Co-based Superalloys Research Articles

Alloying effects on site preference, mechanical properties, and deformation behavior of L12 Co–Ti-based alloys

Alloying plays an important role in controlling the phase stability, mechanical properties, and deformation behavior of ordered intermetallic compounds. In this study, the effects of 3d, 4d, and 5d transition elements on site preference, elastic properties, ideal shear strength, and planar fault energies of L12 Co–Ti-based alloys were systematically investigated by using first-principles calculations. The calculated transfer energy and formation enthalpy indicate that Sc, V, Cr, Y, Zr, Nb, Mo, W, Hf, Ta, and W tend to occupy the Ti site, which reduce the structural stability of L12-Co3Ti. The elastic moduli and ideal shear strength of L12-Co3(Ti,M) increase with average electron density. The electron localization function (ELF) analysis reveals that the Co–M bonds have a stronger covalent character than the Co–Ti bond, which plays an important role in the strengthening of the alloys. The ratio of superlattice intrinsic stacking fault (SISF) to anti-phase boundary (APB) energy of L12-Co3(Ti,M) decreases with increasing atomic number of alloying elements in each period in the range of studied elements, which tends to change the deformation mode from the APB-favored to SISF-favored ones. The APB anisotropy ratio of L12-Co3(Ti,M) decreases with increasing atomic number of alloying elements in each period, which enhances the yield strength anomaly. This study not only sheds insight into the fundamental understanding of phase stability and mechanical behavior of multicomponent L12 compounds, but also provides useful guidelines for designing novel L12-stregnthened Co-based superalloys with superior mechanical properties.

A novel single-crystal L12-strengthened Co-rich high-entropy alloy with excellent high-temperature strength and antioxidant property

Severe intermediate-temperature embrittlement as well as rapid softening and oxidation at high temperatures are significant problems for high-entropy alloys (HEAs) in polycrystalline structures, which greatly limit their applications in modern industries. In this study, a novel Co-rich single-crystal high-entropy alloy (HEA), Co41Ni35Al11·5Ta2·5Cr4Ti6 (at.%), was innovatively developed. After isothermally aged at 1250 °C, this alloy exhibits an ultrahigh volume fraction of cuboidal L12-type nanoprecipitates (∼80%) which is higher than those of previous Co-based superalloys. More importantly, the present HEA shows excellent mechanical performance at both ambient and elevated temperatures, and no intermediate temperature embrittlement and rapid softening were observed. Noticeably, the yield strength can still maintain at ∼800 and ∼500 MPa when tested at 900 and 1000 °C, which are superior to other conventional single-crystal alloys. Moreover, extraordinary oxidation resistance can be achieved at the high temperature of 1000 °C due to the formation of a compact sandwich-structured oxide layer.

Atomistic simulations of diffusion in γ′ -strengthened Co-based superalloys and its connection to selective alumina formation in early-stage oxidation

The suppressed diffusion of Al in ${\ensuremath{\gamma}}^{\ensuremath{'}}$ precipitates of Co-based superalloys is suspected to be one of the reasons for the selective formation of alumina during the early-stage oxidation above $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. In order to scrutinize this assumption, the diffusion properties of Al, Co, and W in a prototype structure of the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase, $\mathrm{L}{1}_{2}\ensuremath{-}{\mathrm{Co}}_{3}(\mathrm{Al},\mathrm{W})$, are investigated in a combined density functional theory (DFT) and kinetic Monte Carlo (KMC) study. A particular challenge in simulating diffusion in this ordered phase is the representation and evaluation of numerous migration pathways on the $\mathrm{L}{1}_{2}$ sublattices. With specified diffusion models for the $\mathrm{L}{1}_{2}$ structure and random solid solution ($\ensuremath{\gamma}$ phase), our KMC results reveal that Al diffusivity in the $\ensuremath{\gamma}$ phase with the same stoichiometric composition is more than two orders of magnitude higher than in the $\mathrm{L}{1}_{2}$ ordered ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase at $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Furthermore, in our rapid thermal exposure experiments for a model Co-based superalloy, the formation of alumina nuclei is observed preferentially in the $\ensuremath{\gamma}$ channels while the Al content is comparable in both channels and precipitates before oxidation. Combining these insights from experiment and simulation, the sluggish diffusion of Al in ${\ensuremath{\gamma}}^{\ensuremath{'}}$ appears to be a key factor in the selective formation of alumina observed in these systems.

Stability and Physical Properties of the L12-γ′ Phase in the CoNiAlTi-System

There is a current interest in Co-based superalloys hardened by a L12-γ′ phase because Co has a higher melting point than Ni and is more resistant against sulfidation attack. However, the Co-Al-W system many of those γ′ hardened Co-based superalloys are based on, has a number of drawbacks. The γ′ phase Co3(Al,W) is not stable at high temperature, the density of the alloys is very high and the oxidation resistance is insufficient. Due to this, there is an ongoing interest to develop γ′-hardened Co-based superalloys based on other systems. Here, first principles calculations are presented to investigate the properties of the γ′ L12-(Co0.5,Ni0.5)3(Al0.5,Ti0.5) phase and related L12 structures. (Co0.5,Ni0.5)3(Al0.5,Ti0.5) exhibits a lower energy of formation than Co3Ti and Co3(Al0.5,W0.5). Nevertheless, Ni3(Al0.5,Ti0.5) has an even lower energy of formation which is further lowered if Ti is enriched on the second sublattice. This finding is supported by analyzing the electronic densities of states. Ni3(Al0.5,Ti0.5) and especially Ni3(Al0.25,Ti0.75) exhibit Fermi levels close to the gap between binding and antibinding states, which is an indicator for stability. In addition to the stability of the γ′-phase in dependence on Ni and Ti content, also the elastic properties were calculated. Ni3(Al0.25,Ti0.75) is less elastic anisotropic and has higher Young’s and shear modulus compared to Ni3(Al0.5,Ti0.5) and (Co0.5,Ni0.5)3(Al0.5,Ti0.5).

Full-field analysis of damage under complex thermomechanical loading

This study combines infrared thermography, digital image correlation and machine learning to measure respectively temperature, strain and damage fields at high temperature. This is applied to thermomechanical fatigue (TMF) testing in presence of severe gradient for typical out-of-phase loading condition. This type of loading is challenging when dealing with thin sheets due to buckling risk induced by high temperature compression. For a Co-based superalloy (Haynes 188), this study demonstrates full-field identification/validation of both behaviour and damage models. Thermal gradient model is the key point in this analysis. With the coupling of TMF measurements by machine learning and DIC, local fatigue micro-crack growth rate and localisation are assessed through jump in displacement. TMF finite element analysis, of loading and damage, validates the whole model framework.

Influence of Ni content on the oxidation behavior of alumina-forming Co-based oxide dispersion strengthened superalloys at 900 °C

This work systematically investigated the effect of Ni additions of 10, 20 and 30 at% on the oxidation behavior of oxide dispersion-strengthened (ODS) Co-20Cr-15Al (at%) superalloys at 900 °C in air. Alumina scale with a duplex structure consisting of outer θ-alumina and inner α-alumina was formed on all tested Co-based ODS superalloys. Ni addition reduced the oxidation rate by inhibiting the growth of the outer θ-alumina during initial oxidation and the inner α-alumina during prolonged exposure. Cr2O3 formation between the interface of the θ- and α-alumina layers was confirmed with the increase of Ni content.

Quantitative analysis and formation mechanisms of non-metallic inclusions in GH4068 alloy prepared by vacuum induction melting

An automatic scanning electron microscope (SEM) analysis system and a self-designed superalloy inclusion analysis module were utilized to characterize the inclusions in the GH4068 Ni–Co-based superalloy prepared by vacuum induction melting (VIM). The morphologies, compositions, and distribution of the inclusions as well as the relationship between the contents of oxygen/nitrogen and inclusion were analyzed. The results indicate that the inclusions in the GH4068 alloy mainly include oxides, carbides, nitrides, carbonitrides and complex inclusions. The oxides are mainly concentrated in the top center of the cast ingots, and their content is higher than that at the bottom and edge. Nitrides and complex inclusions are mainly distributed near the bottom and edge of ingots. The analyses of thermodynamics indicate that the homogeneous nucleation of Al2O3 is spontaneous during the solidification although TiN prefers to wrap on the nucleated particles rather than homogeneously precipitate from the melt. The increase of oxygen/nitrogen content results in the increase of oxides, nitrides and complex inclusions. However, when the concentration of nitrogen is in the range of 3–12 ppm, the increase of nitrogen content shows negligible influence on the content of nitride inclusions.

Effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy

In this work, the effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy for Generation Ⅳ nuclear reactor application was systematically investigated. The results show that different deformed microstructures were observed during cold rolling, including dislocation tangles, slip bands, and microtwins (MTs). The dominant cold rolling deformation mechanism of the alloy gradually changed from planar slipping to deformation twinning, which was a little different from Ni-/Co-based superalloys. A texture transition from Copper {112}< 111 > to Brass {110}< 112 > and Goss {110}< 001 > at 25 % rolling reduction was detected due to the changed deformation mechanism. The standard heat treatment alloy exhibited a yield strength of 213 MPa and good elongation of 54 %. By increasing the rolling reduction up to 55 %, the yield strength was significantly enhanced to 872 MPa, which was attributed to the combined strengthening from both MTs and dislocations. Fine-spaced MTs with coherent interfaces can impede dislocation motion effectively and play a critical role in the strengthening, contributing 75–235 MPa to the yield strength. MTs can also provide ample room for dislocation accumulation, which maintained a reasonable elongation of 12 %. Moreover, due to the texture evolution during cold rolling, the averaged Schmid factor (SF) values for {111}< 110 > slip system exhibited a decreasing trend with the increased rolling reduction, contributing to the progressive increase in yield strength. The present work demonstrates that the formation of MTs at high rolling reductions is an essential reason for the improvement of mechanical properties of the alloy, which also provides a new strategy for designing and optimizing high-performance solid-solution strengthened Ni-Fe-based superalloys.

Quantitative and Qualitative Assessment of As-Cast Microstructure and Microporosity of γ-γ′ Co-based Superalloys

The γ-γ′ Co-based alloys have attracted extensive attention as a new class of materials for high temperature applications. The work is focused on quantitative and qualitative microstructural characterization of selected γ-γ′ type Co-based superalloys obtained by gravity casting. The four Co-based alloys were prepared via vacuum induction melting. The microstructure was analyzed by means of light microscopy and scanning electron microscopy. The microstructural parameters such as porosity and secondary dendrite arm spacing (SDAS) were evaluated by image analysis. The new Co-based superalloys are characterized by proper as-cast microstructure owing to low SDAS and limited microporosity. The values of secondary dendrite arm spacing are comparable for all alloys. The W-free alloys containing Mo and Nb exhibit substantially lower shrinkage porosity compared to those of Co–Al–W and Co–Ni–Al–W. Due to crystallization of Mo and Nb rich phases at lower temperatures, these alloys are characterized by higher solidification range.

First-Principles Calculations of Elastic and Thermodynamic Properties for Multi-component Co-based Superalloys

First-principles calculations were performed to investigate the elastic and thermodynamic properties for multi-component Co-based superalloy systems and explored the effect of alloying on stabilizing the γ′ phase. First, the comparisons were carried out for the γ′ phase in Co3(Al,TM) (TM being transition metals) and Ni3Al systems between the present computational results using the EMTO-CPA method and other available DFT calculations as well as experimental data. The lattice parameters, elastic constants, and Debye temperatures are consistent with experimental results and other calculations. The predicted thermodynamic properties, e.g., the Gibbs free energy, excess entropy, and linear thermal expansion coefficient, agree well with CALPHAD results, experimental results, and other available first-principles calculations. A combination of EMTO-CPA method and Debye–Grüneisen model is utilized in this work to ensure that the alloying effect on the stability of the γ′ phase in a multi-component Co-based system is captured efficiently. This could open the path for designing novel multi-component Co-based alloys based on first-principles calculation. To demonstrate this, predictions for the properties of multicomponent systems were undertaken. Our results show that Ni aids in the stabilization of the (CoNi)3(Al, Mo, Nb) phase.Graphical

Development of a Diffusion Mobility Database for Co-Based Superalloys

To facilitate the development of high-temperature Co-based γ-γ′ superalloys, a Co-Ni based diffusion mobility database is developed for the eight component FCC (Face Centered Cubic) system of Co-Al-W-Ni-Cr-Ti-Ta-Re. A CALPHAD approach is used to represent the temperature and composition dependency of the multicomponent system. The mobility descriptions were based on previous assessment work for the Ni-based superalloys, published experimental and computational data, and established diffusion correlations. The initial mobility descriptions were then refined using additional diffusion couple experimental data, particularly for the Co-Cr, Co-Ta, and Ni-Ta systems. After re-optimizing the descriptions with the new experimental data, the mobility descriptions were validated using a collection of published diffusion couple composition profiles, which were not included in the initial assessment process.

Extreme hardness at high temperature with a lightweight additively manufactured multi-principal element superalloy

Materials are needed that can tolerate increasingly harsh environments, especially ones that retain high strength at extreme temperatures. Higher melting temperature alloys, like those consisting primarily of refractory elements, can greatly increase the efficiency of turbomachinery used in grid electricity production worldwide. Existing alloys, including Ni- and Co-based superalloys, used in components like turbine blades, bearings, and seals, remain a performance limiting factor due to their propensity, despite extensive optimization efforts, for softening and diffusion-driven elongation at temperatures often well above half their melting point. To address this critical materials challenge, we present results from integrating additive manufacturing and alloy design to guide significant improvements in performance via traditionally difficult-to-manufacture refractory alloys. We present an example of a multi-principal element alloy (MPEA), consisting of five refractory elements and aluminum, that exhibited high hardness and specific strength surpassing other known alloys, including superalloys. The alloy shows negligible softening up to 800°C and consists of four compositionally distinct phases, in distinction to previous work on MPEAs. Density functional theory calculations reveal a thermodynamic explanation for the observed temperature-independent hardness and favorability for the formation of this multiplicity of phases.

Alloying effects and effective alloy design of high-Cr CoNi-based superalloys via a high-throughput experiments and machine learning framework

The discovery of a stable γ/γ' microstructure in Co-Al-W-based alloys brought forth possibilities for Co-based superalloys as potential alternatives to Ni-based superalloys in high temperature applications. To date, only a few γ′-strengthened Co-based superalloys with good comprehensive performance have been reported due to the limited knowledge of alloying effects, mainly limited by the “trial and error” approach. In the present work, an accelerated framework integrating a multicomponent diffusion-multiple and machine learning was developed to efficiently and systematically study the effects of alloying elements, e.g. Ni, Al, W, Ti, Ta, Cr, Mo and Nb on the microstructures including phase equilibrium, γ′ volume fraction, γ′ size and γ′ morphology of multicomponent CoNi-based superalloys with high Cr content. The multicomponent diffusion-multiple enabled the collection of a large amount of experimental data relating composition and microstructural parameters, while machine learning enabled the exploration of alloying effects on the microstructural stability and parameters in the compositional space. To validate the efficacy of this approach, two multicomponent CoNi-based superalloys with high Cr addition were designed to possess a γ/γ' two-phase microstructure of relatively high γ' volume fraction, low γ' coarsening rate, with one having a spherical and the other cuboidal γ' morphology. This framework and the insight obtained in this study will be helpful to accelerate the development and application of multicomponent CoNi-based superalloys with high Cr content for engineering applications.

Surface Recrystallization in a Co-Based Superalloy During High Temperature Exposure

Surface Recrystallization in a Co-Based Superalloy During High Temperature Exposure

Development of Cr and Al Pack Cementation Coatings on Co-Based γ/γ′ Superalloys

Co-based superalloys have been developed as candidate materials to replace Ni-based superalloys in hot sections of turbine engines, however, their oxidation resistance is limited. Therefore, in this work chromium and aluminum diffusion coatings were developed via the pack cementation process for novel γ/γ′-strengthened superalloys of the Co–Ni–Al–W system. This alloy system also offers the opportunity to vary the cobalt-to-nickel ratio and the tungsten content systematically to investigate their influence on the coating process. The coating process was shown to successfully enrich the surfaces of such alloys either in aluminum or chromium. For Al, the result was the formation of (Co, Ni)Al on top of an interdiffusion zone, while for chromium a Cr-rich layer in solid solution with the substrate along with the formation of σ-phase were observed. The coating formation mechanisms are discussed for both coatings in the light of activities and phase formation.

Recovery of superlattice stacking faults at high temperatures

Stacking fault shearing is commonly observed in Co-base and CoNi-base superalloys in the intermediate temperature regime between 850 °C to 1000 °C in contrast to Ni-base superalloys. In this work, we report on the significance of recovery processes after aging and creep experiments on predeformed specimens with extreme stacking fault densities. Above 950 °C, superlattice stacking faults start to recover rapidly, whereby intrinsic ones recover more quickly than extrinsic ones. Our results also reveal that the significance of stacking fault shearing in the high temperature creep regime could be higher than that indicated by traditional postmortem investigations due to recovery processes.

CALPHAD informed design of multicomponent CoNiCr-based superalloys exhibiting large lattice misfit and high yield stress

Usually, Co-based superalloys contain a high fraction of W and/or Mo. The refractory elements stabilize the precipitate phase, but cause a high density. In this work, new L12-phase hardened, low-density CoNiCr-based superalloys were developed with the assistance of CALPHAD calculations. Several alloys were studied experimentally and their microstructures, elemental distributions, lattice parameters and the lattice misfit between the γ and γ′ phases were evaluated using scanning electron microscopy, transmission electron microscopy, atom probe tomography and high energy X-ray diffraction. The alloys exhibit a very high misfit, good phase stability and excellent mechanical strength. It was found by hardness tests that a two-step heat treatment improves the strength of the new alloys even further. In comparison with conventional Co-based superalloys, Co–Al–W based superalloys and Co–Ti based superalloys, as well as Ni-based superalloys, i.e. Udimet 720Li, they have a low mass density and high yield stress. They also overcome the problem of a generally low lattice misfit of previous L12-phase hardened CoNiCr-based superalloys with high Ni and Cr contents. As a result, they show cuboidal-shaped precipitates similar to Co–Al–W and Co–Al–Mo based superalloys.

Improving the intermediate- and high-temperature strength of L12-Co3(Al,W) by Ni and Ta additions

The effects of Ni and Ta additions on the mechanical properties in the L12 compound Co3(Al,W), the strengthening phase of Co-based superalloys, have been investigated by compression tests between room temperature and 1000 °C, in order to elucidate the effects of stability of the L12 phase on the mechanical properties. The additions of Ni and Ta, both of which are L12-stabilizers that increase the L12 solvus temperature, increase the yield strength at intermediate and high temperatures. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L12 phase increases. Two factors account for the strength increase at intermediate temperatures: The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increase in both the base strength and the intensity of the yield stress anomaly associated with an increased anti-phase boundary (APB) energy on (111) planes. The strength increase at high temperatures, on the other hand, arises from the increase in the peak temperature due to the increased L12 solvus temperatures. The increased strength of the L12 phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta.

Publisher Correction to: Characterization of the as-cast microstructure and selected properties of the X-40 Co-based superalloy produced via lost-wax casting

Publisher Correction to: Characterization of the as-cast microstructure and selected properties of the X-40 Co-based superalloy produced via lost-wax casting

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.