Deformed Nanocrystalline Materials Research Articles

Synergy of grain boundary sliding and shear-coupled migration process in nanocrystalline materials

An attempt has been made to illustrate a new physical mode of plastic deformation in nanocrystalline materials that synergizes two processes, i.e. grain boundary (GB) sliding and stress-driven shear-coupled GB migration. The latter process incorporates a rotational and a translational plastic flow, in which a normal migration is coupled with a shear. The energy change resulting from the above-mentioned mode was calculated, and showed that the new deformation mode was much more energetically favorable than both the pure GB sliding mode and the cooperative process of GB sliding and migration without a coupling shear. In addition, the new deformation mode considerably enhances the ductility of nanocrystalline materials compared with the other two above-mentioned processes.

Toughening of nanocrystalline materials through shear-coupled migration of grain boundaries

A theoretical model has been proposed to illustrate the effect of shear-coupled migration of grain boundaries on crack growth in deformed nanocrystalline materials. The shear-coupled migration process is initiated by the stress concentration near the tip of a mode I crack. The results obtained show that the coupled shear plays a key role in enhancing the fracture toughness of nanocrystalline materials, and the shear-coupled migration process can serve as an effective toughening mechanism in nanocrystalline materials.

Effect of cooperative grain boundary sliding and migration on emission of dislocations from a crack tip in nanocrystalline materials

Interaction of the cooperative grain boundary sliding and migration with a crack in deformed nanocrystalline materials is investigated using the complex variable method. Effects of the two disclination dipoles produced by the cooperative deformation on the emission of lattice dislocations from the crack tip are theoretically described. The complex form expressions of the stress field and the force field are derived. The critical stress intensity factors for the first dislocation emission are calculated. Influences of disclination strength, grain size, locations of the two disclination dipoles as well as crack length on the critical stress intensity factors are discussed in detail. Results show that, the cooperative deformation has great influence on dislocation emission from the crack tip. In general, the cooperative deformation can promote the lattice dislocation emission from the crack tip, thus improve the toughness of the nanocrystalline materials.

Effect of special rotational deformation on dislocation emission from a semielliptical blunt crack tip in nanocrystalline solids

Abstract

On shear-coupled migration of grain boundaries in nanocrystalline materials

Shear-coupled migration of grain boundary (GB) was theoretically modeled as a generic mode of plastic deformation in nanocrystalline materials. The energy change due to this process that carried both rotational and translational plastic flow through the normal migration and the shear coupled to the said migration, respectively, was calculated. The results obtained revealed that the shear-coupled migration was energetically more favorable than the pure migration of a grain boundary. The translational deformation can turn the initially energetically unfavorable process into a favorable one, and it can decrease the critical stress required to initiate the process of migration by around 30%–80% as compared with that for pure GB migration.

Constitutive model for plastic deformation of nanocrystalline materials with shear band

A phase mixture model was used to study the plastic deformation behaviors in hardening stage of nanocrystalline materials. The material was considered as a composite of grain interior phase and grain boundary (GB) phase. The constitutive equations of the two phases were determined in term of their main deformation mechanisms. In softening stage, a shear band deformation mechanism was presented and the corresponding constitutive relation was established. Numerical simulations have shown that the predications fit well with experimental data. The investigation using the finite-element method (FEM) provided a direct insight into quantifying shear localization effect in nanocrystalline materials.

The effects of the finest grains on the mechanical behaviours of nanocrystalline materials

This article proposes a new constitutive model to account for effects of the finest grains, with sizes ranging from 2 to 4 nm, on the mechanical behaviours of nanocrystalline (NC) materials. In this model, the normal nanograins (ranging from 20 to 100 nm) were treated as though they were composed of a grain interior (GI) and a grain boundary (GB) affected zone (GBAZ). The finest grains were considered to be part of the GBAZ, denoted as super triple junctions (STJs). For the initial plastic deformation stage of the NC materials, a phenomenological constitutive equation was suggested to predict the deformation behaviours of the GBAZ. The formation of GB dislocation (GBD) pileups provides dramatic strain hardening in deformed NC materials and thereby enhances their ductility. Then, the constitutive equations to describe the plastic deformation of the GI and the GBAZ lattice region were established. In this stage, the GBAZ are already saturated with GBD pileups, and GI deformation is the dominant mechanism. Finally, the mechanical model for the NC materials with the finest grains was built using the self-consistent method, and an overall moderate “work hardening,” sustained over a long range of plastic strain, was predicted. The effects of TJs/STJs on the deformation mechanism were quantitatively analysed. The analysis demonstrated that the existence of the finest grains will simultaneously lead to good strength and good ductility.

Dislocationless sliding in a polycluster glass

Abstract At low temperature, T → 0, the yield stress of a perfect crystal is equal to its so called theoretical strength. The yield stress of imperfect crystals is controlled by the stress threshold of dislocation mobility. A non-crystalline solid has neither the ideal structure nor gliding dislocations. Its yield stress depends on the distribution of local critical stresses attributed to each atomic site at which the local inelastic deformation occurs. We describe the exactly solvable model of planar layer strength and sliding with an arbitrary homogeneous distribution of local critical stresses. The kinetics of thermally-activated creep of the sliding layer is described. The sliding activation volume scales with the applied external stress as ∼ωe -β, where β < 1. The proposed model accounts for mechanisms of the low temperature deformation of polycluster metallic glasses, since intercluster boundaries of a polycluster metallic glass are natural sliding layers of the described type. We also discuss applicability of the model to the low temperature plastic deformation of nano-crystalline materials.

Constitutive Model for Large Plastic Deformation of Nanocrystalline Materials

A constitutive model was presented for nanocrystalline metallic materials that can experience large plastic deformation with shear band. The model was composed of two parts for different deformation stage: hardening stage and softening stage. In the hardening stage, the phase mixture model was used, and in the softening stage, a shear band deformation mechanism was proposed. Based on the model presented, numerical simulations were carried out to prove that the predications kept in good agreement with experimental data.

Deformation of nanocrystalline materials in the formalism of coupled mode theory

In the present paper, we suggest to use the coupled mode theory (CMT) to describe the strain behavior of nanomaterials. Our approach is based on the analogy between nanocrystalline and granulated materials, on the similarity in the behavior of granulated materials, concentrated suspensions, and amorphous materials, and on the efficiency of CMT in the description of glass and suspension rheology. We use the successive approximation method for solving the nonlinear integro-differential equation of the CMT and show that already the first approximation describes the main qualitative properties of numerical solutions quite well. The Laplace transform is written out for the second approximation, which permits integrating the correlator over time and writing out the rheological relation implicitly.

Pseudoelastic stacking fault and deformation twinning in nanocrystalline Ni

It is usually believed that the partial dislocation and deformation twin are the results of permanent plasticity in materials. Here, we present in situ atomic-scale observation of reversible stacking fault and deformation twin during loading and unloading in nanocrystalline Ni under high-resolution transmission electron microscopy. The high propensity for the reversibility of the stacking fault and deformation twin is due to the high stacking fault force and small grain size, and will provide an understanding at atomistic scale on the nature of the deformation in nanocrystalline materials.

Elongated nanoscale voids at deformed special grain boundary structures in nanocrystalline materials

A special micromechanism for the formation of elongated nanoscale voids at grain boundaries (GBs) in deformed nanocrystalline materials is suggested and theoretically described. Within our description, the formation of nanoscale voids represents a slow (diffusion-controlled) process driven by release of the elastic energy of GB disclination configurations formed due to GB sliding. It is demonstrated that the nucleation of elongated nanoscale voids at GB disclination dipoles occurs as an energetically favorable process in deformed nanocrystalline Ni and Al 2O 3 (sapphire) in wide ranges of their parameters.

Multiscale constitutive modeling for plastic deformation of nanocrystalline materials

Macroscopic and microscopic constitutive modeling that can display large plastic deformation with shear band were presented for nanocrystalline materials subjected to uniaxial load over a wide strain rate range. The macroscopic model implemented with parameters microscopic meaning was established based on the theory of plastic dissipation energy. The microscopic model based on deformation mechanisms was composed of two parts: hardening and softening stages. In the hardening stage, the phase mixture model was used and a shear band deformation mechanism was proposed in the softening stage. Numerical simulations shown that the predications were in good agreement with experimental data. Finally, a parameter of normalized softening rate was proposed and its characteristics were evaluated quantitatively. It can be concluded that the failure strain could be prolonged when the normalized softening rate decrease through changing the softening path.

Nucleation of nanograins near cracks in nanocrystalline materials

Nucleation of nanograins near crack tips and its role as a special (new) toughening mechanism in deformed nanocrystalline materials are theoretically described. Within our description, stress-driven splitting and migration of grain boundaries occur near crack tips. These stress-driven processes result in nucleation of nanograins and cause a partial stress relaxation near cracks. It is shown that the nucleation of nanograins near crack tips is energetically favorable and effective in enhancing toughness in nanocrystalline ceramic α-Al2O3.

Phase field crystal study of deformation and plasticity in nanocrystalline materials

We introduce a modified phase field crystal (MPFC) technique that self-consistently incorporates rapid strain relaxation alongside the usual plastic deformation and multiple crystal orientations featured by the traditional phase field crystal (PFC) technique. Our MPFC formalism can be used to study a host of important phase transformation phenomena in material processing that require rapid strain relaxation. We apply the MPFC model to study elastic and plastic deformations in nanocrystalline materials, focusing on the "reverse" Hall-Petch effect. Finally, we introduce a multigrid algorithm for efficient numerical simulations of the MPFC model.

Mechanism of the formation of shear microbands under plastic deformation of nanocrystalline materials

The mechanism of deformation localization and formation of shear microbands under plastic deformation of submicrocrystalline and nanocrystalline materials is theoretically discussed in the framework of the dislocation-kinetic approach. An equation of evolution of the density and self-organization of dislocations in these materials is formulated taking into account that the grain boundaries are the main sources, sinks, and barriers to moving dislocations. By solving this equation, it is found that the width of microbands and the distance between them depend on the nanograin size and the degree of plastic deformation. It is also demonstrated that there exists a critical grain size (350 nm in the case of α-Fe) above which no microbands are formed in the nanomaterial. The theoretical results are compared with the data available in the literature.

Self-thickening, cross-slip deformation twinning model

We propose a cross-slip deformation twinning model based upon our experimental in situ deformation of Cu–Ge alloys using a transmission electron microscope. During the deformation, Shockley partials are emitted from the grain boundaries, which encounter an obstacle within the grain and split to nucleate deformation twins in the cross slip as well as primary planes. Upon further deformation a thickening process for twins is directly observed. The proposed model provides a dislocation mechanism for the observed simultaneous twin thickening in both the cross-slip and primary planes. The role of the proposed mechanism in the deformation of nanocrystalline materials is discussed.

On the elastic tensile deformation of 〈1 0 0〉 bicrystal interfaces in copper

The elastic response of 〈1 0 0〉 tilt bicrystal copper interfaces, when subjected to a uniaxial tensile deformation applied normal to the boundary, is examined using molecular dynamics simulations with an embedded-atom method potential. Simulations in this work are designed explicitly to study the discrete atomic motions that occur within the grain boundary region prior to dislocation nucleation. Seven symmetric tilt interfaces with low-order coincident site lattice descriptions are considered: Σ5 (2 1 0), Σ5 (3 1 0), Σ13 (3 2 0), Σ13 (5 1 0), Σ17 (4 1 0), Σ17 (5 3 0) and Σ29 (7 3 0). Simulations indicate that bicrystal boundaries which contain the C structural unit deform via an elastic structural transition which initiates at a critical threshold stress. Furthermore, it is found that the excess interfacial energy decreases during elastic deformation for both Σ5 and Σ29 interfaces, whereas it increases for the Σ13 and Σ17 boundaries. It is suggested that this observation can be attributed to the geometric constraints imposed by the structural units that comprise the Σ5 and Σ29 tilt boundaries, which has significant implications for grain boundary engineering and continuum models of elastic and viscoplastic deformation in nanocrystalline materials.

Special strain hardening mechanism and nanocrack generation in nanocrystalline materials

A special mechanism of strain hardening in deformed nanocrystalline materials (NCMs) is suggested and theoretically described. The mechanism represents generation of disclination dipoles at grain boundaries (GBs) due to GB sliding. It is shown that special strain hardening can effectively suppress plastic flow instability in metallic and ceramic NCMs and thus enhance their ductility. At the same time, the disclination dipoles formed due to GB sliding serve as dangerous stress sources that can induce nucleation of nanocracks, decreasing ductility of NCMs.

Dislocation loop nucleation and plastic deformation of nanocrystalline materials

We propose a three-dimensional model of plastic deformation of a mechanically loaded nanocrystalline material by means of heterogeneous nucleation of loops of complete and partial lattice dislocations and by means of grain boundary dislocations on the already formed dislocation loops. We calculate and compare the energy variations characterizing various versions of dislocation loop nucleation. We discover three basic regions of variation of nanocrystalline material grain dimensions which are characterized by their own types of loop nucleation. We also study the role played by loop nucleation in plastic and superplastic deformations of nanocrystalline materials.