Deformed Nanocrystalline Materials Research Articles

Grain rotation mechanisms in nanocrystalline materials: Multiscale observations in Pt thin films.

Near-rigid-body grain rotation is commonly observed during grain growth, recrystallization, and plastic deformation in nanocrystalline materials. Despite decades of research, the dominant mechanisms underlying grain rotation remain enigmatic. We present direct evidence that grain rotation occurs through the motion of disconnections (line defects with step and dislocation character) along grain boundaries in platinum thin films. State-of-the-art in situ four-dimensional scanning transmission electron microscopy (4D-STEM) observations reveal the statistical correlation between grain rotation and grain growth or shrinkage. This correlation arises from shear-coupled grain boundary migration, which occurs through the motion of disconnections, as demonstrated by in situ high-angle annular dark-field STEM observations and the atomistic simulation-aided analysis. These findings provide quantitative insights into the structural dynamics of nanocrystalline materials.

Temperature effect on the transition between grain boundary migration and sliding

Grain boundary (GB) dynamics plays an important role in the mechanical and physical properties of nanocrystalline metals. In this study, we investigate the temperature effect on GB deformation transition using molecular dynamics simulations of [100] symmetric tilt GBs in Au bicrystals. Different deformation behaviours were revealed in GBs with the same structures at varied temperatures. GB sliding occurs as the predominant deformation mechanism at elevated temperatures, while GB migration dominates at low temperatures. The temperature-dependent critical stresses for GB migration and GB sliding were compared, revealing that temperature alters the critical GB misorientation at which GB migration transitions to GB sliding. Our study provides new insight into GB-mediated plastic deformation in nanocrystalline materials.

Twin interaction with [formula omitted] tilt grain boundaries in BCC Fe : Formation of new grain boundaries

It is well known that the twinning is an important mode of plastic deformation in nanocrystalline materials. As a result, it is expected that the twin can interact with different grain boundaries (GBs) during the plastic deformation. Understanding these twin–GB interactions is crucial for our understanding of mechanical behavior of materials. In this work, the twin interaction with different Σ11 symmetric and asymmetric tilt GBs has been investigated in BCC Fe using molecular dynamics (MD) simulations. The results indicate that twin nucleate from the crack or GB and, its interaction with Σ11 asymmetric tilt GBs leads to the formation of a new GB. This new GB consist of 〈100〉 Cottrell type immobile dislocations. The detailed atomistic mechanisms responsible for this new GB formation have been revealed using atomistic simulations. Interestingly, the new GB formation has not been observed in the case of twin interaction with Σ11 symmetric tilt GBs.

Deformation mechanism and inverse Hall – Petch behavior in nanocrystalline materials

Abstract It is widely accepted that for the very smallest grain sizes (typically below 20 – 30 nm), dislocations play no significant role in the deformation of nanocrystalline materials. However, the grain-boundary mechanisms responsible for the reported decrease in strength with decreasing grain size in this regime (the ‘inverse Hall–Petch effect’) remain unclear. Here, we demonstrate by molecular-dynamics simulation that, in the absence of both grain growth and any dislocations, nanocrystalline fcc metals deform via a mechanism involving an intricate interplay between grain-boundary sliding and grain-boundary diffusion. By quantitatively reproducing the well-known Coble-creep formula for coarse-grained materials, we show that the ‘inverse Hall–Petch effect’ arises from sliding-accommodated grain-boundary diffusion creep. Previous, apparently contradictory, suggestions that GB sliding, on the one hand, or GB-diffusion creep, on the other, are responsible for this behavior can thus be reconciled as originating from one and the same deformation mechanism. We discuss the reasons why we believe that these simulations also capture the room-temperature deformation behavior of nanocrystalline fcc metals in the absence of dislocation nucleation and microcracking.

Enhanced ductility of nanomaterials through cooperative dislocation emission from cracks and grain boundaries

An analytical model is established to explore the cooperative mechanism between the dislocation emission from cracks and grain boundaries driven by grain boundary sliding in deformed nanocrystalline materials. In our model, high local stress concentration nearby the crack actives grain boundary sliding which creates a wedge disclination dipole at the grain boundaries’ triple junctions. The grain size-dependent criterions for the dislocation emission from the crack tip and the grain boundary are respectively derived. Influences of grain boundary sliding and grain size on the cooperative mechanism are discussed. The results show that the dislocation emission from the grain boundary is activated ahead of that from the crack tip for small grain sizes. This can explain that grain boundary sliding can toughen the nanocrystalline materials even though it suppresses dislocation emission from cracks when their grain sizes are relative small, which is because the dislocation emission from grain boundaries is activated. With the increasing grain size, the main dislocation source may transform from grain boundaries to crack tips due to grain boundary sliding. Therefore, the ductility of nanomaterials with different grain sizes can be enhanced through the cooperative dislocation emission from cracks and grain boundaries.

Effect of nanotwin and dislocation pileup at twin boundary on dislocation emission from a semi-elliptical blunt crack tip in nanocrystalline materials

The effects of the nanotwin and dislocation pileup at the twin boundary on lattice dislocation emission from a surface semi-elliptical blunt crack in deformed nanocrystalline materials (NCMs) are investigated. The nanotwin as a stress source can be described by a wedge disclination quadrupole. Using the complex variable method, complex analytic solution of stress field, the dislocation force and the critical stress intensity factors (SIFs) for the first edge dislocation emission were obtained. Then, through numerical calculation, the influence of twin size, twin orientation, twin position, twin strength and dislocation pileup, the curvature radius of blunt crack tip on the critical stress intensity factors were discussed in detail. The results show that the nanotwin and dislocation pileup has significant influence on the dislocation emission from the blunt crack tip. The presence of nanotwin and dislocation pileup will increase the most probable emission angle, and increase the critical SIFs for dislocation emission, making it harder for the dislocation to emit from the blunt crack tip and thus to decrease the toughness of materials produced by dislocation emission.

Effect of nanotwin near a branched crack tip on crack blunting in deformed nanocrystalline materials

A theoretical model is established that mainly addresses the influence of nanotwin near the tip of branched crack on crack blunting in deformed nanocrystalline materials. Using the complex variable method of Muskhelishvili and conformal mapping technique, the solutions of complex potentials are derived in closed forms. The critical stress intensity factors for the emission of first lattice edge dislocation from the branched crack tip are obtained. The effects of typical parameters such as the relative length between the main crack and the branched crack, the angle between their planes as well as the relative thickness of the nanotwin band on the dislocation emission from the branched crack tip are evaluated in detail. The results indicate that the shorter branched crack is more easily induced to blunting through dislocation emission from its tip, while the longer main crack tends to propagate. The main crack shows a shielding effect to the tip of branched crack for the dislocation emission. Furthermore, the larger the relative thickness of the nanotwin band is, the easier is the dislocation emitted from the branched crack tip.

Effect of blunt nanocracks on the splitting transformation of grain boundary dislocation piled up at triple junctions

Considering that grain boundary (GB) deformation is a structural element that hinders crack growth and promotes superplastic deformation in nanocrystalline materials (NCMs), a theoretical model is suggested to describe the effect of the blunt pre-nanocracks with surface stress on the splitting transformation of the first head grain boundary dislocation (GBD) within the pile-up at the triple junctions (TJs) of GBs in mechanically loaded NCMs. The analytic solution of the total energy change that characterizes this process of splitting transformation of GBD is derived quantitatively by the complex variable method, and then, the very beginning plastic deformation occurrence near the nanocrack tip is predicted. We theoretically evaluated the influence of the various parameters of blunt pre-nanocracks, such as nanocrack blunting and length, the characteristics of grain and GBs, such as grain size, the number of GBDs, and GB angles, and the surface stress at critical conditions for such a splitting transformation. Further analyses revealed that the positive (negative) surface stress significantly decreases (increases) the energy and obviously influences the critical conditions for splitting transformation. Our study identified the beneficial role of GB defect structure transformation, namely the splitting transformation, in enhancing the ductility and fracture toughness of NCMs. It also lays the foundation for investigating how the microstructures caused by GB deformation affect the novel mechanical properties of NCMs.

Influence of grain boundary sliding near a nanovoid on crack growth in deformed nanocrystalline materials

A theoretical model is developed that examines the effect of grain boundary (GB) sliding near a nanovoid on crack growth in deformed nanocrystalline materials. By combining complex variable method of Muskhelishvili, superposition principle of elasticity and distributed dislocation technique, the singular integral equations are solved numerically and then the stress intensity factors (SIFs) near the left crack tip are obtained. The influences of the location of the disclination dipole, the dipole arm, the orientation and the relative crack length on the SIFs near the left crack tip are evaluated in detail. The results indicate that the wedge disclination dipole produced by the GB sliding shields mode I crack tip, but anti-shields mode II crack tip. At the same time, surface stresses of the nanovoid characterized by positive surface elasticity and negative surface elasticity both show shielding effect to mode I SIFs of the crack tip, yet have negligible effect on mode II SIFs of the crack tip. Meanwhile, the mode I crack tip is more significantly shielded by the negative residual surface stresses than that of the positive residual surface stresses.

Role of Localized Non-Equilibrium States in Nucleation of Plastic Deformation in Nanocrystalline Materials

A molecular dynamics simulation of the behavior of nanocrystalline materials in the fields of external influences was carried out. Crystallites of the fcc copper and bcc iron under different schemes of mechanical loading were investigated. Revealed specific localized non-equilibrium states served as the mechanism of formation and evolution of partial dislocations in fcc materials and twin growth in bcc materials. These non-equilibrium states were realized on the basis of local transformation of the martensitic type when the nearest surrounding of atoms – the centers of local rearrangements – changed according to the A-B(C) scheme, where A, B and C are types of crystal lattice. The bcc-fcc-bcc local rearrangements during twin growth were typical for bcc iron. The fcc-bcc-hcp and hcp-bcc-fcc local rearrangements during the partial dislocation movement were typical for fcc copper.

Effect of special rotational deformation on the dislocation emission from a branched crack tip in deformed nanocrystalline materials

The problem of the special rotational deformation interacting with an internal crack is studied by developing a theoretical model of deformed nanocrystalline materials. Using the complex variable method of Muskhelishvili and the conformal mapping technique, the expressions of complex potentials and stress fields are obtained analytically. The stress intensity factors (SIFs) near the crack tips and the critical SIFs for the first lattice dislocation emission from the branched crack tip are calculated. The effects of important parameters such as grain size, the length of internal crack, and the angle between the main crack and its branched crack on the critical SIFs for dislocation emission are evaluated in detail. As a result, the special rotational deformation has great influence on the growth of internal crack and the emission of lattice dislocations from the branched crack tip. The disclination quadrupole produced by the special rotational deformation will shield the branched crack tip under a certain condition. Moreover, when the main crack approaches its branched crack, it will stop the emission of lattice dislocations from the branched crack tip.

Stress-dependent deformation behaviour in bulk nanocrystalline titanium–nickel alloys

In this study, the deformation mechanisms operating with stress in bulk nanocrystalline (NC) titanium–nickel with an average grain size below a critical size of 10–20 nm have been investigated. We demonstrate a sequential variation of the deformation mechanism from grain boundary (GB) sliding and grain rotation to grain growth and dislocation activity with the increase of the deformation stress. These deformation mechanisms are different from the previous understanding that below a critical grain size of 10–20 nm, GB sliding and grain rotation govern plastic deformation of NC materials.

Free surface effects on rotational deformation in nanocrystalline materials

Free surface effects on rotational deformation mediated by grain boundary dislocations in nanocrystalline materials are theoretically described. The critical stresses and characteristic geometric parameters for the rotational deformation occurring in nanocrystalline materials near their free surfaces are calculated and compared with those specifying the rotational deformation in bulk regions. The role of the free surface effects in the interpretation of electron microscopy data for plastically deformed nanocrystalline materials is discussed.

Influence of nanoscale amorphization on emission of dislocations from a finite length crack tip in nanocrystalline materials

The interaction between nanoscale amorphization and crack in deformed nanocrystalline materials is investigated. Effects of nanoscale amorphization characterized by three disclination dipoles on the emission of lattice dislocations from the crack tip are theoretically investigated. The critical stress intensity factors (SIFs) for dislocation emission from the crack tip are calculated. The influences of nanoscale amorphization as well as crack length on the critical SIFs are discussed in detail. The results show that nanoscale amorphization can suppress lattice dislocation emission. The most probable angle of dislocation emission decreases with increment of disclination strength and the shorter length crack drivers easier the dislocation emission from the crack tip.

Simultaneous grain boundary motion, grain rotation, and sliding in a tricrystal

Grain rotation and grain boundary (GB) sliding are two important mechanisms for grain coarsening and plastic deformation in nanocrystalline materials. They are in general coupled with GB migration and the resulting dynamics, driven by capillary and external stress, is significantly affected by the presence of junctions. Our aim is to develop and apply a novel continuum theory of incoherent interfaces with junctions to derive the kinetic relations for the coupled motion in a tricrystalline arrangement. The considered tricrystal consists of a columnar grain embedded at the center of a non-planar GB of a much larger bicrystal made of two rectangular grains. We examine the shape evolution of the embedded grain numerically using a finite difference scheme while emphasizing the role of coupled motion as well as junction mobility and external stress. The shape accommodation at the GB, necessary to maintain coherency, is achieved by allowing for GB diffusion along the boundary.

Effect of nanograin boundary sliding on nanovoid growth by dislocation shear loop emission in nanocrystalline materials

A theoretical model is suggested to describe the effect of nanograin boundary (NGB) sliding at triple junctions of grain boundaries on nanovoid growth by incipient emission of dislocation shear loop from its surface in mechanically loaded nanocrystalline materials. In the framework of the model, NGB sliding deformation represents generation of disclination dipoles at triple junctions of NGBs. The analytical expression of critical stress for the first dislocation emission is derived. The effects of NGB sliding deformation, the sizes of nanograin and nanovoid on critical stress and the corresponding most favorable slip plane for dislocation emission are evaluated quantitatively in the deformed nanocrystalline materials. The results indicate that NGB sliding deformation releases, in part, the high stresses near the nanovoid, thereby nanovoid growth is slowed down or even arrested with the increment of the strength of NGB sliding deformation in nanocrystalline solids. There exists a range of nanovoid size to make critical stress keep almost unchanged with the appearance of NGB sliding deformation. There is also a critical nanograin size to make critical stress reach the maximum value, at which dislocation emission and nanovoid growth occur most difficultly. • Suggest a theoretical model to describe effect of NGB sliding on nanovoid growth. • Derive analytical expression of critical stress for the first dislocation emission. • Nanovoid growth is slowed down or even arrested due to NGB sliding deformation. • Effect of NGB sliding deformation on critical stress is evaluated. • Effect of sizes of nanograin and nanovoid on critical stress is studied.

Effect of cooperative grain boundary sliding and migration on dislocation emitting from a semi-elliptical blunt crack tip in nanocrystalline solids

A theoretical model is established to investigate the interaction between the cooperative grain boundary (GB) sliding and migration and a semi-elliptical blunt crack in deformed nanocrystalline materials. By using the complex variable method, the effect of two disclination dipoles produced by the cooperative GB sliding and migration process on the emission of lattice dislocations from a semi-elliptical blunt crack tip is explored. Closed-form solutions for the stress field and the force acting on the dislocation are obtained in complex form, and the critical stress intensity factors for the first dislocation emission from a blunt crack under mode I and mode II loadings are calculated. Then, the influence of disclination strength, curvature radius of blunt crack tip, crack length, locations and geometry of disclination dipoles, and grain size on the critical stress intensity factors is presented detailedly. It is shown that the cooperative GB sliding and migration and the grain size have significant influence on the dislocation emission from a blunt crack tip.

Effects of accommodated grain boundary sliding on triple junction nanovoid nucleation in nanocrystalline materials

A theoretical model is suggested to investigate the effect of grain boundary (GB) sliding with GB diffusion accommodation on triple junction nanovoids nucleation in deformed nanocrystalline materials. Within our description, at high strain rate, nanovoids are nucleated due to GB sliding which is non-accommodated at comparatively low temperature and effectively accommodated by GB diffusion process at elevated temperature. In this paper, the corresponding energy characteristics of the pile-up of GB dislocations caused by GB sliding and GB diffusion are calculated, respectively. Then, the nucleation of nanovoid at triple junctions is studied and the equilibrium radius of nanovoid is obtained. Calculation results show that: the nucleation and the equilibrium radius of the triple junction nanovoid both depend significantly on the shear stress, the length of the pile-up and the GB structures; the accommodated GB sliding results in the suppression of nanovoid generation, and thereby promotes the ductility deformation behavior in a nanocrystalline material.

Influence of nanotwin generation near crack twins on the fracture toughness of nanomaterials

A theoretical model of microscopic mechanisms of the nucleation and development of deformation twins in nanocrystalline materials has been developed. Within the model, we have studied the generation of deformation twins near crack tips, which occurs through multiple nanoscopic shears that represent nanoscopic regions of an ideal plastic shear. It has been shown that the nucleation of such nanotwins near crack tips reduces the high local stresses that arise near these tips. Thus, the generation and development of nanotwins near crack tips increases the fracture toughness of brittle nanocrystalline materials and serves as an efficient mechanism of improving the crack resistance of deformed nanocrystalline materials.

Effect of cooperative nanograin boundary sliding and migration on dislocation emission from a blunt nanocrack tip in nanocrystalline materials

Theoretical model is suggested that describes the effects of the cooperative nanograin boundary sliding and stress-driven nanograin boundary migration (CNGBSM) process on the lattice dislocation emission from an elliptically blunt nanocrack tip in deformed nanocrystalline materials. Within the model, CNGBSM deformation near the tip of growing nanocrack carries plastic flow, produces two dipoles of disclination defects and creates high local stresses in nanocrystalline materials. By using the complex variable method, the complex form expression of dislocation force is derived, and critical stress intensity factors for the first lattice dislocation emission are obtained under mode I and mode II loading conditions, respectively. The combined effects of the geometric features and strengths of CNGBSM deformation, nanocrack blunting and length on critical SIFs for dislocation emission depend upon nanograin size and material parameters in a typical situation where nanocrack blunting and growth processes are controlled by dislocation emission from nanocrack tips. It is theoretically shown that the cooperative CNGBSM deformation and nanocrack blunting have great influence on dislocation emission from blunt nanocrack tip.