Ni-based Superalloy Research Articles

Effects of residual Laves phase on microstructural evolution and mechanical properties of wire arc additive manufactured GH4169 Ni-based superalloy

The GH4169 Ni-based superalloy is prepared by wire arc additive manufacturing and the sizes and morphologies of Laves phases under different heat treatment strategies are obtained to further investigate the influence of residual Laves phases on the tensile properties of the superalloy. After the solution treated at a higher 1150 °C and followed by aging, the Laves phase is completely dissolved, with limited δ phase with varying sizes distributed at the grain boundaries. While for lower 1020 °C solid solution heat treatment temperature, the Laves phase in the sample changes from chain-like to particle-like shapes by partially dissolving, and the needle-like δ phase exists around the particle-like Laves phase. Such residual "Laves + δ" impede high-temperature tensile crack propagation, significantly enhancing plasticity. This mechanism makes the lower temperature solid solution treated sample exhibit similar tensile stress but ∼2 times of elongation when compared to the sample treated by higher solid solution.

Microstructure, Mechanical, and Tribological Properties of Nb-Doped TiAl Alloys Fabricated via Laser Metal Deposition.

TiAl alloys possess excellent properties, such as low density, high specific strength, high elastic modulus, and high-temperature creep resistance, which allows their use to replace Ni-based superalloys in some high-temperature applications. In this work, the traditional TiAl alloy Ti-48Al-2Nb-2Cr (Ti4822) was alloyed with additional Nb and fabricated using laser metal deposition (LMD), and the impacts of this additional Nb on the microstructure and mechanical and tribological properties of the as-fabricated alloys were investigated. The resulting alloys mainly consisted of the γ phase, trace β0 and α2 phases. Nb was well distributed throughout the alloys, while Cr segregation resulted in the residual β0 phase. Increasing the amount of Nb content increased the amount of the γ phase and reduced the amount of the β0 phase. The alloy Ti4822-2Nb exhibited a room-temperature (RT) fracture strength under a tensile of 568 ± 7.8 MPa, which was nearly 100 MPa higher than that of the Ti4822-1Nb alloy. A further increase in Nb to an additional 4 at.% Nb had little effect on the fracture strength. Both the friction coefficient and the wear rate increased with the increasing Nb content. The wear mechanisms for all samples were abrasive wear with local plastic deformation and oxidative wear, resulting in the formation of metal oxide particles.

Effect of coating-substrate microstructure on nucleation and growth of surface microcracks in a PtAl-coated Ni-based SX superalloy with sheet specimens during HCF at 900 °C

The fatigue crack initiation process accounts for the main portion of the high-cycle fatigue (HCF) life. Although coated superalloys are well-known to be prone to surface microcracks, the influence of the coating-substrate microstructure on the damage mechanism during HCF of the coated single crystal (SX) superalloys remains unclear. In the current study, HCF failure and interrupted tests were conducted at 900 °C under a low applied maximum stress on a PtAl-coated third-generation SX superalloy using sheet specimens. The HCF crack initiation was investigated by analyzing the evolution of surface microcracks and coating-substrate microstructure. The results show that the surface microcracks nucleation and growth are controlled by the initial microstructure of the coating-substrate region and the oxidation process during the HCF crack initiation stage. The rapid nucleation and growth of the surface microcracks are promoted by grain boundaries (GBs) in the coating and the interdiffusion zone (IDZ). Comparatively, the surface microcracks are blunted by oxidation in the SX substrate due to the loss of GBs, leading to a slower microcrack growth. However, the surface microcracks can gradually grow further into the substrate by repeating the processes as follows: oxidation at the crack tip, formation of γ′-depleted region due to oxidation, recrystallization at the γ′-depleted region and cracking along the GBs in the recrystallized γ′-depleted region. Subsequently, the surface microcracks, which reach the critical size, propagate along the slip bands during the progressive propagation stage. This study will contribute to improve the HCF durability of the PtAl-coated SX superalloy.

Correction: Comparative Study on the Cleanliness and Mechanical Properties of a Cast and Wrought Ni-Based Superalloy GH4738 Prepared with Revert Addition

Correction: Comparative Study on the Cleanliness and Mechanical Properties of a Cast and Wrought Ni-Based Superalloy GH4738 Prepared with Revert Addition

Comparison of fatigue characteristics and failure analysis of Ni-based superalloy subjected to thermomechanical fatigue under different phase conditions

In this research, the thermomechanical fatigue (TMF) characteristics of a second-generation Ni-based single-crystal superalloy were investigated. The combined effects of temperature cycling and mechanical stress were examined using a strain-controlled approach under different TMF testing conditions. The results indicated that cyclic softening occurred during the early stages during in-phase (IP) loading. In contrast, out-of-phase (OP) tests exhibited cyclic hardening, as evidenced by an increase in the stress range, which intensified at higher mechanical strain amplitudes. This resulted in a shorter fatigue life compared to the IP TMF case. The crack distribution patterns and damage mechanisms on the cross-sectional and fracture surfaces of the failed specimens were analyzed. In the OP TMF tests, oxidation-assisted cracking was identified as the primary damage mechanism. The dominance of oxide layers was a key distinguishing feature. Additionally, twinning-related deformation and topologically close-packed (TCP) phase particles were observed in specimens subjected to a higher mechanical strain of 0.6%, where they ran parallel to the main crack, contributing to a reduction in lifetime. Conversely, the IP TMF cases were mainly dominated by creep damage mechanisms, with some signs of oxidation penetration observed during testing. The application of the Ostergren model to the fatigue life prediction of a Ni-based single-crystal superalloy under both conditions was evaluated. As this model was insufficient, a modified Ostergren model was proposed by incorporating the maximum and amplitude stresses in the power law forms.

Microstructural evolution and diffusion mechanism of MCrAlY coated nickel-based superalloy turbine blades after serviced for 47,000 h

The large-sized industrial gas turbine blades exhibit non-uniform microstructural degradation in various parts due to ultra-long term service in high-temperature and high-pressure environments. Determination of degradation behavior at critical locations is important for the safety of gas turbine and blades refurbishment program. The microstructure characterization on the surface and interface of MCrAlY coated Ni-based superalloy blades have been studied. The microstructure evolution near the coating/ substrate interface was studied by scanning electron microscope (SEM) and the difference in degradation of the main precipitates at several locations were quantified. Surface oxidation and interfacial diffusion were analyzed through energy dispersive spectrum (EDS) and electron probe microanalyzer (EPMA) to clarify the influence of elemental diffusion on microstructure. The results indicated that regional microstructure evolution influenced by the spatial structural characteristics of the blades. The migration of Ni, Al, Cr, and Co dominated the diffusion mechanism during service, and reconstructed the microstructure of the coating/substrate interface and surface.

Effect of Bond Coating Surface Morphology on the TBC System Lifetime

AbstractA study on the effect of bond coating (BC) surface modifications prior to ceramic deposition is presented. Grit blasting and polishing were used to modify the BC surface finish. Thermal barrier coating (TBC) systems were made of a commercial yttria-stabilized zirconia (YSZ) deposited by electron beam physical vapor deposition on a β-(Ni,Pt)Al-coated Ni-based single crystal superalloy. Surface roughness measurements and scanning electron microscopy observations, before and after thermal cycling at 1100 °C, were performed to investigate the influence of the initial surface treatment on the YSZ/BC interface morphology and top coat spallation resistance. Surface states enhancing TBC spallation resistance have been found. In particular, it is shown that rumpling can be avoided even in the presence of phase transformations in the BC, by grinding samples with P600 SiC paper or by applying an “heavy” grit blasting leading to a thinner BC.

Atomic-scale investigation on the mechanical behaviors of the γ′/γ′′ co-precipitates compounds in Ni-based superalloys

The γ′/γ′′ co-precipitation strategy entails excellent mechanical properties for Ni-based superalloys, but the deformation mechanism of γ′/γ′′ co-precipitates Ni-based superalloys at atomic-scale remains unclear. In this work, we studied the effect of loading rate, void size, and temperature on the mechanical behaviors of γ′-Ni3Nb/γ′′-Ni3Nb co-precipitates via molecular dynamic (MD) simulations. The simulation results show that the tensile load rate 1×109 s−1 is critical for the free-void γ′/γ′′ co-precipitates. When the loading rate exceeds 1×109 s−1, a transition occurs in the deformation mechanism, leading to a rapid increase in tensile strength and a subsequent reduction in flow stress until it eventually vanishes (1.35 GPa→0.98 GPa→0 GPa). The presence of voids at the interphase boundary significantly reduces the material's strength, with this weakening effect becoming more pronounced as the void radius increases. Furthermore, the voids facilitate the easier penetration of dislocations into the γ′ and γ′′ precipitated phases along the void peripheries through the interphase boundary. The study also reveals that the trends in Young’s modulus and yield stress for both pre-void and void-free models are similar with increasing temperature, with Young’s modulus decreasing as temperature rises. Notably, at elevated temperatures, γ′′ precipitated phase transitions occur within the γ′/γ′′ co-precipitates, and the yield stress is maximum at 800 K. This study provides valuable insights into the mechanical behavior of γ′/γ′′ co-precipitates and the weakening effects of atomic-scale voids in Ni-based superalloys.

Alumina coatings on Ni-based superalloys: The impact of annealing on heavy oil fouling

As the sweet crude oil reserves decline, refiners must treat sulfur-rich heavy oil, requiring harsher operating conditions, which are detrimental to process equipment. Application of coatings on critical components protects surfaces against sulfidation, corrosion, and fouling, extends the equipment's lifetime, and reduces the frequency of costly turnarounds. In the present work, we coated Inconel 625 and Inconel 718 substrates with amorphous alumina thin films at room temperature using reactive RF magnetron sputtering. Annealing of the deposited coatings at 800, 900, and 1000 °C increased hardness, improved adhesion, and generated crystalline polymorphs, predominantly γ-Al2O3 at lower temperatures, while α-Al2O3 was present at 1000 °C. The annealed substrates formed thermally grown oxides (TGOs), which interacted with the alumina coatings. The TGOs followed grain boundaries in the case of IN718 and a crater-like pattern on IN625. Annealed substrate precipitates generated columnar-like protrusions responsible for inducing crack propagation, which exhibited TGO formation. After 2 h exposure to heavy oil (containing 0.06 g g−1 sulfur) at 450 °C and 11.3 MPa the as-deposited amorphous alumina presented no clear sign of adherent fouling, while the 1000 °C annealed crystalline alumina surfaces presented evidence of fouling.

Insight into elongation and strength enhancement of heat-treated LPBF Ni-based superalloy 718 using in-situ SEM-EBSD

This study presents a detailed investigation into the deformation behaviour and the factors affecting the strength and ductility of laser powder bed fusion (LPBF) Ni-based superalloy 718 (Inconel 718) in both as-printed and heat-treated conditions using in-situ SEM + EBSD. The results show that following post-deposition high homogenization temperatures, the columnar grain structure of the as-printed Inconel 718 alloy successfully transformed to a nearly uniform grain size and the subsequent aging processes facilitated the precipitation of strengthening phases (γ′, γ''). The specimen subjected to heat treatment (HT1) exhibited higher strength but lower ductility, while the as-printed specimen shows higher tensile elongation due to high initial dislocation density. In contrast, the heat-treated (HT2) specimen presented a more optimal combination of strength and ductility. The underlying mechanism for the enhanced tensile properties in the HT2 specimen was nearly homogeneous deformation trend across the grains and ultrafine precipitation of hardening phases. In addition, the twin boundaries were stable in the early deformation stages and maintained their orientation. Multiple slip systems were activated at the high-deformation stage, resulting in a high proportion of geometric necessary dislocations. Further, the interaction of deformation-induced dislocation with twin boundaries transformed them into general boundaries such as high and low-angle grain boundaries, ultimately increasing ductility. On contrary, in the HT1 specimen, the deformation inhomogeneity in grains enabled the strain localization at grain boundaries, which ultimately caused the early formation of cracks.

Stability of the B2 phase among refractory metals

Energy efficiency demands finding new materials for applications at elevated temperatures. Refractory multicomponent alloys with a BCC/B2 microstructure promise to serve as alternatives to the current state-of-the-art, Ni-based superalloys for applications at higher temperatures. With the vast compositional space of these alloys, the first fundamental question is whether the B2 phase can form among refractory metals. In this study, we span through the periodic table to investigate the potential for creating ordered B2 intermetallics among the refractory metals, using first principles calculations, and establish the correlation between the nature of the atomic bonds and stability of the B2 structure. We use our newly developed Multi-Cell Monte Carlo method to explore the extensive compositional space of these alloys, refine the trends in ordering, and pinpoint the most promising combinations for detailed analysis. We discover that B2 phases form exclusively with the inclusion of elements from groups VII and VIII, such as Re, Ru, and Os, and we identify these phases among a total of eleven elements. Further analysis of the electronic structure and bonding characteristics of these B2 systems reveals a pseudogap in the electronic density of states at the Fermi level. This pseudogap is attributed to the hybridization of d orbitals, leading to the formation of strong covalent bonds in the most stable compounds. This finding explains why B2 formation is restricted to elements from groups IV-VIII and V-VII refractory metals.

A new strategy for preparing high strength and high precision diffusion bonding GH536 joints via pulsed current and subsequent heat treatment

GH536 as a typical solid solution strengthened Ni-based superalloy, has been widely used in the preparation of laminated cooling structure. Diffusion bonding, which is highly promising as a joining process in laminated cooling structure, generally requires high strength and high precision. However, the high strength joint fabricated by the conventional hot pressure diffusion bonding (HPDB) often leads to severe deformation of base materials. Here, we developed a new diffusion bonding strategy to overcome this issue in GH536 via pulsed current diffusion bonding (PCDB) and subsequent heat treatment. At low temperature and short time (900 °C/30 min), a joint with a high shear strength of 535 MPa and a low deformation rate of 0.42 % was obtained using the PCDB, and these were 1.60 times and 0.28 times that of the conventional HPDB joint, respectively. Meanwhile, the low bonding temperature and short bonding time hindered the formation and coarsening of carbides, allowing them to be eliminated in a short time heat treatment. The unbonded zones healing, carbide dissolution and recrystallization during heat treatment significantly improved the joint shear strength, and the joint shear strength after heat treatment reached 561 MPa.

Thermal stability and coarsening of eutectic and near-eutectic Ni–Ce alloys

Ni–Ce alloys are currently being studied as a castable alternative to conventional Ni-based superalloys. To understand the evolution of microstructure and mechanical behavior at elevated temperatures, this study investigates the mechanism of coarsening behavior in eutectic and near eutectic Ni–Ce at 900 °C. The as-cast eutectic Ni–Ce consists of a combination of fine lamellar and rod eutectic colonies with coarser boundary regions. Upon annealing, continuous coarsening, via process of globularization, initiates at colony boundaries or primary dendrites where faults (termination and branches) in the lamellar structure are plentiful. Coarsening then proceeds by a combination of fault migration mechanism and lamellar boundary splitting, growing toward the center of the eutectic colonies with increasing annealing time. Coarsening near the center of a colony is extremely slow, indicating the eutectic microstructure itself is very stable. The near-eutectic alloys containing primary dendrites coarsen and stabilize faster compared to the fully eutectic alloy. Moreover, an anomaly is observed in the hardness of the fully eutectic alloy after long exposure at elevated temperature compared to the near-eutectic alloys.

Near-threshold fatigue crack propagation in a Ni-based single crystal superalloy affected by crystallographic anisotropy

Fatigue crack propagation in the near-threshold regime was investigated experimentally and numerically in a Ni-based single crystal superalloy, namely, CMSX-4. Fatigue crack propagation tests were conducted at room temperature using four types of specimens with different primary and secondary crystallographic orientations. The results reveal that the crystallographic orientations strongly affect the fatigue crack propagation behavior, including the crack paths, fatigue crack propagation rates, and fatigue threshold. Crystal plasticity finite element analysis was conducted to quantify the slip activity of an octahedral slip system in front of the crack tip, considering the actual 3D geometry of the crack plane and elastic–plastic anisotropy. The fatigue damage parameter, considering the slip activities of the individual octahedral slip systems, provides reasonable explanation for the fatigue crack propagation rates and crack paths in the near-threshold regime, regardless of the crystallographic orientation. Furthermore, these damage parameters are equivalent at the fatigue thresholds for all specimens with different crystallographic orientation.

An accurate and transferable machine learning interatomic potential for nickel

Nickel (Ni) is a magnetic transition metal with two allotropic phases, stable face-centered cubic (FCC) and metastable hexagonal close-packed (HCP), widely used in structural applications. Magnetism affects many mechanical and defect properties, but spin-polarized density functional theory (DFT) calculations are computationally inefficient for studying material behavior requiring large system sizes and/or long simulation times. Here we develop a “magnetism-hidden” machine-learning Deep Potential (DP) model for Ni without a descriptor for magnetic moments, using training datasets derived from spin-polarized DFT calculations. The DP-Ni model exhibits excellent transferability and representability for a wide-range of FCC and HCP properties, including (finite-temperature) lattice parameters, elastic constants, phonon spectra, and many defects. As an example of its applicability, we investigate the Ni FCC-HCP allotropic phase transition under (high-stress) uniaxial tensile loading. The high accurate DP model for magnetic Ni facilitates accurate large-scale atomistic simulations for complex phase transformation behavior and may serve as a foundation for developing interatomic potentials for Ni-based superalloys and other multi-principal component alloys.

Recovery of Zinc and Rhenium for the Production of Zinc Perrhenates

This study outlines findings from an investigation into the development of a hydrometallurgical process for manufacturing various forms of zinc perrhenate, entirely from waste from recycling and from the Zn–Pb industry. Scraps of Re-bearing Ni-based superalloys and acidic waste, circulating zinc solutions generated during the production of Zn by the electrolytic method and which contain >45 g/dm3 of Zn, Na, Mn, and Mg, were used in the research. In the publication, the conditions for the production of three types of zinc perrhenate, i.e., Zn(ReO4)2·4H2O, Zn(ReO4)2, and Zn(ReO4)2·2H2O, are presented. As a result of the analysis of the obtained results, it was concluded that to obtain the above-mentioned forms of zinc perrhenate, zinc carbonate can be used, precipitated from acidic, waste, and multi-component solutions after their prior neutralization to pH 4.0 and partial purification from Mn, Mg, and Na using metallurgical zinc oxide. Zinc carbonate should be precipitated using Na2CO3 at pH 6.3 and subsequently purified from other impurities, i.e., Mg, Na, and Mn, using aqueous ammonia solutions. As a result, zinc carbonate was obtained, which was used in a reaction with an aqueous solution of HReO4 to produce zinc perrhenate. The precipitated forms of Zn(ReO4)2 were obtained by appropriately drying the crude and hydrated Zn(ReO4)2 to obtain its tetrahydrate, dihydrate, and anhydrous forms, respectively, using drying temperatures of 55, 135, and 185 °C. The developed technology has been submitted for a patent and is an example of a technology founded on the principles of sustainable development, with a particular emphasis on the minimalization of loss of rhenium and zinc at all stages of its realization.

A new test for evaluating the cracking susceptibility of laser cladding coatings and its validity on assessing the effect of laser power

Cracking is one of the most common and destructive defects and Ni-based superalloys are susceptible to hot cracking during laser cladding. In this study, a new test was developed to evaluate the cracking susceptibility of materials during laser cladding, and the crack rate was used as an evaluation criterion for cracking susceptibility. Three groups of different laser powers (1.2 kW, 1.5 kW, and 1.8 kW) were selected to assess the validity of test in terms of effect of laser power on the cracking susceptibility. The cracking rates of coatings were 15.5 %, 20.4 %, and 22.0 %, respectively. It was observed that the cracking rate increased with increasing laser power, as the crack depth also increased due to the effect of the laser power on the substrate with a processed gap. The reinforcements, which were partially melted WC particles and in-situ generated reinforcements, had a similar impact on crack propagation. If there were no microcracks within the reinforcement, the crack bypassed the reinforcement and continued to propagate along the interface between the reinforcement and the matrix. When microcracks were present within the reinforcement, the stress concentration caused the crack to propagate through the microcracks within the reinforcement, crossing the reinforcement and propagating forward. Additionally, a special cored-eutectic microstructure, consisting of inner ceramic particles and fine lamellar eutectics, was found in coatings created using 1.2 kW.

A unified model of tensile and creep deformation for use in niobium alloy materials selection and design for high-temperature applications

There is renewed interest in refractory alloys that possess higher service temperatures than incumbent Ni-based superalloys (e.g., ⪆1100 °C). This study provides a review of the high-temperature constitutive responses of Nb-alloys measured over a wide range of temperatures (≈860 °C < T < ≈1760 °C) and strain rates (≈10–9 s-1< ε˙ < ≈10–1 s-1). Nevertheless, the extant data is sparse and informed materials selection decisions require constitutive expressions to interpolate and reliably extrapolate. The Larson-Miller parameter approach to describe creep-life provides a conservative estimate of material response at the highest temperatures and lowest strain rates, whereas the Sellars-Tegart model describes both steady-state creep and high-temperature tensile test data with a single, universal equation. A minimum flow stress based on the combination of these two models is proposed for design considerations to address the overprediction of strength that can arise from applying one or the other independently. This effort highlights the fact that refractory alloys exhibit strain rate sensitive flow strengths in the temperature range of interest for applications. The roles of alloying, thermomechanical processing, and impurity levels are discussed, and highlight the fact that these advanced Nb-alloys evidence Class 1 (Class A) solute drag controlled creep behavior, except the carbide precipitation strengthened alloy, D-43. In addition, the high-temperature strengths are confirmed to be strongly correlated with alloy melting point.

Repairing weld for directly solidified Ni-based superalloy substrates using directed energy deposition of Inconel 625

Although the maintenance, repair, and operation of gas turbine systems are important issues in enhancing the operation of power units, advanced welding repair processes have limitations in depositing directionally solidified Ni-based superalloy components. To overcome this problem, in this study, a directed energy deposition method was applied to repair directionally solidified components using an Inconel 625 alloy. The rapid cooling rate and small laser beam diameter during directed energy deposition minimized the heat-affected zone and duration in the mushy zone, resulting in crack inhibition at the interface. The first laser scan partially melted the substrate and formed a mixing zone with a chemical composition gradient. By combining the chemical compositional gradient and dislocation accumulation due to repetitive solidification, that is, with the melting cycles of directed energy deposition, the thermal stability decreased, resulting in recrystallization and grain growth in the mixing zone. Consequently, the crack-free continuous interface of the partially repaired components did not induce tensile fracture near the interface. Although the mechanical properties of the IN625 deposit layer were degraded by severe heat treatment of the substrate, the results demonstrated that energy deposition with post-heat treatment is an effective approach in repairing Ni-based superalloy components without cracks and inclusion initiation.

Influence of Oxidizing Atmosphere on the Oxidation of Ni-based Superalloy Rene 65

Influence of Oxidizing Atmosphere on the Oxidation of Ni-based Superalloy Rene 65