See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/325922808 MD-based characterization of plastic deformation in Cu/Ag nanocomposites via dislocation extraction analysis: Effects of nanosized surface porosities and voids Article in Computational Materials Science · June 2018 DOI: 10.1016/j.commatsci.2018.06.018 CITATIONS READS 655 authors: Arash Kardani Abbas Montazeri Khaje Nasir Toosi University of Technology Khaje Nasir Toosi University of Technology 14 PUBLICATIONS 13 CITATIONS 43 PUBLICATIONS 268 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Biosensors View project Deformation Mechanisms of Nanocomposites View project All content following this page was uploaded by Arash Kardani on 23 November 2018 The user has requested enhancement of the downloaded file SEE PROFILE Computational Materials Science 152 (2018) 381–392 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci MD-based characterization of plastic deformation in Cu/Ag nanocomposites via dislocation extraction analysis: Effects of nanosized surface porosities and voids A Kardani, A Montazeri T ⁎ Faculty of Materials Science and Engineering, K.N Toosi University of Technology, Tehran, Iran A R T I C LE I N FO A B S T R A C T Keywords: Cu/Ag nanocomposite Porosity MD simulation Plastic deformation Dislocation Recently, copper-silver nanocomposites (NCs) have been utilized in medical instruments owing to their ability in destroying the bacterial cell wall, which prevents inflammation of the body tissue It has been revealed that introducing nanosized porosities in their structure can lead to an increase in the interfacial surface area with the tissue promoting the quality of treatment However, since yielding and occurrence of plastic deformation are not acceptable in medicine, analyzing the mechanical behavior of these NCs having nanopores is an important challenge Therefore, the focus of this study is to assess the role of porosities on the deformation mechanism of Cu/Ag NCs under uniaxial tensile loading conditions Accordingly, several perfect and defected samples are systematically studied through molecular dynamics simulation It is observed that plastic deformation of perfect sample occurs through twinning For samples with surface voids, this is happened as a result of perfect dislocations gliding Meanwhile, for volumetric porosities, the deteriorating effect is stopped passing the critical void content This is ascribed to the formation of many motionless dislocations such as stair-rod, Hirth and Frank as confirmed via the dislocation extraction analysis Consequently, it is demonstrated that presence of surface voids can be more destructive Introduction In recent years, the use of copper and silver as two antibacterial agents has been extensively interested in medical instruments According to studies conducted by researchers in the field of microbiology, copper and silver prevent inflammation of the body tissue by destroying the bacterial cell wall [1,2] On the other hand, metal matrix nanocomposites (MMNCs) have been extensively implemented due to their excellent mechanical properties and commercialization of their manufacturing processes [3] Accordingly, Cu-Ag nanocomposites (NCs) can be widely utilized in medical implants leading to intensification of the antimicrobial activity Meanwhile, owing to the increased interfacial area, the use of nanoporous implants can accelerate the process of ion exchange at the NC surface, which reduces time of the treatment period [4] Since release and absorption of drugs are increased by reducing the dimensions of the voids to the nanoscale, creation of nanopores can also be considered in terms of the drug delivery issue [5] However, the presence of voids is not always useful Various defects can occur in the structure of a NC over its formation process Surface voids with uncontrollable dimensions or the presence of them in the volume of materials can lead to substantial challenges in ⁎ Corresponding author E-mail address: a_montazeri@kntu.ac.ir (A Montazeri) https://doi.org/10.1016/j.commatsci.2018.06.018 Received 11 April 2018; Received in revised form June 2018; Accepted June 2018 0927-0256/ © 2018 Elsevier B.V All rights reserved their mechanical properties Whereas, the presence of defects in substances with medical applications, causes a high sensitivity Any yield and plastic deformation in the implants are not acceptable [6] Therefore, characterization and analysis of the mechanical behavior of implants are very important Although failure of NCs is a macroscopic phenomenon, this is resulted by atomic-scale structural variations of the sample Therefore, to understand the failure mechanism and to investigate the structural changes during deformation, it is necessary to use a tool that can detect the displacement of atoms in the crystalline structure of material A suitable option for simulating mechanical properties and investigating atomic structural changes of nanostructured materials is utilizing atomic-scale modeling techniques such as molecular dynamics (MD) simulation [7] The plasticity of crystal materials is usually controlled by dislocation slipping According to the Hall-Petch relation, grain boundary prevents the activity of dislocations, which in turn, results in the different mechanisms affecting the plastic deformation of nano-sized grains [8] By reduction of the grain size to below 10 nm, plastic deformation occurs through grain boundary sliding, while increasing this parameter enhances nucleation of partial dislocations and, as a result, the concentration of stacking faults and twins are increased Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri [9,10] According to the previous studies, temperature, strain rate and sample geometry are the most important factors affecting the nucleation and slipping of dislocations [11–13] It has been well established that by increasing the temperature, decreasing the strain rate and reducing the dimensions of the specimen under tension, the stress required for nucleation of dislocations is decreased Subsequently, the yield strength and strain of the samples are reduced According to the study of Zhang et al [14] on the effect of sample geometry, nucleation of dislocations in a cubic sample of single crystal copper occurred easier and faster than that of its cylindrical specimen This was attributed to the much greater stresses in the cube edges compared to the stress distributions in the cylinder wall Study of stacking faults is another subject that has been considered by the researchers [15–20] According to these investigations, stacking faults can change the deformation mechanism by creating twins Also, these areas can change the slip direction of the dislocations Consequently, formation of stacking faults can prevent dislocation movements by changing the crystal lattice structure from FCC to HCP [21] In addition to the external variables such as temperature, strain rate and sample geometry, internal factors have also been studied by researchers They include presence of crystalline defects and voids within the structure, which can influence the deformation mechanism For example, Xu et al [22] demonstrated that presence of voids within the single crystal copper can accelerate the propagation of dislocations from their surroundings In addition, Traiviratana et al [23] showed that most of the dislocations nucleated from the surrounding of a void are the partially dislocation type Based on the previous studies, stacking faults can also be extended from the voids into the material [24–26] Asari et al [27] showed that by increasing the dimensions of the void, internal dislocations need more stress to pass through it Therefore, these imperfections can also appear as an obstacle for slipping of dislocations In general, position and amount of defects can be considered as the most important factors affecting the mechanical properties of the samples So far, significant parts of studies have been focused on the developments of single phase nanostructures However, the NCs deformation has some remarkable differences compared to other materials Most of the researches in this field focus on the effect of the second phase on the matrix deformation For example, in the research of Sun et al [28], influence of the second phase geometry and its distribution was investigated It was found that the presence of particles with spherical shape and random distribution can lead to formation and expansion of the stacking faults According to the conducted studies, the interface of nanoparticles (NPs) with matrix has a significant contribution in determining the mechanical properties of NCs In addition, difference between the atomic radius of the second phase and matrix, as well as the increase of temperature, can weaken the strength of the interface [23,29,30] Mathiazhagan et al [31] showed that dislocations can be emitted from the interface The amount of reinforcement phase is also very effective in the nucleation and movement of dislocations [32] In this study, to investigate the deformation mechanism of Cu-Ag NCs, first, the perfect nanocomposite sample is examined through uniaxial tensile test using MD simulations Then, the tensile procedure is repeated under the same conditions in the presence of surface and volume voids at different amounts This is followed by comparing the results with those of the perfect sample Accordingly in the second part, details of MD simulation and sampling techniques are introduced Then, in the third section, the results of the tensile test are presented for all samples Also, the governing deformation mechanisms are systematically described in this part Finally, results of the research are summarized and the conclusions are made Fig Schematic illustration of the uniaxial tensile test in LAMMPS open-source LAMMPS code developed by Sandia National Laboratory [33] In order to determine the interatomic interactions between copper–copper, silver-silver and copper-silver atoms in the force and energy fields, Embedded Atom Model (EAM) function was implemented This potential function has been successfully used in many previous studies to simulate copper-silver systems [34–37] Simulations were carried out using the canonical ensemble (NVT) at a constant temperature of 300 K The Nose-Hoover thermostat was used to maintain the simulation temperature at 300 K The use of this thermostat causes fewer fluctuations than other methods [38] The initial velocities were determined based on the Maxwell-Boltzmann distribution at the desired temperature In addition, velocity-Verlet integration algorithm [39] with a time-step of fs was used for solving equations of motion through time Since the initial configuration of the samples may not be in the equilibrium state, the system was relaxed to 100,000 steps (i.e., 200 ps) before loading After relaxation, according to the quasi-statics method introduced by authors [40], the left side of the representative volume element (RVE) was kept fixed during imposing the uniaxial tensile loading This was followed by applying the incremental displacement on the other side to obtain the desired strain Fig presents schematic illustration of a typical RVE under uniaxial tension At each loading step, a displacement of 0.1 Å was applied along the sample length After each displacement, the system was relaxed for 30,000 steps Free boundary conditions (BC’s) were considered in direction of the axial stretch Also, periodic BC’s were imposed in the lateral directions As shown in Fig 2a, rectangular RVE with the dimensions of 10 × × nm in the x, y, and z directions, respectively, was constructed to resemble the copper-reinforced with Ag and the porous samples The basic MD cells were created in two steps First, the NC matrix consists of Cu atoms were created using the built-in tools in LAMMPS guided by the specific metal lattice parameters At a later stage, a central hole with the diameter of nm was included to accommodate the Ag atoms (Fig 2b and c) In all of the simulations, there was no overlapping between silver inclusion and copper atoms as depicted in Fig It is noted that all graphical illustrations have been made using OVITO software [41] The geometrical characteristics of silver nanoparticle and the RVE were chosen so that a sample containing 2.6% mass fraction of Ag was achieved in each case To model porous samples, some voids with the diameter of nm were distributed at the surface or in the volume of the copper matrix (See Fig 4) The volume fraction (VF) for a void with the mentioned diameter was about 0.32% Therefore, the VF for a surface void was half of this amount for the volumetric pore Accordingly, to produce different cases having various VF of these porosities, the corresponding amount of them could be introduced in the RVE as shown in Table The centro-symmetry (CS) parameter [42] was used to identify the partial dislocations, twinning and stacking faults For the specified atom i , this parameter is defined as follows: N /2 Details of MD simulation CS = ∑ i=1 In this research, MD simulation method was used for analyzing the effect of surface and volumetric porosities on the deformation mechanism of Cu-Ag NCs All simulations were conducted utilizing the ⎯→ ⎯ ⎯→ ⎯ | Ri + Ri + N /2 |2 (1) where N is the number of nearest neighbors of atom i equal to 12 for the ⎯→ ⎯ ⎯→ ⎯ FCC metals Also, Ri and Ri + N /2 are vectors from the central atom to a 382 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri Fig Construction steps of the perfect NC sample: (a) Building the rectangular copper matrix, (b) Creation of the central void, and (c) Accommodation of the Ag nanoparticle Fig Perfect Cu/Ag nanocomposite sample: (a) initial configuration, (b) after relaxation Fig Implantation of voids into samples with: (a) surface and (b) volumetric porosities 383 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri particular pair of nearest neighbors in the crystal lattice The CS value is for a system without any defect Meanwhile, positive values demonstrate presence of defects and free surfaces within the crystalline structure Dislocations were identified implementing a dislocation extraction analysis (DXA) developed by Stukowski and Albe [43] Additionally, the color classification based on a pattern matching algorithm was employed to distinguish between defected and perfect crystalline lattice This algorithm acts on the basis of common neighbor analysis (CNA) approach [44] Table Characteristics of nanoporous samples having different volume and surface voids content Voids content (%) Number of voids in the volume Number of voids at the surface 1.3 2.5 5.0 16 16 32 Results and discussion 3.1 Perfect NC under tension: Mechanical behavior and deformation mechanism In order to examine the mechanical properties and the underlying deformation mechanism, the perfect sample was first put under the uniaxial tension as illustrated in Fig Fig depicts the corresponding stress–strain curve of this sample from which, different mechanical properties including could be achieved As seen, the stress–strain relationship in the elastic region is almost linear This is followed by the saw-tooth fluctuations in the plastic deformation zone For the sake of simplicity, the reader may refer to [45] that documents in detail the physical mechanism governing this stress fluctuation Elastic modulus of this sample at the atomistic scale was obtained from slope of the stress–strain curve at low strains (i.e., < 3%) as 64.8 GPa Additionally, the yield strength defined as the stress at which the first plastic deformations occurred was found to be 11.1 GPa These values were in accordance with the results of similar studies presented in Table demonstrating the validation of the tensile testing procedure implemented in the present study To capture the mechanism affecting deformation of the NC sample after the yield point, we further proceeded to extract all types of dislocations within the crystalline structure of the model This was achieved through the DXA algorithm provided by the OVITO software [43] through the atomistic snapshots Based on the DXA results, when the material is in the elastic region, there are no dislocations observed in the structure Meanwhile, passing the yield point, the first partial dislocations are nucleated as illustrated in Fig Deng et al [47] showed that the first dislocations of a single crystal material are nucleated from the specimen surface This was not the case happened in the present sample in which, dislocations were nucleated from the NP/ matrix interface (See Fig 7) It arises because Ag and Cu atoms have 12% difference in terms of the lattice misfit As this parameter is larger than 5%, interface of the Ag/Cu sample seems to be semi-coherent rather than the coherent one [48] Accordingly, passing the yield point, the interfacial energy of the semi-coherent interface would be increased Therefore, this mismatch in the interface region resulted in the local stress concentration in this area, which in turn, reduced the stress required for dislocation nucleation As revealed in Fig 8, placement of Ag NP in the Cu matrix increased the local stress value up to 13 GPa Similarly, in the research conducted by Amigo et al [35], it was shown that presence of four Ag atoms in the Cu crystalline lattice enhances the local stress up to GPa in the interfacial zone According to Fig 7, a significant proportion of dislocations are Shockley partial ones These dislocations alter the atomic order in the Fig Various stages of the uniaxial tensile loading: (a) ε = 0, (b) ε = 0.2, (c) ε = 0.8, and (d) ε = 1.1 Fig Stress–strain curve of the perfect NC sample showing some stress fluctuations after the yield point Table Yield strength and Young's modulus of the perfect sample obtained in the present work in comparison with the ones given in the literature Study Case Amigo [35] Xu [22] Xie [12] Zhan [46] Present work Cu Cu Cu Cu Cu single single single single single crystal and Ag impurities crystal with volumetric voids crystal crystal crystal and Ag NP 384 Yield strength (GPa) Young's modulus (GPa) 10.7 6.5 11 11.1 61 86.6 64.5 63 64.8 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri Fig Dislocations emission in the perfect NC sample: (a) Nucleation of partial dislocations surrounding the Ag nanoparticle at ε = 0.15 (corresponds to the yield point in Fig 6), (b) Spreading of dislocations into the neighbor regions at ε = 0.2, and (c) Propagation of the plastic deformation at ε = 0.3 (configurations have been characterized by DXA) Fig von Mises stress distribution in the perfect nanocomposite sample Meanwhile, in the case of NCs as previously discussed, due to the large lattice misfit in the interfacial region, this area is prone to initiate the first stacking faults in the sample In other words, in the competition between the free surfaces and interface area, stacking faults are initially nucleated at the interface Accordingly, the twinning formation occurs in this case via the interface-dependent plasticity This issue has been illustrated in Fig 11 It should be pointed out that the formation of twins from the stacking faults can be a better way to reduce the energy imposed on the system (Tadmor et al [52]) This resulted in the reduction of stacking faults as a consequence of converting them to twins Formation of twins would also manifest itself in the zigzag behavior observed in the plastic deformation region of the sample as presented in Fig This special behavior was also reported by Zhan et al [46] for tension of the copper single crystal nanowire FCC lattice creating stacking faults having the HCP structure [49,50] To further investigate the issue, CNA results of the perfect NC sample at the various stages of the deformation have been presented in Fig They include the yield point, the deformations occurred at the selective strains of 0.2 and 0.4, and also at the start of the necking stage In the yield strain, stacking faults were formed around the silver nanoparticle Then, their percent in the sample were increased up to the strain of 0.4 After the necking stage, the size of these areas decreased significantly This was ascribed to the formation of twins from the stacking faults as demonstrated in Fig 10 As seen, stacking faults observed at 4058 ps corresponding to the strain of 0.51 are ready to creation of twins The corresponding snapshot at 4123 ps (in accordance with ε = 0.53) distinguishes the twins atoms from the other crystalline regions as presented in Fig 10c This phenomenon was also studied by Jiang et al [51] in the case of homogeneous single crystal copper It should be emphasized that there is a fundamental difference between formation of twins in the single crystal metal nanowires compared to that for the case of NC samples In the former, the deformation occurs through the surface-nucleated plasticity in which, stacking faults needed to form twins are initially created from the free surface of the samples 3.2 Porous nanocomposite under tensile: Role of volumetric porosities on the results The stress–strain curve of the volumetric porous samples has been illustrated in Fig 12 Results show that, as expected, the yield strength 385 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri Fig Evolution of partial dislocations and stacking fault areas (HCP atoms) in the perfect sample: (a) at the yield point, (b) ε = 0.2, (c) ε = 0.4, and (d) at the initial of necking stage Fig 10 Steps of twin formation from stacking faults: (a) HCP atoms in the stacking fault regions, (b) Creation of a twin nucleus by overlapping of stacking faults, and (c) Twined vs untwined regions Fig 11 Formation of a twinning fault in the nanocomposite sample through the interface-dependent plasticity mechanism (Stacking faults, as the main sources of twinning formation, are initially nucleated in the interface region) 386 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri containing 2.5% volumetric porosities To explore the mechanism governing this behavior, we further studied the dislocation length for the samples introduced in Table during their total deformation (See Fig 13) This analysis included various types of dislocations, namely of Shockley, stair-rod, Hirth, and Frank As seen, the Shockley dislocation of these two samples is close to each other Meanwhile, the other dislocations show a significant enhancement for the sample having 5% voids According to a research conducted by Martinez et al [53], passing the yield point and before significant deformation of the sample, two dislocations combine to each other creating a stair-rod dislocation as stated in Eq (2): 1 〈2 1〉 + 〈1 〉 → 〈1 0〉 6 (2) This type of dislocations is usually seen as tetrahedral in the crystalline structure It is noted that slipping of the stair-rod dislocations with the Burgers vector of 〈1 0〉 is not possible in the tetrahedron corners Accordingly, as shown in Fig 14, they must decompose into two Shockley dislocations during the imposed deformation requiring the energy [54] Therefore, it was concluded that high amount of stairrod dislocations is one of the most important reasons for preventing mechanical properties reduction in the sample with 5% volumetric voids compared to the less defected case To further proceed with the underlying mechanism, the formation of other dislocations such as Hirth and Frank were also investigated employing the DXA analysis The former takes place when two perfect dislocations with Burgers vectors perpendicular to each other slide across the slip planes (See Fig 15) This collision resembles the Hirth lock as stated in the following equation [25]: Fig 12 Stress–strain curve of NC samples having different VFs of volumetric porosities and Young's modulus were decreased with increasing the voids in the bulk material This was attributed to the local stress concentrations in the vicinity of porosities which in turn, led to easier nucleation of dislocations It is noted that the reduction was much more noticeable for samples having more than 2.5% VF of volumetric porosities To facilitate better comparisons, Table lists mechanical properties of Cu/ Ag nanocomposite samples in the presence of different amounts of these voids along with the results of the perfect specimen Surprisingly, it was found that the amount of mechanical properties reduction in the case with 5% VF of voids is very similar to the sample 1 1 〈1 0〉 + 〈1 0〉 → 〈2 1〉 + 〈2 1〉 + 〈1 0〉 2 6 (3) Since the Hirth sliding direction does not correspond to the close- Table Effects of the amount of volumetric porosities on the mechanical properties of the NC sample along with the results of the perfect case Case Yield strain Yield strength (GPa) Young's modulus (GPa) Perfect nanocomposite sample Sample with1.3%VF of volumetric porosities Sample with 2.5%VF of volumetric porosities Sample with 5%VF of volumetric porosities 0.15 0.13 0.12 0.12 11.1 9.2 8.8 8.4 64.8 61 59.6 52.1 Fig 13 DXA results representing the influence of void density on the dislocation length for various types of dislocations 387 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri Fig 14 Formation of the stair-rod dislocation at 4141 ps and its decomposition into two Shockley partial dislocations at the snapshot of 4299 ps (Configurations were characterized by DXA) Fig 15 Hirth lock and two Shockley partial dislocations obtained from the interaction of two perfect dislocations in the sample with 5%VF of volumetric porosities (The snapshot was taken at the time-step of 3246 ps corresponds to the strain of 0.4) packed slip direction, it is unable to slide and so, is called as the Hirth lock [55] Occurring this phenomenon in the crystalline structure can also limit the movement of other dislocations Accordingly, increasing the number of Hirth dislocations could be one of the inhibitor factors of further reduction in the mechanical properties of the sample containing 5% volumetric voids In addition to the discussed factors, Frank dislocations were also studied for the aforementioned sample As demonstrated in Fig 16, this type of dislocations was created in the form of a loop, which could be moved only through climbing based on the atomic diffusion [56,57] Since diffusion is a thermally activated process happened at the elevated temperatures, increasing the number of Frank dislocations at the ambient temperature could prevent further reduction in the mechanical characteristics of the sample having 5% volumetric voids 3.3 Surface porosities: An in-depth study towards the underlying mechanism Fig 16 Frank dislocation loop observed at the time-step of 4532 ps corresponds to ε = 0.57 in the sample with 5%VF of volumetric porosities After examining the effect of volumetric porosities, the next step involved analysis of mechanical behavior of the nanocomposite sample in the presence of surface porosities We focused on answering the important and vital question - which type of porosities are more destructive in terms of mechanical characteristics of Cu/Ag NCs? To answer this, the tensile procedure was repeated under the same conditions for samples having different amounts of surface voids The corresponding stress–strain curves have been shown in Fig 17 This was followed by comparing the results with those of the perfect sample as listed in Table Our results revealed that in comparison with volumetric porosities (See Table 3), surface defects play a critical role in weakening the mechanical properties of NC samples To further 388 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri result, advances the yield point Furthermore, Cui and Chen explored the yield response of the metallic films containing nanovoids via MD simulations [59] It was shown that yield stress of the samples decreases to much lower values ascribed to the initiation of dislocations nucleation shortly prior to the yield point These results are in line with the data reported in Fig 18 As seen, increasing the void content of the samples causes a significant increase in their dislocation density Consequently, their yield response deteriorates as reported in Tables and We would also like to draw the reader's attention to the significant difference between the results reported for these two cases According to Table 4, in the surface defected samples, yield strength and Young's modulus decreased monotonically with increasing the percentage of surface porosity Contrary to the case containing volumetric voids, here, creating 5% VF of surface voids significantly reduced the mechanical properties of the specimen compared to those of the case having 2.5% surface porosities As seen in Fig 17, for the latter case, passing the yield point A, there was observed a drop up to point B After B, the curve demonstrated an increase towards point C in which, the tensile stress reached the yield point value The same phenomenon was also observed more intensely for the sample with 5% surface voids To explore the underlying mechanism affecting this special behavior, let’s take a closer look at the dislocations creation and emission in the case with 5% VF of surface voids (See Fig 19) Based on Fig 19a, when the sample reached the yield point at the strain of 0.12 (i.e., point D), in addition to the NP/matrix interface, some types of dislocations were also nucleated from the surface voids leading to the declined trend in the curve This weakening effect was terminated at point E (corresponds to the strain of 0.13) in which, these new dislocations were stopped This was also ascribed to the formation of perfect dislocations as a result of partial dislocations combination happened around the voids as seen in Fig 19b Finally, further loadings led to the formation of new dislocations During this process, there was a rise in the curve up to point F To further analysis of the issue, the CNA results have been presented Fig 17 Stress–strain curve of NC samples having different VF of surface porosities examine the issue, the approach proposed by Begau et al [58] was implemented to quantify the local dislocation density for the introduced samples For a visual representation of the results, the reader may refer to Fig 18 that shows the variations of this parameter at the yield stage As revealed, dislocation density is higher for the cases with surface porosities This would manifest itself in the lower values reported for the yield strain and stress of these samples compared to the corresponding data for the samples having volumetric pores Regarding the effects of nanovoids within the metallic samples on their yield response, there are several publications in the literature For example, it has been demonstrated that dislocations begin to nucleate from the void surface prior to the maximum stress (i.e., the yield point) [22] As previously discussed, presence of voids in the sample leads to increase of the normal stresses around the porosities This in turn, promotes the dislocation nucleation from the voids surface and as a Table Effect of the amount of surface porosities on the mechanical properties of the NC sample along with the results of the perfect case Case Yield strain Yield strength (GPa) Young's modulus (GPa) Perfect nanocomposite sample Sample with 1.3% VF of surface porosities Sample with 2.5% VF of surface porosities Sample with 5% VF of surface porosities 0.15 0.13 0.12 0.11 11.1 9.3 8.6 6.1 64.8 61 56.5 44.2 Fig 18 Comparison of the dislocation density at the yield point for perfect and defected samples 389 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri Fig 19 Snapshots of the sample with 5% VF of surface voids: (a) creation of new dislocations around the surface voids, (b) stopping of partial dislocations with the formation of perfect ones, and (c) nucleation of new dislocations due to further imposed loadings (Configurations were characterized by DXA) Fig 20 Variations of the fraction of HCP atoms (ƞ) during deformation for the perfect sample Fig 22 Variations of the fraction of HCP atoms (ƞ) during deformation for the sample having 5% VF of volumetric porosities Fig 21 Variations of the fraction of HCP atoms (ƞ) during deformation for the sample having 5% VF of surface porosities contribution of perfect dislocations In other words, for this sample, presence of more sites for nucleation of partial dislocations increased the density of these dislocations in the materials structure Consequently, a significant part of these dislocations were combined together resulting in the formation of perfect dislocations Glide of these dislocations accelerated the failure of the specimen, which in turn, caused a significant reduction in the mechanical properties of the sample containing surface voids as discussed before To facilitate a better comparison, Fig 22 depicts the corresponding curves for the case of NC sample having 5% VF of volumetric pores As seen, regions with partial dislocations are more pronounced in this case compared to the sample with surface porosities Additionally, the reverse trend is observed for perfect dislocations as thoroughly inspected in 3.2 Conclusion for the perfect NC along with the data corresponds to the sample with 5% VF of surface voids (See Figs 20–22) In these curves, ƞ denotes the ratio of atoms in the HCP structure to the total atoms of the sample Additionally, figures inset shows the parameter ξ expressing contribution of perfect dislocations out of all ones in each loading step As seen in Fig 20, in the case of perfect sample, there were not observed stacking faults in the elastic region due to lack of partial dislocations Passing the yield point (i.e ε = 0.15 in accordance with Fig 6), the first stacking faults were formed through nucleation of partial dislocations Consequently, increasing the external tensile loading would lead to enhancement of the dislocations density, which in turn, promoted the beginning of necking in the sample These findings were in a good accordance with the results of Sun et al [28] Additionally, due to the negligible portion of parameter ξ in this case, it was concluded that partial dislocation was the main factor dominating plastic deformation of the perfect sample Meanwhile, for the surface defected sample as seen in Fig 21, the ratio of atoms in the stacking faults was significantly decreased compared to that of the perfect sample This was accompanied by the incremental trend found for ξ illustrating a rise in the In summary, due to the potential usage of Cu/Ag nanocomposites in medical implants, we documented in details the deformation mechanism of these NCs under tensile loading conditions via numericalbased MD simulations Moreover, it has been well-established that existence of nanosized voids within their structure promotes the quality of treatment through enhancing the interfacial surface area Accordingly, the main goal of the current study was to follow variations in the mechanical properties of these NCs in the presence of different amounts of surface and volumetric porosities The underlying mechanism governing this issue was also deeply addressed in this paper through microstructural characterization provided by means of the dislocation extraction analysis (DXA) The results showed that in the case of perfect nanocomposite sample, due to the presence of weak adhesion in the copper/silver interfacial area, the local stresses in this region were increased compared to other areas Consequently, the interface converted to a place for nucleation and release of partial dislocations In the following, stacking faults resulted from the partial dislocations were 390 Computational Materials Science 152 (2018) 381–392 A Kardani, A Montazeri converted to twinning as demonstrated by the zigzag behavior observed in the plastic deformation region In the next stage, by investigating stress–strain curve of the samples having different amounts and types of porosities, the pore content and distribution type showed its effect intensely Our simulations revealed that presence of volumetric voids causes a decrease in the mechanical characteristics of the perfect NC sample This was ascribed to the local stress concentrations in the vicinity of porosities which in turn, led to easier nucleation of dislocations Meanwhile, the weakening effect was stopped after the critical void content This was attributed to the formation of stair-rod, Hirth and Frank dislocations as confirmed by DXA results It was concluded that these dislocations were locked during plastic deformation of the samples with volumetric porosities which in turn, led to prevent the movement of other dislocations Additionally, our findings revealed that in samples containing surface voids, the reduction of mechanical properties was more significant than their counterparts having volumetric voids This was attributed to the activation of new sites for nucleation of dislocations at the sample surface in addition to the matrix/ nanoparticle interface It was 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