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Influences of grain size and temperature on the mechanical properties of nano-polycrystalline Ni-Co alloy were investigated with molecular dynamics simulations. It is found that the critical grain size of Hall-Petch relationship is 4.3 nm, at which the maximum flow stress of 4.83 GPa is obtained. In samples with the d > 4.3 nm, the average flow str...
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... the grains are formed, in which the blue line represents the grain boundary (GB). Then, an orientation is defined for each crystal grain, and the all crystal grains expand in three directions in space with deleting atoms outside the crystal grain. Finally, a polycrystalline model is obtained. The modeling and visualization process is shown in Fig. 1. A nickel unit cell was constructed and subsequently increased threefold to become a nickel supercell. After that, a part of nickel atoms (red) were randomly selected to be replaced with cobalt atoms (blue), followed by polycrystallization. The Ni-Co polycrystal models were obtained to simulate the tensile process. The potential ...
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... effect on the generation and disappearance of GB atoms under the conditions that the grain sizes are small. Secondly, for the purpose of describing the motion of GB atoms, the movement evolution diagram is collected in Fig. 9. In order to facilitate observation, some grain boundaries (red-labeled atoms) are selected as observation objects in Fig. 9a1-a4. One can clearly see the position and distribution information of the marked atoms during the stretching process. To compare with grain boundaries and intragranular atoms, the corresponding images are shown in Fig. 9b1-b4, in which the blue lines indicate the change in the region of movement of the atoms marked. It can be observed that ...
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... Fig. 9. In order to facilitate observation, some grain boundaries (red-labeled atoms) are selected as observation objects in Fig. 9a1-a4. One can clearly see the position and distribution information of the marked atoms during the stretching process. To compare with grain boundaries and intragranular atoms, the corresponding images are shown in Fig. 9b1-b4, in which the blue lines indicate the change in the region of movement of the atoms marked. It can be observed that the higher energy of its own makes the marked atoms more prone to motion. As the stretching proceeds, some atoms move inward and become the atoms inside the grains, as shown in Fig. 9a2 and b2. Individual atoms can also ...
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... the grain size exceeds the critical size of 4.3 nm, the deformation mechanism of the material during stretching is different from the previous one. Fig. 10 shows the schematic diagrams of the Ni-30%Co sample with grain size of 10.5 nm during the stretching process. The atomic configuration shown in the figure is mainly a face-centered cubic structure. Fig. 10a shows the initial configuration after relaxation, where no defects are found except grain boundary regions. When the strain comes ...
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... size exceeds the critical size of 4.3 nm, the deformation mechanism of the material during stretching is different from the previous one. Fig. 10 shows the schematic diagrams of the Ni-30%Co sample with grain size of 10.5 nm during the stretching process. The atomic configuration shown in the figure is mainly a face-centered cubic structure. Fig. 10a shows the initial configuration after relaxation, where no defects are found except grain boundary regions. When the strain comes to 0.05 in Fig. 10b, the stacking fault in GB of the initial position, indicated by yellow arrow in white dotted ellipse, can be observed and this is because some dislocations slip to the GB to be absorbed ...
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... shows the schematic diagrams of the Ni-30%Co sample with grain size of 10.5 nm during the stretching process. The atomic configuration shown in the figure is mainly a face-centered cubic structure. Fig. 10a shows the initial configuration after relaxation, where no defects are found except grain boundary regions. When the strain comes to 0.05 in Fig. 10b, the stacking fault in GB of the initial position, indicated by yellow arrow in white dotted ellipse, can be observed and this is because some dislocations slip to the GB to be absorbed by the GB or leave stacking faults, further leaving the steps when they slip to the surface. During the stretching process, the GB may break due to the ...
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... they slip to the surface. During the stretching process, the GB may break due to the movement of the grain boundary atoms and the rotation of the grain. Meanwhile, the dislocation near the GB, shown by white arrow in white dotted ellipse, may move on the main slip surface and be obstructed at the GB, resulting in the rise of stress. At = 0.07 in Fig. 10c, the stacking faults also appear at other different GBs, as demonstrated in the yellow curve, indicating that a large number of dislocations move to the GB position to be absorbed by it. This will lead to the concentration of stress at the GBs and increase the plastic deformation resistance, which is in accordance with the results in ...
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... position to be absorbed by it. This will lead to the concentration of stress at the GBs and increase the plastic deformation resistance, which is in accordance with the results in Fig. 4a. With the increase of strain, the most stacking faults in GBs disappear and the GB containing the stacking faults in the initial position also is invisible in Fig. 10d. Accompanying the further increase of strain in Fig. 10e, the emerging stacking faults can be observed elsewhere indicated by white arrows. At this time, dislocations will be generated inside the polycrystalline, and the secondary slip system also starts. It is worth noting that the ISF transforms the extrinsic stacking fault (ESF) ...
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... of stress at the GBs and increase the plastic deformation resistance, which is in accordance with the results in Fig. 4a. With the increase of strain, the most stacking faults in GBs disappear and the GB containing the stacking faults in the initial position also is invisible in Fig. 10d. Accompanying the further increase of strain in Fig. 10e, the emerging stacking faults can be observed elsewhere indicated by white arrows. At this time, dislocations will be generated inside the polycrystalline, and the secondary slip system also starts. It is worth noting that the ISF transforms the extrinsic stacking fault (ESF) displayed by white arrows, and this is because a partial ...
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... by white arrows, and this is because a partial dislocation is emitted on the adjacent parallel sliding surface in the intrinsic stacking fault (ISF). As the strain increases, some dislocations are also emitted on the adjacent parallel sliding plane in the ESF, which induces the ESF to form deformation twin tissue. When the strain is 0.13 in Fig. 10f, the deformation twin tissue propagates and is confined in the grain, and finally disappears under the shear stress. Fig. 10(e-f) shows the process of ESF first forming twins and then gradually disappearing, and the schematic diagram is illustrated in Fig. 10(g). From Fig. 10(g1), we can clearly see that the atoms on both sides of the ...
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... stacking fault (ISF). As the strain increases, some dislocations are also emitted on the adjacent parallel sliding plane in the ESF, which induces the ESF to form deformation twin tissue. When the strain is 0.13 in Fig. 10f, the deformation twin tissue propagates and is confined in the grain, and finally disappears under the shear stress. Fig. 10(e-f) shows the process of ESF first forming twins and then gradually disappearing, and the schematic diagram is illustrated in Fig. 10(g). From Fig. 10(g1), we can clearly see that the atoms on both sides of the twin boundary are symmetrical, so it can be determined as a twin structure. In Fig. 10(g2-g3) the twin structure gradually ...
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... ESF, which induces the ESF to form deformation twin tissue. When the strain is 0.13 in Fig. 10f, the deformation twin tissue propagates and is confined in the grain, and finally disappears under the shear stress. Fig. 10(e-f) shows the process of ESF first forming twins and then gradually disappearing, and the schematic diagram is illustrated in Fig. 10(g). From Fig. 10(g1), we can clearly see that the atoms on both sides of the twin boundary are symmetrical, so it can be determined as a twin structure. In Fig. 10(g2-g3) the twin structure gradually disappears as the stretching progresses. On the whole, in the plastic deformation process, the dislocation glide and growth of deformation twin play an ...
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... and finally disappears under the shear stress. Fig. 10(e-f) shows the process of ESF first forming twins and then gradually disappearing, and the schematic diagram is illustrated in Fig. 10(g). From Fig. 10(g1), we can clearly see that the atoms on both sides of the twin boundary are symmetrical, so it can be determined as a twin structure. In Fig. 10(g2-g3) the twin structure gradually disappears as the stretching progresses. On the whole, in the plastic deformation process, the dislocation glide and growth of deformation twin play an important role in this polycrystalline alloy, accompanied by the fracture of individual grain boundaries. Nevertheless, the small amount of dislocations ...
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... addition, there is great interest in the formation of stacking fault and deformation twins, and the evolution process is as follows. The structure state and evolution process of stacking fault are found and shown in Fig. 11. It can be observed that different faults are present when the defect-free atoms are removed, as indicated by white arrows in Fig. 11a. The stacking fault is initiated on the grain boundary and gradually extends to the grain interior, and finally intersects with the other grain boundary, as shown in Fig. 11b-e. The deformation twin is ...
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... there is great interest in the formation of stacking fault and deformation twins, and the evolution process is as follows. The structure state and evolution process of stacking fault are found and shown in Fig. 11. It can be observed that different faults are present when the defect-free atoms are removed, as indicated by white arrows in Fig. 11a. The stacking fault is initiated on the grain boundary and gradually extends to the grain interior, and finally intersects with the other grain boundary, as shown in Fig. 11b-e. The deformation twin is also initiated from the grain boundary under the action of external forces in Fig. 12a. This is because the plastic deformation with ...
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... stacking fault are found and shown in Fig. 11. It can be observed that different faults are present when the defect-free atoms are removed, as indicated by white arrows in Fig. 11a. The stacking fault is initiated on the grain boundary and gradually extends to the grain interior, and finally intersects with the other grain boundary, as shown in Fig. 11b-e. The deformation twin is also initiated from the grain boundary under the action of external forces in Fig. 12a. This is because the plastic deformation with uniform slip occurs at the same time between a series of adjacent surfaces in some lattices, resulting in a twinning relationship between the slip part and the non-slip part. As ...
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... atoms are removed, as indicated by white arrows in Fig. 11a. The stacking fault is initiated on the grain boundary and gradually extends to the grain interior, and finally intersects with the other grain boundary, as shown in Fig. 11b-e. The deformation twin is also initiated from the grain boundary under the action of external forces in Fig. 12a. This is because the plastic deformation with uniform slip occurs at the same time between a series of adjacent surfaces in some lattices, resulting in a twinning relationship between the slip part and the non-slip part. As the deformation increases, the part of fault gradually grows, as shown in Fig. 12c. The ISF interacts with the ...
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... under the action of external forces in Fig. 12a. This is because the plastic deformation with uniform slip occurs at the same time between a series of adjacent surfaces in some lattices, resulting in a twinning relationship between the slip part and the non-slip part. As the deformation increases, the part of fault gradually grows, as shown in Fig. 12c. The ISF interacts with the emerging partial dislocation to form the ESF in Fig. 12c and this further develops to deformation twins in Fig. ...
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... with uniform slip occurs at the same time between a series of adjacent surfaces in some lattices, resulting in a twinning relationship between the slip part and the non-slip part. As the deformation increases, the part of fault gradually grows, as shown in Fig. 12c. The ISF interacts with the emerging partial dislocation to form the ESF in Fig. 12c and this further develops to deformation twins in Fig. ...
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... series of adjacent surfaces in some lattices, resulting in a twinning relationship between the slip part and the non-slip part. As the deformation increases, the part of fault gradually grows, as shown in Fig. 12c. The ISF interacts with the emerging partial dislocation to form the ESF in Fig. 12c and this further develops to deformation twins in Fig. ...
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... explore the influence of temperature on the deformation mechanism of nano-polycrystalline NiCo alloys under uniaxial tension, the mechanical deformation behavior of samples with grain size of 7.6 nm in the temperature range of 300-1100 K was carried out in Fig. 13, as well as the percentage of atoms in the grain boundary and grain interior. From Fig. 13a, it can be seen that the polycrystalline alloy has a great dependence on temperature. With the increase of temperature, the maximum tensile stress gradually decreases, but the maximum tensile strain increases little by little. The reason is that ...
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... explore the influence of temperature on the deformation mechanism of nano-polycrystalline NiCo alloys under uniaxial tension, the mechanical deformation behavior of samples with grain size of 7.6 nm in the temperature range of 300-1100 K was carried out in Fig. 13, as well as the percentage of atoms in the grain boundary and grain interior. From Fig. 13a, it can be seen that the polycrystalline alloy has a great dependence on temperature. With the increase of temperature, the maximum tensile stress gradually decreases, but the maximum tensile strain increases little by little. The reason is that the thermal motion of atoms intensified when the temperature increases, they may move from ...
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... motion of atoms intensified when the temperature increases, they may move from one equilibrium position to another, and a new slip system appears, which is conducive to the increase in plasticity. At the same time, the fraction of GB atoms increases by degrees and the fraction of grain interior (GI) atoms decreases gradually, as indicated in Fig. 13b. This indicates that after the increase of temperature, the activity of atoms may increase, breaking through the interatomic force and spreading to the surrounding area, which results in the increase of disordered atoms at the GBs and the decrease of atom at the GI. In order to illustrate this problem, a single grain and its ...
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... the interatomic force and spreading to the surrounding area, which results in the increase of disordered atoms at the GBs and the decrease of atom at the GI. In order to illustrate this problem, a single grain and its surrounding region are intercepted for investigation. The initial configuration of atoms at different temperatures is shown in Fig. 14. The grey and green parts represent the GBs and the atoms within the crystal, respectively. Fig. 14a shows the initial configuration after relaxation at 300 K, in which the GB and GI of the crystal are clearly divided, and the four sides of the grain at the middle position are shown by the orange arrows. With the increase of ...
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... atoms at the GBs and the decrease of atom at the GI. In order to illustrate this problem, a single grain and its surrounding region are intercepted for investigation. The initial configuration of atoms at different temperatures is shown in Fig. 14. The grey and green parts represent the GBs and the atoms within the crystal, respectively. Fig. 14a shows the initial configuration after relaxation at 300 K, in which the GB and GI of the crystal are clearly divided, and the four sides of the grain at the middle position are shown by the orange arrows. With the increase of temperature, there are more disordered atoms in the GB by brown arrows and new emerging gray atoms also appear ...
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... after relaxation at 300 K, in which the GB and GI of the crystal are clearly divided, and the four sides of the grain at the middle position are shown by the orange arrows. With the increase of temperature, there are more disordered atoms in the GB by brown arrows and new emerging gray atoms also appear in the GI in the initial configuration in Fig. 14b. The effect will become more apparent with the loading temperature increasing, as indicated by blue, pink and red arrows, respectively, in Fig. 14c-e. This leads to the increase of the atomic fraction of GB and the decrease of the atomic fraction of GI, which is accordance with the results in Fig. 13 and the previous report [28]. Fig. ...
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... by the orange arrows. With the increase of temperature, there are more disordered atoms in the GB by brown arrows and new emerging gray atoms also appear in the GI in the initial configuration in Fig. 14b. The effect will become more apparent with the loading temperature increasing, as indicated by blue, pink and red arrows, respectively, in Fig. 14c-e. This leads to the increase of the atomic fraction of GB and the decrease of the atomic fraction of GI, which is accordance with the results in Fig. 13 and the previous report [28]. Fig. 15 illustrates the evolution of atomic structure during stretching at two different temperatures of 300 K and 1100 K. The stress level in GB is much ...
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... in the GI in the initial configuration in Fig. 14b. The effect will become more apparent with the loading temperature increasing, as indicated by blue, pink and red arrows, respectively, in Fig. 14c-e. This leads to the increase of the atomic fraction of GB and the decrease of the atomic fraction of GI, which is accordance with the results in Fig. 13 and the previous report [28]. Fig. 15 illustrates the evolution of atomic structure during stretching at two different temperatures of 300 K and 1100 K. The stress level in GB is much higher than that in the GI from Fig. 15a2-f2. At a strain of 0.06, there are stacking faults in both conditions in Fig. 15b1 and e1, however, compared ...
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... in Fig. 14b. The effect will become more apparent with the loading temperature increasing, as indicated by blue, pink and red arrows, respectively, in Fig. 14c-e. This leads to the increase of the atomic fraction of GB and the decrease of the atomic fraction of GI, which is accordance with the results in Fig. 13 and the previous report [28]. Fig. 15 illustrates the evolution of atomic structure during stretching at two different temperatures of 300 K and 1100 K. The stress level in GB is much higher than that in the GI from Fig. 15a2-f2. At a strain of 0.06, there are stacking faults in both conditions in Fig. 15b1 and e1, however, compared with regular stacking fault (yellow ...
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... increase of the atomic fraction of GB and the decrease of the atomic fraction of GI, which is accordance with the results in Fig. 13 and the previous report [28]. Fig. 15 illustrates the evolution of atomic structure during stretching at two different temperatures of 300 K and 1100 K. The stress level in GB is much higher than that in the GI from Fig. 15a2-f2. At a strain of 0.06, there are stacking faults in both conditions in Fig. 15b1 and e1, however, compared with regular stacking fault (yellow arrows) at low temperature, the irregular stacking faults (purple arrows) occur at high temperature. In addition, under high temperature the intense atomic movement increases the interatomic ...
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... which is accordance with the results in Fig. 13 and the previous report [28]. Fig. 15 illustrates the evolution of atomic structure during stretching at two different temperatures of 300 K and 1100 K. The stress level in GB is much higher than that in the GI from Fig. 15a2-f2. At a strain of 0.06, there are stacking faults in both conditions in Fig. 15b1 and e1, however, compared with regular stacking fault (yellow arrows) at low temperature, the irregular stacking faults (purple arrows) occur at high temperature. In addition, under high temperature the intense atomic movement increases the interatomic distance, leading to a decrease in the interatomic bonding force, which eventually results ...
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... the irregular stacking faults (purple arrows) occur at high temperature. In addition, under high temperature the intense atomic movement increases the interatomic distance, leading to a decrease in the interatomic bonding force, which eventually results to the lower stress. The results are also good accordance with the previous calculation in Fig. 13. When the strain comes to 0.12, a comparison of the atomic configurations in Fig. 15c1 and f1 shows that the disorder degree of GB atoms increases accompanying GB melting (brown square frame) and at the same time the stacking faults disappear at high temperature. Generally, a large part of the microstructure is GBs, which are connected ...
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... under high temperature the intense atomic movement increases the interatomic distance, leading to a decrease in the interatomic bonding force, which eventually results to the lower stress. The results are also good accordance with the previous calculation in Fig. 13. When the strain comes to 0.12, a comparison of the atomic configurations in Fig. 15c1 and f1 shows that the disorder degree of GB atoms increases accompanying GB melting (brown square frame) and at the same time the stacking faults disappear at high temperature. Generally, a large part of the microstructure is GBs, which are connected with grains in different arrangement directions and transition from one arrangement mode to ...
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... further clarify the deformation mechanism, the diagram of dislocation density at different temperatures is exhibited in Fig. 16. Dislocation refers to the total length of dislocations contained in a unit volume crystal, or the number of dislocation lines crossing the unit cross-sectional area. The continuous propagation of dislocations will lead to the stacking of dislocations, thus increasing the strength of the material, while the density of dislocation is ...
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... addition, the surface characteristics of alloy samples are also different at different temperatures after tensile fracture and dimples of different sizes are formed on the sections, as shown in Fig. 17. The higher the temperature is, the greater the dimple on the section is. As can be seen from the picture in Fig. 17a, at a low temperature of 300 K, the number of small dimples reaches eight. With the temperature increasing, the number of dimples on the section gradually decreases, and a large dimple is formed at 1100 K. Considering ...
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... addition, the surface characteristics of alloy samples are also different at different temperatures after tensile fracture and dimples of different sizes are formed on the sections, as shown in Fig. 17. The higher the temperature is, the greater the dimple on the section is. As can be seen from the picture in Fig. 17a, at a low temperature of 300 K, the number of small dimples reaches eight. With the temperature increasing, the number of dimples on the section gradually decreases, and a large dimple is formed at 1100 K. Considering that the intrinsic properties of materials used at different temperatures are consistent, the grain size is uniform to ...
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... In addition, as the degree of atomic disorder increases, the grain boundary melts, and the density of partial dislocations decreases. Samples at high temperatures have lower resistance to grain growth than those at low temperatures, causing the particle size to grow during high calcination temperature [34]. A previous study by Zhang et al. [6] suggested that adding metal oxide could enhance SSA because of its smaller particle size than LSCF. ...
... Relative reports indicate that grain sizes typically make a considerable impact on dislocation nucleation and propagation [108]. Fig. 18a shows the density variation of the SPDs in Fe 50 Mn 30 Co 10 Cr 10 with various grain sizes. ...
Medium/high-entropy alloys (MEAs/HEAs) have become one of the primary research highlights in the material field due to their superior microstructure and mechanical properties. By means of molecular dynamic (MD) simulation, the tensile behavior of polycrystalline Fe80-xMnxCo10Cr10 (x=20, 30, 40, 50) MEAs was investigated under uniaxial stretching. The effects of various strain rates (1 × 10⁸ s⁻¹, 1 × 10⁹ s⁻¹, 1 × 10¹⁰ s⁻¹) and deformation temperatures (77 K, 300 K, 600 K, 800 K) on tensile properties and deformation mechanisms of Fe80-xMnxCo10Cr10 (x=30, 40) MEAs were explored, and the interaction of dislocation defects with stacking fault including intrinsic and extrinsic stacking fault (ISF/ESF) as well as the evolution of dislocation and cluster defects were further discussed. Moreover, the relationship between grain size and flow stress of polycrystalline Fe50Mn30Co10Cr10 MEAs was discussed. Results indicated that Fe40Mn40Co10Cr10 and Fe50Mn30Co10Cr10 alloys exhibited higher yield stresses and Young's moduli. The yield stresses, Young's moduli and average flow stresses of Fe40Mn40Co10Cr10 and Fe50Mn30Co10Cr10 MEAs increased with an increase in strain rates from 1 × 10⁸ to 1 × 10¹⁰ s⁻¹. Besides, the deformation temperature produced a considerable influence on its mechanical properties. The phase transformation, grain boundary glide, dislocation slip and twinning formation together accounted for the plastic deformation behavior of Fe80-xMnxCo10Cr10 MEAs under uniaxial stretching on an atomic scale.
... The yield stress and Young's modulus of the Cu/Ni, Al/Ni, Ni/Ti, and Cu/Co alloys are lower than the Ni-Co alloy. In general, the yield strength and Young's modulus in this study are similar to the other reports [28], [65]. Lu et al. [28] reported slightly lower values as they investigated the single-crystal Ni-Co alloy. ...
... Lu et al. [28] reported slightly lower values as they investigated the single-crystal Ni-Co alloy. In comparison, the result in Ref. [65] presented the polycrystalline Ni-Co alloy. ...
This report investigates the deformation behaviors and mechanical properties of Ni/Co multilayers under tensile tests by using molecular dynamics (MD) simulations. The effects of orientations, layer thicknesses, temperature, and strain rates, on the tensile strength, phase transformation, dislocation density, stress-strain relationships, and Young's modulus of the Ni/Co multilayers are examined. The results show that reducing layer thickness leads to an increase in dislocation density. The dislocation density of the vertical sample is higher than the horizontal sample. The FCC structure of the Ni layers is strongly transformed to HCP, BCC, and amorphous structures during uniaxial tension. On the contrary, only a minor percentage of the HCP structure of Co layers is changed to FCC or amorphous structure. For both the horizontal layers and vertical layers specimens, stacking faults appear from the interface and then expand towards the Ni layers. Notably, the yield strength of the horizontal sample is higher than the vertical one. When increasing the strain rate from 10⁸ to 1010 s⁻¹, the yield stress of the vertical varies slightly from 9.1 GPa to 10.6 GPa. While in the horizontal direction, the yield strength oscillates around 9.1-12.6 GPa. Increasing the strain rate results in a growth in the yield strength. Moreover, increasing the temperature leads to the reduction of Young's modulus and tensile strength.
