well as the uniaxial thermo-mechanical fatigue behavior of the nickel-base superalloy IN738LC was investigated at high temperatures using standardized and advanced test technologies. Small scale specimens’ tests were carried out in order to study the specimen size effect and biaxial-planar tests under equibiaxial and shear loading to investigate the influence of the multiaxial stress state on the isothermal fatigue behavior both at 750°C. Standardized specimens were used to determine the uniaxial isothermal and thermo-mechanical fatigue behavior under in-phase and out-of-phase loading in the temperature range between 750°C and 950°C. The standard specimens’ results particularly served as reference data as well as for the verification of a recently developed lifetime prediction model for thermo-mechanical fatigue loading. The small scale specimens’ results were in good agreement with standardized specimens. The comparison of fatigue lifetimes of uniaxial and biaxial-planar tests showed that the equivalent strain hypothesis according to von Mises is applicable to correlate the fatigue life of the equibiaxial loading case within a scatter band of factor two, whereas the shear loading case was predicted conservatively. Furthermore, the recently developed lifetime prediction model correlated the fatigue lives of the isothermal and thermo-mechanical tests under uniaxial and biaxial-planar loading as well as literature data. Finally, the deformation and crack growth mechanism were studied.

Application of a multi-component mean field model to the coarsening behaviour of a nickel-based superalloy

A multi-component mean field model has been applied to predict the particle evolution of the γ′ particles in the nickel based superalloy IN738LC, capturing the transition from an initial multimodal particle distribution towards a unimodal distribution. Experiments have been performed to measure the coarsening behaviour during isothermal heat treatments using quantitative analysis of micrographs. The three dimensional size of the γ′ particles has been approximated for use in simulation. A coupled thermodynamic/mean field modelling framework is presented and applied to describe the particle size evolution. A robust numerical implementation of the model is detailed that makes use of surrogate models to capture the thermodynamics. Different descriptions of the particle growth rate of non-dilute particle systems have been explored. A numerical investigation of the influence of scatter in chemical composition upon the particle size distribution evolution has been carried out. It is shown how the tolerance in chemical composition of a given alloy can impact particle coarsening behaviour. Such predictive capability is of interest in understanding variation in component performance and the refinement of chemical composition tolerances. It has been found that the inclusion of misfit strain within the current model formulation does not have a significant affect upon predicted long term particle coarsening behaviour. Model predictions show good agreement with experimental data. In particular, the model predicts a reduced growth rate of the mean particle size during the transition from bimodal to unimodal distributions.

Microstructural characteristics of the nickel-based alloy IN738LC and the cobalt-based alloy Mar-M509 produced by selective laser melting

This study investigates selective laser melting (SLM) of the nickel based superalloy IN738LC and the cobalt based alloy Mar-M509, and identifies the influence of process and material parameters on the resulting microstructure. Comprehensive microstructural characterization was performed using electron backscattered diffraction analysis. Significant differences between IN738LC and Mar-M509 were observed with respect to grain size, grain shape and texture sharpness. Alloy IN738LC exhibits coarse and elongated grains with a sharp texture and thus a pronounced mechanical anisotropy. Alloy Mar-M509 shows smaller grains with only moderate structural and mechanical anisotropy. The different microstructural and mechanical characteristics are attributed to the different recovery and recrystallization behavior of IN738LC and Mar-M509. The high stacking fault energy (SFE) of IN738LC results in pronounced recovery of lattice defects without affecting the basic grain structure, whereas the low SFE in Mar-M509 favors recrystallization with the effect of significant grain refinement and weakening of the solidification texture. The effect of microstructure and the structural anisotropy on the orientation-dependent values of the Young’s modulus and the mechanical properties are further discussed.

火山灰堆積したNI基超合金IN738LCの高温腐食特性

火山灰堆積したNI基超合金IN738LCの高温腐食特性

Investigations on the microstructure and crack formation of IN738LC samples processed by selective laser melting using Gaussian and doughnut profiles

This paper contributes to a better understanding of processing the nickel-based superalloy IN738LC using selective laser melting (SLM). Initially, the basic workability of IN738LC using SLM is demonstrated. Subsequently, a comparison between a Gaussian and a doughnut profile is carried out, and clear correlations between the choice of process parameters and the resulting imperfections depending on the chosen laser beam profile are shown. Electron backscatter diffraction (EBSD) measurements show a significant influence of scan strategy and build direction on the texture of the sample, independent of the used laser beam profile. Regardless of the laser beam profile, two contradicting trends complicate the defect-free processing of IN738LC, i.e. a reduction in the crack density can only be realized with increasing porosity. Through microstructural investigations, observed hot cracks are identified to be solidification cracks. Based on a broad experimental study in combination with a numerical solidification study, a theory about the crack initiation mechanism is presented. Finally, by using atom probe tomography (APT), the element zirconium is confirmed as a possible reason for the occurrence of solidification cracks.

Prediction of thermo-mechanical fatigue life of IN738 LC using the finite element analysis

This paper describes a study we performed on predicting the thermo-mechanical fatigue (TMF) life of the nickel-based superalloy IN738LC, which is the main material of gas turbine blades. Gas turbine blades experience mechanical fatigue and thermal fatigue as a result of centrifugal force and flames. Furthermore, gas turbine blades experience low cycle fatigue due to frequent startups and shutdowns. In the case of low cycle fatigue behavior that a large plastic deformation mainly occurs, the fatigue life can be decided by the plastic strain amplitude. The hysteresis loop of the material should be derived to obtain the plastic strain. However, precise experimental techniques are required to derive the hysteresis loop, which, in turn, require extensive time and effort. Thus, it is necessary to develop a simple method to obtain the hysteresis loop without the TMF test. In this study, we performed the TMF test and obtained the hysteresis loop and fatigue life curves. We also derived the hysteresis loop and predicted the fatigue life through finite element analysis (FEA). The analysis results were verified by comparison with the experimental results. As a result, the fatigue life of the IN738LC can be predicted with FEA.