Atomistic Deformation Mechanisms Research Articles

Revealing the atomistic deformation mechanisms of face-centered cubic nanocrystalline metals with atomic-scale mechanical microscopy: A review

Nanocrystalline metals have many functional and structural applications due to their excellent mechanical properties compared to their coarse-grained counterparts. The atomic-scale understanding of the deformation mechanisms of nanocrystalline metals is important for designing new materials, novel structures and applications. The review presents recent developments in the methods and techniques for in situ deformation mechanism investigations on face-centered-cubic nanocrystalline metals. In the first part, we will briefly introduce some important techniques that have been used for investigating the deformation behaviors of nanomaterials. Then, the size effects and the plasticity behaviors in nanocrystalline metals are discussed as a basis for comparison with the plasticity in bulk materials. In the last part, we show the atomic-scale and time-resolved dynamic deformation processes of nanocrystalline metals using our in-lab developed deformation device.

Atomistic deformation mechanisms of amorphous/polycrystalline metallic nanolaminates

A series of multilayer amorphous Cu50Zr50/Cu nanolaminates with consideration of grain boundary characteristics in the Cu layers were constructed and compressed to investigate the atomistic mechanisms of yielding and plastic deformation behavior using large-scale atomistic simulations. The results revealed that yielding occurs initially in the Cu layers through lattice dislocations, while plastic deformation in the amorphous layers is induced by the transfer of dislocation plasticity from the Cu layers, mainly at the intersections of the crystalline-amorphous interfaces and grain boundaries. Similar to the roles of defects-like secondary phases, the Cu layers serve as sites for heterogeneous nucleation of embryonic shear bands, as well as barriers to their propagation into mature ones. The coupled interplay between the crystal plasticity and the glassy plasticity in the nanolaminates promotes a more homogeneous redistribution of plastic deformation, providing a kind of hardening mechanism. In addition, our simulations also demonstrate a transition of the deformation mode from localized to homogeneous-like deformation by tailoring the relative volume fraction of the Cu layers. The findings provide more detailed atomistic information for understanding the underlying deformation mechanisms that are difficult to obtain by post-mortem observations and are useful for optimizing the structure of amorphous/crystalline metallic nanolaminates.

In situ atomistic deformation mechanisms of twin-structured nanocrystal Pt

The atomic-scale in situ tensile tests of twin-structured Pt (d = ~25 nm) nanocrystals were conducted using a home-made device in a high-resolution transmission electron microscope. We showed that the twin-structured Pt nanocrystals exhibit as large as 21.9% homogeneous elongation before crack emergence. The atomic-scale and time-resolved in situ investigation indicated that, full and partial dislocations that intersect with coherent twin boundaries (TBs), as well as TB migration resulting from partial dislocations glide adjacent to the TBs, and the rapid migration of incoherent TBs all contribute the large homogeneous elongation capability of the twin-structured nanocrystals.

Fracture of β-SiC bulk with a void of different shapes under different loading modes

Molecular dynamics simulations using the Tersoff potential are performed to explore the mechanical properties of β-SiC bulk with a void of elliptic and circular shapes under different loading conditions. For a given void volume fraction, the geometric properties of elliptic and circular voids are differentiated by their aspect ratios. We find that, the critical stress related to the material failure under simple shear is the largest for the circular void whereas smaller for the elliptic void. Moreover, elliptic void rotates during the shear. Further analysis by comparing the critical stresses under uniaxial tension and deformation reveals that the presence of tensile stress in the direction perpendicular to the stretching direction has little impact on the final fracture stress. Finally, it is shown that the fracture-stress surfaces of voided β-SiC bulk for a specified void aspect ratio is in the rectangular shape. For a given void volume fraction, vertexes of fracture-stress surfaces for different void aspect ratios constitute a linear relationship between stresses in two directions. This study may help us understand and reveal the underlying atomistic deformation mechanisms of void geometry-dependent mechanical properties of β-SiC bulk.

Size effect and atomistic deformation mechanisms of hierarchically nanotwinned fcc metals under nanoindentation

Molecular dynamics simulations have been performed to investigate the atomistic deformation mechanisms of hierarchically nanotwinned Cu under nanoindentation. When the grain size (d) and the spacing of primary twins (lambda (1)) are fixed, the hardness is observed to increase with decreasing spacing of secondary twins (lambda (2)) until a critical lambda (2), and then decrease with further decreasing lambda (2). The size effect of lambda (2) on the plastic area beneath the indenter is observed to be exactly opposite to the trend of the size effect on the hardness. There exist two plastic zones beneath the indenter: the severe plastic zone and the moderate plastic zone. In the severe plastic zone, high density of dislocation networks are observed and the deformation mechanisms are independent of lambda (2). The deformation mechanisms in the moderate plastic zone are highly dependent on the lambda (2), which is the origin of the size effect on the hardness. Below the critical lambda (2), the deformation mechanisms are dominated by the softening mechanisms with decreasing lambda (2): (i) detwinning of secondary twins and (ii) nucleation and propagation of partial dislocations with a small angle to the boundaries of secondary twins. Above the critical lambda (2), the deformation mechanisms are dominated by the strengthening mechanisms with decreasing lambda (2): partial dislocations are blocked by the boundaries of primary twins or secondary twins.

Atomistic tensile deformation mechanisms of Fe with gradient nano-grained structure

Large-scale molecular dynamics (MD) simulations have been performed to investigate the tensile properties and the related atomistic deformation mechanisms of the gradient nano-grained (GNG) structure of bcc Fe (gradient grains with d from 25 nm to 105 nm), and comparisons were made with the uniform nano-grained (NG) structure of bcc Fe (grains with d = 25 nm). The grain size gradient in the nano-scale converts the applied uniaxial stress to multi-axial stresses and promotes the dislocation behaviors in the GNG structure, which results in extra hardening and flow strength. Thus, the GNG structure shows slightly higher flow stress at the early plastic deformation stage when compared to the uniform NG structure (even with smaller grain size). In the GNG structure, the dominant deformation mechanisms are closely related to the grain sizes. For grains with d = 25 nm, the deformation mechanisms are dominated by GB migration, grain rotation and grain coalescence although a few dislocations are observed. For grains with d = 54 nm, dislocation nucleation, propagation and formation of dislocation wall near GBs are observed. Moreover, formation of dislocation wall and dislocation pile-up near GBs are observed for grains with d = 105 nm, which is the first observation by MD simulations to our best knowledge. The strain compatibility among different layers with various grain sizes in the GNG structure should promote the dislocation behaviors and the flow stress of the whole structure, and the present results should provide insights to design the microstructures for developing strong-and-ductile metals.

Mesoscale mechanics of twisting carbon nanotube yarns.

Fabricating continuous macroscopic carbon nanotube (CNT) yarns with mechanical properties close to individual CNTs remains a major challenge. Spinning CNT fibers and ribbons for enhancing the weak interactions between the nanotubes is a simple and efficient method for fabricating high-strength and tough continuous yarns. Here we investigate the mesoscale mechanics of twisting CNT yarns using full atomistic and coarse grained molecular dynamics simulations, considering concurrent mechanisms at multiple length-scales. To investigate the mechanical response of such a complex structure without losing insights into the molecular mechanism, we applied a multiscale strategy. The full atomistic results are used for training a coarse grained model for studying larger systems consisting of several CNTs. The mesoscopic model parameters are updated as a function of the twist angle, based on the full atomistic results, in order to incorporate the atomistic scale deformation mechanisms in larger scale simulations. By bridging across two length scales, our model is capable of accurately predicting the mechanical behavior of twisted yarns while the atomistic level deformations in individual nanotubes are integrated into the model by updating the parameters. Our results focused on studying a bundle of close packed nanotubes provide novel mechanistic insights into the spinning of CNTs. Our simulations reveal how twisting a bundle of CNTs improves the shear interaction between the nanotubes up to a certain level due to increasing the interaction surface. Furthermore, twisting the bundle weakens the intertube interactions due to excessive deformation in the cross sections of individual CNTs in the bundle.

Scaling laws and deformation mechanisms of nanoporous copper under adiabatic uniaxial strain compression

A series of large-scale molecular dynamics simulations were conducted to investigate the scaling laws and the related atomistic deformation mechanisms of Cu monocrystal samples containing randomly placed nanovoids under adiabatic uniaxial strain compression. At onset of yielding, plastic deformation is accommodated by dislocations emitted from void surfaces as shear loops. The collapse of voids are observed by continuous emissions of dislocations from void surfaces and their interactions with further plastic deformation. The simulation results also suggest that the effect modulus, the yield stress and the energy aborption density of samples under uniaxial strain are linearly proportional to the relative density ρ. Moreover, the yield stress, the average flow stress and the energy aborption density of samples with the same relative density show a strong dependence on the void diameter d, expressed by exponential relations with decay coefficients much higher than -1/2. The corresponding atomistic mechanisms for scaling laws of the relative density and the void diameter were also presented. The present results should provide insights for understanding deformation mechanisms of nanoporous metals under extreme conditions.

Mechanical anisotropy and origin of shear plastic deformation of tetragonal B4C4

Based on first-principle calculations, we fundamentally investigate the anisotropic mechanical properties and atomistic deformation mechanism of the recent proposed ultra-incompressible tetragonal t-B4C4. The obtained results reveal that the t-B4C4 exhibits a pronounced elastic anisotropy according to the distributions of elastic moduli along the principal crystal orientations. It is found that the ideal shear strength in the direction of dominates the shear plastic deformation for t-B4C4. The lattice instability of the t-B4C4 can be revealed by the bond lengths and electron localization function as functions of shear strain. The results show that the failure modes of t-B4C4 can be attributed to the breaking of C-B bonds in the C2B8 double quadrangular pyramid.

Tensile deformation mechanisms of the hierarchical structure consisting of both twin-free grains and nanotwinned grains

A series of large-scale molecular dynamics simulations have been performed to investigate the tensile properties and atomistic deformation mechanisms for the nanostructured Cu with three typical microstructures: the hierarchical structure consisting of both twin-free grains (d = 70 nm) and grains with bundles of smaller nanotwins (d = 70 nm, λ = 10 nm), the fully nanograined structure and the fully nanotwinned structure. The average flow stress of the hierarchically structure is found to be higher than that calculated by rule of mixture. As compared with that of fully nanograined structure, the strength for the twin-free grains in the hierarchical structure is promoted and gives extra hardening due to the increased dislocation density and dislocation behaviours. It is also found that the nanotwin bundles are more deformable than the twin-free grains in the hierarchical structure according to the deviatoric strain invariant contour. This indicates that the fully nanograined structure cannot only be strengthened to a higher level, but also obtain better ductility by embedded with stronger bundles of smaller nanotwins. Thus, a superior strength–ductility synergy could be obtained in this kind of hierarchical structures, and this novel strategy has also been implemented in bulk austenitic steels or copper by recent experiments.

Atomistic deformation mechanisms in twinned copper nanospheres.

In the present study, we perform molecular dynamic simulations to investigate the compression response and atomistic deformation mechanisms of twinned nanospheres. The relationship between load and compression depth is calculated for various twin spacing and loading directions. Then, the overall elastic properties and the underlying plastic deformation mechanisms are illuminated. Twin boundaries (TBs) act as obstacles to dislocation motion and lead to strengthening. As the loading direction varies, the plastic deformation transfers from dislocations intersecting with TBs, slipping parallel to TBs, and then to being restrained by TBs. The strengthening of TBs depends strongly on the twin spacing.

Twin boundary spacing effects on shock response and spall behaviors of hierarchically nanotwinned fcc metals

Atomistic deformation mechanisms of hierarchically nano-twinned (NT) Ag under shock conditions have been investigated using a series of large-scale molecular dynamics simulations. For the same grain size d and the same spacing of primary twins λ1, the average flow stress behind the shock front in hierarchically NT Ag first increases with decreasing spacing of secondary twins λ2, achieving a maximum at a critical λ2, and then drops as λ2 decreases further. Above the critical λ2, the deformation mechanisms are dominated by three type strengthening mechanisms: (a) partial dislocations emitted from grain boundaries (GBs) travel across other boundaries; (b) partial dislocations emitted from twin boundaries (TBs) travel across other TBs; (c) formation of tertiary twins. Below the critical λ2, the deformation mechanism are dominated by two softening mechanisms: (a) detwinning of secondary twins; (b) formation of new grains by cross slip of partial dislocations. Moreover, the twin-free nanocrystalline (NC) Ag is found to have lower average flow stress behind the shock front than those of all hierarchically NT Ag samples except the one with the smallest λ2 of 0.71 nm. No apparent correlation between the spall strength and λ2 is observed in hierarchically NT Ag, since voids always nucleate at both GBs and boundaries of the primary twins. However, twin-free NC Ag is found to have higher spall strength than hierarchically NT Ag. Voids can only nucleate from GBs for twin-free NC Ag, therefore, twin-free NC Ag has less nucleation sources along the shock direction when compared to hierarchically NT Ag, which requiring higher tensile stress to create spallation. These findings should contribute to the understandings of deformation mechanisms of hierarchically NT fcc metals under extreme deformation conditions.

Mechanical properties and atomistic deformation mechanism of spinel-type BeP2N4

The anisotropic mechanical properties and atomistic deformation mechanism of incompressible γ-BeP2N4 were comprehensively investigated by first-principles calculations. According to the dependence of the Young’s modulus on different directions in crystal, the γ-BeP2N4 exhibits a well-pronounced anisotropy which may impose certain limitations and restrictions on its applications. The ideal strength calculations demonstrated that γ-BeP2N4 shows substantially lower ideal shear strength than superhard c-BN and diamond, suggesting that it cannot be intrinsically superhard as claimed in the previous studies. Furthermore, the origin of the lattice instability of γ-BeP2N4 under large shear strain that occurs at the atomic level during plastic deformation can be attributed to the breaking of P–N bonds in PN6 octahedrons.

Size effects of primary/secondary twins on the atomistic deformation mechanisms in hierarchically nanotwinned metals

A series of large-scale molecular dynamics simulations have been performed to investigate the tensile properties of nanotwinned (NT) copper with hierarchically twinned structures (HTS). For the same grain size d and the same spacing of primary twins λ1, the average flow stress first increases as the spacing of secondary twins λ2 decreases, reaching a maximum at a critical λ2, and then decreases as λ2 becomes even smaller. The smaller the spacing for λ1, the smaller the critical spacing for λ2. There exists a transition in dominating deformation mechanisms, occurring at a critical spacing of λ2 for which strength is maximized. Above the critical spacing of λ2, the deformation mechanisms are dominated by the two Hall-Petch type strengthening mechanisms: (a) partial dislocations emitted from grain boundaries (GBs) travel across other GBs and twin boundaries (TBs); (b) partial dislocations emitted from TBs travel across other TBs. Below the critical spacing of λ2, the deformation mechanism is dominated by the two softening mechanisms: (a) Partial dislocations emitted from boundaries of the primary twins travel parallel to the TBs of the secondary twins, leading to detwinning of the secondary twins; (b) Boundaries of the primary twins shift entirely, leading to thickening in one part of primary twins and thinning in the other part of primary twins. The present results should provide insights to design the microstructures for reinforcing the mechanical properties in the NT metals with HTS.

Relationship between layered crystal structure and mechanical properties of M3AlN (M = Zr and Hf): A first-principles investigation

Bonding character, elastic mechanical parameters, ideal strengths, and atomistic shear deformation mechanisms of M3AlN (M = Zr and Hf) were studied by first-principles method. M3AlN exhibits layered chemical bonding character due to the alternately stacking of relatively soft Al–M and strong N–M covalent bonds. The second-order elastic constants and mechanical parameters of M3AlN were reported for the first time. The stress–strain relationships for different deformation modes were studied and the ideal shear and tensile strength were obtained. M3AlN ceramics are predicted to be “quasi-ductile” layered nitrides based on the low shear-modulus-to-bulk-modulus ratios, positive Cauchy pressure (c12–c44), and lower ideal shear strength compared to ideal tensile strength. Investigation of the atomistic shear deformation mechanism of Hf3AlN shows that stretching of soft Al–Hf bonds and relatively weak bridge N–Hf1 bonds dominate the shear deformation; while the rigid N–Hf2 bonds resist against the applied shear strain. Chemical bonding characteristics and shear deformation mechanism of M3AlN are similar with those of other “quasi-ductile” ceramics, such as MAX phases, LaPO4 monazite, and γ-Y2Si2O7. The results further suggest that M3AlN nitrides should be quasi-ductile and damage tolerant.

Shear deformation, ideal strength, and stacking fault formation of fcc metals: A density-functional study of Al and Cu

Ab initio density-functional calculations have been used to study the response of two face-centered-cubic metals (Al and Cu) to shearing parallel to the close-packed (111) planes along two different directions, $[11\overline{2}]$ and $[\overline{1}10]$. Two different types of deformations---affine and alias---have been investigated. Under an affine shear deformation, all atoms are shifted parallel to the shearing direction by a distance proportional to their distance from the fixed basal plane. In the alias regime, only the top layer is displaced in the shearing direction. In both regimes, calculations have been performed with (pure shear) and without (simple shear) relaxation. For a pure alias shear, due to the interaction between the atoms, the displacement propagates through the sample; this is certainly the most realistic description of the shearing processes. In the pure alias regime, shear deformation, theoretical shear strength, and stacking fault formations may be described on a common footing. For small strains (in the elastic region), affine and alias shears lead to very similar results. Beyond the elastic limit, relaxation has a strong influence of the response on an applied shear strain. The elastic shear moduli are significantly larger for Cu than for Al, but a much higher shear strength is calculated for Al, although the shear strength is limited by the occurrence of a stacking fault instability before the stress maximum is reached. Under $⟨\overline{1}10⟩$ {111} shear the analysis of the atomistic deformation mechanism shows that in this case the formation of a stacking fault leads to a splitting of the $\frac{1}{2}[\overline{1}10]$ dislocation into two partial Shockley dislocations. Due to the repulsive interaction between the atoms in adjacent close-packed planes, the atoms in the top A layer move along $\frac{1}{6}[\overline{2}11]$ to a position directly above the B layer such that the stable intrinsic stacking fault configuration is the same for both slip systems. The analysis of the variation in the lattice parameters under strain reveals significant differences in the relaxation behavior of both metals: Al is very stiff, but Cu is rather soft along the $⟨112⟩$; in-plane relaxation is very strong for Cu but modest for Al. This much stronger relaxation explains that while the differences in the unstable stacking fault energies of both metals are only modest, the intrinsic stacking fault energies differ by as much as a factor of 4. A detailed comparison of the response to shear and tensile deformations has been performed. A phonon instability of the uniaxial tensile deformation along the [110] direction has been explained by the close connection with the shear system $⟨11\overline{2}⟩$ {111}.

Mechanical properties and atomistic deformation mechanism of γ-Y 2Si 2O 7 from first-principles investigations

The theoretical mechanical properties and atomistic shear deformation mechanisms of γ-Y 2Si 2O 7, one of the most refractory silicates and potentially useful as a high-temperature structural ceramic, were investigated using first-principles calculations. The material shows low shear moduli to bulk modulus ratios, as well as a low ideal shear strength to tensile strength ratio. The unusual low shear deformation resistance of γ-Y 2Si 2O 7 originates from the inhomogeneous strength of its chemical bonds. The Y–O bond is weaker and readily stretches and shrinks; and Si–O bond is stronger and more rigid. The relative softer YO 6 octahedron positively accommodates shear deformation by structural distortion, while the Si 2O 7 pyrosilicate unit is more resistant to deformation. The reported shear-load-bearing mechanism is quite similar to those found in the “quasi-ductile” LaPO 4 monazite and ternary layered carbides (the so-called MAX phases), and can endow γ-Y 2Si 2O 7 with quasi-ductility and damage tolerance.

Hypervelocity impact induced deformation modes in α-alumina

Hypervelocity impact deformation mechanisms of α-alumina are studied using 540×106 atom molecular dynamics simulation on massively parallel computers. The projectile impact on the (0001) surface of α-alumina at 18km∕s exhibits a fundamentally different symmetry of the deformation patterns from those under lower strain rates. The simulation reveals a sequence of atomistic deformation mechanisms following localized melting and amorphization. These include pyramidal slips, basal slips and twins, rhombohedral twins, and twins along the {01¯11}. Some of these deformation patterns are not observed under lower impact velocities.

The Effect of Defects on the Mechanical Behavior of Silver Shape Memory Nanowires

We present atomistic simulations of the uniaxial tensile deformation of silver shape memory nanowires to investigate the effects of initial defects on the resulting thermomechanical behavior. In particular, the focus of the work is on investigating the unique atomistic deformation mechanisms that are observed during the tensile loading as a result of the initial defects, while correlating that behavior to the measured mechanical properties of the shape memory nanowires. In particular, wires with initial defects show a non-constant stress state during the 110 / 111 to 100 / 100 reorientation due to the presence of multiple propagating twin boundaries, as well as reductions in transformation stresses and strains due to the presence of the initial defects. Under most circumstances, the wires with initial defects still tend to exhibit complete reversibility between the 110 / 111 and 100 / 100 orientations, and thus the shape memory effect. Comparisons are made to defect-free shape memory nanowires to illustrate the relative mechanical performance of each structure.