Co-based Superalloys Research Articles

Effect of CaO deposit on the oxidation kinetics of CoCrAlY alumina forming alloys

MCrAlY (M = Ni and/or Co) alloys are commonly utilized as protective coatings for a variety of metallic substrates, including Ni- and Co-based superalloys due to their outstanding resistance to corrosion and oxidation in harsh environmental conditions. In such environments, external deposits have the potential to compromise the oxidation behavior of these alloys. This work assesses the oxidation kinetics of three different CoCrAlY alloys that can form alumina, with and without CaO deposits. Thermal gravimetric analysis experiments were carried out at 1100 °C for 50 h in dry-air conditions. It was observed that the presence of CaO deposits resulted in an accelerated oxidation rate and hindered the formation of alpha-alumina. Upon exposure to CaO, the examination of cross-sections revealed the formation of varying layers of calcium aluminates through the chemical reaction between Al2O3 and CaO. The characterized alloys exhibited a significant increase in oxidation rate during the initial stage, and therefore, an analysis at short oxidation times (9 min) is included in this study. These results indicate the formation of calcium chromate and suggest a potential link to the content of the gamma phase, which could be responsible for the accelerated oxidation during the early oxidation stages.

Microstructure Control and Hot Cracking Prevention During Laser Additive Manufacturing of Cobalt-Based Superalloy

Hot cracking is a frequent and severe defect that occurs during laser additive manufacturing of superalloys. In this work, a pulsed-wave (PW) laser modulation process was employed to control the solidification microstructure and reduce the hot cracking susceptibility of laser additive manufactured cobalt-based superalloy. The effects of continuous-wave (CW) and PW laser processing modes on the dendrite morphology, element segregation, eutectic phase, and hot cracking of fabricated Co-based superalloys were investigated. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to characterize the microstructural characteristics of samples. A two-color pyrometer was used to measure the molten pool temperature variation under different laser processing modes. The results show that coarse columnar dendrites, chain-like eutectic carbides, and hot cracks were observed in the CW sample. In contrast, the fine equiaxed crystals, discrete eutectic carbides, and low-level residual stresses were obtained to avoid hot cracks, owing to the high cooling rate and the periodic melting and solidification of the molten pool under the PW laser processing mode. This work provides a new method for controlling solidification structure and hot cracking of laser additive manufactured Co-based superalloy.

Effect of the synergistic effect of Cr and Mo on the solidification microstructure and mechanical properties of NiAl-based high-entropy alloys

To balance the strength and toughness of NiAl-based alloys by coupling multiple strengthening mechanisms, two novel high-entropy alloys (HEAs), Ni0.35Al0.3 (CoFe)0.2Cr0.15-xMox (x=0,0.05), were prepared, and their mechanical properties were explored. First, the addition of Mo induces the shape of the precipitate to change from spherical to acicular. Ni0.35Al0.3(CoFe)0.2Cr0.1Mo0.05 has excellent high-temperature mechanical properties. Foremost, the strength is better than that of most Ni-based superalloys, Co-based superalloys and Ni-Co-based superalloys at 873 K. Notably, its yield strength is greater than 1400 MPa. When the temperature reaches 1123 K, the yield strength is still greater than 600 MPa. Moreover, the acicular precipitated phase maintains a co-lattice relationship with the matrix. The synergistic effect of solid solution strengthening and precipitation strengthening significantly enhances the strain hardening ability of the Ni0.35Al0.3 (CoFe)0.2Cr0.1Mo0.05 alloy.

A new understanding of phase transformation in vacuum electron beam welding of NS163 Co-based superalloy and AISI 410L stainless steel: Based on in situ observation and variant selection

This study establishes a link between crystallographic variants and mechanical properties at both the edge and center regions of NS163 Co-based superalloy wires and AISI 410L stainless steel plates welded joints. The thermal cycle of vacuum electron beam welding was simulated using in situ laser confocal microscopy to clarify the martensitic transformation process. Results indicate that martensite preferentially nucleates at grain boundaries, maintaining the Kurdjumov-Sachs orientation relationship with the parent austenite. Most variant boundaries in these regions correspond to variants within the same crystal packet, with V1/V3&V5 emerging as dominant pairs. At the edge, the increased cooling rate and temperature gradient amplify the driving force for martensitic transformation, fostering the generation of diverse variants. Conversely, lower cooling rate at the center raises the martensitic transformation temperature and expands variant selection. The study notes significant dislocation slip during micropillar compression, with the edge of weld exhibiting finer martensite laths and dense dislocations, which enhances strength (∼1279 MPa) compared to the center (∼1040 MPa), aligning with the results obtained via nanoindentation. The observed "size effect" results in a twice strength as measured by micropillar compression compared to nanoindentation. Additionally, staggered Bain groups at the edge include a greater number of high angle grain boundaries, indirectly improving toughness. This research aligns with recent literature and aids in the development of compositional design and machining techniques for heterogeneous welds.

Facilitated the discovery of new γ/γ′ Co-based superalloys by combining first-principles and machine learning

Superalloys are indispensable materials for the fabrication of high-temperature components in aircraft engines. The discovery of a novel class of γ/γ′ Co-Al-W alloys has ignited a surge of interest in Co-based superalloys, with the aspiration to transcend the inherent constraints of their Ni-based counterparts. However, the conventional methodologies utilized in the design and advancement of new γ/γ′ Co-based superalloys are frequently characterized by their laborious and resource-intensive nature. In this study, we employed a coupled Density Functional Theory (DFT) and machine learning (ML) approach to predict and analyze the stability of the crucial γ′ phase, which is instrumental in expediting the discovery of γ/γ′ Co-based alloys. A dataset comprised of thousands of reliable formation (Hf) and decomposition (Hd) energies was obtained through high-throughput DFT calculations. Through regression model selection and feature engineering, our trained Random Forest (RF) model achieved prediction accuracies of 98.07% for Hf and 97.05% for Hd. Utilizing the well-trained RF model, we predicted the energies of over 150,000 ternary and quaternary γ′ phases within the Co-Ni-Fe-Cr-Al-W-Ti-Ta-V-Mo-Nb system. The energy analyses revealed that the presence of Ni, Nb, Ta, Ti, and V significantly reduced the Hf and the Hd of γ′, while Mo and W deteriorate the stability by increasing both energy values. Interestingly, although Al reduces the Hf, it increases Hd, thereby adversely affecting the stability of γ′. Applying domain-specific screening based on our knowledge, we identified 1049 out of >150,000 compositions likely to form stable γ′ phases, predominantly distributed across 11 Al-containing systems and 25 Al-free systems. Combining the analysis of CALPHAD method, we experimentally synthesized two new Co-based alloys with γ/γ′ dual-phase microstructures, corroborating the reliability of our theoretical prediction model.

Short fatigue crack growth sensitivity to thermo-mechanical fatigue loading

Most high-temperature components are subject to out-of-phase thermomechanical fatigue (OP-TMF), which induces crack growth at low temperatures. However, OP-TMF has been little studied in the context of short cracks. This study focuses on the experimental sensitivity of OP-TMF loading conditions playing on temperature range, gradient, and dwell time for thin sheet superalloy specimens. The material of interest is a Co-based superalloy, HA188. It is widely used in combustion chambers.The experimental analysis is based on full-field measurements for temperature, strain and damage by infrared thermography, digital image correlation and high resolution images from 300 to 900°C. The main conclusion is that the temperature gradient, together with the temperature amplitude, largely determines the strain amplitude and subsequent fatigue crack growth rate (FCGR) of short cracks. In situ measurements of damage and crack closure were obtained using supervised machine learning based on images. This clarifies that crack closure is only partial and that the crack network growth rate is consistent with the individual short crack growth rate. Finally, 3D finite element analysis considering realistic temperature field and strain energy based FCGR model was able to evaluate the fatigue life in this context. It is shown that the OP-TMF FCGR is very close to the FCGR of the maximum temperature of the TMF cycle due to partial crack closure.

Creep rafting fracture and strain properties of overheating Co-based superalloy: Crystal plasticity phase-field simulation

The creep fracture and rafting of Co-based superalloys under thermal loading are critical properties for the long time working at high temperature, the crystal plasticity phase-field (CPPF) model is developed by incorporating the creep damage theory. The study of creep in Co-5Al-14V (at.%) superalloy in states of isothermal, continuous heating and overheating is realized. The rafting mechanism and evolution kinetics of γ′–Co3(Al, V) phase are analyzed in processes of directional coarsening and dissolution fracture under thermal loading stress, the incomplete P-type rafting morphology with low volume fraction of γ′ phase shows a reduced creep resistance. The creep strain has a steep rise for the rafting fracture and degeneration induced by overheating stress. The overheating followed by cooling triggers the recovery of γ′ rafts and improves the creep resistance. In addition, the dissolution and fracture of γ′ rafts are driven by the chemical potential, while the rafting and re-precipitation of γ′ phase are driven by the elastic potential under the creep stress. At the tertiary creep stage, the damage variable and creep strain increase in the order of isothermal creep, overheating and continuous heating. The studies show the creep fracture and strain properties in stress-diffusion mechanisms by coupling experiment morphology and CPPF simulation, the relationship of creep fracture and rafting properties of superalloys is established.

Insights into the multimodal γ′ precipitation in Co-based superalloys

The mechanical properties of precipitation-strengthened Co-based superalloys are determined by the γ/γ′ two-phase microstructure features. This study investigates the precipitation behavior of multimodal γ′ phases in a Co-based superalloy from both thermodynamic and kinetic perspectives. The effects of secondary and tertiary γ′ phases were examined using different cooling methods from 900 °C to 1000 °C. The results reveal that water cooling from 900 °C inhibited the precipitation of tertiary γ′ phase, whereas other samples exhibited nanoscale tertiary γ′ phases within the γ matrix channels. Finally, the influence of tertiary γ′ phase on chemical composition, microstructure evolution, and mechanical properties in Co-based superalloys is elucidated.

Magneto-chemical effects in the elastic properties of Co3Al-based compounds

The thermodynamic and mechanical properties of the L12Co3Al type of compounds are fundamental for understanding and designing Co-based superalloys. In these systems, both compositional and magnetic changes can occur upon service due to the elevated temperature. Here, using first-principle calculations, we study the bulk properties of three families of Co3Al-based compounds: (Co1−xNix)3Al (0⩽x⩽0.5), Co3(Al1−yWy) (0⩽y⩽1), and (Co0.5Ni0.5)3(Al0.5TizTa0.5−z) (0⩽z⩽0.5). The calculated lattice constants, Curie temperatures, and formation energies show good agreement with the limited available theoretical and experimental data. Our results reveal the impact of chemistry and magnetism on the elastic parameters. We find that both chemical composition and magnetic state alter the elastic parameters and the elastic anisotropy, which in turn makes the predictions based on common ductile–brittle criteria challenging. We separate the volume and chemical effects for both ferromagnetic and paramagnetic states and show that in most cases, the chemical effect gives the dominant contribution to the alloying trends in the elastic parameters. The present findings reveal the complex relationship between alloying elements and elastic parameters in the Co3Al-based precipitates, providing insights into their mechanical properties for engineering applications.

Evaluation of critical temperatures of as-cast Co-based superalloys as a function of alloying elements and their effect on microstructure

Co-based superalloys are of great interest in the aeronautical and aerospace industry due to their superior thermal performance compared to Ni-based superalloys. This work explores the influence of elemental additions (Ni, Al, Cr, Si, and Ti) on the critical temperatures (liquidus, solidus, solvus, and eutectic) and microstructural features of several Co systems fabricated by vacuum induction melting until the final Co-30Ni-10Al-4Cr-2Si-2Ti (wt%) composition was obtained. Differential Scanning Calorimetry results indicated high thermal stability concerning the γ' phase formation. Si and Ti additions suppressed the eutectic presence in the proposed superalloy, which is desirable to avoid the incipient melting of components. Instead, the present work exhibits a low-density Co-based superalloy conformed of a continuous γ/β microstructure with γ' precipitating uniformly. These characteristics make the Co-30Ni-10Al-4Cr-2Si and Co-30Ni-10Al-4Cr-2Si-2Ti superalloys promising candidates to be subjected to high temperatures in comparison with current Co-based superalloys.

Microscopic mechanism of the L12–D019 phase transformation in a Co-base single crystal superalloy

In γ′-strengthened superalloys based on the Co-Al-W system, the stability of the γ′ phase is often limited, as it is found to transform into other phases such as B2 and D019 after long-term annealing. To explore the details behind the annealing-induced transformation from the metastable L12-γ′ phase to the thermodynamically stable D019-χ phase in the single-crystalline Co-base superalloy ERBOCo-VF60 (Co79.8Al8.9W9.0Ta2.3 in at.%), scanning transmission electron microscopy and atom probe tomography are employed. Due to the structural similarity between the L12 and D019 structures, coherent plate-shaped χ precipitates are formed in the γ/γ′ microstructure parallel to {111} planes through a shear-based transformation mechanism. While the χ phase precipitates slowly during isothermal aging, its formation can be significantly accelerated locally by coating the superalloy with a Cr-rich layer, which indirectly stabilizes the χ phase as revealed by thermodynamic calculations. This procedure allowed us to obtain an intermediate (incomplete) state of χ-phase formation in terms of both crystal structure and composition. By characterizing the partial dislocations at the tip of growing χ precipitates, the microscopic details of the shear-based cubic-to-hexagonal transformation from L12 to D019 are uncovered. The compositional aspect of the transformation involves a significant diffusion-mediated enrichment of W in the χ phase, accompanied by a simultaneous W depletion in the γ′ phase, leading to its transformation to the γ phase.

Effects of microstructures and double aging on high temperature yield strength in Ni–Co-base superalloy produced by a powder metallurgy route

Microstructural features and their effects on yield strength at high temperatures in the P/M HGN200 Ni–Co-base polycrystalline superalloy, subjected to a standard double aging treatment, have been clarified using both Thermo-Calc® and a unified approach. The matrix γ possesses a very fine grain size due to the used processes and pinning by primary γ'. The calculated antiphase boundary energy is very high, resulting in high yield strength. Dispersion of fine tertiary γ′ phases is remarkable to increase the yield strength compared with those of primary and secondary γ′; the contribution of the primary γ′ to the yield strength is negligible. The stoichiometric composition of primary and secondary γ′ is (Ni,Co)3(Ti,Al) and that of tertiary γ′ is (Ni,Co,Cr)3(Ti,Al) and the excellent high temperature strengthening is related to homogeneous dispersion of both the secondary and tertiary γ′.

Co–Ni–Al–W γ/γ′ superalloy with Cr and Ti additions fabricated via laser fusion of elemental powders

A Co-based superalloy (Co-0.20Ni-0.11Al-0.07W-0.04Cr-0.03Ti, mole fraction) was synthesized by laser powder-bed fusion of a blend of elemental powders. The as-built alloy shows a γ-FCC matrix where the high-melting Ti and Cr powders are fully dissolved, but the refractory W particles are only partially melted. Full dissolution of the W particles is achieved after homogenization at 1150 °C, resulting in a homogeneous, single-phase γ microstructure. Upon aging at 900 °C, a two-phase γ/γ′ microstructure forms, with a high-volume fraction of sub-micron γ′ precipitates with cuboidal shape. Compared to a control quaternary alloy without Cr and Ti fabricated by the same method, the present alloy has increased γ′ fraction and microhardness after aging for 300 h, maintaining a strong γ′ strengthening effect without forming γ′-depleted zone at grain boundaries. Micrometer-size γ′-precipitates at grain boundaries also shift the transition between diffusional and dislocation creep to lower stress levels, by inhibiting diffusional creep and grain-boundary sliding. However, overall creep resistance is lowered, consistent with a reduction of lattice mismatch due to Cr additions and rafting of γ′ precipitates. Furthermore, Cr and Ti additions improve oxidation resistance at 900 °C, due to the formation of a continuous Cr-, Ti- and Al-rich oxide layer.

Research on microstructure and properties enhancement of TiN/GH5188 Co-superalloy composite fabricated by laser powder bed fusion

Improving mechanical properties effectively in Laser Powder Bed Fusion (LPBF) is a substantial challenge. Utilizing metal matrix composites with customizable TiN nanoparticle proportions presents an efficient strategy for fabricating high-strength LPBF parts. This study employed LPBF to fabricate TiN/GH5188 Co-based superalloy composites with alternative TiN nanoparticle contents (0, 0.5, 1, and 1.5 wt%). The effect of nano-TiN particles on the microstructure, texture evolution, and mechanical properties of TiN/GH5188 composites was investigated. Notably, the presence of nano-TiN acts as a reinforcing particle, facilitating the precipitation of the BCC strengthening phase, resulting in substantial strength enhancement. As the TiN content increases, the yield strength and microhardness consistently rise. At a 1.5 wt% TiN addition, a significant enhancement is observed, with yield strength rising by 22.0 % and microhardness increasing by 16.9 %. Although the aggregation of TiN precipitates into larger particles reduces ductility, the appropriate addition of TiN can effectively mitigate the microstructural anisotropy and induce a transition in the texture toward a non-preferred orientation, while maintaining high strength and ductility. A TiN/GH5188 composite containing 1 wt% TiN exhibits a yield strength and tensile strength of 822.4 MPa and 1131.8 MPa, respectively, accompanied by a microhardness of 334.3 HV. The elongation remains at 29.3 %, while exhibiting a non-preferred orientation (111) texture. Compared to GH5188, the yield strength, tensile strength and microhardness are increased by 17.6 %, 7.2 % and 12.1 %, respectively. This study demonstrates the significance of LPBF in producing high-performance TiN/GH5188 Co-superalloy composites and elucidates the potential mechanisms of TiN in improving microstructure and enhancing properties.

Effect of the Mn and Cr contents on the oxidation and creep resistance at 1100°C of cast cantor–based HEAs

Cast High Entropy Alloys (HEAs) derived from the Cantor’s composition may represent alternative substitutional solutions to the superalloys which are constituted for more than half of the critical elements nickel and cobalt. Recently, MC–alloyed HEAs constituted by a Cantor’s type matrix associated to an efficient interdendritic network of MC carbides, have demonstrated promising creep resistance at elevated temperatures. Regrettably, they also show poor high temperature oxidation resistance. This bad oxidation behavior is possibly due to the deleterious effect of manganese and to a too low content in chromium. New alloys were elaborated and tested in oxidation at 1100°C with thermogravimetry follow–up and metallographic characterisation, and their creep behavior was controlled. With less Mn and more Cr than the original alloys, these new alloys demonstrated significant progress in oxidation resistance. These changes in chemical compositions did not modify their creep resistances which were globally maintained, for the carbide-free alloy as well as for the MC-containing ones. These Mn-decreased and Cr-increased MC-containing alloys can be considered as low-cost alternative to polycrystalline Ni or Co-based superalloys for working in moderate conditions of corrosive fluids and applied mechanical stresses.

High temperature oxidation behavior of a Ni–Co-based superalloy

Oxidation behavior of a Ni–Co-based superalloy prepared by vacuum induction melting (VIM) plus electron beam smelting layered solidification technology (EBSL) and VIM plus electro slag remelting (ESR) at 1180 °C was investigated. The predominant oxides from the outer layer to the inner layer are TiO2, Cr2O3, (Al, Ti)-rich oxide and Al2O3, respectively. The (Al, Ti)-rich oxide is considered to be Al2Ti4O9, which is formed by TiO2 and Al2O3. At high temperature, the external oxides experienced significant spalling, particularly in ESR-alloy, which indicated that the EBSL-alloy is more preferable in terms of oxidation resistance. This can be attributed to the presence of finer grains in EBSL-alloy, which facilitates the diffusion of elements and promotes the rapid formation of oxidation scales on the surface of the alloy. Additionally, the presence of a TiO2 layer cover on Cr2O3 reduces the degree of spalling of oxides in EBSL-alloy.

First-principles prediction of the Co–Al phase diagram including configurational, vibrational and magnetic contributions

Co-based superalloys have attracted attention to replace Ni-based superalloys in high temperature structural applications because of their higher melting temperature and corrosion resistance. Strengthening is provided by Co3(Al, X) (X = W, Cr, Mo, Ni) phases and optimization of their properties requires detailed information about the free energies of the different phases and of the phase diagram of the Co–Al system. This is achieved in this paper through first principles calculations in combination with statistical mechanics. Configurational entropic contributions to the free energy were included through Monte Carlo simulations using the cluster expansion formalism. The vibrational entropy of each phase was determined through the length-stiffness relationship while the magnetic entropy was also included through the Heisenberg Hamiltonian. The computed phase diagram is compared with the currently accepted experimental phase diagram and the different contributions to the stability of each phase are analyzed independently. The potential of this strategy to predict phase diagrams of magnetic systems is clearly established.

Effect of Mo on the coherent interface features of γ′-phase strengthened Co-based superalloys

Molybdenum (Mo) addition was investigated for its influence on the interfacial characteristics of γ′-phase strengthened Co-based superalloys with γ/γ′ dual phases. Advanced characterization techniques, including scanning transmission electron microscopy (STEM), atom probe tomography (APT), and high-resolution X-ray diffraction (HR-XRD), were employed for this purpose. Mo addition was found to increase the compositional and structural widths of the γ/γ′ interface. This was accompanied by segregation of Mo and Co at the interface, attributed to the reduction of interfacial energy. Additionally, Mo addition led to a decrease in the lattice misfit between the γ-matrix and γ′-phases. Those results enhanced interface stability, contributing to improved high-temperature stability. This study offers valuable insights into the mechanisms of alloying and the microstructural stability of Co-based superalloys.

Insight into element segregation mechanisms during creep in γʹ-strengthened Co-based superalloy by elastoplastic phase-field simulation

The element segregation accompanying the creep process has been shown to significantly affect the deformation resistance of the superalloys. However, the processing and mechanism of element segregation are still unclear. This paper investigated the concentration evolution of a model Co–9Al–9W (at. %) alloy during 900 ​°C/275 ​MPa using developed ternary elastoplastic phase-field model coupled with CALPHAD method and crystal plasticity model. The results of simulation show that co-depletion of Al and W element occurs in γʹ precipitate and in γ side at γ/γʹ interface, and this depletion is gradually increasing with the accumulation of plastic strain. From the perspective of changes of driving force of element diffusion, it is found that these segregation phenomena are attributed to the high elastic potential caused by the large local plastic strain. In addition, the effects of these segregations on creep property are also predicted. The current research provides a new method for exploring the mechanism of element diffusion.

Phase-field approach assisted solution heat treatment efficient design for novel multicomponent Co-based superalloys

Fast homogenization without incipient melting is the key to designing solution heat treatment (SHT) process for single crystal superalloys. In this paper, a multicomponent phase-field model coupled with solidus temperature is developed for dynamically tracking incipient melting temperature, TIM, of alloy during SHT. The simulation process runs completely automatically only by inputting the composition information of as-cast specimen. Based on the obtained TIM-time profile, the holding temperature and the time of SHT are determined. Applying this approach to Co–30Ni–11Al–4W–1Ta–4Ti–5Cr single crystal specimen, a gradual increased TIM from 1247 °C to 1293 °C within 72,000 s is obtained, and thus a stepwise SHT process with a short time of less than 20 h is designed. The experimental verification shows that after SHT, the segregation coefficient of the slowest diffusing element W decreased from more than 2 to 1.065, the content of harmful secondary phase has decreased to below 0.1%, and no incipient melting occurs.