IN718 Superalloy Research Articles

Elemental mapping and quantitative characterization of dendrite structure in IN718 supperalloy based on micro beam X-ray fluorescence and EPMA

Micro beam X-ray fluorescence (μ-XRF) has been applied to characterize the quantitative distribution of elements in the dendrite structure of IN718 superalloy for the first time with the advantage of simple sample preparation, no requirement for conductivity, high spatial resolution and non-destructive surface. Dendrite structure is an important microstructure in cast polycrystalline superalloys such as IN718 superalloy. In this paper, the distribution characterization of Ni, Cr, Fe, Nb, Mo and Ti in the dendrite structure of superalloy was performed by μ-XRF with the improvement of the quantitative method. The elemental mapping with higher spatial resolution in the dendrite structure was obtained by electron probe micro-analysis (EPMA). The elemental content distribution on primary dendrite arm, secondary dendrite arm and inter-dendritic zone of superalloy was investigated and the segregation ratios of different elements have been obtained. It was found that the elemental distribution in dendrite structure of IN718 alloy was inhomogeneous and the content of Nb, Mo and Ti was much higher than that on dendrite arm and the elements of Fe,Cr and Ni have the opposite distribution tendency. Severe micro segregation of Nb has been revealed on the dendrite structure. The minimum Nb value of 2.90 wt%, the mean Nb value of 4.90 wt% and the maximum value of 8.84 wt% in the tested IN718 superalloy sample determined by μ-XRF have good agreement with the composition data of key nonequilibrium point in the Pseudo-binary solidification constitution diagram of Inconel 718. The content distribution of alloy elements (Mo, Nb and Ti) presents a sine distribution between the secondary dendrite arm and inter-dendritic region and the average SDAS value obtained from the line content distribution figure is about 70–80 μm. The segregation ratio of different elements has obtained and the SR values of Nb and Ti are much greater than 1. There were some large precipitates particles containing Nb and Ti elements distributed on the inter-dendritic zone, which led to the fluctuation of Nb and Ti contents.

Microstructure characteristics and surface oxidation behaviour of SLM WC/IN718 composite

ABSTRACT In this work, WC/Inconel 718 composite was fabricated by selective laser melting (SLM) the IN718 superalloy powders decorated with surface embedded WC powders. Also, the composite was heat-treated by a designed double-aging regime for microstructure tuning. The resulting microstructure and surface oxidation behaviour of the as-printed and the heat-treated composites were studied. The results revealed that the homogeneous microstructure was obtained, and the obvious weld beads structure with fine dendrites was detected on the surface in the SLM composite. After the double-aging heat treatment, the matrix showed a confined growth from 21.81 to 26.14 μm and the weld bead structure disappeared. XRD and EBSD analysis confirmed the formation of γ, γ′, γ′′, WC, (Nb,M)C, δ and Laves phase in the composites before and after heat-treatment. The high-density GND accommodated the strain incompatibility and the internal residual stress in different constituents. Furthermore, the underlying mechanism of oxidation behaviour was discussed.

The Hardness Evolution of Cast and the High-Cycle Fatigue Life Change of Wrought Ni-Base Superalloys after Additional Heat Treatment.

Concerning the use of modern technologies and manufacturing systems in the production of high-stress components from Ni-base superalloys and the optimization of the production process, knowledge of the microstructure–mechanical properties relationship is very important. The microstructure of Ni-base superalloys is very closely related to the chemical composition. With the high number of alloying elements, various phases are presented in the structure of Ni-base superalloys, which have a predominantly positive effect on the mechanical properties, but also phases that reduce, in particular, the heat resistance of these materials. The aim of the presented paper is the quantification of structural parameters of two types of cast alloys, ZhS6K and IN738, where the effect of dwell at 10 and 15 h at 800 °C on the change in morphology and volume fraction of the γ′-phase precipitate was studied. The detected changes were verified by the Vickers hardness test. The IN718 superalloy was chosen as a representative of the wrought superalloy. This alloy was also annealed for 72 h at a temperature of 800 °C, and the quantification of structural parameters was performed by EDS mapping and TEM analysis. Another partial goal was to assess the effect of changes in the volume fraction of the γ′-phase and δ-phase on the change in the high-cycle fatigue life of superalloy IN 718. This superalloy was tested by dynamic cyclic loading with cycle asymmetry parameter R = −1 at an ambient temperature of 22 ± 5 °C and at a temperature of 700 ± 5 °C and with cycle asymmetry parameter R < 1 (three-point bending load) after annealing at 700 °C/72 h. The results of the quantitative analyses and fatigue tests will be further used in optimizing the design of Ni-base superalloy components by modern technologies such as additive technologies for the production of turbine blades and implemented within the philosophy of Industry 4.0.

Effect of post-processes on the microstructure and mechanical properties of laser powder bed fused IN718 superalloy

The post-processing on the additively manufactured component is of huge interest as the key to tailor the microstructure to obtain certain mechanical properties. In this present study, the effects of hot isostatic pressing, as well as heat treatment on the microstructure, phase configuration and mechanical properties of laser powder bed fused (LPBF) IN718 superalloy were systematically investigated. Three different post-processes were studied such as hot isostatic pressing (HIP), heat treatment (HT), and HIP followed by HT (HIP+HT). The HIP process effectively eliminated the Laves phase remained in the as-built microstructure and brought uniformly distributed super fine γ″ precipitates in nano-meter size. In the heat-treated microstructure, larger γ″ precipitates were promoted directly from the as-built material. In comparison the HIP+HT process caused a moderate growth of γ″. In the latter two cases, the developed γ″ significantly strengthened the material. Yield strength of IN718 was increased from 738 MPa in as-built condition to 1015 MPa and 1184 MPa after HT and HIP+HT, respectively. On the contrary the ductility in the as-built IN718 condition was reduced by more than 40% after HT and HIP+HT. This can be compared to an increase in the ductility by almost 30% when subjected the as-built specimens to only HIPping. Finally, the correlation between microstructure evolution and mechanical properties is discussed in detail.

Microstructure and oxidation resistance of CoNiCrAlY coating manufactured by laser powder bed fusion

In this study, an IN718 superalloy matrix and its CoNiCrAlY coating were prepared using the laser powder bed fusion (LPBF) technique. The microstructure and isothermal oxidation properties of the CoNiCrAlY coating at 1100 °C were studied. The results showed that the CoNiCrAlY coating prepared by LPBF was dense, uniform, and crack-free. The coating was mainly composed of the γ phase, the β phase, and some Y2O3 microparticles. The microstructure along and perpendicular to the building direction consisted of columnar dendrites and equiaxed grains, respectively, with a strong (100) 〈001〉 cubic texture. The coating showed a low thermal growth oxide layer growth rate and a thin oxide layer during isothermal oxidation for 1–100 h. Cr2O3 oxides were initially formed on the surface of the coating. With an increase in the oxidation time, the α-Al2O3 scale and (Co,Ni)Cr2O4 and CoNiO2 mixed oxides were formed on the surface of the coating.

The fracture behaviors of the selective laser melting processed IN738 superalloy by in-situ tensile SEM

In this study, the ductility and fracturing behavior of IN738 superalloy, which was produced by selective laser melting, were investigated by the in-situ scanning electron microscopy (SEM) method at 700 °C. The results showed that the cavities nucleate mainly at the grain boundaries and inter-dendritic region (IDR). In these areas, sites with high-stress concentrations, including γ-γ 'eutectic and TiC carbide, are more prone to crack formation and propagation than other areas. With increasing strain to 27%, the IDR cracks propagated into the core-dendritic regions (CDR) and eventually caused the specimen to fracture. This was due to the presence of cavities that had formed in the CDR due to the outflow of moving dislocations from misorientation.

Analysis of Solidification and Liquation Cracks in the Electron Beam Welding of IN738 Superalloy

Analysis of Solidification and Liquation Cracks in the Electron Beam Welding of IN738 Superalloy

Effects of Thermally Assisted Warm Laser Shock Processing on the Microstructure and Fatigue Property of IN718 Superalloy

Effects of Thermally Assisted Warm Laser Shock Processing on the Microstructure and Fatigue Property of IN718 Superalloy

Characterization of fracture behavior of a nickel-based using Charpy instrumented in different conditions of heat treatment and evaluation temperatures

Since gas turbine blades work in acute conditions, the study of their mechanical behavior, including impact, is one of the most important tasks recommended. The aim of this study was to investigate the impact behavior of IN738 superalloy welded by pulsed laser under different heat treatment conditions and different temperatures. The results of this study showed that the lowest fracture toughness was related to the specimens that were welded under casting conditions. This was due to the formation of liquation cracks during welding and due to the reaction between the (Ti,Ta)C carbide, γ-γ′ eutectic, and γ′ phase with the superalloy matrix. As the test temperature increased from ambient temperature to 600 °C, the impact toughness increased. This is attributed to the free presence of dislocations, which causes ductile behavior in the alloy. As the temperature increased further to 770 °C, the fracture toughness increased due to the increased strength of the superalloy yield due to locking of dislocations. At temperatures above 770 °C, the toughness of the superalloy was reduced again due to the decrease in strength due to the passage of dislocations through the obstacles.

A stochastic scan strategy for grain structure control in complex geometries using electron beam powder bed fusion

Spatial control of microstructure within a three-dimensional component has been a dream of materials scientists for centuries. However, limitations in traditional manufacturing processes prevent detailed control over the distribution of microstructures in a single part. Here, we demonstrate the ability to control grain structure and crystallographic texture during metal additive manufacturing for arbitrary cross-sections of a practical size, with profound implications for the design and optimization of next-generation products. The key to this advance is a new geometry agnostic scan path algorithm that manipulates the spatial distribution of solidification conditions. Utilizing a fundamental understanding of solidification dynamics and a model of the heat transfer during processing, we have designed this algorithm to manipulate the natural competition between epitaxial dendrite growth and grain nucleation. With this algorithm, we successfully controlled the grain structure of Ni-based superalloy IN718 in the shape of the Mona Lisa.

Effects of increasing powder layer thickness on the microstructure, mechanical properties, and failure mechanism of IN718 superalloy fabricated by laser powder bed fusion

Effects of increasing powder layer thickness on the microstructure, mechanical properties, and failure mechanism of IN718 superalloy fabricated by laser powder bed fusion

Effect of post-processing on microstructure and mechanical properties of Alloy 718 fabricated using powder bed fusion additive manufacturing processes

PurposeThis study aims to investigate additive manufacturing of nickel-based superalloy IN718 made by powder bed fusion processes: powder bed fusion laser beam (PBF-LB) and powder bed fusion electron beam (PBF-EB).Design/methodology/approachThis work has focused on the influence of building methods and post-fabrication processes on the final part properties, including microstructure, surface quality, residual stresses and mechanical properties.FindingsPBF-LB produced a much smoother surface. Blasting and shot peening (SP) reduced the roughness even more but did not affect the PBF-EB surface finish as much. As-printed PBF-EB parts have low residual stresses in all directions, whereas it was much higher for PBF-LB. However, heat treatment removed the stresses and SP created compressive stresses for samples from both PBF processes. The standard Arcam process parameter for PBF-EB for IN718 is not fully optimized, which leads to porosity and inferior mechanical properties. However, impact toughness after hot isostatic pressing was surprisingly high.Originality/valueThe two processes gave different results and also responses to post-treatments, which could be of advantage or disadvantage for different applications. Suggestions for improving the properties of parts produced by each method are presented.

Evolution behavior of γ″ phase of IN718 superalloy in temperature/stress coupled field

Evolution behavior of γ″ phase of IN718 superalloy in temperature/stress coupled field

Fatigue crack propagation behavior of IN718 superalloy aged with temperature/stress coupled field

Fatigue crack propagation behavior of IN718 superalloy aged with or without stress was investigated. The alloy aged with stress exhibited a higher fatigue crack growth rate and a lower fatigue threshold compared to that without stress. Both the restricted dislocation planar slip and the decreased difference in orientation of the activated slip system between adjacent grains were promoted by coarse γ″ phases, which increased the local plastic deformation and reduced the crack deflection. The accelerated coarse γ″ phase caused by applied stress was the key factor for the degradation of fatigue crack propagation resistance of the alloy aged with stress.

Selective laser melting of IN738 superalloy with a low Mn+Si content: Effect of energy input on characteristics of molten pool, metallurgical defects, microstructures and mechanical properties

IN738 superalloy with excellent high-temperature performance is of huge interest for hot-end component applications in aerospace, but its additive manufacturing via selective laser melting (SLM) is rather challenging due to the unsolved high crack susceptibility. Here, for the purpose of improving the formability of IN738 in SLM, Mn + Si content was then strictly controlled in raw powder. An orthogonal experiment was then carried out under different energy inputs to investigate the characteristics of the molten pool, metallurgical defects, microstructures, and mechanical properties of IN738 by SLM. The results showed that high laser power was more likely to shape large melting depth and depth-to-width ratio under the same linear energy density (LED), rather than scanning speed. Four morphologies of the molten pool were observed in the entire LED range. At lower LEDs, hot cracks were usually accompanied by irregular pores due to the stress concentration at the sharp corner of pores; at higher LEDs, cracks were located at the interface of grain boundaries with large misorientation angles. Energy dispersive spectrometry results revealed that the presence of Al-rich oxides and high Mn + Si content at the interface were the main reasons for cracking. The Scheil-Gulliver solidification modeling was used to examine the cracking susceptibility of Si and Mn elements. In addition, both dendrites and cellular dendrites were found in columnar grain structures. By reducing the Mn + Si content and optimizing process, crack-free and nearly fully dense samples of IN738 were obtained by SLM with excellent mechanical properties (ultimate tensile strengths = 1206 MPa, yield strength = 866 MPa and elongation = 16.5%), which were much higher than the mechanical properties of conventionally manufactured IN738.

Effects of Grain Refinement on the Low-Cycle Fatigue Behavior of IN792 Superalloys

The Ni-based IN792 superalloy is widely applied as a component in industrial turbines and aircraft engines due to its good high-temperature properties and excellent corrosion resistance. Since these components have to suffer from cyclic thermal and mechanical stresses during service, the high-temperature fatigue failure becomes one of the major factors affecting their service lives. Grain refinement has been considered as an effective way to improve the mechanical performance of superalloys. However, due to the complexity of alloy composition, microstructure and service condition, there is no unified theory about the influence of grain refinement on the fatigue performance and fracture mechanism of superalloys. In the present research, the IN792 superalloy with different grain sizes was manufactured. Then, fully reversed, strain-controlled, low-cycle fatigue (LCF) tests with four different total strain amplitudes were carried out on the alloy at 700 °C and 800 °C to clarify the effects of grain refinement on its LCF behavior. The results show that grain refinement improved the fatigue life significantly, which is mainly attributed to increasing the grain boundary content and refining MC carbides, eutectic structures and dendritic structures. During fatigue test under lower strain amplitude, the alloy exhibits a pronounced initial fatigue hardening followed by a continuous well-defined stability stage, which is caused by the formation of dislocation networks and coarsening of primary γ’ phases. However, during fatigue test under higher strain amplitude, the alloy exhibits continuous hardening response because the dislocations could shear primary γ’ phases, which could give rise to resistance to dislocation movement. In addition, the fracture surface observation shows that the fatigue fracture mode is mainly affected by the total strain amplitude. Under lower total strain amplitude, the fatigue microcracks mainly initiate at the porosities near the specimen surface, while under higher total strain amplitude, the fatigue microcracks tend to form at the interior of the specimen.

BEHAVIOR OF APSED TBCS SUBJECTED TO COOLING MEDIA IN THERMAL CYCLE SYSTEMS

In this study, first, yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) with CoNiCrAlY bond coats both were fabricated on IN718 superalloy substrate, using air plasma spraying (APS) techniques. In order to understand the actual microstructural morphological behavior, phase stability, and thermal cycling behaviors of 8YSZ TBCs were characterized by field emission spectroscopy equipped scanning electron microscopy (FESEM), differential scanning calorimetry (DSC) and thermal cycling test. DSC results confirmed the basic reason of selecting the high cycling temperature. The crack initiation and propagation under thermal stresses due to temperature differences from maximum temperature of 1100∘C to cooling medium; here, air cooling and water quenching are used. Arising from either edges or corners it was observed serious problem which is restraining thermal cycling lifetime of TBCs, indicating failure mechanism is independent from operating parameters of thermal cycling test. However, in comparison to water-cooled thermal fatigue life, air-cooled thermal fatigue lifetime was 2.36 times better.

Occurrence of a brass texture and elastic anisotropy in laser blown powder processed superalloy IN718

Challenges are encountered with the laser-blown-powder directed-energy-deposition (LBP-DED) of high-performance alloys such as IN718. Complications that arise include the formation of undesired phases, irregular microstructure, and pronounced texture leading to anisotropic elastic properties. To mitigate these issues, deposition parameters and post-processing treatments must be carefully selected to achieve the desired properties in the final product. In this study it is shown that a Brass texture component ({110} <211>) can be achieved in LBP-DED IN718. EBSD analyses indicate that this texture is enhanced through partial recrystallisation following selected heat treatments, increasing the degree of microstructural anisotropy. Interestingly, resonant ultrasound spectroscopy (RUS) results indicate a decrease in the elastic anisotropy with recrystallisation. This is hypothesised to be a result of the pronounced Brass texture leading to pseudo-isotropic elastic properties along the cubic axes. Further insight into the kinetics of this process have been obtained with in-situ high temperature RUS measurements.

Contribution of hot isostatic pressing on densification, microstructure evolution, and mechanical anisotropy of additively manufactured IN718 Ni-based superalloy

Microstructural defects and mechanical anisotropy are regarded as the main obstacles in additively manufactured, laser powder bed fusion (LPBF) IN718 superalloy, decelerating industrial development of the related components. Hot isostatic pressing (HIP) is an effective solution to overcome these challenges. In this study, standard heat treatment (HT) with and without prior HIPing was applied to the LPBF IN718 and the impacts of the HIP operation on microstructures and room- and high-temperature mechanical anisotropy were studied. Observations showed that intrinsic pores and large contour defects were significantly closed by HIPing and consequently the relative density enhanced from 99.50% to 99.96%. The highly distorted columnar structure of the HT specimen was broken down due to recrystallization, and an equiaxed morphology with about 5-time coarser grain size appeared in the HIP + HT specimens. Although too many transgranular precipitates were observed in the HT microstructure, they were just formed at grain boundaries when HIP was carried out before HT. Furthermore, the formation of nano-scale γʺ/γʹ/γʺ co-precipitates was favored by HIPing which had a positive effect on strengthening. It was also found that prior HIPing effectively mitigated mechanical anisotropy both at room- and high-temperatures by influencing several factors. The degree of room-temperature mechanical anisotropy was reduced from 11.6% (HT) to 3.5% (HIP + HT) by HIPing and mitigated from 4% to 2% at 650 °C. While the fracture mechanism was orientation-dependent in the HT samples, it was completely ductile for the HIP + HT samples in all circumstances.

Microstructures and mechanical properties evolution of IN939 alloy during electron beam selective melting process

Microstructure formation and transformation during electron beam selective melting (EBSM) is a unique process that has rarely been investigated, but can significantly influence the mechanical properties of parts. In this study, defect-free IN939 superalloy specimens were prepared and characterized using the EBSM method. It was found that the microstructures of the EBSM IN939 specimens consisted of primary, middle, and coarse γ' phases, and the ƞ phases had a gradient attribute. Transmission electron microscopy, selected-area electron diffraction, and energy dispersive spectroscopy tests were performed at different specimen heights, as well as a series of tensile tests. The results showed that the yield strength was influenced by the gradient microstructure. Higher yield strengths were obtained at the top of the fabricated specimens. Post-treatment was adopted to homogenize the gradient microstructure. Fracture analyses after the tensile test demonstrated the effects of different strengthening phases on the fracture behavior.