Cobalt-based Superalloy Research Articles

Microstructure and mechanical properties of L12-strengthened Co–Ni–Fe-based superalloys

The effect of partial Fe substitution for Ni on microstructure, lattice misfit, oxidation, creep, and yield strength are investigated for six W-free cobalt-based superalloys with compositions Co-(30-x)Ni-xFe-yCr-5Al–3V–2Ti-1.5Nb-1.5Ta-0.08B (at.%), with three levels of Fe (x = 12, 14, or 18, replacing Ni) and two levels of Cr (y = 4 or 8, replacing Co). Alloys with 12 and 14% Fe exhibit mostly γ/γʹ microstructures after aging up to 1000 h at 850 °C, while those with 18% Fe display additional intermetallic phases. All alloys show positive lattice misfit from 0.56 to 0.34%, with Fe substitutions reducing lattice misfit by ∼0.02% per 1 at.% Fe. Increasing Fe from 12 to 18% reduces oxidation rates for both levels of Cr. Substitution of Ni by Fe also increases solvus temperature but reduces creep resistance at 850 °C as well as yield strength below 800 °C. By contrast, increasing Cr from 4 to 8 at.% improves both oxidation and creep resistance at 850 °C.

Effect of internal nitridation on microstructure and mechanical properties of cobalt-based superalloy under high-temperature nitrogen atmosphere

Effect of internal nitridation on microstructure and mechanical properties of cobalt-based superalloy under high-temperature nitrogen atmosphere

Tool wear and surface integrity in liquid nitrogen clean cutting of cobalt-based superalloy GH605 with AlTiN coated tools

The tool wear is severe and the machined surface quality is low in dry cutting of cobalt-based superalloy. The traditional coolant is harmful to the environment and not in line with sustainable development. The liquid nitrogen (LN2) clean cutting of cobalt-based superalloy GH605 using AlTiN coated tools is proposed in this work. The tool wear mechanism and the machined surface integrity are investigated. The results show that, with the decrease of LN2 injection temperature, the sharp wear stage of the tool can be delayed. The injection temperature directly affects the tool life. Compared with dry cutting, the tool life is increased by 42.5% at LN2 injection of −190 °C. Under the lower liquid nitrogen condition, the coating peeling is reduced on the rake face. The decrease of LN2 injection temperature can effectively reduce the damaged area of the flank face and inhibit the oxidation wear. The injection temperature between −30 °C and −150 °C can significantly reduce the surface roughness. The minimum surface roughness value at the injection temperature of −90 °C is 0.486 μm, which is 39.4% lower than that at dry cutting condition. When the LN2 injection temperature is below −90 °C, the thickness of the white layer is less than half of that in dry cutting condition. The hardness of the machined surface is enhanced and the residual stress is reduced at injection temperature below −150 °C. This work demonstrates the possibility of improving the machinability of cobalt-based superalloy by optimizing the LN2 injection temperature.

Effect of carbides on the electrochemical dissolution behavior of solid-solution strengthened cobalt-based superalloy Haynes 188 in NaNO3 solution

The electrochemical dissolution behavior of cobalt-based superalloy Haynes 188 was observed by an in-situ observation device. The results showed that at low current density, a dense oxide film was formed on the carbide surface to protect its dissolution. But at high current density, carbide dissolved and produced CO bubbles until falling off. The dissolution of carbide at high current density resulted in the reduction of the high ten-point mean roughness (Rz) and the improvement of surface quality. A dissolution model under different current densities was established to reveal the mechanism of influence of carbide on surface quality under different current densities.

Dynamic Strain Aging Behavior of Cobalt-Based Haynes188 Superalloy

Dynamic Strain Aging Behavior of Cobalt-Based Haynes188 Superalloy

Effect of high temperature exposure on microstructure, mechanical and tribological properties of cold sprayed NiCoCrAlTaY coatings

NiCoCrAlTaY coating is widely used in high temperature oxidation resistance, gas erosion protection and gas path sealing of hot-end parts of aeroengines and gas turbines. In this study, NiCoCrAlTaY coatings were prepared on cobalt-based superalloy substrates by high pressure cold spray technology. The microstructure, mechanical property and tribological behaviors of the coatings before and after thermal exposure at 1050 °C were systemically studied. The results showed that NiCoCrAlTaY coatings with very dense structure could be obtained by high-pressure cold spraying with nitrogen as carrier gas. The coating remained completely intimate with the substrate without any cracking and spalling after thermal exposure at 1050 °C for 500 h. Due to elemental diffusion at high temperatures, the coating formed a four-layer structure, including alumina film on the surface, β-NiAl phase depletion layer near the surface, intermediate layer containing β-NiAl phase and β-NiAl phase depletion layer near the substrate. With the prolongation of thermal exposure at 1050 °C, the thicknesses of β-NiAl phase depletion layers both near the surface and the substrate increased. The thickness and content of the intermediate β-NiAl phase layer decreased gradually during thermal exposure. Besides, the average hardness values of the coating rapidly decreased by about 30 % after thermal exposure at 1050 °C for 5 h. In particular, the hardness of the β-NiAl phase depletion layer near the surface substrate was lower than that of the intermediate area containing β-NiAl phase. Due to the microstructure and hardness evolution, the wear resistance of the coating decreased significantly after thermal exposure. The friction coefficient of the coating increased from 0.2 to 0.5, and the volume wear rate of the thermally exposed coating was about 4.5 times higher than that of the as-sprayed coating. The results of this study provide guidance for design and application of high-performance MCrAlY coatings.

Microabrasive wear behavior of different stellites obtained by laser cladding and casting processes

Cobalt superalloys are used in several applications, mainly involving corrosive environments and high temperatures, in which wear and corrosion resistance are key factors in their performance. These alloys can be produced by a variety of manufacturing processes. Among these, the technology of laser cladding coatings has emerged as a promising and economically viable alternative to produce coatings from cobalt-based alloys. One of the applications of these alloys is in the production of components to obtain second-generation ethanol. In this application, about 8% of the processed material is composed of micrometric abrasive particles, mainly silica, among others, which cause wear of components. In order to better understand the tribological behavior of these alloys in microabrasion, three coatings of commercial cobalt-based superalloys, Stellite 1, 6 and 12, were produced by laser cladding. Additionally, two cast alloys, Stellite 250 and an experimental Stellite alloy, were also evaluated. Microabrasion tests using a statistical design of experiments were performed with SiO2, Al2O3 and SiC abrasives in a suspension of distilled water. Statistical analysis revealed that the wear coefficients of these alloys are influenced by both the abrasives and the alloy. The as-cast Stellite 250 alloy showed the highest wear coefficients, while the coated Stellite 1 and the as-cast experimental Stellite showed the lowest wear coefficients. The predominant wear micromechanism was scratching, except for Stellite 6 and 12 coated alloys in SiC tests, in which predominant mechanism were mixed and rolling, respectively. The presence of secondary micromechanism of rolling wear was also observed and associated with the granulometric distribution of the abrasives. The analysis of wear severity allowed the comprehension of the granulometric distribution influence of abrasive particles on wear.

Influence of the Chromium Content on the Characteristics of the Matrix, the Tantalum Carbides Population, and the Hardness of Cast Co(Cr)-0.4C-6Ta Alloys

The mechanical and chemical behaviors of cast cobalt-base superalloys are governed by the carbides and by a reactive element, which is often chromium. The content of this later element, which is efficient in resisting hot oxidation and also hot corrosion, may have consequences on the melting temperature, microstructure, and mechanical properties at high temperatures and at room temperature. Seemingly, the effect of chromium content on the microstructure and properties of cast equi-axed Co-Cr-Ta-C superalloys containing TaC as single reinforcing carbide and in high-enough quantities to achieve a high level of creep resistance has not been the subject of previous investigations. The present work is devoted to the exploration of this influence of Cr content on the as-cast microstructure of a model alloy in this category, as well as on its microstructure transformations at high temperatures. The work aims to help rate the Cr content to achieve the best characteristics in machinability and high-temperature properties. This is of great importance for fabricability (production cost) and sustainability in service (long enough lifetime performance). A series of six alloys derived from a rather well-known alloy and presenting various Cr contents were thus elaborated by casting. Their microstructures were investigated in their as-cast state as well as in an aged state resulting from a 4-day stage at 1400 K. Vickers indentation was also carried out to study how hardness may evolve with Cr content. It was seen that the higher the Cr content, the lower the solidus temperature, the coarser the TaC population, the harder the alloy, and the higher the risk of brittleness. In order to reach the best compromise, the preferred Cr weight content range, as identified by this work, is 20–30%; indeed, for such Cr contents: (1) the matrix is austenitic, then dense, and then hard and not brittle, and thus is mechanically resistant and tough; (2) the TaC carbides are script-shaped and resistant against morphology changes at high temperatures, and thus efficiently preserve interdendritic cohesion for a long time, and, consequently, (3) the alloys are machinable, have expected good toughness, and can be resistant against creep deformation as well as oxidation and corrosion at high temperatures thanks to the Cr content, allowing for chromium-forming behavior.

High-Temperature Tensile Properties of a Cobalt-Based Co-20Cr-15W-10Ni Superalloy with a Bimodal Grain Structure

Cobalt-based superalloys are common materials for the manufacturing of various components used in aerospace applications. Conventional cobalt-based superalloys with a unimodal grain structure generally exhibit low strength and ductility at high temperatures. A bimodal grain structure of a cobalt-based superalloy, Co–20Cr–15W–10Ni (CCWN), was designed to achieve both high strength and ductility at high temperatures. The deformation behavior and tensile properties of a CCWN alloy with unimodal fine-grain (FG), coarse-grain (CG), and bimodal (FG/CG) structures were investigated at 900 °C. The microstructures and substructures after high-temperature deformation were examined via electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to determine the deformation mechanisms. The microstructural observation showed that the bimodal grain structure consisted of FG and CG domains. During high-temperature deformation at 900 °C, the FG structure was mainly deformed by dynamic recrystallization (DRX), maintaining a similar FG structure. The CG structure was mainly deformed by DRV, resulting in a small amount of DRX grains and a large amount of dynamic recovery (DRV) grains. However, the bimodal grain structures were mainly softened via DRX and transformed into a new bimodal structure, ultrafine grain (UFG) and FG. The FG domains tended to deform via dislocations, and the CG domains via twinning. The high-temperature tensile tests revealed that the bimodal-structured alloy exhibited both higher strength and ductility than those of the alloy samples with unimodal FG or CG structure. This is associated with the newly developed UFG/FG structures in the bimodal grain-structured samples during high-temperature deformation. This work may provide new insight into the development of high-temperature alloys with bimodal grain structures.

Performance of eco-benign lubricating/cooling mediums in machining of superalloys: A comprehensive review from the perspective of Triple Bottom Line theory

Nickel, titanium, and cobalt-based superalloys are utilized in several engineering applications. The unique material properties, for instance, low thermal conductivity and high strength, make it a challenge to machine them easily. The utilization of appropriate lubricating technology is crucial for increasing the machinability of superalloys. Superalloys are frequently machined using mineral oils as lubricants. However, various biological and environmental issues could be caused by these mineral oils. To eliminate the harmful consequences, it's indeed necessary to switch to the use of environmentally friendly lubricants/coolants. Sustainable lubricating/cooling mediums use a minimum quantity of lubricants, liquid nitrogen, dry ice, vegetable oil, nanofluid and compressed air as a cutting fluid, which is thought to be environmentally beneficial and biodegradable. In this review paper, a comprehensive analysis of different eco-benign lubricating/cooling methods used in superalloy machining specifically for turning, milling, drilling, and grinding, etc. is presented. In comparison to traditional wet machining, it has been discovered that these methods produce significantly enhanced surface morphology and tool life, lower cutting temperatures, and cutting forces. The current paper also discusses the benefits and drawbacks of various lubrication/cooling strategies for enhancing superalloy machinability. Following that, for the first time, a review is conducted from the perspective of the Triple Bottom Line, demonstrating that the majority of previous surveys focus on the environmental and economic dimensions of lubrication/cooling techniques used in machining, so more emphasis should be put on the social dimension. In summary, this paper presents a detailed collection of information concerning eco-friendly lubrication/cooling systems that would assist new researchers in recognizing the latest progress as well as potential future research areas on the sustainability elements of superalloy machining.

Interplay between solidification microsegregation and complex precipitation in a γ/γ’ cobalt-based superalloy elaborated by directed energy deposition

This study examines the segregations and the precipitation appearing during the solidification path of a Co-Ni-Al-W-Ta-Ti-Cr γ/γ’ cobalt-based superalloy processed by directed energy deposition (DED). Observations reveal characteristic elements partitioning in the liquid during additive manufacturing. Due to this microsegregation, complex multiphase precipitation occurs and the various precipitates are identified and characterized in a cobalt-based superalloy fabricated by DED. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to investigate the spatial distribution and nature of the various phases detected in the as-fabricated microstructure. Energy dispersive X-ray spectrometry (EDS), wavelength dispersive X-ray spectrometry (WDS), and electron energy loss spectroscopy (EELS) are coupled with a fine analysis of the diffraction patterns to identify the different phases decorating the interdendritic regions. These characterizations allow the identification of different submicronic precipitates: Al 2 O 3 , (Ta,Ti)(N,C), HfO 2 , Cr 3 B 2 and (Ti,Zr,Hf) 2 SC. The solidification sequence is discussed at light of the experimental results. This work provides a first understanding of the interplay between solidification segregations and the formation of second phase precipitation in a cobalt-based superalloy processed by DED. • A cobalt-based superalloy was processed by Direct Energy Deposition. • Solidification segregations lead to the precipitation of interdendritic phases. • Automated crystallographic orientation mapping and chemical analyses were used. • Various interdendritic phases have been identified: oxides (Al 2 O 3 and HfO 2 ), carbonitrides (Ta, Ti)(N, C), sulphocarbides (Ti, Zr, Hf) 2 SC, and borides Cr 3 B 2 . • The solidification path can be established based on the experimental observations.

On the mechanism of carbide network oxidation and subsequent passivation interference in a high carbon-containing cobalt-based superalloy

Oxidation is a life-limiting challenge for components made from cobalt-based superalloys used in high temperature service. However, further elucidation of the oxidation mechanisms is required if new strategies to prolong service life are to be effective. In this study, the mechanism responsible for the preferential oxidation of the carbide network was revealed in the cobalt-based superalloy, Co-101. It was found that the preferential oxidation of the carbide network provided regions of low oxygen partial pressure, where gaseous chromium species could be evolved. This volatilisation resulted in enhanced wastage of chromium in the local microstructure surrounding the oxidised carbide network. Critically, this depletion of chromium from the matrix near the surface of the alloy could inhibit passivation of the alloy surface, a crucial requirement for long duration high-temperature service.

Creep behavior and microstructural characterization of cobalt-based superalloy

Creep behavior and microstructural characterization of cobalt-based superalloy

Tensile, hardness and metallurgical analysis of laser cured Co-Cr-Mo alloy samples

ABSTRACT Co-Cr-Mo is a cobalt-based superalloy widely used in medical and aerospace applications. Direct metal laser sintering is an additive manufacturing technology which is used for fabrication of more complex and customised parts. The knowledge of mechanical properties is essential for the mechanical engineer working with materials. In the present study, the samples are modelled using NX software and are converted into STL files for fabrication. Laser power, scan speed and porosity are taken as input parameters. The orientation is selected as vertical (length in Z-direction) and samples are fabricated. The fabricated samples are then tested for their tensile strength and hardness. The microstructure of the fabricated samples is analysed by capturing the scanning electron microscopy images. The results obtained state that samples fabricated using medium laser power, low scan speed and medium porosity exhibit high yield strength, and the samples fabricated with high laser power, medium scan speed and low porosity exhibit high ultimate tensile strength and hardness. The reasons are discussed in detail.

Accelerating design of novel Cobalt‐based superalloys based on first-principles calculations

The discovery of ordered γ' phase in Co-Al-W based superalloys opens up a pathway for the development of novel Cobalt-based superalloys. γ' phase from binary compounds was calculated by first-principles methods, and the third alloying elements were also considered to stabilize and strengthen γ' phase, the ordered γ' phase with higher stability and more excellent mechanical properties was confirmed herein in novel Cobalt-based superalloys. The formation energies and lattice parameters of Co-X (X = Al, Mo, Nb, Ta, Ti, V, W) and Co-Ti-Z (Z = Al, V, Cr, Mo, Ta, W) were calculated, the results show that X = Ti and Z = Mo are better choices. The doping of Mo can stabilize the ordered L12-γ' phase and improve the mechanical properties of the γ' phase when Mo occupies Ti sites. The occupying preference of alloying elements in Co3Ti lattice is analyzed, Al and Cr prefer to occupy Co sites in L12-Co3Ti lattice, while V, Mo, Ta and W prefer to occupy Ti sites. The as-cast microstructure of the five superalloys is typical dendrite with γ/γ' two-phase region in the inter-dendrite. Besides, segregation phenomenon can be observed in the as-cast microstructure of the superalloys, and Ti prefers to enriched in the inter-dendrite, while other elements tend to be distributed in the dendritic arms.

Enhanced resistance to gas tungsten arc weld heat-affected zone cracking in a newly developed Co-based superalloy

The occurrence of heat-affected zone (HAZ) cracking and the effect of pre-weld microstructural modifications on HAZ cracking susceptibility of newly developed cobalt-based superalloy welded by gas tungsten arc welding (GTAW) process is investigated. Microstructural examination shows that a primary cause of HAZ cracking is the sub-solidus liquation of MC-type carbide and γ′ precipitates, which are identified by transmission electron microscopy (TEM) as present in the material before the welding process. Total crack length (TCL) analysis on pre-weld heat-treated samples reveals that pre-weld hardness does not control the HAZ cracking susceptibility during welding. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with HAZ TCL analysis confirms that grain boundary (GB) elemental segregation of Boron (B) controls the cracking susceptibility during welding. In addition, it is observed that reduction in grain size aids the alloy's resistance to HAZ cracking. Accordingly, a new pre-weld heat treatment that couples reduction in GB segregation of B with small grain size, is found to effectively suppress the HAZ cracking not only after welding, but also after post-weld heat treatment (PWHT). Furthermore, the new pre-weld heat treatment produces tensile properties, after PWHT, that are comparable to those of the alloy without the welding operation, which demonstrates that the pre-weld heat treatment prevents the deleterious effects of welding on tensile properties of the new superalloy.

Effects of Ni and Cr additions on γ + γ’ microstructure and mechanical properties of W-free Co–Al–V–Nb–Ta-based superalloys

The effect of Ni and Cr additions on microstructure, lattice misfit, oxidation, and creep properties are investigated for six W- and Mo-free cobalt-based superalloys with compositions Co-xNi-5Al-yCr-3V–2Ti-1.5Nb-1.5Ta-0.08B (at.%), where x = 10, 20, or 30 and y = 4 or 8. In all alloys, the γ + γ′ microstructure is stable for up to 1000 h upon aging at 850 °C. As Ni increases from 10 to 30 at.%, the γ′ area fraction increases from 32 to 49% in the three low-Cr alloys, while remaining constant (45%) for the high-Cr alloys. All alloys show positive γ/γ′ lattice misfits of 0.8 ± 0.2%, consistent with γ’ rafting observed after creep. Both Cr and Ni additions increase oxidation resistance, more so for 8% Cr than for 30% Ni, as expected from the high stability of chromium oxide. Increasing Cr from 4 to 8 at.% slightly increases creep resistance at 850 °C and increasing Ni from 10 to 30% has a similar effect.

Double minimum creep processing and mechanism for γʹ strengthened cobalt-based superalloy

The double minimum creep, characterized by two creep rate minima, in Co-based superalloy is investigated using a phase-field model coupled with a crystal plasticity model. The constitutive modeling, based on the dislocation slip theory considering dislocation interaction, is applied to simulate microstructural evolution and deformation behavior. Rafting process commences at the beginning of creep until the global minimum of creep rate is reached, demonstrating a strengthening effect from N-type rafts under compressive creep. The high shear strain rate of (111)[01¯1] slip system in the intersections of horizontal and vertical γ channels leads to a slight increase of creep rate after the first local minimum. The evolution of stress field shows that the softening effect is the combined effect of the increase of resolved shear stress and the decrease of hardening stress in the intersections. Further, these changes in stress are primarily caused by the dislocation annihilation and the inhomogeneous plastic deformation. This study indicates that the intermediate local softening stage during creep may be eliminated if the initial inter-distance between γʹ precipitates is decreased.

Accelerated Design of High γ′ Solvus Temperature and Yield Strength Cobalt-Based Superalloy Based on Machine Learning and Phase Diagram

This study combines machine learning and a phase diagram to accelerate the design of a cobalt-based superalloy with a composition of Co-30Ni-10Al-6Ta (at%). The results show that Co-30Ni-10Al-6Ta alloy exhibits high γ′ solvus temperature (1,215 °C) and high yield strength (1,220 Mpa at 25 °C), which is comparable with commercial nickel-based polycrystalline superalloy M-Mar-247. Moreover, the wide processing window and excellent γ′ phase stability make it lucrative for further applications at high temperatures. Meanwhile, the alloy design method also provides a new idea for efficiently realizing the preparation of high-performance alloys.

Flow behavior and strain rate sensitivity assessment of γ and γʹ phases in Co–Al–W-based superalloy using experimental and computational approaches

The recently developed precipitation-hardened cobalt-based superalloys can be considered as a new category of elevated temperature materials. An attempt was made in this research to estimate the elastoplastic characteristics of γ and γʹ phases embedded in Co–Al–W superalloy by using nanoindentation data and constitutive equations. The elastic modulus and hardness were initially utilized to determine the yield stress, which will be subsequently used to characterize the flow behavior in the plastic region. The proposed computational model has been verified with quantitative and semi-quantitative results, calculated by finite element analysis (FEA). It was perceived that both γ and γʹ phases in the investigated superalloy exhibited the ductile load-bearing response as γʹ strengthening phase displayed higher yield stress compared to the γ matrix. The work hardening rate and work hardening exponent were also identified to be higher for γʹ rather than γ phase due to the presence of an anti-phase boundary, which consequently promotes the strain hardening mechanism. The load–displacement curves in both phases illustrated pop-ins which marks the transition from elastic to plastic deformation. Finally, the elastic and plastic data of both constituent phases were incorporated into the FEA paradigm to simulate the indentation projected area. Based on the indentation morphology, the FEA simulation results were further validated with experimental nano-mechanical data. The results revealed that the load–displacement curve acquired from the numerical simulation of the Berkovich indenter has an excellent agreement with the obtained nanoindentation test. The primary purpose of this research is to recognize the correlation between the strain-rate sensitivity of the hardness and the flow stress using FEA results integrated with the analytical modeling.