Ni Superalloy Research Articles

Friction stir welding of Haynes 282 Ni superalloy by using a novel hemispherical tool

In the present investigation, friction stir welding (FSW) of a gamma prime (γ′) strengthened Haynes 282 nickel-base superalloy by using a novel hemispherical tool is documented. A joint efficiency of ~ 96% was achieved in the as-welded condition, which further increased to ~ 100% after two-step post-weld aging heat treatment. An extremely fine (~ 2 μm) grained microstructure was observed in the FSWed region compared with the coarse (~ 48 μm) grain base metal region. The carbides (MC and M23C6) and γ′ precipitates are identified as the important microstructural constituent phases observed in the welded region. The aging heat treatment resulted in the precipitation of the γ′ strengthening phase and altered the morphology of the M23C6 carbide phase in the welded region from a continuous film to discrete particles at the grain boundaries. The preliminary results concerning tool life suggest that no significant tool wear occurs at least up to a welding distance of 200 mm. Therefore, the novel hemispherical tool design allows the successful FSW of high-temperature application materials without any significant tool wear or high heat generation.

Correlation between microstructure properties and the effect of WC addition on the interface, hardness and corrosion performance of thermally sprayed NiCrBSi-WC coating

ABSTRACT Ni super-alloy is based on a powder coating of NiCrBSi – WC from flexicord wire feedstock material. The role of the CMoAlNi thick layer on the microstructure and interface properties of the coating was examined. The microstructure and interface properties were analyzed for both NiCrBSi-WC and NiCrBSi-WC/CMoAlNi thick layer using optical microscopy (OM), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD). The mechanical property was evaluated by a Vickers hardness and nano-indentation measurement under 3N and 100mN, respectively. The performance of corrosion interface coatings was investigated in 3.5% of bentonite solution. The applied layer has a flame-spray morphology with unmelted and deformed particles associated with voids and porosities. In contrast, after CMoAlNi under-layers, the interface is a more compact boundary structure with no porosity or cracks. The as-sprayed NiCrBSi-WC coating exhibits high hardness (HIT≈6 GPa), and high Young's modulus (EIT≈ 240 GPa) The ability to resist cracking is significantly higher for the NiCrBSi-WC coating concerning the NiCrBSi-WC/CMoAlNi coating, 7.21 compared to 2.88 (GPA−3) x106. The electrochemical potentiodynamic polarisation results show that the NiCrBSi-WC coating's corrosion resistance is higher than that of the NiCrBSi-WC / CMoAlNi coating.

The effect of stress state and He concentration on the dislocation loop evolution in Ni superalloy irradiated by Ni+ & He+ dual-beam ions: In-situ TEM observation and MD simulations

In-situ TEM observation was conducted during Ni+&He+dual-beam irradiation to monitor the evolution of dislocation loops accompanied by He bubbles in the Ni-based alloy GH3535. Two distinct evolutions of dislocation loops, driven by residual stresses, were observed within the monitored grains. Hence, molecular dynamics (MD) simulations were employed to reveal the effects of stress magnitude and direction on loop evolution, including size, number density, type and variation. The simulations revealed that the presence of compressive stress reduced the formation energy of perfect dislocation loops, thus promoting their formation. Stress state was found to influence the preferential orientation of the loops, and compressive stress resulted in a decreased number density of dislocation loops but an increase in their size. This establishes a clear relationship between stress state and magnitude and the evolution of dislocation loops during ion beam irradiation. Additionally, the nature and characteristics of dislocation loops were quantified to explore the effects of He concentrations on their evolution. The higher He concentration not only promotes the nucleation of dislocation loops, leading to their higher number density, but also facilitates the unfaulting evolution by increasing the stacking fault energy (SFE). Moreover, the accumulation of He in the lower-He-concentration sample led to the growth of dislocation loops in multiple stages, explaining their nearly identical average sizes when compared to the higher-He-concentration sample.

Microstructural damage assessment in alloy 617M near high cycle fatigue threshold at elevated temperature

AbstractHigh cycle fatigue (HCF) behavior of Ni superalloy 617M is investigated at 973 K and R‐ratio −1 on a resonance‐based fatigue testing system at 85 Hz frequency. The alloy experiences an abrupt drop in fatigue life within a narrow domain of 2.5–5 MPa near its fatigue strength at 320 MPa. Electron microscopy and diffraction techniques were employed to thoroughly analyze the nominal fatigue damage. The characterization revealed the significance of precipitation of secondary phases M23C6, Ti (C, N), and γ′ phase in dictating the HCF strength of the alloy. Cyclic loading at high temperature causes γ‐matrix hardening and secondary phase precipitation synergistically strengthening the material beyond its yield strength. Conjunctively, dynamic strain aging was also seen to play a major role in the evolution of fatigue damage. The work highlights the collective contribution of γ′‐phase precipitation, carbides, and dynamic strain aging and their influence on the HCF behavior of alloy 617M.

Effect of the Cooling Rate on the Microstructure of Directionally Solidified Casting (CMSX-4 Ni Superalloy)

Effect of the Cooling Rate on the Microstructure of Directionally Solidified Casting (CMSX-4 Ni Superalloy)

Investigations on the Surface Integrity and Wear Mechanisms of TiAlYN-Coated Tools in Inconel 718 Milling Operations.

Inconel 718 is a Ni superalloy with superior mechanical properties, even at high temperatures. However, due to its high hardness and low thermal conductivity, it is considered a difficult-to-machine material. This material is widely used in applications that require good dimensional stability, making the milling process the most used in machining this alloy. The wear resulting from this process and the quality of the machined surface are still challenging factors when it comes to Inconel 718. TiAlN-based coating has been used on cutting tools with Yttrium as a doping element to improve the process performance. Based on this, this work evaluated the machined surface integrity and wear resistance of cutting tools coated using Physical Vapor Deposition (PVD) HiPIMS with TiAlYN in the end milling of Inconel 718, varying the process parameters such as cutting speed (vc), feed per tooth (fz), and cutting length (Lcut). It was verified that the Lcut is the parameter that exerts the most significant influence since, even at small distances, Inconel 718 already generates high tool wear (TW). Furthermore, the main wear mechanisms were abrasive and adhesive wear, with the development of a built-up edge (BUE) under a125 m/min feed rate (f) and a Lcut = 15 m. Chipping, cracking, and delamination of the coating were also observed, indicating a lack of adhesion between the coating and the substrate, suggesting the need for a good interlayer or the adjustment of the PVD parameters.

Dispersoid coarsening and slag formation during melt-based additive manufacturing of MA754

We have assessed the structural evolution and dispersoid coarsening behaviors of the oxide dispersion-strengthened superalloy MA754 during two different melt-based additive manufacturing techniques – metal laser powder bed fusion (PBF-LB/M) and directed energy deposition (DED). The mechanically alloyed MA754 powder posed challenges for both processes due to its irregular flaky morphology and large particle size. Successful consolidation with PBF-LB/M required increasing the layer height, decreasing the scanning speed, and increasing the laser power relative to typical Ni superalloy printing parameters. The resulting materials contained residual porosity and large Y-Al-oxide slag inclusions which formed in situ. The more prolonged thermal excursion during DED resulted in even larger, mm-scale slag inclusions, which spanned several build layers. In both PBF-LB/M and DED, these inclusions grew at the expense of nanoscale dispersoids, depleting the material of this strengthening phase. These observations motivate alternative approaches for preparing dispersion-strengthened powder feedstocks besides mechanical alloying and highlight the deleterious effects of Al microalloying on dispersoid stability and structure.

Robust additive manufacturable Ni superalloys designed by the integrated optimization of local elemental segregation and cracking susceptibility criteria

To achieve an effective design of additively manufacturable Ni superalloys with decent service performance, a hybrid computational design model has been developed, where the strategy to tailor local elemental segregations was integrated within a scheme of minimizing the cracking susceptibility. More specifically, the phase boundary of primary NbC / γ matrix was introduced into the design routine to tune the spatial distribution of critical solutes at an atomic scale, thereby inhibiting the formation of borides and segregation-induced cracking. Based on the output of the design, new grades of Ni superalloy have been developed with excellent additive manufacturability, as confirmed by the robustness of printing parameters in fabricating low-defect-density samples. The capability of the phase boundaries to evenly distribute boron atoms was validated experimentally, and the cracking induced by uncontrolled boron segregation at grain boundaries was effectively prevented. The newly designed alloys showed good tensile properties and decent oxidation resistance at different service temperatures, which are comparable to those of conventionally produced superalloys. The finding that phase boundaries can be employed to prevent undesirable clustering of boron atoms can be extended to manipulate the distributions of other critical elements, which provides a new path for designing novel Ni superalloys with balanced printability and mechanical properties.

Analysis of the high cracking resistance of a Co Ni superalloy during laser additive manufacturing

A newly developed CoNi-based superalloy is demonstrated to be tolerant of a wide range of laser powder bed fusion print conditions, with limited defect content and high relative densities. These promising traits motivated investigation of the structural uniformity in samples printed at the upper end of the scan velocity processing window (>▪). A new misorientation metric, starting reference orientation deviation (SROD), was calculated from ▪-scale 3D microstructure information to examine structure evolution during epitaxial growth of columnar grains. Misorientations of >20° were observed within the large columnar grains that grew through multiple build layers. The SROD observations highlight the CoNi superalloy's ability to plastically deform to high levels, as evidenced by misorientation gradients, across the range of temperatures experienced in laser additive manufacturing. Misorientation gradients within the columnar grain structure are high relative to Ni base superalloys, allowing for strengthening with high volume fractions of L12 precipitates.

Influence of Near-Surface Severe Plastic Deformation on the Corrosion Behavior of GTD-111 Nickel Superalloy in Hydrofluoric Acid Solution

High-energy shot peening (HESP) as a common near-surface severe plastic deformation (NS-SPD) was used to create a severely deformed surface with ultrafine grains and dense crystallographic defects (e.g., grain boundaries, dislocations, and twins) on GTD-111 Ni superalloy. The fluoride-induced corrosion performance of HESPed GTD-111 and its solution-annealed counterpart is comparatively studied using immersion tests, grazing-incidence x-ray diffraction analysis, electrochemical techniques, and glow discharge optical emission spectroscopy (GDOES). As supported by the immersion tests and electrochemical measurements, HESPed GTD-111 exhibits corrosion film with higher resistance and lower passivity current density at the expense of a higher initial corrosion rate. Both samples suffer pitting corrosion; however, the solution-annealed one shows deeper and larger pits. The dense distribution of crystallographic defects on the surface of the HESPed sample significantly increases the diffusion of alloying elements to the corrosion front. The GDOES depth profiles reveal that (i) a thicker corrosion film with a higher contribution of alloying elements (namely, Cr, Ti, Co, and Al) is developed on the HESPed sample, and (ii) the corrosion films formed on the solution-annealed and HESPed samples consist of an outer F-rich part and an inner O-rich region. The protective mechanism of NS-SPD is discussed by a physical model.

High-Throughput Investigation of Multiscale Deformation Mechanism in Additively Manufactured Ni Superalloy

In this paper, Inconel 718 (IN718) superalloy was processed by laser powder-bed fusion additive manufacturing (L-PBFAM), followed by heat treatment. High-resolution nanoindentation was used to investigate the complex deformation mechanisms that occurred at various length scales in both conditions. The nanoindentation elastoplastic maps show a strong crystal orientation dependency of modulus and hardness, which is attributed to the high mechanical anisotropy of IN718. The hardness map effectively resolves complex microscale strength variation imparted due to the hierarchical heat distribution associated with the thermal cycles of L-PBFAM. The disproportionately high hardening effect of Nb, Mo-rich chemical segregations and Laves phases in dendritic structures is also observed. The heat treatment resulted in a 67% increase in yield strength (from 731 MPa in the L-PBFAM condition to 1217 MPa in the heat-treated condition) due to the activation of multiple precipitation-strengthening mechanisms. The nanoindentation mapping of a heat-treated sample delineates the orientation-dependent hardness distribution, which apart from high mechanical anisotropy of the alloy, is also contributed to by a high degree of coherency strengthening of the D022 γ″-precipitates oriented parallel to the <001> crystal plane of the γ-matrix. The mean hardness of the sample increased from 13.3 GPa to 14.8 GPa after heat treatment. Evidence of extensive deformation of twin networks and dislocation cells was revealed by transmission electron microscopy of the deformed region under the nanoindentation tip.

Graph neural network modeling of grain-scale anisotropic elastic behavior using simulated and measured microscale data

Here we assess the applicability of graph neural networks (GNNs) for predicting the grain-scale elastic response of polycrystalline metallic alloys. Using GNN surrogate models, grain-averaged stresses during uniaxial elastic tension in low solvus high-refractory (LSHR) Ni Superalloy and Ti 7 wt%Al (Ti-7Al) are predicted as example face-centered cubic and hexagonal closed packed alloys, respectively. A transfer learning approach is taken in which GNN surrogate models are trained using crystal elasticity finite element method (CEFEM) simulations and then the trained surrogate models are used to predict the mechanical response of microstructures measured using high-energy X-ray diffraction microscopy (HEDM). The performance of using various microstructural and micromechanical descriptors for input nodal features to the GNNs is explored through comparisons to traditional mean-field theory predictions, reserved full-field CEFEM data, and measured far-field HEDM data. The effects of elastic anisotropy on GNN model performance and outlooks for the extension of the framework are discussed.

Advanced time-resolved characterization of Stress Assisted Grain Boundary Oxidation of 718 Ni superalloy

Nickel superalloys are used for harsh condition application cases as they have high chemo-thermomechanical stability. However, they can suffer from embrittlement due to stress corrosion cracking. This effect is difficult to observe as it can take place at long time scales. Here we propose a feasible experiment to study stress assisted grain boundary oxidation, a phenomenon that has similar mechanisms involved that can take place in laboratory compatible time scales. We observed the event using phase and diffraction contrast tomography while applying monotonic loading of the sample at 650 °C. This initial analysis shows that the experimental setup is a good candidate for the study of such degradation mechanism.

Microstructure of a coated single crystalline René N5 part repaired by epitaxial laser deposition

Laser Metal Deposition has been applied to the repair and manufacture of single crystalline Ni-based superalloys components. The extensive research carried out so far showed that single crystalline depositions of the same alloy of the substrate, or similar, can be produced in appropriate conditions. However, repair is often performed on previously refurbished parts, combining materials with different compositions and structures. In this work we investigated the deposition of René N4 Ni superalloy on a René N5 single crystalline part previously coated by TIG welding with René 195. Deposition was performed on the (100) surface of the single crystalline substrate. The results show that the multilayer deposits inherit the single crystalline structure and orientation of the original crystal when the material is laid down on the single crystalline regions of the substrate. Above the polycrystalline René 195 a heavily textured columnar dendritic solidification structure develops, similar to the structure of directionally solidified cast alloys. Above the dilution zone of the first deposited layer, both the chemical composition and the microstructure of the deposited René N4 alloy remain constant across all the deposited layers showing that repairing is feasible even on heterogeneous substrates. Several heat treatments aimed at developing a suitable γ/γ’ microstructure within the deposited material were tested and an optimized heat treatment method consisting of a solution step, followed by two ageing steps at specific temperatures was devised. • Over the single crystal, the deposit maintains the orientation of the substrate. • Above the coating, solidification structure is similar to a directionally cast alloy. • The coating does not interfere with the deposit orientation over the single crystal. • A three-stage heat treatment is required to obtain the γ/γ’ microstructure.

Graphene oxide decorated spherical powder for Ni superalloy with high yield strength and ductility

Graphene and its analogous 2D nanomaterials demonstrate high potential as reinforcements in bulk metallic materials with high performance, but homogenous dispersion within metal matrices remains considerable challenging. Here we report the successful fabrication of Ni superalloys from graphene oxide (GO)-decorated spherical Ni superalloy powder. The GO-reinforced Ni superalloys show an exceptional combination of yield strength and tensile ductility. The high yield strength arises from collective effects of specific microstructures, including good interfacial bonding between GO and the matrix, uniformly-distributed precipitates, and GO-induced twin boundaries. The good ductility is attributed to a sustainable work hardening ability of GO-reinforced Ni superalloys by the addition of GO, enhanced by and dislocation-precipitate interaction and stacking fault hardening mechanisms. This work reveals a novel strategy to obtain GO reinforced metallic materials with high strength and high ductility.

A Melt Pool Temperature Model in Laser Powder Bed Fabricated CM247LC Ni Superalloy to Rationalize Crack Formation and Microstructural Inhomogeneities

This study of the laser powder bed fusion (LPBF) of γ′-strengthened Ni superalloy CM247LC focuses on the development of a melt pool temperature model to predict crack density within the alloy. This study also analyzes spatter and elemental evaporation, which might cause defects and inhomogeneities, at different melt pool temperatures. The melt pool temperature model provides more accurate predictions than the widely used energy density model. Spatter particles were collected and characterized to study their sizes and chemical compositions, compared with the virgin powder, recycled powder, and as-built samples, to probe the impact of their entrapment into the melt pool. This study also investigated Al evaporation, revealing that its extent does not correlate with the laser energy density and is believed to be rather limited by comparing the chemistry of the virgin powder and the build. Last, the impact of LPBF process parameters on the formation of these inhomogeneities, and accordingly crack formation, was studied using finite element analysis by estimating the maximum melt pool temperature and correlating it with the formation of the microstructural inhomogeneities. The morphology of the various cracking modes was associated with the process parameters.

Failure analysis of a butterfly valve made with nickel aluminum Bronze (NAB) and manganese aluminum Bronze (MAB)

Butterfly valves installed at the seawater system used to cool the fresh water of cooling medium in an offshore oil platform type Floating Production Storage and Off-loading (FPSO) failed in an unexpected way after periods of three to six months of operation. When closed, the valve faces were kept in contact with water injection on one side, and with untreated seawater on the other side. Moreover, they were totally exposed to seawater when opened. One failed valve was sent for metallurgical failure analysis and the damage appeared in the valve body in the position of sealing. The body and the disc of the failed valve were made of a cast Nickel Aluminum Bronze alloy (NAB) type ASTM B148 UNS C95800, and the sealring attached to the disk was made of Ni superalloy 625 with polytetrafluoroethylene (PTFE). The investigations conducted in the failure analysis revealed that the material of the seat area was different from the material of the body, since a large ring of NAB was welded to the body, using a Manganese Nickel Bronze (MAB) alloy as feed metal. The failure was associated to three distinct problems. First, several welding defects were observed. Second, an adequate post weld heat treatment was not performed in the seat part, leading to the presence of β retained phase (β’) which decreases the corrosion reistance of NAB and MAB alloys. Third, galvanic corrosion also ocurred since the Ni superalloy 625 is nobler than the NAB, and they were in close contact. Additional corrosion tests revealed that NAB alloy was very susceptible to de-alloying in 3.5 %NaCl solution.

High-Temperature Corrosion of APS- and HVOF-Coated Nickel-Based Super Alloy under Air Oxidation and Melted Salt Domains.

Various thermal spraying approaches, such as air/atmospheric plasma spraying (APS) and high-velocity oxy-fuel (HVOF) spraying, are widely employed by plants owing to their flexibility, low costs and the high surface quality of the manufactured product. This study focuses on the corrosion behavior of a Ni superalloy coated with powder Cr3C2-25NiCr through APS and HVOF at 950 °C under air oxidation and Na2SO4 + 0.6V2O5 molten salt environments (MSE). The results show that HVOF-deposited Ni superalloys have higher hardness and bond strength than the respective APS coating. The thermo-gravimetric probe reveals that the Ni superalloys exposed to an oxidizing air environment has a minor mass gain compared to those under the MSE domain for both non-coated and coated samples, in line with the parabola curvature rate oxidizing law. The Ni superalloys show good corrosion resistance but poor oxidation resistance in APS-deposited Ni superalloys under the MSE. HVOF-coated Ni superalloys in both environments exhibit better corrosion resistance and lower mass gain than APS-coated superalloys. The excellent coating characteristics of HVOF-coated Ni superalloys lead to their better high-temperature corrosion performance than APS.

Effect of Microsegregation on High‐Temperature Microstructure Evolution in Rapid Solidification Processed Nb‐Rich Ni Superalloys

Rapid solidification processes are used for coating, repairing, and manufacturing of metal alloy components in numerous industries. However, many alloys experience microsegregation of elements at subgrain boundaries at the termination of solidification, which have potential consequences for phase transformations at elevated temperatures if used without homogenization heat treatments. Herein, the effect of this microsegregation and solidification microstructure on the coarsening kinetics of the δ phase in a Nb‐rich Ni superalloy is evaluated. Microsegregation in rapid solidification processed (RSP) alloy 718 results in a greater availability of nucleation sites for the precipitation of the δ phase, resulting in smaller precipitate sizes. Coarsening of the δ phase is found to follow a Lifshitz–Slyozov–Wagner model, which is used to determine that Nb diffusion is the limiting factor in δ phase coarsening. With the formation of the δ phase, and the concurrent coarsening and dissolution of the γ″ strengthening phase, a decrease in both tensile strength and microhardness is observed. An improved understanding of the influence of microsegregation on phase transformations allows for a more informed application of non‐homogenized, RSP alloys in industry.

Effect of Pre-Post TIG Welding Heat Treatment on Cast NI Superalloy

The effect of a pre (before) and post (after) heating welding treatment on the microstructure and mechanical properties of the scrap blades made of cast INC738LC superalloy is the main goal of the present investigation. The filler used in TIG welding was a INC 625 solution hardened superalloy as the proposed solution for hot cracking of the INC738LC cast superalloy in literature. The TIG welding was processed with respect to the constantly optimized parameters (current, voltage, speed, gas flux rate and number of passes) to make a mechanical properties comparison between the as received superalloy and the welded superalloy with heat treated specimens. The characterization techniques employed in this study are hardness measurements, tensile tests, optical microscopy and scanning electron microscopy. We found that the proposed preheating improves the TIG welding of the INC 738 LC superalloy specimens and the post welding heat treatment enhances its mechanical properties.