Coordinated grain boundary deformation governed nanograin annihilation in shear cycling

Grain growth and shrinkage are essential to the thermal and mechanical stability of nanocrystalline metals,which are assumed to be governed by the coordinated deformation between neighboring grain boundaries(GBs)in the nanosized grains.However,the dynamics of such coordination has rarely been reported,especially in experiments.In this work,we systematically investigate the atomistic mechanism of coordinated GB deformation during grain shrinkage in an Au nanocrystal film through combined state-of-the-art in situ shear testing and atomistic simulations.We demonstrate that an embedded nanograin experiences shrinkage and eventually annihilation during a typical shear loading cycle.The continu-ous grain shrinkage is accommodated by the coordinated evolution of the surrounding GB network via dislocation-mediated migration,while the final grain annihilation proceeds through the sequen-tial dislocation-annihilation-induced grain rotation and merging of opposite GBs.Both experiments and simulations show that stress distribution and GB structure play important roles in the coordinated defor-mation of different GBs and control the grain shrinkage/annihilation under shear loading.Our findings establish a mechanistic relation between coordinated GB deformation and grain shrinkage,which reveals a general deformation phenomenon in nanocrystalline metals and enriches our understanding on the atomistic origin of structural stability in nanocrystalline metals under mechanical loading.     

Grain boundary complexions and pseudopartial wetting

The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.     

Size and microstructure effects on the mechanical behavior of FCC bicrystals by quasicontinuum method

The effects of structure and size on the deformation of <110> tilt bicrystals in copper are investigated by concurrent multiscale simulations at zero temperature. In the simulation of eleven grain boundary (GB) structures, a direct relation is shown between structural units and sliding at GBs. We find that GB sliding operates by atom shuffling events localized on one particular type of structural units, which are present in the GB period. When this type of unit is absent, the GB deformation process occurs by migration, or GB-mediated nucleation of partial dislocations with no sliding, depending on the initial GB configuration. The elastic limit causing sliding is found to vary slightly at zero temperature, but no correlation was obtained with the GB energy at equilibrium. Additionally, both modulus of rigidity, and elastic limit remain constant as the bicrystal size varies from 1 nm up to 25 nm. However, differences in the stress relaxation after sliding are observed with respect to the size.     

Grain growth prediction with inclination dependence of 〈1 1 0〉 tilt grain boundary using multi-phase-field model with penalty for multiple junctions

Multi-phase-field (MPF) model with a higher-order term representing energetic penalty for multiple junctions was proposed to predict the grain growth accompanying the inclination dependence of grain boundary (GB) energy and mobility. The inclination effect was introduced on the basis of GB energy obtained from molecular dynamics (MD) simulations. The preliminary grain growth simulation of an isolated grain surrounded by Σ3 GB certified that the analytical equilibrium shape was well reproduced. The augmented higher-order term added to conventional MPF model could improve convergence and stability of numerical calculations around triple junction (TJ) region even if there exists the large GB energy gap at the TJ. Moreover, the present MPF model can realize well the Young’s relation with no GB inclination effect and further extend to the case with that effect. For the polycrystalline grain growth simulations with the GB energy distribution according to the misorientation angle of Al 〈1 1 0〉 tilt GB, Σ3 GB inclination lead the weak anisotropy characterized by Σ3{111} twin boundary. Besides, the inclination dependence can effectively drive the GBs with low GB energy like the low-angle GB during grain growth.     

Solid-state wetting transitions at grain boundaries

A previously developed model of the dependence of grain boundary (GB) segregation on GB character has been exercised to investigate solid-state wetting transitions at GBs, and their anisotropy. In the case of binary systems displaying a solid-state miscibility gap, it is shown that the wetting transition temperature for precipitates at a GB is anisotropic, and is inversely related to GB energy. The model also allows calculation of prewetting transitions and associated excursions in adsorption off phase coexistence. These transitions are first order below a prewetting critical point (T_PWC), and higher order at temperatures above T_PWC. Investigation of the prewetting behavior provides the means for construction of the two-dimensional phase diagram of a GB.     

Grain boundary diffusion: recent progress and future research

Grain boundary (GB) diffusion often controls the evolution of structure and properties of engineering materials at elevated temperatures. A knowledge of diffusion characteristics of GBs and deep fundamental understanding of this phenomenon are critical to many materials applications. In this paper we give an overview of boundary diffusion theory with emphasis on the interpretation of concentration profiles measured in diffusion experiments. We consider the most important situations encountered in boundary diffusion experiments, such as diffusion in the B and C regimes and diffusion in the presence of segregation. We also discuss the recent progress in the atomistic interpretation of GB diffusion. We conclude with an outlook for future research in this area.     

An intrinsic correlation between driving force and energy barrier upon grain boundary migration

The migration of grain boundary (GB), which plays a key role in the microstructural evolution of polycrystalline materials, remains mysterious due to the unknown relationship between GB mobility associated with specific geometry and external conditions (e.g. temperature, stress, etc., hence the thermodynamic driving force). Combining the rate equation of GB migration with molecular dynamics simulations, an intrinsic correlation between driving force and energy barrier for the migration of various types of GBs (i.e. twist, symmetric tilt, asymmetric tilt, and mixed twist-tilt) is herein explored, showing the decrease of energy barrier with increasing thermodynamic driving force.     

Wetting and premelting of triple junctions and grain boundaries in the Al–Zn alloys

Phase transitions in grain boundaries (GBs) and GB triple junctions (TJs) can change drastically the properties of polycrystals. The GB and TJ wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid and solid phases are in equilibrium. The GB and TJ wetting tie-lines can continue in one-phase area of the bulk phase diagram as a GB or TJ solidus line. This line represents the GB or TJ premelting phase transition. The structure and composition of grain boundaries and GB triple junctions were studied by high-resolution electron microscopy and analytical transmission electron microscopy in the Al–5 at.% Zn polycrystals and by differential scanning calorimetry (DSC) in the Al–7.5 at.% Zn polycrystals. Between bulk solidus and GB or TJ solidus the metastable Zn-rich β_m-phase was observed in the GB triple junctions of quenched samples. This phase appears neither in the samples annealed above the bulk solidus nor in those annealed below the GB solidus. Zn-content in this β_m-phase corresponds to that of bulk liquidus. This is a structural indication that if the melt wets the GBs or TJs, the GB (or TJ) solidus line appears in the bulk phase diagram, and the liquid-like phase exists in GBs and TJs between bulk solidus and GB (or TJ) solidus lines. The structural observation of this phase is also supported by our data obtained by means of DSC.     

Amelioration of weld-crack resistance of the M951 superalloy by engineering grain boundaries

Fusion weld is a portable and economical joining and repairing method of metals.However,weld cracks often occur during the fusion weld of Ni-base superalloys,which hinder the applications of fusion weld on this kind of materials.In this work,the effects of microstructures of grain boundaries(GBs)of the prototype M951 superalloy on its weldability were investigated.The precipitated phases,the elemental segregations on GBs,and the morphologies of GBs can be largely altered by regulating the cooling rates of pre-weld heat treatments.With decreasing the cooling rate,chain-like M23X6 phase precipitates along the GBs,accompanying segregations of B,and GBs becomes more serrated in morphology.During fusion weld,the engineered GBs in the M951 superalloy with a low cooling rate favor the formation of the continuous liquid films on GBs,which together with the serrated GB morphology significantly prevents the formation of weld cracks.Our findings imply that the weld-crack resistance of the superalloys can be ameliorated by engineering GBs.     

Continuum metrics for deformation and microrotation from atomistic simulations: Application to grain boundaries

Motivated by a desire to incorporate micro- and nanoscale deformation mechanisms into continuum mechanical models of material behavior, we apply recently developed volume-averaged metrics to the results of atomistic simulations to investigate deformation and microrotation in the vicinity of grain boundaries. Three-dimensional bicrystalline structures are employed to study the inelastic deformation behavior under uniaxial tension and simple shear at a temperature of 10 K. Each bicrystal is constructed by molecular statics followed by thermal equilibration under NPT using an embedded atom method potential for copper. Strain is imposed in each simulation cell at a constant 10⁹ s⁻¹ strain rate applied perpendicular and parallel to the grain boundary plane for tension and shear, respectively. A variety of grain boundary deformation mechanisms arise and the resulting deformation and microrotation fields are examined. We also include an analysis showing how microrotation varies as a function of distance from the grain boundary with increasing strain for different grain boundary deformation mechanisms. This work demonstrates that critical interface behavior can be extracted from atomistic simulations using volume-averaged metrics, offering a potential avenue for translating fundamental information to continuum theories of grain boundary deformation in polycrystalline materials.     

Grain boundary optimization induced substantial squareness enhancement and high performance in iron-rich Sm-Co-Fe-Cu-Zr magnets

Increasing iron content has been witnessed an essential method to improve the remanence of 2:17-type Sm-Co-Fe-Cu-Zr magnets,however,the inferior squareness factor accompanied with the increased iron content turns into a neck sticking problem.In this work,the grain boundary optimization induced sub-stantial squareness enhancement from 63.4％to 91.4％,and consequently an excellent maximum energy product of 32.63 MGOe have been achieved in iron-rich Sm-Co-Fe-Cu-Zr magnets via tuning solution process.It is clearly revealed that the grain boundary(GB)phases as well as the micro-twins'density in grain interiors can be controlled and interprets the enhancement mechanism of squareness.     

Combined atomistic and mesoscale simulation of grain growth in nanocrystalline thin films

We have combined molecular-dynamics (MD) simulations with mesoscale simulations to elucidate the mechanism and kinetics of grain growth in nanocrystalline palladium with a columnar grain structure. The conventional picture of grain growth assumes that the process is governed by curvature-driven grain-boundary (GB) migration. Our MD simulations demonstrate that, at least in a nanocrystalline material, grain growth can also be triggered by the coordinated rotations of neighboring grains so as to eliminate the common GB between them. Such rotation–coalescence events result in the formation of highly elongated, unstable grains which then grow via the GB migration mechanism. These insights can be incorporated into mesoscale simulations in which, instead of the atoms, the objects that evolve in space and time are discretized GBs, grain junctions and the grain orientations, with a time scale controlled by that associated with grain rotation and GB migration and with a length scale given by the grain size. These mesoscale simulations, with physical insight and input materials parameters obtained by MD simulation, enable the investigation of the topology and long-time grain-growth behavior in a physically more realistic manner than via mesoscale simulations alone.     

Crystallographic features of phase transformations in solids

The paper reviews the current knowledge and understanding of the crystallographic features of phase transformations in solid materials – metals, ceramics and alloys. It covers both of the broad classes of phase transformations in crystalline solids – martensitic or ‘displacive’ and ‘diffusional’ or ‘reconstructive’. The factors that govern the crystallographic features of these two classes of transformations are compared and contrasted. This provides an appropriate basis for examining the ‘diffusional–displacive’ transformations that appear to exhibit the characteristics of both classes. After a brief summary of the considerable body of experimental data available on the crystallographic characteristics of these various types of phase transformation, the different models/theories advanced to account for these observations are discussed. The main emphasis is on those models/theories that are capable of predicting, rather than just rationalising or explaining, these crystallographic features. The review purposely adopts a unifying approach and attempts to reconcile the controversy that has on occasions existed between the ‘displacive’ group and the ‘diffusional’ group – particularly in respect of the ‘diffusional–displacive’ transformation. Developing a comprehensive understanding of the crystallographic features of all classes of phase transformations is obviously the ultimate goal. The review concludes by assessing how close we are to this final achievement, identifies the gaps in current knowledge and suggests future work.     

Effect of boron and sulphur on the electronic structure of grain boundaries in Ni

The first-principles discrete variational method is employed to study the effect of boron and sulphur on the electronic structure of the Ni grain boundary (GB). The calculated results show that boron does not strongly influence (only slightly decreases) the bonding between the atoms of the metal. In addition, B forms the strong bonding state with its neighbouring metal atoms. Our study also indicates that S strongly decreases the bonding between the atoms of the metal, and that the bonding tendency between S and the atoms of the metal across the GB plane is very weak. The calculations of environment-sensitive embedding energies show B has the strong site-competition ability and can successfully drive out S from the GB region. We conclude that the influence of impurities segregating on the GBs is closely associated with their effects on: (i) the decrease of the bonding between the atoms of the metal due to the presence of impurities; (ii) the bonding between the impurity atom and the atoms of the metal; and (iii) the site-competition ability of impurity atoms.     

Effect of Interstitial Hydrogen on Cohesive Strength of Al Grain Boundary with Mg Segregation

The effect of interstitial hydrogen on the cohesion of the Al ∑ =11（113） grain boundary （GB） is investigated based on the thermodynamic model of Rice-Wang using the first-principles density tunction calculation. I he results indicate that interstitial H behaves as an embrittler from ＂strengthening energy＂ analysis. The reduced GB cohesion due to the presence of H at the GB is attributed to the low affinity between H and Al, and the weakened bonding of Al atomic pairs perpendicular to GB plane.     

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.     

Correlation of Magnetic Properties of Co/Cr Bilayer Thin Films with Grain Boundary Diffusion

The microstructure and magnetic properties of Co/Cr bilayer films were examined before and after postdeposition annealing by using transmission electron microscopy (TEM), X-ray diffraction (XRD) technique and vibrating sample magnetometer (VSM). A model of grain boundary (GB) Cr-rich phase growth involving GB diffusion derived from the Cr underlayer was proposed to elucidate the kinetics of the paramagnetic Crrich phase growth along Co GBs within the Co layer. The correlation of the GB Cr-rich phase formati...     

Controlling the strength of Zr(10 1 2) grain boundary by nonmetallic impurities doping: A DFT study

Impurity segregation even small amounts,can drastically change the cohesive properties of the grain boundaries(GB),eventually leading to intergranular embrittlement and failure of the materials,thereby effectively controlling the types and the concentrations of the impurity is very important.In this work,the nonmetallic impurities(C,H,O,N) segregation and their effects on the strength of Zr(10 1 2) GB were thoroughly investigated using first-principles calculations based on density functional theory.A comprehensive analysis of the interstitial configurations and the relative site energies indicating that C,N and O overwhelmingly prefer the octahedral sites,only H,prefers to reside in the tetrahedral sites.Moreover,the strengthening/embrittlement potency of impurity atoms on the GB was estimated using both the Rice-Wang model and first-principles tensile test calculations.The results show that all impurities,exhibit a strong segregation tendency near the GB region.The segregation of C,N and O has a remarkable strengthening effect on strength of the GB,whereas the presence of impurity H weaken the GB.Most importantly,the underlying mechanism of the strength change of the GBs due to the segregation of impurities was profoundly discussed by charge density and the bond lengths analyses,revealing that the strengthening effect especially for C-doped GB,mainly comes from an enhancement of the charge density across the GB plane.In the end,we expect that our results will be certainly useful for future theoretical and experimental investigations on Zr and its alloys.     

Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study

Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.     

Computer modeling of grain boundaries in Ni3Al

Computer simulation of symmetric tilt grain boundaries (GB) Σ=5 [1 0 0] (0 1 2) and Σ=5 [1 0 0] (0 1 3) in alloy Ni₃Al was carried out. The energy of GB was calculated out by a method of construction of γ-surface with using Morse's empirical central-force potentials. Investigations shown that GB have several steady states: one is stable and other – metastable. These states differ by energy and atomic structure GB. It is shown, that GB in model CSL are unstable, the stabilization is achieved by additional displacement on some vector along plane of defect. Directions and value of potential barriers of GB slippage are calculated.