Segregation Of Atoms Research Articles

Atomic diffusion, segregation, and grain boundary migration in nickel-based alloys from molecular dynamics simulations

Grain boundary diffusion and metal mobility in alloys control material performance in many applications and yet remain poorly understood at a mechanistic level. With advances in accessible time and length scales for computational molecular simulations, and recent force field developments, we now possess tools to help unravel those mechanisms. Using large-scale molecular dynamics simulations, here we examined vacancy-mediated diffusion processes in Ni-5Cr alloy with low and high-energy grain boundaries. We show that atomic diffusion inside the grain boundary plane is about four times higher than bulk diffusion, at any temperature, and exhibits a typical Arrhenius behavior with a very small energy barrier (∼0.8 eV for Cr and 0.7 eV for Ni within 1300–1600 K). Additionally, the fastest diffusing species inverts; Cr diffusion was faster than Ni in the bulk but slower in the grain boundaries. This is attributed to the creation of high cohesive energy clusters of Cr at the grain boundary. Grain boundary migration was also observed to be temperature dependent and appears to be two times higher in the 5% Cr alloy than in pure Ni, highlighting the important role of the alloying element on grain boundary motion.

Effect of ultrasonic impact treatment on the properties of CoCrFeNiCu high-entropy alloy coatings on steel by induction cladding

The aim of this study is to improve the comprehensive properties of CoCrFeNiCu high entropy alloy coating (HEAc) prepared by induction cladding by means of ultrasonic impact treatment (UIT). After UIT, the interdendritic segregation of Cu atoms on the surface of HEAc was inhibited, which made the dual-phase structure consisting of FCC1 Cu-rich phase and FCC2 Cu-depleted phase transform to a single-phase FCC. High density of the dislocation and the sub-grain boundary produced by UIT have hardened the surface of the HEAc, which strengthened microhardness, elastic modulus and wear resistance. The combination of the soft region and the hard region in the HEAc/substrate specimen with UIT made the strength increase but the elongation decrease. The inhibition effect of UIT on the Cu-rich phase alleviated the inhomogeneity of the passivation film, while the grain refinement and high-density dislocation promoted the growth of the passivation film, hindered the invasion of corrosive ions, and effectively improved the corrosion resistance of the HEAc surface.

Vacancies-mediated atomic diffusion controlled by the interfacial structure of FCC/BCC HEA multilayers

The radiation-induced segregation is one of the major concerns for nuclear materials, as it can damage the service performance of the material. Eutectic high entropy alloy (HEA) multilayer is considered a potential nuclear material for improving the irradiation tolerance of materials, due to its outstanding irradiation tolerance and mechanical properties. Abundant heterogeneous interfaces in eutectic HEA multilayer can interact with irradiation-induced vacancies, thus resulting in changes in the behavior of atomic diffusion and segregation. In the current work, to explore the effects of the heterogeneous interfacial structure on atomic diffusion and segregation, we have investigated the evolution of the elemental distribution in FCC/BCC HEA multilayers with coherent and semicoherent interfaces under vacancy flow in detail. It is found that the segregation and atomic diffusion behaviors of eutectic HEA multilayer are sensitive to the heterogeneous interfacial structure. The semicoherent interface promotes the interface diffusion and interface segregation due to the vacancy annihilation at interface and the intrinsic excess volume of interface. Whereas, the coherent interface is benifcial to the intragranular diffusion because it is transparent for the diffusion of vacancy.

First-Principle Investigation of the Interface Properties of the Core-Shelled L12-Al3M (M = Sc, Zr, Er, Y) Phase

The interface structure and segregation behavior of L12-Al3M (M = Sc, Zr, Er, Y) phases were investigated based on first-principles calculations. The results showed that the order of the thermodynamically stable interface was Al3Zr/Al > Al3Sc/Al > Al3Er/ Al > Al3Y/Al. The interfaces of Al3Sc/Al3Zr, Al3Er/Al3Zr, and Al3Y/Al3Er obtained negative interfacial energies and low coherent strain energies and were favorable to form a clear interface. Zr atom tended to segregate to the first atomic layer on the Al side of the Al/Al3Sc, Al/Al3Er, and Al/Al3Y interfaces. The driving effect of the Zr atom segregation to the Al3Y shows was stronger than that to Sc and Er atoms, whereas the high coherent strain energy hindered the formation of Al3Y/Al3Zr interface. Er atom tended to segregate at the Al/Al3Y interface and accelerated the formation of core-shelled Al3Y/Al3Er. Furthermore, the formation of the double core-shelled Al3Y/Al3Er/Al3Zr was discussed.

Twin suppression by atomic scale engineering of precipitate-matrix interfaces

This work aims to investigate the influence of solute segregation at particle-matrix interfaces on the twin activity of aged Mg alloys. With that purpose a binary Mg-8Al alloy (wt%) and two ternary Mg-8Al-1 Zn and Mg-8Al-1Ag alloys (wt%) were first homogenized and then aged in order to obtain comparable precipitate distributions. Particle-matrix interfaces in the binary alloy were observed to be clean, while Zn and Ag atomic segregation was clearly present at the interfaces of the ternary alloys. Room temperature micropillar compression tests were carried out in grains with favorable orientations for tensile twinning in the homogenized and the aged conditions for the three materials under investigation. The activation of twinning and slip was then characterized by electron microscopy. This study shows that while in the binary alloy precipitation did not lead to any major changes in the twinning activity and, in particular, it did not prevent the formation of thick twin lamellae encompassing the entire width of the tested micropillars, a drastic reduction of the twin activity took place in the aged ternary alloys. In the latter, straining led to the formation of narrow twin lamellae belonging to a single variant, and occasionally arrested at the interior of the micropillars. We propose that the drastic reduction of the twin volume fraction in the aged ternary alloys may be attributed to the decrease of the particle-matrix interface energy due to Zn and Ag segregation, which would hinder re-nucleation at such interfaces, thus suppressing twin propagation.

Roles of cooling rate of undercooled austenite on isothermal transformation kinetics, microstructure, and impact toughness of bainitic steel

At present, the application of extra thick super bainite steel has attracted considerable attention. In this paper, medium carbon bainite steel is taken as the research object, and the effects of the cooling rate of undercooled austenite on the transformation kinetics, microstructure and mechanical properties of bainite steel are studied systematically. The results show that the incubation period of bainite shortens and the transformation kinetics accelerates with the decrease of the cooling rate of undercooled austenite. The segregation of solute atoms at a slow cooling rate shortens the incubation period. When bainite nucleates and grows by an autocatalytic method, the size of the untransformed austenite is large and the strain compatibility is strong, leading to low activation energy of its autocatalytic nucleation and accelerating the kinetics of bainite transformation. The size of block structure and the length of bainite sheaves decrease with the increase of cooling rate. The impact toughness of the sample with a cooling rate of 30 °C/s is twice as high as that of the sample with a cooling rate of 0.3 °C/s. As the smallest unit to control the toughness of bainite steel, block structure affects the impact toughness. The impact toughness of bainite increases with the decrease of its block size. The content of film-like retained austenite increases with the increase of cooling rate, which is also one of the factors of excellent impact toughness of bainite at high cooling rate.

Novel method to achieve synergetic strength–ductility improvement of Al–Cu alloy by in situ TiC–TiB2 particles via direct reaction synthesis

Insufficient performance, especially at high temperatures, limits the extensive application of cast Al–Cu alloys. This study proposes a novel method of direct reaction synthesis (DRS) for the simultaneous synthesis and dispersion of hybrid-sized in situ TiC–TiB2 particles in the melt to prepare a trace-particle-reinforced high-performance cast Al–Cu5.5–Mn0.45–Ti0.3–V0.3–Cr0.2–Zr0.14–B0.04 alloy. The particulate-reinforced alloy achieved strength–ductility synergetic enhancement at room and high temperatures. In comparison to the Al–Cu matrix alloy, the yield strength of 0.7 wt% (TiC–TiB2)/Al–Cu alloy at 298 K (529 MPa), 493 K (214 MPa), and 533 K (183 MPa) had an increase of 28%, 18% and 35%, meanwhile their elongation increased by 4%, 50% and 62%, respectively. The yield strength enhancement primarily benefited from the thermal mismatch between the particles and matrix, and Orowan strengthening. The strength–ductility performance improvement is due to the optimization of the microstructure: significantly refined α-Al grains with smaller and more dispersed θʹ precipitates. Further, the improvement mechanism of the microstructure is due to the action of the nanoparticles on the heterogeneous nucleation stimulation and dendrite growth inhibition of α-Al during the solidification process. Meanwhile, the segregation of the Cu atoms was inhibited by the particles. The precipitation was significantly promoted because of more uniformly distributed Cu atoms, and more defects induced by particles improved diffusion of the Cu atoms the number and density of the precipitation sites. The results of this study provide an economical and accessible synthesis strategy for industrial manufacturing.

Investigation of the nucleation and growth behavior of Ti2AlC and Ti3AlC nano-precipitates in TiAl alloys

Abstract In this work, the nucleation and growth behavior of Ti2AlC and Ti3AlC nano-precipitates in Ti–44Al–1.2C alloys were systematically studied by observing their distribution, morphology, and interface atomic structure. The experiment results show that the needle-like C atom segregation zones in TiAl alloys are the nucleation site of Ti2AlC, and the long axis direction of segregation zones is parallel to the TiAl(111) plane. The rod-like Ti2AlC nano-precipitates mainly distribute at the TiAl/Ti3Al interface, and the orientation relationship between them is [ 1 ̅ 01 \bar{1}01 ]TiAl//[ 11 2 ̅ 0 11\bar{2}0 ]Ti2AlC//[ 11 2 ̅ 0 11\bar{2}0 ]Ti3Al, (111)TiAl//(0001)Ti2AlC//(0001)Ti3Al. The needle-like Ti3AlC nano-precipitates distribute in TiAl with the orientation relationship of [001]Ti3AlC//[001]TiAl, (100)Ti3AlC//(100)TiAl, (020)Ti3AlC//(020)TiAl, and (110)Ti3AlC//(110)TiAl during the nucleation stage. After growing into the rod-like, the orientation relationship between Ti3AlC precipitates and TiAl is [ 1 1 ̅ 0 1\bar{1}0 ]Ti3AlC//[ 1 1 ̅ 0 1\bar{1}0 ]TiAl, (001)Ti3AlC//(001)TiAl, (220)Ti3AlC//(220)TiAl, and (111)Ti3AlC//(111)TiAl. Both the needle- and rod-like Ti3AlC precipitates preferentially grow along the [001]TiAl direction. The crystal structure and lattice mismatch between Ti2AlC and Ti3AlC nano-precipitates and the TiAl matrix determine their distribution, morphology, and interface atomic structure.

Evaluation of chemical structure and Si segregation of Al/Si(111)

Control of diffusion and segregation of Si atoms through a thin metal layer from a stacked Si structure is one of the effective techniques to grow two-dimensional (2D) or ultrathin Si crystals. We have studied the formation of the Al layer with a flat surface and high crystallinity on a wet-cleaned Si(111) wafer by thermal evaporation in order to use it as a growth template for ultrathin Si crystals. Then, the impacts of the annealing in N2 ambient on the surface flatness, the Al crystallinity, and the chemical bonding features for the Al/Si(111) structure were investigated. A formation of a sub-nanometer Si layer on the Al(111) surface using Si segregation with keeping surface flatness was demonstrated by the control of annealing temperature.

Effect of Interfacial Schemes on the Optical and Structural Properties of InAs/GaSb Type-II Superlattices.

Incorporating an intentional strain compensating InSb interface (IF) layer in InAs/GaSb type-II superlattices (T2SLs) enhances device performance. But there is a lack of studies that correlate this approach's optical and structural quality, so the mechanisms by which this improvement is achieved remain unclear. One critical issue in increasing the performance of InAs/GaSb T2SLs arises from the lattice mismatch between InAs and GaSb, leading to interfacial strain in the structure. Not only that but also, since each side of the InAs/GaSb heterosystem does not have common atoms, there is a possibility of atomic intermixing at the IFs. To address such issues, an intentional InSb interfacial layer is commonly introduced at the InAs-on-GaSb and GaSb-on-InAs IFs to compensate for the strain and the chemical mismatches. In this report, we investigate InAs/GaSb T2SLs with (Sample A) and without (Sample B) InSb IF layers emitting in the mid-wavelength infrared (MWIR) through photoluminescence (PL) and band structure simulations. The PL studies indicate that the maximum PL intensity of Sample A is 1.6 times stronger than that of Sample B. This could be attributed to the effect of migration-enhanced epitaxy (MEE) growth mode. Band structure simulations understand the impact of atomic intermixing and segregation at T2SL IFs on the bandgap energy and PL intensity. It is observed that atomic intermixing at the IFs changes the bandgap energy and significantly affects the wave function overlap and the optical property of the samples. Transmission electron microscopy (TEM) measurements reveal that the T2SL IFs in Sample A are very rough compared to sharp IFs in Sample B, indicating a high possibility of atomic intermixing and segregation. Based on these results, it is believed that high-quality heterostructure could be achieved by controlling the IFs to enhance its structural and compositional homogeneities and the optical properties of the T2SLs.

Role of extrusion rate on the microstructure and tensile properties evolution of ultrahigh-strength low-alloy Mg-1.0Al-1.0Ca-0.4Mn (wt.%) alloy

Mg-1.0Al-1.0Ca-0.4Mn (AXM1104, wt.%) low alloy was extruded at 200 °C with an extrusion ratio of 25 and different ram speeds from 1.0 to 7.0 mm/s. The influence of extrusion rate on microstructure and mechanical properties of the AXM1104 alloy was systematically studied. With the increasing of extrusion rate, the mean dynamically recrystallized (DRXed) grain size of the low alloy and average particles diameter of precipitate second phases were increased, while the degree of grain boundary segregation and the intensity of the basal fiber texture were decreased. With the rising of extrusion rate from 1.0 to 7.0 mm/s, the tensile yield strength (TYS) of the as-extruded AXM1104 alloy was decreased from 445 MPa to 249 MPa, while the elongation to failure (EL) was increased from 5.0% to 17.6%. The TYS, ultimate tensile strength (UTS) and EL of the AXM1104 alloy extruded at the ram speed of 1.5 mm/s was 412 MPa, 419 MPa and 12.0%, respectively, exhibiting comprehensive tensile mechanical properties with ultra-high strength and excellent plasticity. The ultra-high TYS of 412 MPa was mainly due to the strengthening from ultra-fine DRXed grains with segregation of solute atoms at grain boundaries. The strain hardening rate is increase slightly with increasing extrusion speed, which may be ascribed to the increasing mean DRXed grain size with rising extrusion speed. The higher strain hardening rate contributes to the higher EL of these AXM1104 samples extruded at higher ram speed.

Effect of annealing temperature on the evolution of copper-rich phases in antimicrobial stainless steels

The effects of different annealing temperatures on the microstructures and properties of 304 stainless steel containing 3.52 Cu (mass fraction %) and 430 stainless steel containing 1.47 Cu (mass fraction %) were studied with the aids of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) instruments, and the evolution mechanism of Cu-rich phase was explored. The results show that the microstructure of 430Cu stainless steel consists of polygonal ferrite, while 304Cu stainless steel consists of irregular austenite. The average grain size of both steel increases with increasing annealing temperature. The mechanism of Cu-rich phase precipitation in 430Cu at different annealing temperatures is: segregation of Cu atoms → Cu precipitation → congregation and growth → spherical ε-Cu → ellipsoidal → long rod. 304Cu Cu-rich phase precipitation mechanism: supersaturated solid solution → direct formation of FCC structure of Cu-rich phase → slow growth, and co-lattice with the matrix. In order to obtain a good match between strength and plasticity, the optimal annealing temperature range for 430Cu and 304Cu stainless steel is 800 °C∼900 °C.

Discontinuous segregation patterning across disconnections

Twinning is a frequent deformation mechanism in nanocrystalline metals, and segregation of solute atoms at twin boundaries is a thermodynamic process that plays an important role in the stability and strengthening of these materials. In pristine, defect-free twin boundaries, solute segregation generally follows a single- or multilayer patterned coverage of solutes that is uniformly and symmetrically distributed at segregation sites across the boundary. However, when a disconnection, a type of interfacial line defect, is present at the twin boundary, we report a possible discontinuity of the segregation patterns across this defect for a broad range of binary alloys. The change of segregation pattern is explained by a break of the local symmetry across the disconnection terraces. The characteristics of this change are dictated by the orientation of the dislocation content sitting at the step region of the disconnection and its synergistic/antagonistic interactions with the step character. These findings not only advance our understanding of the origin of the interface segregation phenomena and the key contribution from interfacial defects, but they also shed light on applications for tailoring atomically precise interfacial structures to design alloys with emerging properties.

Improving the short-wave infrared response of strained GeSn/Ge multiple quantum wells by rapid thermal annealing

In this work, the evolution of structural, optical and optoelectronic properties of coherently strained Ge0.883Sn0.117/Ge multiple quantum wells (MQWs) grown by molecular beam epitaxy under rapid thermal annealing (RTA) is systematically investigated. The MQW structure remains fully-strained state with RTA at 400 °C or below and disrupts at higher annealing temperatures due to Sn segregation and interdiffusion of Ge and Sn atoms. The GeSn well layers exhibit the strongest absorption in 2.0–2.4 μm after annealing at 400 °C and become transparent above 1.8 μm after RTA at 600 °C or beyond due to serve Sn segregation. Owing to improved crystal quality after RTA at 400 °C, the dark current of the fabricated metal-semiconductor-metal photodetector is effectively lowered by more than two times. Additionally, the responsivities at 1.55 and 2.0 μm are improved by 4.15 and 3.78 folds, respectively, compared to those of the as-grown sample. The results can be an insightful guidance for the development of high-performance short-wave infrared photonic devices based on Sn-containing group-IV low-dimensional structures.

DFT approach for predicting the pH-potential-dependent durabilities of Pt-skinned Pt-M (M = Ni, Co, and Ir) alloys for fuel cell cathodes

Pt-skinned Pt-M (M = Ni, Co, and Ir) alloy catalysts have been reported as cathode catalysts for polymer electrolyte membrane fuel cells (PEMFCs) that are durable and highly efficient toward the oxygen reduction reaction (ORR). While density functional theory (DFT) studies have confirmed that these catalysts are efficient, they did not clearly demonstrate the superior durability imparted by the Pt-skin layer. Herein, we introduce an atomic segregation model based on density functional theory calculated (DFT-calculated) vacancy formation energies that overcomes these limitations. Solvation-corrected segregation energies reveal that the Pt/PtNi(111), Pt/PtCo(111), and Pt/PtIr(111) surfaces, which can contain very low amounts of Pt, are highly resistant to atomic segregation from the alloy catalyst. The influence of the Pt-skin layer of the examined Pt/PtM catalysts was closely investigated by considering the PEMFC operating conditions, including solvation, the oxygen-rich environment of the cathode, the electrical double-layer, pH, potential, and temperature. As a result, we demonstrated that the Pt/PtCo catalyst exhibits superior durability with a high vacancy formation energy for the Pt-skin layer.

Severe elastic modulus shift of deformed CoNiCrMo alloy under relative low-temperature exposure

Elastic modulus (EM) is usually considered to be a relatively stable physical quantity; even phase transformation only results in a little change in EM, while this work demonstrates that it is not so straightforward. Internal friction tests reveal that the EM of a 60% cold-worked CoCrMoNi (MP35N) alloy increases by over 17% after exposed at $100\text{--}700{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, with no change in phase or grains. Analysis of nanoscale structure of the alloy by high-resolution transmission electron microscope shows that high-density nanosized unstable stacking faults and incoherent nanotwins are formed in the cold-worked MP35N alloy, which severely weaken the bonding between the atom planes and decrease the EM. During the exposure process, the unstable fault interfaces and incoherent nanotwins transform into coherent nanotwins or annihilate to decrease the system energy, which repairs the crystal structure and increases the interatomic bonding force and the EM. The segregation of solute atoms at the twin boundaries and stacking faults also contribute a little to the increase in EM. These mechanisms should also suit for the faced centered cubic (FCC)-structured alloys with low-level stacking-fault energy.

Substructure and crystallography of lath martensite in as-quenched interstitial-free steel and low-carbon steel

In this study, the substructures and crystallographic features of as-quenched lath martensite in interstitial-free (IF) steel and low-carbon steel (0.2C steel) were investigated using focused ion beam-scanning electron microscopy serial sectioning and advanced transmission electron microscopy. A three-dimensional analysis revealed that the morphology of the so-called lath, the smallest structural unit, exhibited a wedge-shaped plate instead of the ideal lath shape. The habit plane orientations were distributed from (5 7 5)A to (1 1 1)A in the IF steel, whereas they were near to that predicted based on the phenomenological theory of martensite crystallography in the 0.2C steel. In the IF steel, well-developed dislocation cells, tangled dislocations, and curved dislocations could be observed. Conversely, straight dislocations and cementite formation were observed in the 0.2C steel. Moreover, the orientation changed significantly inside a given lath in both steels. The lath boundary comprised regularly spaced steps with (0 1 1)M atomistic terrace planes. Interfacial dislocations corresponding to the discontinuity of the (0 1 1)M lattice planes could be observed at steps, and these dislocations were introduced to compensate for the misorientation between adjacent laths. Additionally, carbon segregations were observed at the lath and block boundaries in the 0.2C steel. Our observation results on the dislocation structures, cementite formation, and segregation of carbon atoms on the lath and block boundaries indicated that the substructure changed significantly during quenching. Based on our experimental results, we discuss the origin of the substructures and crystallographic features of lath martensite.

Low-Temperature Deformation and Fracture of Cr-Mn-N Stainless Steel: Tensile and Impact Bending Tests

The paper presents the results of tensile and impact bending tests of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel in temperatures ranging from −196 to 20 °C. The steel microstructure and fracture surfaces were investigated using transmission and scanning electron microscopes, as well as X-ray diffraction analysis. The steel experiences a ductile-to-brittle transition (DBT); however, it possessed high tensile and impact strength characteristics, as well as the ductile fracture behavior at temperatures down to −114 °C. The correspondence between γ–ε microstructure and fracture surface morphologies was revealed after the tensile test at the temperature of −196 °C. In this case, the transgranular brittle and layered fracture surface was induced by ε-martensite formation. Under the impact bending test at −196 °C, the brittle intergranular fracture occurred at the elastic deflection stage without significant plastic strains, which preceded a failure due to the high internal stresses localized at the boundaries of the austenite grains. The stresses were induced by: (i) segregation of nitrogen atoms at the grain boundaries and in the near-boundary regions, (ii) quenching stresses, and (iii) reducing fcc lattice volume with the test temperature decrease and incorporation of nitrogen atoms into fcc austenite lattice. Anisotropy of residual stresses was revealed. This was manifested in the localization of elastic deformations of the fcc lattice and, consequently, the stress localization in <100>-oriented grains; this is suggested to be the reason of brittle cleavage fracture.

“Non-equilibrium” grain boundaries in additively manufactured CoCrFeMnNi high-entropy alloy: Enhanced diffusion and strong segregation

Grain boundary diffusion in an additively manufactured equiatomic CoCrFeMnNi high-entropy alloy is systematically investigated at 500 K under the so-called C-type kinetic conditions when bulk diffusion is completely frozen. In the as-manufactured state, general (random) grain boundaries are found to be characterized by orders-of-magnitude enhanced diffusivities and a non-equilibrium segregation of (dominantly) Mn atoms. These features are explained in terms of a non-equilibrium state of grain boundaries after rapid solidification. The grain boundary diffusion rates are found to be almost independent on the scanning/building strategy used for the specimen’s manufacturing, despite pronounced microstructure differences. Grain boundary migration during diffusion annealing turned out to preserve the non-equilibrium state of the interfaces due to continuous consumption of the processing-induced defects by moving boundaries. Whereas the kinetic “non-equilibrium” state of the interfaces relaxes after annealing at 773 K, the non-equilibrium segregation is retained, being further accompanied by a nano-scale phase decomposition at the grain boundaries. The generality of the findings for additively manufactured materials is discussed.

Suzuki hardening and segregation in Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloys

A Co0.95Cr0.8Fe0.25Ni1.8Mo0.475 high-entropy alloy was heavily cold-rolled and subsequently aged at 773 K. Aging-induced Suzuki hardening was observed. High-angle annular dark-field scanning transmission electron microscopy imaging and the corresponding energy dispersive X-ray spectroscopy elemental distribution mapping suggested the enrichment of Co atoms at stacking faults (SFs), which pinned down partial dislocations and enhanced the resistance of the alloy to plastic deformation. Furthermore, the enrichment of Co and Mo atoms at SFs was observed in a low dislocated, strain-aged specimen. The significant difference in the SF density of the specimens led to difference in segregation. The local stacking fault energy (SFE) with the segregation of Co and Mo atoms was calculated as −30.6 mJ/m2 at the aging temperature. The negative SFE did not restrict the SF expansion, indirectly resulting in the formation of wide SFs.