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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Effect of homogenization heat treatment on the evolution of carbonitrides in powder bed fusion processed INCONEL 718

Inconel 718 fabricated using laser powder-bed fusion, contains precipitates in the as-built condition that can be coarsened by high-temperature heat treatments. In this study, two homogenization heat treatment regimens were applied to discern the impact of heat treatment conditions on the growth of complex M(C, N) carbonitrides. In the first heat treatment, the samples underwent heat treatment between 1050 °C and 1200 °C for 0.5 h. In the second heat treatment, the samples were held at a constant temperature of 1150 °C, with varied holding times from 0.5 h to 8 h. Heat treatments dissolved the unstable Laves phase, but the carbonitrides persisted in the structure regardless of temperature and duration. Changes in the compositions, lattice parameters, and sizes of M(C, N) carbonitrides were measured using transmission electron microscopy and high-energy synchrotron X-ray diffraction. The results show an Nb enrichment at carbonitrides while Nb loss at the matrix with increased homogenization temperature. The findings suggest the optimum heat treatment conditions to achieve a homogeneous structure and controlled carbonitride size are between 1000 and 1050 °C for up to 2 h.

IN178↗
DOE OSTI2020

Characterization of precipitation in gradient Inconel 718 superalloy

Inconel 718 (IN718) superalloy is a well-established material for the application of aircraft engine and power generator due to its excellent performance at high temperatures. The outstanding mechanical properties of IN718 are largely determined by the precipitates. Here, we investigated the precipitation behaviors and the corresponding mechanical properties of the heat-treated gradient IN718 alloy. The microscopy studies reveal that high-density hexagonal η phase with chemistry of Ni 3 (Al,Ti,Nb) formed in the nanograined IN718 alloy after annealing at 700 °C. Additionally, the compressive stress introduced during severe plastic deformation may have prohibited the formation of γ” phase while accelerated the precipitation of γ’ phase, consequently favoring the precipitation of η phase. The corresponding evolution of mechanical behavior of gradient IN718 alloy is investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Powder bed binder jet 3D printing of Inconel 718: Densification, microstructural evolution and challenges

Traditional manufacturing of Inconel 718 components from castings and thermomechanical processing routes involve extensive post processing and machining to attain the desired geometry. Additive manufacturing (AM) technologies including direct energy deposition (DED), selective laser melting (SLM), electron beam melting (EBM) and binder jet 3D printing (BJ3DP) can minimize scrap generation and reduce lead times. While there is extensive literature on the use of melting and solidification based AM technologies, there has been limited research on the use of binder jet 3D printing. In this paper, a brief review on binder jet additive manufacturing of Inconel 718 is presented. In addition, existing knowledge on sintering of Inconel 718 has been extended to binder jet 3D printing. We found that supersolidus liquid phase sintering (SLPS) is necessary to achieve full densification of Inconel 718. SLPS is sensitive to the feedstock chemistry that has a strong influence on the liquid volume fraction at the processing temperature. Based on these results, we discuss an empirical framework to determine the role of powder particle size and liquid volume fraction on sintering kinetics. In conclusion, the role of powder packing factor and binder saturation on microstructural evolution is discussed. The current challenges in the use of BJ3DP for fabrication of Inconel 718, as well as, extension to other metal systems, are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Report on Evolution of Inconel 718 Following HFIR Irradiation

The report presents the microstructure and mechanical properties of 3D printed Inconel 718 after irradiation in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) to assess its potential use as a structural material. The structural components near the outlets of several proposed reactor cores will experience significant neutron fluxes and outlet coolant temperatures ranging from the hot standby temperature of 300 °C to nearly 550 °C at the center of the part. These components must support the core in appropriate loading conditions and require structural analysis at relevant temperatures. In FY21, three heat treatments were designed and conducted to simplify the microstructure and to determine how each precipitating phase contributed to the overall strength. In FY21, baseline mechanical properties were measured from uniaxial tensile tests on subsize SS-J2 specimens at room temperature and at elevated temperatures of 300, 450, and 600 °C to serve a comparison to the irradiated properties. Four capsules containing 3D printed Inconel 718 were inserted into HFIR in FY21 for a matrix of two temperatures and two doses. The lower of the two doses was available for characterization in FY22. Multiple heat treatments of Inconel 718 irradiated to nominal conditions of 2 displacements per atom (dpa) at either 300 or 600 °C were strained with uniaxial tensile tests at the Irradiated Material Examination and Testing (IMET) Facility to discern the mechanical properties. Transmission electron microscopy was performed to correlate the observed mechanical properties with nanoscale features. The initially homogenous AM718-HM increased in strength at both irradiation temperatures based on a high density of nanometer-scale radiation-induced cavities at lower temperature and nucleation and growth of γ" precipitates at higher temperatures. The precipitate-hardened AM718-HT2 showed very small differences in strength before and after irradiation: the contribution to strength from γ" precipitates was replaced with dislocation loops. Because the radioactivity of the nickel superalloys from neutron activation limited the scope of the analysis, a feasibility study examined the possibility of an ultra-miniature specimen geometry, colloquially SS-Tiny (SS-T), for mechanical property determination using the nonirradiated Inconel 718. This study found an overestimation of ductility from the SS-T geometry with yield strength and ultimate tensile strength slightly above the SS-J2 geometry: this could be contributed to a reduction in specimen thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Characterization of Properties of Laser Powder Bed Fusion 3D-Printed Inconel 718 for Centrifugal Turbomachinery Applications

Here, this paper presents the results of a comprehensive effort to characterize the properties of Inconel 718 produced by a form of laser powder bed fusion (LPBF) additive manufacturing (AM) or 3D-printing, subsequently subjected to hot isostatic pressing (HIP) and heat treatment according to standards F3055-14a and AMS 5663, respectively. Material property data, while broadly available for traditional Inconel 718 presentations (e.g. forgings or castings), is currently lacking for the 3D-printed material. It is expected that while limited in size, the experimental data sets presented provide sufficient information to glean the capability of LPBF Inconel 718. These include: 1) Chemical composition, electron backscatter diffraction (EBSD), and x-ray energy dispersive spectroscopy (XEDS) characterization of 3D-printed material structure; 2) Tensile properties — 0.2% yield stress, ultimate stress, modulus of elasticity, and elongation to failure — based on 108 samples, as functions of temperature and sample print orientation; 3) Creep rupture data including the Larson-Miller parameter, based on 21 samples; and 4) High cycle fatigue data based on 21 samples as a function of temperature. Results are compared to available standards and/or data for forged, cast, and other AM Inconel 718. A key observation of this study, based on the EBSD results, is that while the material appears to approach full recrystallization following heat treatment, there is a detectable fraction of the material that does not fully recrystallize, resulting in a material with mechanical properties (e.g. yield stress, creep rupture) measurably lower than those of forgings, but higher than those of castings.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2020

Laser powder bed fusion of Inconel 718 on 316 stainless steel

The joining of Inconel 718 and 316 stainless steel has notable industrial importance. However, traditional arc-welding has many issues. In particular, the formation of the detrimental Nb-rich Laves phase in the heat affected zone can promote crack initiation and propagation, which leads to poor mechanical properties. The small heat input and the rapid solidification of laser powder bed fusion (PBF) has the potential to suppress this problem. Despite the infancy of the multi-material PBF, it is of interest to investigate the relationship among the process parameter, the microstructure at the materials joining point, and the mechanical performance. In this study, we used high-speed x-ray imaging to in-situ monitor the printing of Inconel 718 tracks on a 316H SS substrate via laser powder bed fusion, followed by post-build characterization and testing with SEM, XRD and nanoindentation. We observed that a higher linear energy density resulted in a more diffused interface and a smaller compositional gradient, but as a trade-off, generated more keyhole pores and thermal cracks. Furthermore, we also showed that the built layer exhibited chemical inhomogeneity at two length scales: a tens-of-mu m-scale compositional vortex due to the melt flow, and a sub-mu m-scale Nb-rich dendritic structure due to the non-equilibrium solidification that led to the formation of the Laves phase. It was inferred that the chemical inhomogeneity may benefit the mechanical properties at the interface by providing more interlocking between the two materials and also by limiting the effect of the brittle Laves phase to potentially short range.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure Development in Electron Beam-Melted Inconel 718 and Associated Tensile Properties

During the electron beam melting (EBM) process, builds occur at temperatures in excess of 800°C for nickel-base superalloys such as Inconel 718. When coupled with the temporal differences between the start and end of a build, a top-to-bottom microstructure gradient forms. Characterized in this study is the microstructure gradient and associated tensile property gradient that are common to all EBM Inconel 718 builds. From the characteristic microstructure elements observed in EBM Inconel 718 material, the microstructure gradient can be classified into three distinct regions. Region 1 (top of a build) and is comprised of a cored dendritic structure that includes carbides and Laves phase within the interdendritic regions. Region 2 is an intermediate transition zone characterized by a diffuse dendritic structure, dissolution of the Laves phase, and precipitation of δ needle networks within the interdendritic regions. The bulk structure (Region 3) is comprised of a columnar grain structure lacking dendritic characteristics with δ networks having precipitated within the grain interiors. Mechanically at both 20°C and 650° C, the yield strength, ultimate tensile strength, and elongation at failure exhibit the general trend of increasing with increasing build height.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Synergistic effect of microstructure and defects on the initiation of fatigue cracks in additively manufactured Inconel 718

Fatigue cracks in additively manufactured (AMed) Inconel 718 (IN-718) in machined surface condition often initiate from persistent slip bands (PSBs) unlike other popular AM alloys such as 17-4 PH stainless steel or Ti-6Al-4V, where fatigue crack initiation is exclusively from volumetric defects; therefore, a competition between PSB- vs. defect- mediated crack initiation clearly exists. To shed light on the factors governing the competition, this study investigates the characteristics of cyclic strain localization, PSB formation, and crack initiation via crystal plasticity (CP) modeling of cyclic loading on polycrystalline aggregates which are then validated by experiments. In this work, a physics-based, free slip distance (FSD) dependent slip strength evolution law is proposed, which is shown to enable the CP model to simulate the heterogeneous strain distribution in IN-718. Implementing a crack initiation criterion based on strain contrast, the locations and lives for crack initiation can also be calculated. It is shown that both FSD and resolve shear stress influence the strain localization and crack initiation behaviors. The distribution of a localization parameter calculated based on the multiplication of FSD and Schmid factor within a grain is found to correlate well with the locations of PSBs. The maximum values of the localization parameters within a microstructure are shown to correlate well with the experimentally obtained crack initiation lives. The presence of volumetric defects in IN-718 generally do not impact strain localization behavior unless their size is large compared to the grain size.

42 ENGINEERING↗
DOE OSTI2019

An Investigation of the Wear Performance of Aged Inconel 718 Superalloy by the Fuzzy Logic Approach

In this study, the effects of δ, γ′ and γ″ phases on the microstructure and wear behavior of Inconel 718 alloy, produced by powder metallurgy technique, were analyzed experimentally and a fuzzy logic model was developed for the weight loss prediction after the wear tests. Aging process was carried out in two stages: firstly at 720 °C for 8 h and then at 620 °C for 6-10 h. Wear tests were performed at five different sliding distances (400-2000 m) for 1 ms{sup −1} sliding speed under 30 to 60 N loads. Weight loss prediction model was developed by using fuzzy logic method. SEM images of Inconel 718 alloy aged for 6, 8 and 10 h at 620 °C show intermetallic precipitates formed around particle boundaries. XRD examinations showed orthorhombic δ (Ni{sub 3}Nb), BCT γ″ (Ni{sub 3}Nb), FCC γ′ Ni{sub 3}(Al,Ti) hexagonal Laves (Fe{sub 2}Nb) phases formed in the structure with the same chemical composition and different crystal lattice. Accurate estimates were obtained in the developed fuzzy logic model. It was illustrated that this fuzzy logic model can be used successfully for the prediction of weight loss in wear tests of Inconel 718 alloy that was produced by powder metallurgy method.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Mechanical behavior of post-processed Inconel 718 manufactured through the electron beam melting process

Here, the electron beam melting (EBM) process was used to fabricate Inconel 718. The microstructure and tensile properties were characterized in both the as-fabricated and post-processed state transverse (T-orientation) and longitudinal (L-orientation) to the build direction. Post-processing involved both a hot isostatic pressing (HIP) and solution treatment and aging (STA) to homogenize the microstructure. In the as-fabricated state, EBM Inconel 718 exhibits a spatially dependent microstructure that is a function of build height. Spanning the last few layers is a cored dendritic structure comprised of the products (carbides and Laves phase) predicted under equilibrium solidification conditions. With increasing distance from the build's top surface, the cored dendritic structure becomes increasingly homogeneous with complete dissolution of the secondary dendrite arms. Further, temporal phase kinetics are observed to lead to the dissolution of the strengthening γ"γ" and precipitation of networks of fine δ needles that span the grains. Microstructurally, post-processing resulted in dissolution of the δ networks and homogeneous precipitation of γ'"γ'" throughout the height of the build. In the as-fabricated state, the monotonic tensile behavior exhibits a height sensitivity within the T-orientation at both 20 and 650 °C. Along the L-orientation, the tensile behavior exhibits strength values comparable to the reference wrought material in the fully heat-treated state. After post-processing, the yield strength, ultimate strength, and elongation at failure for the EBM Inconel 718 were observed to have beneficially increased compared to the as-fabricated material. Further, as a result of post-processing the spatial variance of the ultimate yield strength and elongation at failure within the transverse direction decreased by 4 and 3× respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of anisotropy and texture on the low cycle fatigue behavior of Inconel 718 processed via electron beam melting

Here in this study, the impact of texture (columnar/equiax grain structure) and influence of material orientation on the low cycle fatigue (LCF) behavior of hot isostatic pressed (HIP) and heat-treated Inconel 718 fabricated through electron beam melting (EBM) is investigated. Material was tested both parallel and perpendicular (transverse) to the build direction. In all instances, the EBM HIP and heat-treated Inconel 718 performed similarly or exceeded the LCF life of wrought Inconel 718 plate and bar stock under fully reversed strain-controlled loading at 650 °C. Amongst the textures, the columnar grains oriented parallel to the build direction exhibited the highest life on average compared to the transverse columnar and equiax EBM material. Further, in relation to the reference wrought material the parallel columnar grain material exhibited a greater life. While a negligible life difference was observed in the equiax grained material between the two orientations, a consistently lower accumulated inelastic strain was measured for the material loaded parallel to the build direction than the transverse orientation. Failure of the parallel columnar material occurred in a transgranular manner with cracks emanating from the surface whereas the transverse columnar material failed in a intergranular manner, with crack growth occurring through repeated rupture of oxide at the crack-tip. Finally, in the case of the equiax material, an influence of material orientation was not observed on the failure mechanism with crack propagation occurring through a combination of debonded/cracked carbides and void formation along twin boundaries resulting in a mixture of intergranular and transgranular crack propagation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of the As-Forged and Heat-Treated Microstructure on the Room Temperature Anisotropic Ductile Fracture of Inconel 718

The purpose of the present work was to investigate the effect of primary carbides and the δ-phase on the anisotropic ductile fracture of Inconel 718 in the forging process. Inconel 718 alloys were prepared by VIM + VAR processes with various carbon contents (0.009 and 0.027 wt.%). Then, the alloys were forged and annealed at temperatures of 980 and 1030 °C. The room temperature mechanical anisotropy of the alloys was evaluated at the longitudinal direction (LD) and transverse direction (TD). Tensile and impact tests were used to characterize the mechanical properties of the specimens. The microstructural characterization and the fractography of the alloys were carried out by FE-SEM. The obtained results showed that the fracture strain and the impact energy in the TD were 30-50% lower than the LD. The fracture was accelerated by the δ-phase, leading to the reduction of impact energy in the longitudinal and the lateral directions up to 50%. The low-carbon alloy indicated similar characteristics in both the LD and the TD. Aligned carbides changed the fracture path from a zigzag path in the LD to a fibrous path in the TD, while the δ-phase created a flat fracture path. The shear lip area ratio in the tensile fracture cross section was decreased by reducing ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Microstructure, stored energy, and stability of H/He-filled nanocavities in low temperature irradiated Inconel 718

The microstructure, trapped transmutation gases, stored energy, and mechanical behavior of samples from an irradiated Inconel 718 proton beam window were characterized using transmission electron microcopy, thermal desorption spectrometry (TDS), differential scanning calorimetry (DSC), and tensile testing. In the as-irradiated condition the microstructure contained a high number density of 1–3 nm gas-filled nanocavities. Emissions of trapped gases, H and He, during TDS correlated with peaks of the energy release curves from DSC examinations, which suggest these gases were likely stored in highly stable defect traps. The stored energy from radiation damage saturated at doses of a few dpa and did not increase with increasing radiation dose, but the amount of stored H and He increased with increasing dose. Effects of post-irradiation annealing were studied as well. After exposure to 700 °C, the nanocavities grew only slightly to 2–4 nm in diameter, but after exposure to 900 °C, the cavities grew to 10–20 nm in diameter and electron energy-loss spectroscopy showed these cavities contained a core of He surrounded by a shell of H. Further, this study demonstrated that the irradiation defect structures containing H and He were remarkably stable during irradiation and after exposure up to 700 °C. The effect of the irradiation temperature, defect mobility, and interaction of H, He, and irradiation defects on mechanical behavior provides insight into the processes responsible for the unusual recovery in ductility with increasing radiation dose observed in Inconel 718 after high energy proton and spallation neutron irradiation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of Mo-rich Fillers in Pulsed Current Gas Tungsten Arc Welding of Inconel 718 for Improved Strength and Hot Corrosion Resistance

The formation of Laves phase in the fusion zone of Inconel 718 welds is considered to be detrimental and requires major attention, as this phase deteriorates the mechanical properties and high-temperature corrosion resistance. This present study addresses the use of current pulsing technique on the gas tungsten arc welding of 5-mm-thick Inconel 718 plates. Nb-free fillers, namely ERNiCrMo-10 and ERNiCrMo-14, are chosen for joining these plates. Microstructure studies revealed the segregation of Mo-rich phases at the inter-dendritic regions of the fusion zone of both the weldments. Further, the occurrence of detrimental Laves phase was minimized while employing Mo-rich fillers. It is inferred from the tensile studies that the fractures occurred in the parent metal for ERNiCrMo-10 filler and at the fusion zone while employing ERNiCrMo-14. Room-temperature Charpy impact studies also affirm that the impact toughness of ERNiCrMo-10 welds is significantly greater than that of ERNiCrMo-14. Hot corrosion studies were performed on the coupons by exposing them to a synthetic molten salt environment comprising Na{sub 2}SO{sub 4} + 60%V{sub 2}O{sub 5} at 800 °C for 50 h. Owing to the presence of higher amounts of Cr{sub 2}O{sub 3} and NiO, both fusion zones exhibited better hot corrosion resistance than the parent metal. The novel outcome of the study reiterates that with the use of current pulsing and Mo-rich fillers, the detrimental phase shall be controlled.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Strain energy-based fatigue failure analyses of LB-PBF Inconel 718: Effect of build orientation

Additively manufactured Inconel 718 parts fabricated in three build orientations (horizontal, vertical, and angled at 45 degrees) are tested in fully-reversed bending fatigue mode. Damping measurements via the Impulse Excitation technique are used to assess the defects content in the microstructure and fractography analysis is performed to detect the root cause of failure. Two micro- and macro-scale methodologies of fatigue damage estimation based on the dissipated heat per cycle and the statistical estimation of micro-plastic strain energies are used to assess fatigue damage. It is shown that at a given stress level, the vertically built specimens endure the most fatigue damage before failure while the diagonally built ones sustain the least. As a result, the fatigue life is estimated for arbitrary load amplitudes via a microstructure-sensitive algorithm with acceptable accuracy as compared to the experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Heat Treatment on Microstructure Evolution and Mechanical Properties of Selective Laser Melted Inconel 718 Alloy

The microstructure characteristics of the selective laser melted Inconel 718 alloy under as-deposited, homogenization + solution + aging (HSA) and solution + aging (SA) conditions were studied. The anisotropy and heterogeneity of mechanical properties under different conditions were also investigated. Under the as-deposited condition, the morphology and size of the grains are heterogeneous. The dendrite structures which grow nearly perpendicular to the molten pool traces, accompanying interdendritic Laves phase, can be observed within the grains. The dendrite structure completely disappeared, and the Laves phases embedded in the interdendritic regions also dissolved into the matrix to precipitate γ′ and γ″ phases after heat treatments. Fully recrystallized grains were obtained under the HSA condition, while only incomplete recrystallized grains were obtained under the SA condition. However, the characteristics of γ′ and γ″ phases are very similar under the HSA and SA conditions. Significant improvement in strength after heat treatments was due to the dissolution of undesirable Laves phase and the precipitation of γ′ and γ″ phases. For all the three conditions, different tensile properties were observed depending on the orientation of the specimens. The scatter of mechanical properties is notable, and heat treatments increased the scatter of mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

In-situ radiation response of additively manufactured modified Inconel 718 alloys

Here in this study, a novel alloy of modified Inconel 718 produced by laser powder bed fusion is studied before and after in-situ Kr irradiation up to 3 dpa at 200 and 450 °C. Before irradiation, the microstructure consists of dislocation cells having a misorientation angle less than 5° and with an average size of ~500 nm. There are also second phase particles of MC type carbides, Laves phase and oxides such as Y-O, Y-(Ti)-Al-O. While the microstructure consists of stacking fault tetrahedra, faulted and perfect loops after irradiation at 200 °C, dislocation loops are the primary defects at 450 °C. With increasing dose, the size of the defects remains similar at 200 °C while it increases at 450 °C. This has been attributed to the existence of vacancy type defects at 200 °C and the different defect transport mechanisms at different temperatures. Moreover, matrix and second phase particle compositions seem to be similar after irradiation. The sink strengths of the structures have been calculated and superior radiation resistance of this alloy has been attributed to the existence of fine cell boundaries stabilized by the second phase particles produced by additive manufacturing.

36 MATERIALS SCIENCE↗