Ni-base Superalloy IN738LC Research Articles

Microstructure and Tensile Properties of Ni-Base Superalloy IN738LC according to Solidification Rate and Heat Treatment

The strength of Ni-base superalloys mainly depends on the γ' precipitates that improve the strength of the materials at high temperatures. The presence of γ' particles within the matrix restricts dislocation movement, and optimized heat treatments can tailor the size, shape, and volume fraction of γ'. In this study the effects of solidification rate and solution temperature on the tensile properties of IN738LC superalloy were investigated. The secondary dendritic arm spacing of casting materials with different diameters was measured and the solidification rate of the casting materials was derived by comparing the results of the solidification microstructure obtained from a directional solidification experiment. The D17 material, which had a faster solidification rate, showed higher values of tensile strength and yield strength than the D60 material, which had a slower solidification rate. The study also concluded that the monomodal γ' precipitates in the S80 material have higher tensile strength and yield strength at room temperature and 760℃ than the bimodal γ' precipitates in the S20 material. As for the deformation behavior at 760℃, an isolated stacking fault was observed in the S20 material only within the large γ’ precipitates. In the S80 material, the high dislocation density increased the yield strength due to the strong interaction between dislocations and fine γ’ precipitates.

The influence of γ′ morphology and size on stress rupture properties in Ni-base superalloy IN738LC

This study investigates the influence of γ′ precipitate morphology and size on the creep deformation of the cast superalloy IN738LC under conditions of 760–982 °C and 78–674 MPa. Two microstructures were obtained by different heat treatments. The analysis of the experiments show that, according to the applied stress at 760 °C and 871 °C, the S1A material, with a bimodal distribution with large cuboidal γ′ and small spherical γ′, exhibited the longer creep life compared to those with a monomodal distribution with spherical γ′ precipitates. In the creep test at 760 °C, the S1A material exhibited a stacking fault within the primary cuboidal-shaped γ′ phase. The main deformation mechanism observed was isolated stacking fault (ISF) formation. Conversely, for the S2A material, an extended stacking fault (ESF) was observed, where dislocations continuously traversed several γ′ and γ phases. At 871 °C in low-stress creep tests, the S1A alloy displayed a dislocation network resulting from thermal activation, with localized stacking faults. In the S2A alloy, three-dimensional dislocations generated by climb were primarily observed around spherical γ′ precipitates, accompanied by localized stacking faults. At 982 °C, both S1A and S2A alloys exhibited rafting under lower stress, displaying similar lifetimes. The formation of a regular dislocation network at the interface between γ′ and γ matrix and rafting of γ′ precipitate proved beneficial in improved creep resistance of the alloy.

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.

DSC investigation of size-dependent γ′ dissolution and re-precipitation kinetics in Ni-based superalloy IN738LC

ABSTRACT The novelty of the work is related to confirming using DSC of previous study by the authors, the determining dependency of the gamma prime dissolution in a Ni-based superalloy on precipitate size. In the present work, the effect of γ′ precipitate size, volume fraction and distribution on γ′ precipitate dissolution and re-precipitation behaviour in polycrystalline IN738LC alloy were investigated. The alloy was subjected to different heat treatments including standard ageing and cooling from 1200°C (10 min soaking) at rates of 3000°C/min and 2.5°C/min. The DSC results of these samples were used to calculate the activation energies and kinetics for the dissolution and re-precipitation of the γ′ precipitates. The activation energies for γ′ precipitation and dissolution under different conditions were determined from the Kissinger equation. The small, monomodal precipitate size distribution sample (3000°C/min) exhibited lower activation energy during dissolution than one with a larger, monomodal (2.5°C/min) and bimodal precipitate size (standard aged) distribution samples. Therefore, it is concluded that precipitate size has a direct influence on eventual precipitate kinetics, both in terms of dissolution and re-precipitation under service conditions.

The effect of boron and zirconium on the microcracking susceptibility of IN-738LC derivatives in laser powder bed fusion

The effect of boron (<0.01 to 0.03 wt%) and zirconium (<0.01 to 0.07 wt%) on the microcracking susceptibility of the γ’-strengthened Ni-base superalloy IN-738LC during laser powder bed fusion (LPBF) was studied using custom designed powder grades. It was found that both elements have a strong effect on the microcracking susceptibility, the microcracks are located at high angle grain boundaries based on EBSD measurements and crack density increases with the content of both elements. High crack density in the material with high boron and zirconium content corresponds to a large fraction of intergranular decohesion facets exhibiting a dendritic morphology on the fracture surface, typical for solidification cracking. Investigation of the fracture surface chemistry by X-ray photoelectron spectroscopy (XPS) indicates that considerable amounts of B and Zr are present in oxide state. Auger electron spectroscopy (AES) confirms that both elements are segregated to the intergranular decohesion facets on the fracture surface. Thin layers of B- and Zr-containing oxide on the microcrack surfaces were indicated by atom probe tomography (APT) as well. Hence, it is suggested that the cracking susceptibility of the studied alloying system is caused by formation of B- and Zr-containing oxide at high-angle grain boundaries during solidification.

Creep Damage Process and Crystal Misorientation Measurement of Ni-based Superalloy IN738LC with Multiple Round Notches

Creep Damage Process and Crystal Misorientation Measurement of Ni-based Superalloy IN738LC with Multiple Round Notches

Microstructure and hot corrosion behavior of hot dip siliconized coating on Ni-base superalloy IN738LC

Nickel-based superalloys are widely used at elevated temperature applications because of their high corrosion and oxidation resistance characteristics as well as high stability. To improve the hot corrosion resistance of Nickel-based superalloys, different coatings are applied. In this study, nickel-base superalloy Inconel 738LC was coated via a novel hot-dip diffusion siliconizing process and the corrosion behavior was investigated by XRD, SEM and EDS analyses. A sever degradation and poor hot corrosion resistance was detected by the uncoated sample, while the siliconized coated sample possessed high corrosion resistance. It was figured out that the high hot corrosion resistance of the coated sample was due to the formation of SiO2 protective scale in the surface layer which protects the substrate elements in the hot corrosion environment.

Effect of solution temperature of rejuvenation heat treatment on the stability of γ′ precipitates in Ni-base superalloy IN738LC during long-term heating

This study aims to investigate the effect of time and temperature of complete and partial solutionizing heat treatment on the coarsening of γ′ precipitates in rejuvenation treatment of the used gas turbine blade IN738LC worked for 60000 h. During long-term heating (simulated under the service conditions). After the sample preparation and rejuvenation treatment, long-term heating was carried out at 900 °C and 1000 °C for 500 to 2500 h. The SEM results showed that after rejuvenation with increasing temperature and time, γ′ precipitates coarse in all the samples and the coarse γ′ precipitates appear with spherical morphology. It was observed that the amount of secondary γ′ precipitates decreases with the increase in temperature. When the solution treatment temperature drops, the amount of coarsened γ′ precipitates has increased, which shows that the temperature rise results in a lower coarsening rate and smaller precipitate size.

Magnetic Property Changes of CoNiCrAlY Coating Under Cyclic Oxidation and Hot Corrosion

In this paper, a non-destructive method for evaluation of a CoNiCrAlY coating applied to gas turbine blades has been studied. The specimens from cast Ni-base superalloy IN738LC were coated with Co–34Ni–18.6Cr–8.7Al–0.5Y by low pressure plasma spray (LPPS) technique. The XRD peaks indicated that CoNiCrAlY coatings are composed of β-NiAl and γ-Co (Ni, Cr) matrix with small amounts of Al2O3 and γ′-Ni3Al. The surfaces of the coated specimens were covered with thin salt mixture of Na2SO4 + l0 wt% NaCl. Afterward, the specimens were maintained at 850 °C for 24 h, cooled to room temperature (as one cycle) and then reintroduced in the furnace. The magnetic susceptibility of coating, as one of the magnetic properties, was periodically determined after each cycle. The results showed that the magnetic susceptibility of Co–34Ni–18.6Cr–8.7Al–0.5Y coating is related to the phase compositions and microstructures. The magnetic susceptibility typically increases with depletion of β-NiAl phase and decrease in the coating thickness during cyclic oxidation and hot corrosion. Therefore, measurement of magnetic susceptibility can be employed as a non-destructive test (NDT) method to estimate the thickness or residual lifetime of CoNiCrAlY coatings.

Optimisation of selective laser melting parameters for the Ni-based superalloy IN-738 LC using Doehlert’s design

PurposeThe Ni-based superalloy IN-738 LC is known to be susceptible to porosity and different types of cracking during the build-up process and, thus, challenging to manufacture using selective laser melting (SLM). Determining a feasible set of operating parameters for SLM of nickel-based superalloys involves new approach to experimental design based on the Doehlert method that assists in determining an optimal (feasible) set of operating parameters for SLM of IN-738 LC powder alloy.Design/methodology/approachThe SLM parameters are evaluated in terms of their effectiveness in obtaining the microstructure with a porosity content of <0.5 per cent and without micro-cracking. The experimental approach is exemplified with the Doehlert matrix response variable, relative density, by comparing Archimedes method with microstructural assessments of pores and cracks from image analysis. The effect of heat treatment (HT) and hot isostatic pressing (HIP) on the microstructure of the SLMed IN-738 LC powder alloy has been examined and the consequential tensile response characterised.FindingsBy using optimised process parameters (low heat input, medium scanning speed and small hatching distance) which provides medium energy density, samples of IN-738 LC with a macroscopic porosity <0.5 per cent and free of micro-cracks can be manufactured by SLM. The results indicate that HIP of SLMed material did not lead to a noticeable effect on mechanical properties compared to HT of SLMed material suggesting that the level of both porosity and crack density might be already below the detection limit for the mere heat-treated material.Originality/valueSLM processing parameters (power, scan speed, hatching distance) for IN-738 LC were successfully optimised after only 14 experiments using Doehlert design. Two independent methods, Archimedes method and image analysis, were used in this study to assess relative density of SLM-produced samples with sets of processing parameters showing coherency in prediction with predicted response by Doehlert design.

The effect of preheating on microstructure and mechanical properties of laser solid forming IN-738LC alloy

The microstructure and microhardness of Ni-base superalloy IN-738LC fabricated by laser solid forming (LSF) with the preheating was investigated. The results showed that with the increase of the preheating temperature (T0) (25°C, 800°C, 900°C and 1050°C), the total crack length decreased significantly, the segregation of Ti and Al worsened, and the volume fraction of equiaxed grain increased, as well as the size of blocky carbides, γ-γ′ eutectic and γ′ particles. The high-angle grain boundaries, at which the continuous liquid film can be easily formed, were found to be susceptible to cracking. Both the concentration of Ti and Al and the increase of dendrite spacing contribute to the growth of carbides and γ-γ′ eutectic in the interdendrite while the size of the γ′ phases was largely affected by preheating. It was interesting to find that γ′ phase exhibited a bimodal distribution in the bottom of the deposit with the preheating at 1050°C. The average microhardness of the deposits without preheating and with preheating at 800°C and 900°C is about at the same level, while the deposit with preheating at 1050°C has the lowest average value. The crack-free deposit can be obtained when T0 was up to 1050°C, and its room temperature tensile properties are superior to cast IN-738LC alloy.

Tailoring the texture of IN738LC processed by selective laser melting (SLM) by specific scanning strategies

Selective laser melting (SLM) is an emerging technology of additive manufacturing, which is used to directly produce metallic parts from thin powder layers. This study aims at correlating laser scanning strategies with the resulting textures and corresponding anisotropy of the elastic behavior of bulk materials. Tensile test specimens made of the γ’-containing Ni-base superalloy IN738LC were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy-specimens). Their bulk mechanical properties were determined at room temperature and at 850°C. Specimens were investigated in the ‘as-built’ condition and after recrystallization heat treatment. SEM-based electron backscatter diffraction (EBSD) was applied to measure their crystallographic preferred orientations (texture) and to correlate the anisotropy of Young's modulus with the texture of the material. It is shown that the applied laser scanning strategies allow to tailor the crystallographic texture locally. The possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. A recrystallization heat treatment reduces the degree of crystallographic texture and thus the elastic anisotropy by abundant annealing twinning. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.

Hot Deformation Processing Map and Microstructural Evaluation of the Ni-Based Superalloy IN-738LC

Hot deformation behavior of the Ni-based superalloy IN-738LC was investigated by means of hot compression tests over the temperature range of 1000-1200 °C and strain rate range of 0.01-1 s−1. The obtained peak flow stresses were related to strain rate and temperature through the hyperbolic sine equation with activation energy of 950 kJ/mol. Dynamic material model was used to obtain the processing map of IN-738LC. Analysis of the microstructure was carried out in order to study each domain’s characteristic represented by the processing map. The results showed that dynamic recrystallization occurs in the temperature range of 1150-1200 °C and strain rate of 0.1 s−1 with the maximum power dissipation efficiency of 35%. The unstable domain was exhibited in the temperature range of 1000-1200 °C and strain rate of 1 s−1 on the occurrence of severe deformation bands and grain boundary cracking.

A study of the LCF characteristics of the Ni-based superalloy IN738LC

In this study, low cycle fatigue (LCF) tests of IN738LC were conducted under high-temperature conditions. The tests were performed with various mechanical strain amplitudes and the relationship between strain amplitude and fatigue life was obtained. In addition, an equation for LCF life prediction was derived from the test results. After obtaining a strain-life diagram from the LCF test results, additional LCF tests were conducted according to the extent of damage. The LCF tests were taken at various degrees of damage in order to observe the change in mechanical properties the severity of damage. For this purpose, indentation tests were conducted and the relationship between hardness and damage was obtained. To nondestructively assess the low cycle fatigued IN738LC, the subharmonic nonlinearity parameter was first introduced because of the high attenuation properties of IN738LC. Until now, the subharmonic technique has been applied only to crack evaluation problems. This technique is presented as an effective tool for assessing low cycle fatigued IN738LC in this paper.

遮熱コーティングを施工したNi基超合金IN738LCの高温低サイクル疲労損傷挙動

Damage behaviors of a thermal barrier coated (TBCed) Ni-base superalloy IN738LC were studied under high temperature low-cycle fatigue (LCF) condition. In this work, the HVOF process was selected as the spray process for bond coat. The CoNiCrAlY alloy was coated by 100 μm in thickness as the bond-coat on the Ni-base superalloy IN738LC substrate. And the 8YSZ was coated by 250 μm in thickness as the top-coat on the bond-coat. The LCF tests of TBCed IN738LC were carried out at 900℃ in air. The LCF lives of the TBCed IN738LC were compared with those of bare IN738LC and metal protective coated IN738LC specimen. The results of the LCF tests at 900℃ indicated that the LCF strength of the TBCed IN738LC is better than those of bare and metal protective coated IN738LC. The effect of thermal oxidation on the LCF life of the TBCed IN738LC was also investigated. The effect of thermal oxidation on the LCF life of the TBCed IN738LC was not significant. It was revealed from an observation of the damage morphologies that the cracking morphologies by LCF loadings were changed by the thermal oxidation.

コールドスプレー施工したNi基超合金&lt;i&gt;IN738LC&lt;/i&gt;材のスプレー後熱処理条件の最適化

コールドスプレー施工したNi基超合金&lt;i&gt;IN738LC&lt;/i&gt;材のスプレー後熱処理条件の最適化

Geometrical influence for creep damage evaluation of IN738LC using electron backscatter diffraction

The Ni-based superalloy IN738LC is an outstanding heat-resistant material, but it does have the disadvantage of being brittle. Therefore creep degradation such as deformation of components or initiation of microscopic creep voids only appear at the end of its available creep life. To date, electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy has been used for damage evaluation. Some results showed that the misorientation was correlated with the magnitude of macroscopic plastic or creep strains. In this study, by using the EBSD method, the characteristics of the change and distribution tendencies of crystallographic misorientation on creep life have been investigated. In addition, the geometrical influence of stress concentrated areas for misorientation analysis has been investigated, which is important to evaluate damage in practical components like blades. As a result, the grain reference orientation deviation parameter which corresponds to the spread of creep strain has been found to be useful to estimate creep life among the misorientation parameters. Also, misorientation analysis allows the evaluation of creep damage before the appearance of obvious defects without being subjected to the geometrical influence of the stress concentrated areas.

Thermal shock properties and microstructure investigation of LVPS and HVOF-CoNiCrAlYSi coatings on the IN738LC superalloy

The thermal shock behavior and microstructural investigation of CoNiCrAlYSi coatings applied by Low Vacuum Plasma Spray (LVPS) and High-Velocity Oxy Fuel (HVOF) techniques on the Ni-base superalloy IN738LC were examined. Thermal shock resistance was investigated by forced water quenching of coated samples from high temperature (1100°C) to room temperature. Thermal shock life time of coatings were determined by measuring the weight gain of the specimens at regular intervals for a duration of 65cycles. Characterizations of the coatings and oxide scales were investigated by SEM equipped with EDS and XRD analyses. Oxidation kinetic obtained from weight measurement and morphological observations indicate better thermal shock resistance of LVPS than HVOF coatings. Larger grain size of protective oxides on the HVOF coatings led to more thermal stresses emanated from severe thermal cycling and spallation of the coatings.

Low Cycle Fatigue Behavior of Ni-Base Superalloy IN738LC at Elevated Temperature

High strength nickel-base super alloys have been used in turbine blades for many years because of their superior performance at high temperature. The prediction of fatigue life for superalloys is important for improving the efficiency. In this study, low cycle fatigue tests are performed the variables of total strain range, and room and elevated temperature. The relations between plastic and total strain energy densities and number of cycles to failure are examined in order to predict the low cycle fatigue life of IN738LC super alloy. The fatigue life is evaluated by the Coffin-Manson equation, also the predicted lives by plastic and total strain energy density are compared with experimental results.

Creep damage evaluation of IN738LC based on the EBSD method by using a notched specimen

The Ni-base superalloy IN738LC developed as a heat-resistant material has excellent high temperature strength, but at the same time it has a brittle property. As a result, any evidence of creep degradation such as deformation of components or initiation of creep voids has hardly made an appearance by the end of its available creep life. This paper investigates the possibility that creep damage can be evaluated by using the electron backscatter diffraction (EBSD) method to analyze misorientation which is considered to be caused by creep strain. The use of an in-situ observational testing machine with a notched specimen allowed creep-EBSD round tests which consisted of creep tests in the in-situ obser- vational testing machine and EBSD observations using scanning electron microscopy (SEM) alternately against an identical specimen. The outcome of these experiments was that crystal orientation change tendencies corresponding to the creep strain were continuously observed. It has been found that there was a good correlation between the creep strains and the misorientations referenced to the average orientation in the grain, the parameter of Grain Reference Orientation Deviation (GROD) defined by TSL EBSD system. Consequently, this study shows that the creep damage could be estimated by analyzing misorientations using the EBSD system automatically. Also, it has been found that crystal orientation changes appeared around notches in the direction of maximum shearing stress, which implies that the misorientation development was caused by the crystallographic lattice rotation.