Molecular dynamics simulation on mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with {111} texture during tensile deformation

Based on molecular dynamics (MD) simulation, the mechanisms of plastic anisotropy in nanotwinned polycrystalline copper with {111} texture during tensile deformation were systematically studied from the aspects of Schmid factor of the dominant slip system and the dislocation mechanism. The results show that the Schmid factor of dominated slip system is altered by changing the inclining angle of the twin boundaries (TBs), while the yield stress or flow stress does not strictly follow the Schmid law. There exist hard and soft orientations involving different dislocation mechanisms during the tensile deformation. The strengthening mechanism of hard orientation lies in the fact that there exist interactions between the dislocations and the TBs during plastic deformation, which leads to the dislocation blocking and reactions. The softening mechanism of soft orientation lies in the fact that there is no interaction between the dislocations and the TBs because only the slip systems parallel to the TBs are activated and the dislocations slip on the planes parallel to the TBs. It is concluded that the plastic anisotropy in the nanotwinned polycrystalline copper with {111} texture is aroused by the combination effect of the Schmid factor of dominated slip system and the dislocation mechanism.     

Deformation characteristics and stress–strain response of nanotwinned copper via molecular dynamics simulation

In this research parallel molecular dynamics (MD) simulations have been performed to study the deformation behavior of nanocrystalline copper samples with embedded nanotwins under approximately uniaxial tensile load. Simulation results reveal that twin boundaries (TBs) act as obstacles to dislocation movements that lead to the strengthening of nanotwinned structures. However, easy glide of dislocations parallel to the TBs contribute primarily to the plastic strain or ductility of these materials. At higher deformation stages, the strengthening effects reach a maximum when abundant dislocations begin crossing the TBs. Due to this highly anisotropic plastic response of the grains, a random polycrystalline sample will show combined properties of ductility and strength. The strengths of the nanotwinned models are found to exhibit an inverse relationship with the twin width and temperature. We also investigate the relation between the deformation behavior in different grains, their orientation with respect to the loading direction, and ultimately the observed response of nanotwinned structures.     

Stress relaxation and the structure size-dependence of plastic deformation in nanotwinned copper

Stress-relaxation experiments were performed on nanotwinned Cu to characterize the twin size-dependence of the activation volume and mobile dislocation density. We find that the variation of activation volume as a function of twin lamellae thickness can be captured well by a Hall–Petch-type relation. This structure size-dependence is interpreted to arise from a transition of the rate-controlling mechanism from intra-twin to twin boundary-mediated processes with decreasing twin thickness. Furthermore, we find that the exhaustion rate of mobile dislocations reduces with decreasing twin thickness. Such a twin size-dependence is attributed to the increased strain-hardening rate associated with a high density of coherent twin boundaries. Our results demonstrate that twin boundary-mediated dislocation processes can effectively promote the strain hardening and preserve mobile dislocations, leading to ultrahigh strength while retaining ductility in nanotwinned Cu.     

In-situ TEM study of dislocation-twin boundaries interaction in nanotwinned Cu films

Epitaxial thin films of nanotwinned face-centered cubic metals such as Cu possess an unprecedented combination of high hardness and high electrical conductivity due to the unique structure of nanometer-spaced coherent twin boundaries. Recent studies of in-situ nanoindentation in a transmission electron microscope have provided new insights on the deformation behavior of nanotwins that are reviewed here. In particular, two unit processes are highlighted: first, stress-induced migration of Σ3 {112} incoherent twin boundary that leads to de-twinning of nanotwins; second, twinning dislocation can be multiplied at Σ3 {111} coherent twin boundary.     

Strength, strain-rate sensitivity and ductility of copper with nanoscale twins

We present a comprehensive computational analysis of the deformation of ultrafine crystalline pure Cu with nanoscale growth twins. This physically motivated model benefits from our experimental studies of the effects of the density of coherent nanotwins on the plastic deformation characteristics of Cu, and from post-deformation transmission electron microscopy investigations of dislocation structures in the twinned metal. The analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating the twin boundary as an internal interface and allowing special slip geometry arrangements that involve soft and hard modes of deformation. This model correctly predicts the experimentally observed trends of the effects of twin density on flow strength, rate sensitivity of plastic flow and ductility, in addition to matching many of the quantitative details of plastic deformation reasonably well. The computational simulations also provide critical mechanistic insights into why the metal with nanoscale twins can provide the same level of yield strength, hardness and strain rate sensitivity as a nanostructured counterpart without twins (but of grain size comparable to the twin spacing of the twinned Cu). The analysis also offers some useful understanding of why the nanotwinned Cu with high strength does not lead to diminished ductility with structural refinement involving twins, whereas nanostructured Cu normally causes the ductility to be compromised at the expense of strength upon grain refinement.     

Recent Studies on the Microstructural Response of Nanotwinned Metals to In Situ Heavy Ion Irradiation

Nanotwinned metals are potential radiation-tolerant materials because they contain high-density coherent and incoherent twin boundaries that may serve as sinks to radiation-induced defects. The behavior of nanotwinned metals subject to ex situ and in situ irradiation remains however largely unexploited. This article offers an overview of the recent studies on the microstructural response of nanotwinned metals to in situ heavy ion irradiation, focusing on the interactions of defect clusters with twin boundaries and the radiation-induced twin boundary migration. Several radiation-tolerant nanotwinned metals are also highlighted.

     

Nanostructure and surface effects on yield in Cu nanowires

The yield strengths of nanomaterials are highly sensitive to their internal and surface structures. However, it is difficult to identify a priori which structural feature will govern plastic yield. We employ very large scale molecular dynamics simulations to explicitly identify the relevant yield mechanisms for Cu nanowires with four distinct, experimentally realizable nanostructures: single crystal (SC), nanotwinned single crystal (NTSC), nanocrystal (NC) and nanotwinned nanocrystal (NTNC). By characterizing the deformation at the yield point on the atomic scale, our simulations elucidate the effects of surface defects, nanotwins and grain boundaries on the commencement of yield and reveal several critically important features of the yielding process. First, the initial yields in all nanowires occur via dislocation nucleation at different characteristic nanostructural features. SC and NTSC nanowires yield via dislocation nucleation from surfaces or surface defects, while NC and NTNC nanowires yield via dislocation nucleation from grain boundary triple junctions. Second, our simulations highlight the relative potency of stress concentrators arising from different imperfections in modulating the yield strength of nanowires. Grain boundary triple junctions are as effective as surface defects at acting as stress concentrators. However, the higher density of triple junctions in NC and NTNC nanowires renders these structures considerably weaker than their SC and NTSC counterparts. Third, the presence of nanotwins only marginally enhances the yield strength of nanocrystalline Cu nanowires, which is in line with experimental observation in NTNC Cu nanowires but contrary to that in bulk ultrafine-grain nanotwinned Cu. The reason for this divergent behavior is that in nanowires yield strength is governed by dislocation nucleation from triple junctions in contrast to dislocation propagation in the bulk. Finally, excellent agreement is obtained between the relative yield strengths, stress–strain behavior and dislocation nucleation conditions of nanowires in our simulations and existing experimental data. This suggests that our predicted atomistic processes controlling yield in our simulations may also control yield in experiments.     

Strengthening an austenitic Fe–Mn steel using nanotwinned austenitic grains

An austenitic Fe–25Mn steel with a low stacking fault energy was subjected to dynamic plastic deformation (DPD) followed by thermal annealing. The as-DPD sample is structurally characterized by a mixed nanostructure consisting of nanosized grains with an average size of 43 nm and bundles of nanoscale twins (with an average twin/matrix lamella thickness of 5 nm), as well as some dislocation structures, which exhibits a high yield strength of about 1470 MPa but a limited tensile ductility. Thermal annealing leads to static recrystallization (SRX) of the nanostructures forming a hierarchical structure of nanotwinned grains embedded in microsized SRX grains, owing to the higher thermal stability of the nanotwinned bundles than that of nanosized grains. With an increasing number of SRX grains the yield strength and ultimate tensile strength drop while the tensile ductility increases. The calculated yield strength of the nanotwinned grains is about 1.5 GPa, much lower than that determined from Hall–Petch strengthening extrapolated to the nanoscale. Work hardening rates of the nanotwin grains, comparable with that of the microsized grains, are higher than that of the original coarse grained sample. The micrograined austenitic Fe–Mn samples strengthened by nanotwinned grains exhibit enhanced strength–ductility synergy in comparison with the deformed samples. A combination of a 977 MPa yield strength with a uniform elongation of 21% is achieved in the annealed samples, well above that of the deformed samples.     

Dislocation–dislocation and dislocation–twin reactions in nanocrystalline Al by molecular dynamics simulation

We use massively parallel molecular dynamics simulations of polycrystal plasticity to elucidate the intricate dislocation dynamics that evolves during the process of deformation of columnar nanocrystalline Al microstructures of grain size between 30 and 100 nm. We analyze in detail the mechanisms of dislocation–dislocation and dislocation–twin boundary reactions that take place under sufficiently high stress. These reactions are shown to lead to the formation of complex twin networks, i.e. structures of coherent twin boundaries connected by stair-rod dislocations. Consistent with recent experimental observations, these twin networks may cause dislocation pile-ups and thus give rise to strain hardening.     

Plastic anisotropy and associated deformation mechanisms in nanotwinned metals

Anisotropic plastic deformation in columnar-grained copper in which preferentially oriented nanoscale twins are embedded is studied by experimental testing, crystal plasticity modeling and molecular dynamics simulations. The dominant deformation mechanism can be effectively switched among three dislocation modes, namely dislocation glide in between the twins, dislocation transfer across twin boundaries, and dislocation-mediated boundary migration, by changing the loading orientation with respect to the twin planes. The controllable switching of deformation mechanisms not only leads to a marked dependence of yield strength on loading orientation, but also induces a strong orientation dependence of strain hardening that can be critical for retaining tensile ductility. These results demonstrate a new route for tailoring both nanostructure and loading to control the deformation mechanisms in order to achieve the desired mechanical properties in engineering materials.     

Comparisons between homogeneous boundaries and heterophase interfaces in plastic deformation: Nanostructured Cu micropillars vs. nanolayered Cu-based micropillars

Both the homogeneous boundaries and the heterophase interfaces play important roles in crystalline plasticity as they often serve as obstacles for dislocation motion, as well as dislocation sources/sinks. In this work, microcompression tests were carefully performed to explicitly identify the relevant plasticity mechanisms of nanostructured micropillars with five distinct nanostructures: (i) Cu nanotwinned multicrystalline micropillars; (ii) Cu nanocrystalline multicrystalline micropillars; (iii) Cu/X (X = Cr, Zr) nanotwinned nanolayered micropillars; (iv) Cu/X nanocrystalline nanolayered micropillars; and (v) Cu/Cu–Zr crystalline/amorphous nanolayered micropillars. By characterizing their stress–strain response and evolution of strain-rate sensitivity (SRS) with strain, our findings elucidate the effects of homogeneous boundaries, heterophase interfaces and their coupling effects on the plastic yield, and reveal the fundamentally different roles these perform in the rate-limiting process of nanomaterials. In sharp contrast to the normal strain-dependent SRS in nanostructured Cu micropillars with homogeneous boundaries that monotonically decreases with increasing strain, nanolayered Cu-based micropillars with heterophase interfaces exhibit inverse strain-dependent SRS that monotonically increases with increasing strain. These expected (normal) and unexpected (inverse) SRSs are quantitatively explained by a dislocation model in terms of the strain-related dislocation mean free path. These findings provide valuable insights into our understanding of the fundamental roles that homogeneous boundaries and heterophase interfaces play in plastic deformation.     

Ballistic performance of nanocrystalline and nanotwinned ultrafine crystal steel

Because of their remarkable mechanical properties, nanocrystalline metals have been the focus of much research in recent years. Refining their grain size to the nanometer range (<100 nm) effectively reduces their dislocation mobility, thus achieving very high yield strength and surface hardness—as predicted by the Hall-Petch relation—as well as higher strain-rate sensitivity. Recent works have additionally suggested that nanocrystalline metals exhibit an even higher compressive strength under shock loading. However, the increase in strength of these materials is generally accompanied by an important reduction in ductility. As an alternative, efforts have been focused on ultrafine crystals, i.e. polycrystals with a grain size in the range of 500 nm to 1 μm, in which “growth twins” (twins introduced inside the grain before deformation) act as barriers against dislocation movement, thus increasing the strength in a similar way as nanocrystals but without significant loss of ductility. Due to their outstanding mechanical properties, both nanocrystalline and nanotwinned ultrafine crystalline steels appear to be relevant candidates for ballistic protection. The aim of the present work is to compare their ballistic performance against coarse-grained steel, as well as to identify the effect of the hybridization with a carbon fiber-epoxy composite layer. Hybridization is proposed as a way to improve the nanocrystalline brittle properties in a similar way as is done with ceramics in other protection systems. The experimental campaign is finally complemented by numerical simulations to help identify some of the intrinsic deformation mechanisms not observable experimentally. As a conclusion, nanocrystalline and nanotwinned ultrafine crystals show a lower energy absorption than coarse-grained steel under ballistic loading, but under equal impact conditions with no penetration, deformation in the impact direction is smaller by nearly 40%. This a priori surprising difference in the energy absorption is rationalized by the more important local contribution of the deviatoric stress vs. volumetric stress under impact than under uniaxial deformation. Ultimately, the deformation advantage could be exploited in the future for personal protection systems where a small deformation under impact is of key importance.     

Deformation, structural changes and damage evolution in nanotwinned copper under repeated frictional contact sliding

Nanotwinned metals have the potential for use as structural materials by virtue of having a combination of high strength as well as reasonable ductility and damage tolerance. In the current study, the tribological response of nanotwinned copper has been characterized under conditions of repeated frictional sliding contact with a conical tip diamond indenter. Pure ultrafine-grained copper specimens of fixed grain size (∼450 nm), but with three different structural conditions involving relatively high, medium and negligible concentrations of nanotwins, were studied. The effects of twin density and number of repetitions of sliding cycles on the evolution of friction and material pile-up around the diamond indenter were studied quantitatively by depth-sensing instrumented frictional sliding. Cross-sectional focused ion beam and scanning electron microscopy observations were used to systematically monitor deformation-induced structural changes as a function of the number of passes of repeated frictional sliding. Nanoindentation tests at the base of the sliding tracks coupled with large-deformation finite-element modeling simulations were used to assess local gradients in mechanical properties and deformation around the indenter track. The results indicate that friction evolution as well as local mechanical response is more strongly influenced by local structure evolution during repeated sliding than by the initial structure. An increase in twin density is found to result in smaller pile-up height and friction coefficient. Compared to the low-density nanotwinned metal, high-density nanotwinned copper showed significantly higher resistance to surface damage and structural changes, after the initial scratch. However with an increase in the number of sliding passes, the friction coefficient and rate of increase of pile up for all specimens acquire a steady value which does not change significantly in subsequent scratch passes. The frictional sliding experiments also lead to the striking result that copper specimens with both a high and low density of nanotwins eventually converge to a similar microstructure underneath the indenter after repeated tribological deformation. This trend strongly mirrors the well-known steady-state response of microcrystalline copper subjected to uniaxial cyclic loading. General perspectives on contact fatigue response of nanotwinned copper are developed on the basis of these new findings.     

Indentation of nanotwinned fcc metals: Implications for nanotwin stability

Cyclic nano- and microindentation, along with indentation creep, were performed on nanotwinned Cu with two twin structures, and on nanotwinned Ag. The results provide evidence that nanotwinned face-centered cubic (fcc) structures are more stable than their nanocrystalline counterparts. The results are put in the important context of the available body of theoretical study of nanotwinned fcc metals, and in particular in the context of the theoretical forecasts of Kulkarni and Asaro [Kulkarni Y, Asaro RJ. Acta Mater 2009;57:2711]. It is shown, for example, that, as predicted, nanotwinned Ag displays performance comparable, if not superior, to nanotwinned Cu.     

Lattice correspondence during twinning in hexagonal close-packed crystals

The correspondence of the dominant slip modes in parent and twin structures in hexagonal closed-packed crystals is considered within the framework of the theory of deformation twinning. The correspondence matrices, which provide a link between the parent and twin lattices, have been worked out for compound twinning modes and selected metals. Geometrical considerations suggest that in most cases deformation twins should inherit a harder dislocation substructure than that of the corresponding parent. Possible mechanisms of twin hardening are discussed.     

Twin stability in highly nanotwinned Cu under compression,torsion and tension

Twin stability under four distinct mechanical loading states has been investigated for highly nanotwinned Cu containing parallel nanotwins ～40 nm thick. Observed deformation-induced microstructural changes under tension, compression, tension–tension fatigue and torsion are qualitatively compared in order to assess twin stability as a function of the loading direction and stress. It is observed that the twins are very stable although small microstructural changes vary with deformation mode. Shear bands, deformation-induced grain growth and detwinning are also discussed.     

Thermally activated deformation of two- and three-variant nanotwinned L10 Au–Cu–Pt

Activation volumes characteristic of room temperature compressive plastic deformation in Au- 45.2 at.% Cu- 1.7 at.% Pt, a near-AuCu alloy that orders to a fully L1₀ structure, are measured using the repeated stress-relaxation method. Four different ordering treatments (540 and 10⁵s at either 250 or 400 °C) are conducted on samples prior to deformation, leading to a classical two-variant polytwin structure after ordering at 400 °C, or alternatively a three-variant highly twinned structure at 250 °C. In both structures, {110} twin boundaries only a few tens of nanometres apart separate variants and constitute the main barriers to dislocation glide. All four ordered nanotwinned microstructures have an initial effective activation volume V_eff of 100–120 b³; however, the two structures differ strongly as they work harden. The two-variant polytwin structure yields a straight line on a Haasen plot, showing that work hardening is associated with an increase in slip obstacle density, most likely dislocation debris at intervariant boundaries. The three-variant structure on the other hand shows a nearly constant (slightly decreasing) activation volume. This suggests that there is little obstacle accumulation when three variants are present, likely because plastic deformation occurs by slip of superdislocation only.     

Repeated frictional sliding properties of copper containing nanoscale twins

Bulk dynamic plastic deformation (DPD) materials comprise a composite structure of nanoscale twin bundles and nanoscale grains. The tribological properties of DPD-processed pure nano-Cu have been investigated in this study and compared with conventional coarse-grained (CG) Cu under both monotonic and repeated frictional sliding. We demonstrate that DPD nano-Cu and CG Cu exhibit steady-state mechanical characteristics after repeated frictional sliding that are similar to those seen in nanotwinned (NT) Cu produced by pulsed electrodeposition.     

Are some nanotwinned fcc metals optimal for strength,ductility and grain stability?

Here we investigate whether certain face-centered cubic metals display a superior behavior of nanotwinned structures compared to others. We also address the question of an optimal lamella thickness that yields maximum strength and stability. Our analysis of the intrinsic stacking fault energies, γ_sf, and the unstable stacking fault energies, γ_us, of Al, Pd, Cu and Ag, as well as our atomistic simulations of dislocation–twin boundary interactions in these metals, suggests an optimal behavior of nanotwinned Pd and Ag as competitive to Cu, and hence a special utility in their synthesis and further exploration. Our results also indicate that the influence of twin–twin interactions may lead to a loss of strength below a critical value of twin lamella thickness.