Formation Of Dislocation Networks Research Articles

Microstructural evolution in doped high entropy alloys NiCoFeCr-3X (X=Pd/Al/Cu) under irradiation

Commonly studied equatomic single-phase FCC high entropy alloys based on 3d transition metals like NiCoFeCr do not provide adequate strength and radiation resistance at high doses for nuclear structural applications. In the current study, the major alloying effects like lattice distortion, ordering and clustering tendencies were investigated by adding low concentration of Pd, Al, or Cu respectively to study the doping effects on the ion irradiation response of NiCoFeCr alloy. The alloys were irradiated with 3 MeV Ni2+ ions at 500°C to a fluence of 1 × 1017/cm2 at a beam flux of approximately 2.8 × 1012 ions/cm2/s. The microstructural evolution upon irradiation i.e., formation of dislocation networks, radiation induced segregation and precipitation, and void formation were studied in detail. Post-irradiation characterization results showed that a Pd addition leads to a high void nucleation rate but controlled void growth, which may be attributed to increased lattice distortion. In Al added HEA, our microstructural analysis indicates that radiation induced ordered L12 precipitates do not affect void swelling significantly. Cu addition led to Cu precipitation that drastically suppressed dislocation density and void swelling of the alloy. Additionally, a model was developed to qualitatively describe the trend in void swelling of typical FCC alloys under ion irradiation. This model was able to qualitatively explain the suppression and reappearance of void swelling in ion irradiated alloys that generally occurs near the region with peak implanted ion concentration.

Applying an alternating current to strontium titanate minimizes the change in oxidation state caused by flash sintering

This study estimated the amount of oxygen deficiency in SrTiO3 polycrystals that were flash sintered under direct current (DC) and alternating current (AC) electric fields, mainly by using the fine structural change of the Ti L23-edges collected by electron energy-loss spectroscopy. The DC electric field corresponded to formation of the maximum amount of oxygen deficiency at approximately δ = 0.07 as SrTiO3−δ, which corresponds to a lattice expansion of roughly 0.1% as a lattice constant of SrTiO3. Furthermore, the oxygen deficiency was distributed unevenly. The formation of dislocation networks in DC-flash-sintered SrTiO3 polycrystals might be related to the lattice strain field caused by this nonuniform oxygen deficiency. In contrast, an AC electric field did not correspond to the oxygen deficiency of SrTiO3, which is similar to the results in thermally sintered SrTiO3 polycrystals.

Misfit and the mechanism of high temperature and low stress creep of Ni-base single crystal superalloys

The present work proposes a new elementary deformation mechanism which governs high temperature and low stress creep of single crystal superalloys (SXs), where the misfit between the γ- and the γ′-phase plays a central role. In the coherent two phase SX microstructure, there is a tendency to minimize the overall elastic strain energy. This is accomplished by the formation of dislocation networks in the γ-phase close to the γ/γ’-interfaces. The stress fields of the network dislocations and misfit stresses accommodate each other to keep the overall strain energy of the system at a minimum. Previous work has shown that dynamic recovery is associated with knitting-out reactions, where dislocations from the network shear the γ’-phase and annihilate with dislocations of opposite sign on the other side of the γ’-phase region. Due to the presence of the misfit this results in an increase of elastic strain energy which is counteracted by coupled knitting-in reactions where newly arriving γ-channel dislocations re-establish the minimum energy configuration. Here we provide microstructural evidence for knitting-out and knitting-in reactions and show that while these reactions clearly occur, dislocation network spacings stay constant. The misfit between the γ- and the γ′-phase accounts for a constant network spacing. Dislocation networks are not static but represent dynamic steady state equilibrium structures in an evolving microstructure. Knitting regular networks requires climb processes which are suggested to be rate controlling. This new view of high temperature and low stress creep mechanism of SXs allows to rationalize previous results published in the literature.

Evolution of local misorientations in the γ/γ'-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method.

The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the γ- and γ'-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850°C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at γ/γ'-interfaces and the filling of γ-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented γ/γ'-neighborhoods, on a size scale above characteristic γ/γ'-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (<200 μm). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature. RESEARCH HIGHLIGHTS: Microstructure evolution during [001] tensile creep of Ni-based single-crystalline alloy. Application of RVB-EBSD technique, focused on angular misorientations between γ/γ' phases, with accuracy of 0.01°. Separation of γ/γ' phases using experimental post-processing of raw EBSD data.

Effect of high temperature annealing on the microstructure evolution and hardness behavior of the Inconel 625 superalloy additively manufactured by laser powder bed fusion

Additive manufacturing of Inconel 625 components attracts great interest due to its ability to produce parts with complex geometries that are needed for high-temperature applications in the aerospace, energy, automotive and chemical industries. To take full advantage of the potential of additive manufacturing, an in-depth understanding of the effects of prolonged high-temperature annealing on microstructure and hardness evolution is needed. Previous research in this field has mainly focused on a limited range of temperature and time. This study aims to determine the effect of prolonged high-temperature annealing on the evolution of intermetallic phases and carbides, as well as changes in the dislocation substructure of Inconel 625 superalloy additively manufactured by laser powder bed fusion subjected to stress relief annealing and subsequent isothermal annealing at a temperature up to 800 °C for 5–500 h. The microstructure development is correlated with hardness behaviour. It is determined that the microstructure evolution proceeds in four stages with temperature and time increase. In the initial stress-relieved condition, a cellular microstructure with nano-sized precipitates of the Laves phase and NbC carbides at the cell walls occurs, and hardness is equal to 300 HV10. In the 1st stage of the microstructure evolution, the γ'' phase particles precipitate on the cell walls, which results in hardening up to 383 HV10 in the specimen annealed at 700 °C for 5 h. The 2nd stage involves the precipitation of the γ'' phase both on the cell walls and inside the cells, as well as the formation of dislocation networks, which contribute to the softening effect and hardness drop to 319 HV10. In the 3rd stage, at temperature 700 and 800 °C, the δ phase, M23C6 carbides, and the Laves phase precipitate and grow, and the subgrain boundaries are formed. The hardness is in the range of 340–350 HV10 and is higher than in the 2nd stage. In the 4th stage, as the annealing time is increased at a temperature of 800 °C, the δ phase and M23C6 carbides coagulate, and the Laves phase particles spheroidize or partially dissolve. Very intense precipitation and growth of the hard δ phase particles provide an increase in hardness to 402 HV10. As a result of systematic studies, the various strengthening and softening mechanisms acting during high-temperature annealing are determined.Graphical abstract

Microstructural investigation and creep behavior of long-term exposed directionally solidified GTD-111 nickel based superalloy

In this research, the effect of the long-term exposure on the creep behavior and microstructure of a directionally solidified GTD-111 precipitation-hardened Ni-based superalloy extracted from a turbine blade, after 76,000 exposure hours, without any interval rejuvenations was investigated. The microstructure observation results demonstrated that there are considerable changes in the microstructure features such as the decomposition of MC carbide, coarsening and spheroidization of primary γ′ (increasing size from 600 to 750 nm and circularity factor from 0.6 to 0.78), dissolution of secondary γ′, and formation of huge deleterious topological closed packed (TCP) phases which profoundly impact its mechanical properties. Furthermore, the rafting phenomenon occurred during the long-term operation at the perpendicular to the loading direction. The formation of dislocation networks can be related to the dislocation cross-slipping in the narrow γ channels, and transition electron microscopic observations confirmed that limited dislocations can shear the γ′ phases. The results showed that the deformation mechanism of the exposed alloy during the steady state creep is dislocation cross slipping and climbing. Meanwhile, the creep behavior was investigated in the longitudinal direction by Larson-Miller, Monkman-grant, and modified Norton-Baily model at stress ranging from 150 to 310 MPa and temperature of 871–982 °C. Long-term exposure to DS GTD-111 significantly decreases its creep properties due to severe microstructural degradation. Based on the fracture analysis of the crept specimens, the main fracture mechanism was an interdendritic fracture, and also the decomposed MC and TCP phases were revealed on the fracture surface.

Creep Behavior and Deformation Mechanism of a Third-Generation Single Crystal Ni-Based Superalloy at 980 °C

As the primary choice for aero-engine turbine blades, creep resistance is an important mechanical property for the developing third-generation single crystal Ni-based superalloys. The creep behavior of the superalloy in the [001] orientation was studied at 980 °C under a series of stress levels, accompanied with scanning electron microscope (SEM) and transmission electron microscope (TEM) observation to investigate the microstructure and deformation mechanism. The deformation mechanism of the alloy is found to be dislocation gliding, propagating and forming a dislocation network in the γ/γ′ interface. Dislocation networks could hinder the movement of dislocation and decrease the creep rate to a constant during the steady-creep stage. The formation of dislocation networks was analyzed due to the interaction of &lt;110&gt; {111} dislocations. Then dislocations cut into γ′ phases as individual &lt;110&gt; super-dislocations, anti-phase boundary dislocation pairs, and stacking faults. The &lt;110&gt; super-dislocation in the γ′ phase may cross-slip into the {001} plane from the {111} plane to form Kear–Wilsdorf locks, which could inhibit dislocations from gliding or cross-slipping and then enhance the creep resistance.

Molecular dynamics simulation of dislocation network formation and tensile properties of graphene/TiAl-layered composites

Alternating stacked graphene/TiAl (Gr/TiAl) composites exhibit excellent mechanical properties because of their high strength, high Young's modulus, and the two-dimensional atomic structure of graphene. Herein, a molecular dynamics approach was used to investigate the uniaxial tensile properties of Gr/TiAl composites after rapid solidification. The results of the simulation show that after rapid solidification, the composites were more crystallizable and were accompanied predominantly by a Shockley type dislocation network, with large periodic hexagonal superlattices (also known as the Moiré pattern) of ∼12.519 and 10.092 Å. Increasing the tensile load activates dislocation emission, which enhances the interaction between dislocations and numerous dislocations, forming a large number of entangled dislocation nodes. This increases resistance to the motion of the remaining dislocations and creating a strengthening effect. The spacing between graphene layers has a substantial effect on the tensile strength and Young's modulus of the Gr/TiAl composites. The composites with smaller layer spacing exhibited better performance than those with larger layer spacing. Because of the dislocation-blocking mechanism between Gr/TiAl interfaces, graphene blocks the propagation of dislocations and takes up most of the load, yielding composites with high Young's modulus, tensile strength, and breaking strain than pure TiAl.

An in-situ TEM investigation of microstructure evolution in Cr-coated zirconium alloy under heavy ion irradiation: Simultaneous evolution of both Cr coating and Zr-4 matrix

Cr-coated zirconium (Zr) alloy is one of the potential candidate materials for accident tolerant fuel (ATF) cladding. However, little attention has been paid in the simultaneous microstructure evolution of Cr coating and Zr alloy matrix under irradiation, which is necessary for comprehensive evaluation of the alloy irradiation performance. In the present work, in-situ ion irradiation experiment was conducted to capture the major features during simultaneous microstructure evolution of Cr coating and Zr alloy at various temperatures. It is found that at 300 ℃ the size and number density of dislocation loops increase with irradiation dose, but at higher temperature the tendency seems different, which is related to the formation of dislocation networks. Besides, the extent of precipitate amorphization in Zr alloy matrix decreases with temperature due to the recovery of irradiation defects. The present work may provide useful insights into the irradiation property of the Cr-coated Zr alloy for ATF applications.

Rotation of the dislocation grid in multilayer FeSe films and visualization of electronic nematic domains via orbital-selective tunneling

Understanding the interplay of structural and electronic symmetry breaking in Fe-based high temperature superconductors remains of high interest. In this work we grow strain-patterned multilayer FeSe thin films in a range of thicknesses using molecular beam epitaxy. We study the formation of electronic nematic domains and spatially-varying strain using scanning tunneling microscopy and spectroscopy. We directly visualize the formation of edge dislocations that give rise to a two-dimensional edge dislocation network in the films. Interestingly, we observe a 45 degree in-plane rotation of the dislocation network as a function of film thickness, yielding antisymmetric strain along different directions. This results in different coupling ratios between electronic nematic domains and antisymmetric strain. Lastly, we are able to distinguish between different orthogonal nematic domains by revealing a small energy-dependent difference in differential conductance maps between the two regions. This could be explained by orbital-selective tip-sample tunneling. Our observations bring new insights into the dislocation network formation in epitaxial thin films and provide another nanoscale tool to explore electronic nematicity in Fe-based superconductors.

Evolution of Symmetrical Grain Boundaries under External Strain in Iron Investigated by Molecular Dynamics Method

In the present work, the evolution of atomic structures and related changes in energy state, atomic displacement and free volume of symmetrical grain boundaries (GB) under the effects of external strain in body-centered cubic (bcc) iron are investigated by the molecular dynamics (MD) method. The results indicate that without external strain, full MD relaxations at high temperatures are necessary to obtain the lower energy states of GBs, especially for GBs that have lost the symmetrical feature near GB planes following MD relaxations. Under external strain, two mechanisms are explored for the failure of these GBs, including slip system activation, dislocation nucleation and dislocation network formation induced directly by either the external strain field or by phase transformation from the initial bcc to fcc structure under the effects of external strain. Detailed analysis shows that the change in free volume is related to local structure changes in these two mechanisms, and can also lead to increases in local stress concentration. These findings provide a new explanation for the failure of GBs in BCC iron systems.

Microstructure Evolution of Reactor Pressure Vessel A508-3 Steel under High-Dose Heavy Ion Irradiation

The microstructure evolution of nuclear reactor pressure vessel A508-3 steel irradiated by heavy ions up to 1.5 dpa was studied by transmission electron microscopy (TEM). According to the TEM analysis, black dots were widely distributed in the irradiated A508-3 steel, with a high density of 1.782 × 1022/m3. A large number of dislocations with Burgers vectors &lt;100&gt; were formed in the irradiated A508-3 steel and tangled together, leading to the formation of dislocation networks. The number density of black dots at 1.5 dpa was 3.5 times higher than that at 0.08 dpa, and the corresponding average size showed an 8% increase. The higher density of dislocation defects led to a significant increase in hardness from 3.0 GPa at 0.08 dpa to 4.2 GPa at 1.5 dpa. The elastic modulus showed a slight increase and less dependence on the irradiation dose.

In-situ TEM investigation of dislocation loop evolution in Al-forming austenitic stainless steels during Fe+ irradiation: Effects of irradiation dose and pre-existing dislocations

Alumina-forming austenitic (AFA) stainless steel with excellent properties has been regarded as a promising candidate of fuel claddings in Supercritical carbon dioxide (S-CO2) gas cooled nuclear reactor. In current work, in-situ irradiation with 400 keV Fe+ in transmission electron microscope were performed on AFA stainless steel to investigate the influences of irradiation dose and pre-existing dislocation lines on loop evolution at 773 K. The irradiation damage caused by irradiation-induced loops was also analyzed. In-situ TEM observation displayed the evolution and characteristics of dislocation loops, including initiation, migration, aggregation, growth, annihilation, and combination. Both Frank loops with b = 1/3<111> and perfect loops with b = 1/2<110> were formed, and at 0.54 dpa, 1/2<110> loops accounted for the majority and was up to 64.6%. Pre-existing dislocation lines obviously affected not only the evolution and distribution of dislocation loops, but also the degree of irradiation hardening. At the same irradiation dose, not only the size and number density of dislocation loops in the region with dislocation lines were smaller than those in the region without dislocation lines, but also the increment of yield strength and irradiation damage was lower before the formation of dislocation network. Increasing pre-existing dislocation density should be beneficial to improve the irradiation resistance of materials, but it needed to be comprehensively considered with the mechanical properties of the material. Al component played an important role in irradiation behavior of AFA steels and the corresponding mechanism was discussed subsequently.

Microstructure evolution of compressed micropillars investigated by in situ HR-EBSD analysis and dislocation density simulations

With decreasing system sizes, the mechanical properties and dominant deformation mechanisms of metals change. For larger scales, bulk behavior is observed that is characterized by a preservation and significant increase of dislocation content during deformation whereas at the submicron scale very localized dislocation activity as well as dislocation starvation is observed. In the transition regime it is not clear how the dislocation content is built up. This dislocation storage regime and its underlying physical mechanisms are still an open field of research. In this paper, the microstructure evolution of single crystalline copper micropillars with a $\langle1\,1\,0\rangle$ crystal orientation and varying sizes between $1$ to $10\,\mu\mathrm{m}$ is analysed under compression loading. Experimental in situ HR-EBSD measurements as well as 3d continuum dislocation dynamics simulations are presented. The experimental results provide insights into the material deformation and evolution of dislocation structures during continuous loading. This is complemented by the simulation of the dislocation density evolution considering dislocation dynamics, interactions, and reactions of the individual slip systems providing direct access to these quantities. Results are presented that show, how the plastic deformation of the material takes place and how the different slip systems are involved. A central finding is, that an increasing amount of GND density is stored in the system during loading that is located dominantly on the slip systems that are not mainly responsible for the production of plastic slip. This might be a characteristic feature of the considered size regime that has direct impact on further dislocation network formation and the corresponding contribution to plastic hardening.

Dislocation-related conductivity in Au(In)/Cd1–xZnxTe(x = 0, 0.1) Schottky contacts

Dislocation-related conductivity is studied in Schottky contacts Au(In)/Cd1-xZnxTe (x = 0, 0.1) prepared on the surface of single crystals modified by multiple irradiation with a ruby laser and mechanical polishing. The contacts were examined by measuring the DC current as a function of the applied bias and temperature as well as the photoelectric response. It is shown that both methods of surface modification result in p-to-n conversion of the conductivity type of the surface layer. The charge transfer in contacts is explained by the formation of dislocation networks buried under the surface. A model of two potential barriers is proposed for the interpretation of the photovoltaic response in contacts. Their existence is associated with compressive strains in the modified surface layer caused by dislocations, which leads to an increase in the band gap and the formation of a heterostructure.

The effect of deviations from precise [001] tensile direction on creep of Ni-base single crystal superalloys

Low temperature (1023 K) high stress (800 MPa) tensile creep behavior of the superalloy single crystal ERBO-1 (CMSX-4 type) is investigated. Three loading directions are compared: precise [001] and 15 ° deviations from [001] towards [111] and [011]. It is found that creep rates ε˙ scale as ε˙[001]→[111]>ε˙[001]>ε˙[001]→[011]already in the early stages of creep (ε≤1%), where dislocation network formation and planar fault intersections cannot rationalize the observed rate effects. An analysis based on Peach-Köhler force calculations suggests, that fast creep rates are observed, when dislocations from two octahedral systems, which are required to react and form the leading part of a planar fault ribbon in the γ’-phase, experience similar driving forces. Creep data, micromechanical calculations and TEM results are in good qualitative agreement. From a technological point of view, the results show that while 15 ° deviations from [001] towards [011] can be tolerated, deviations towards [111] must be avoided.

Precipitation of Cu- and Nb-rich phases and its strengthening effect in 17Cr ferritic stainless steel during high-temperature creep process

In the present study, the evolution and strengthening effect of precipitates in 17Cr ferrite stainless steel during the creep process are investigated. The precipitates are characterized by scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy. Moreover, the interactions between the precipitates and the grain boundary and dislocation are quantitatively analysed. The results show that Cu- and Nb-rich phases are formed during the creep process. The stress concentration at the tip of the precipitates, the stacking fault at the precipitate edge and the internal dislocation can promote the nucleation of the Nb-rich phase. The Nb-rich precipitates can provide the driving force for the nucleation of Cu-rich particles through stacking fault, dislocation entanglement and internal dislocation. At 700 °C, as the creep time increases, the pinning effect of precipitates on the grain boundary increases linearly, while the pinning effect on the dislocation decreases. As the creep temperature increases, a peak pinning effect on the grain boundary is observed at 750 °C, and the pinning effect of the precipitates on dislocation decreases gradually inside the grain. The formation of dislocation networks at grain boundaries can delay pinning effect reduction of the Nb-rich phase, and the main blocking mechanism of precipitates on dislocation movement is bypassing. The precipitation strengthening effects of Cu- and Nb-rich phases are also compared with each other.

Effects of Grain Refinement on the Low-Cycle Fatigue Behavior of IN792 Superalloys

The Ni-based IN792 superalloy is widely applied as a component in industrial turbines and aircraft engines due to its good high-temperature properties and excellent corrosion resistance. Since these components have to suffer from cyclic thermal and mechanical stresses during service, the high-temperature fatigue failure becomes one of the major factors affecting their service lives. Grain refinement has been considered as an effective way to improve the mechanical performance of superalloys. However, due to the complexity of alloy composition, microstructure and service condition, there is no unified theory about the influence of grain refinement on the fatigue performance and fracture mechanism of superalloys. In the present research, the IN792 superalloy with different grain sizes was manufactured. Then, fully reversed, strain-controlled, low-cycle fatigue (LCF) tests with four different total strain amplitudes were carried out on the alloy at 700 °C and 800 °C to clarify the effects of grain refinement on its LCF behavior. The results show that grain refinement improved the fatigue life significantly, which is mainly attributed to increasing the grain boundary content and refining MC carbides, eutectic structures and dendritic structures. During fatigue test under lower strain amplitude, the alloy exhibits a pronounced initial fatigue hardening followed by a continuous well-defined stability stage, which is caused by the formation of dislocation networks and coarsening of primary γ’ phases. However, during fatigue test under higher strain amplitude, the alloy exhibits continuous hardening response because the dislocations could shear primary γ’ phases, which could give rise to resistance to dislocation movement. In addition, the fracture surface observation shows that the fatigue fracture mode is mainly affected by the total strain amplitude. Under lower total strain amplitude, the fatigue microcracks mainly initiate at the porosities near the specimen surface, while under higher total strain amplitude, the fatigue microcracks tend to form at the interior of the specimen.

In situ transmission electron microscopy study and molecular dynamics simulation of dislocation loop evolution in FeCrAl alloys under Fe+ irradiation

FeCrAl alloys with excellent comprehensive properties are the most promising candidates to replace zirconium alloy fuel claddings. In our study, the dislocation loop evolution including initiation, migration, merging, growth, annihilation, and reaction in a FeCrAl alloy was investigated by using in situ transmission electron microscopy during 400 keV Fe+ irradiation. The mechanism induced the growth of dislocation loops including the absorption of high-mobility point defects and defect clusters and the merging of two or more dislocation loops of different sizes. In the initial stage of irradiation, the loop density was relatively stable and the loop size increased rapidly with the increase in irradiation dose; however, owing to the formation of dislocation networks, the loop density decreased significantly in the later stage of irradiation. Both b = 1/2<111> and b = <100> dislocation loops were formed in the FeCrAl alloy. The ratio of <100> loops was 49% after irradiation with 0.14 dpa at 723 K. Molecular dynamics simulations displayed the reaction of dislocation loops with different Burgers vectors and sizes. Although the presence of alloying elements (Cr and Al) would prohibit or delay the interaction process, loop merging continued owing to the atomic rearrangement of dislocation loops.

Investigation of microstructure evolution and mechanical properties of multi-precipitation Ni–Co base superalloys

The microstructure evolution and mechanical properties were investigated in a multi-precipitation Ni–Co base superalloy system with different chemical compositions (wt. %) of Ni30–Co33–C0.02 (1#) and Ni28.5-Co34.5-C0.01 (2#). The interaction of multi-precipitates and its influence on the creep behavior were studied in correlation with the evolution of deformed microstructure. The multi-precipitates mainly consist of γ′-Ni3Al phase, β-NiAl phase, carbides, and η phase. After a long term aging, both of the β and γ′ phase possess a strong thermal stability in the two superalloys, while the number density of η precipitates has increased a lot (volume fraction: 0.02%–0.21%) in the 2# Ni–Co alloys compared with that in the 1# Ni–Co alloys. The room temperature mechanical properties of both Ni–Co alloys almost remain stable such as ultimate tensile strength (1260–1280 MPa) and total elongation (22–24%) even after a long aging time of 1500 h. However, different from the stress-free aging process, the γ′ precipitate in the 2# Ni–Co alloy shows a higher rafting rate compared with that in the 1# Ni–Co alloy after the creep (903 K, 450 MPa) for 500 h. Thus, the 2# Ni–Co alloy exhibits a significantly higher creep strain rate and an advanced accelerating creep stage than the 1# Ni–Co alloy. The differences of the creep properties are mainly attributed to a change in deformation mechanism from dislocation gliding to dislocation climbing over the rafted γ′ phase during the steady-state stage. A high stacking fault energy (SFE) of the matrix results in strong stress concentration and dislocation network formation in the γ/γ′ interfaces, contributing to an accelerated creep strain rate of the 2# Ni–Co alloy. In contrast, sustainable precipitation hardening from spherical MC carbides can delay the steady-state stage to a very long life until 1500 h in the 1# Ni–Co alloy.