Grain Boundary Kinetics Research Articles

The conflicts between strength and ductility of bimodal Ti-5553 alloy with fine equiaxial prior β grains

In the paper, homogeneous bimodal Ti-5553 alloy with the prior-β grain size of 15 μm is successfully fabricated by elaborate thermo-mechanical processing in α+β field, solution and ageing, in order to explore the combination of strength and ductility accessible to β-titanium alloys. The strengthening curve of ageing at 500°C comprises the incubation period, the strengthening period and the stable period. The attainable yield strength of the alloy aged at 500°C is 1430 MPa, associating with a receivable tensile elongation of 9.5%. During the ageing at 620°C the strength of the bimodal alloy first increases with ageing time, then reaches a maximum value of 1079 MPa after 2 h and finally remains constant within the ageing period from 2 h to 32 h. It is unexpected that the tensile elongation of bimodal alloys aged at 620°C are the same as that of the solution-treated alloy. Interestingly, the ageing at 700°C dose not exhibit the strengthening effects but could make the alloy more ductile. Within the ageing temperature scope of 500°C∼700°C, the precipitating kinetics of grain boundary α (αGB) precipitation is more active than that of the intragranular α (αIntra) precipitation. However, αGB does not have the precipitation strengthening effects. The yield strength of bimodal Ti-5553 alloy depends on the dislocation density of β phase and the size effects of prior β grains. By exploring the tensile damage sources, we find that the ductility of bimodal Ti-5553 alloy of is influenced by the plasticity of αGB and the stress concentration of β/αIntra interface. The combination of strength and ductility of high-strength bimodal Ti-5553 alloy might be further improved by optimizing the thickness of αGB.

Segregation‐controlled densification and grain growth in rare earth‐doped Y2O3

AbstractCation doping of Y2O3 is an established approach for tailoring densification and grain growth during sintering. However, the segregation of doped cations to the grain boundary and their impact on processing are still not completely understood. Segregation can be driven by electrostatic effects due to charge mismatch with the host lattice or elastic effects induced by ion size mismatch. While segregation is caused by thermodynamics, it impacts diffusion and the kinetics of grain boundaries during densification and microstructure evolution. In this study, we utilize two isovalent dopants (La3+ and Gd3+), that is we focus on the elastic component of segregation. We investigate the densification as well as the grain growth kinetics of both doped and undoped Y2O3 during field‐assisted sintering/spark plasma sintering (FAST/SPS). While Gd3+ is showing no significant effect on densification, La3+ resulted in a strongly reduced sintering activity. Furthermore, the analysis of the grain growth behavior during sintering and on predensified samples revealed a decrease in the grain growth coefficient, with La3+ having the strongest impact. The structure and chemistry at the grain boundary were observed by aberration‐corrected TEM. While no structural change was caused by doping, the chemical analysis showed a strong segregation of La3+ to the grain boundary, which could not be observed for Gd3+. The results indicate that segregated La3+ causes a drastic decrease in grain boundary migration rates through solute drag as well as much slower sintering kinetics, likely caused by a decrease in the grain boundary self‐diffusion due to segregation. This study further underlines the importance of the elastic contribution to cation segregation and establishes a clear relationship to grain growth and sintering kinetics, which are both decreased by segregation.

A general mechanism of grain growth ─Ⅰ. Theory

The behaviors of grain growth dominate the formation of the microstructure inside polycrystalline materials and thus strongly influence their practical performances. However, grain growth behaviors still remain ambiguous and thus lack a mathematical formula to describe the general evolution despite decades of efforts. Here, we propose a new migration model of grain boundary (GB) and further derive a mathematical expression to depict the general evolution of grain growth in the cellular structures. The expression incorporates the variables influencing growth rate (e.g. GB features, grain size and local grain size distribution) and thus reveals how the normal, abnormal and stagnant behaviors of grain growth occur in polycrystalline systems. In addition, our model correlates quantitatively GB roughening transition with grain growth behavior. The general growth theory may provide new insights into the GB thermodynamics and kinetics during the cellular structure evolution.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Summary Grain boundaries (GBs) determine the properties of polycrystals, and tailoring the GB structure offers a promising method for the discovery and engineering of new materials. However, GB structures are far from well understood because of their structural complexity and limitations of conventional projection imaging methods. Here, we decipher three-dimensional atomic structure and crystallography of GBs in nanometals using atomic-resolution electron tomography. Unlike conventional descriptions, whereby they are either straight or curved planar planes with one-dimensional translational symmetry, we show that the high-angle GBs completely lose translational symmetry due to undulated curvature related to configurations of structural units. Moreover, we directly visualize kinks and jogs at the single-atom scale in dislocation-type GBs and investigate their mobilities. Our findings bring new insights to the conventional wisdom of GBs and show the importance of developing methods to include the non-planar nature of GBs to statistically evaluate the behavior of GBs in modeling studies.

S-PFM model for ideal grain growth

The phase-field method is increasingly used for studying grain growth in metallic alloys. Such an approach offers a thermodynamically consistent framework for studying microstructure evolution during grain growth without the need to explicitly track the interface positions. However, the numerical solution of the models requires a grid spacing much smaller than the interface width. This leads to computationally very intensive simulations, especially when a large number of grains is required to accurately measure statistical quantities. The recently proposed S-PFM approach provides a new inherently discrete formulation of phase field models where the interface width can be as small as the grid spacing, thus drastically improving the numerical performances of the method. Here, we show that this approach can be extended to a multi-phase field model for ideal grain growth. Then, we perform two dimensional simulations to analyse in detail the kinetics of both grain boundaries and triple junctions. We compare our model to a classical phase field formulation and we demonstrate that, for a prescribed accuracy, the memory requirement and simulation times are both reduced by a factor of 4D, where D is the space dimension. Finally, we perform a large-scale simulation of grain growth in two dimensions and show that the method is able to reveal the specific shape of the grain size distribution in the scale-invariant regime displaying two peaks around the mean grain size and quantitative measurements of the underlying topological classes are performed.

Shear-assisted grain coarsening in colloidal polycrystals

Grain growth under shear annealing is crucial for controlling the properties of polycrystalline materials. However, their microscopic kinetics are not well understood because individual atomic trajectories are difficult to track. Here, we study grain growth with single-particle kinetics in colloidal polycrystals using video microscopy. Rich grain-growth phenomena are revealed in three shear regimes, including the normal grain growth (NGG) in weak shear melting-recrystallization process in strong shear. For intermediate shear, early stage NGG is arrested by built-up stress and eventually gives way to dynamic abnormal grain growth (DAGG). We find that DAGG occurs via a melting-recrystallization process, which naturally explains the puzzling stress drop at the onset of DAGG in metals. Moreover, we visualize that grain boundary (GB) migration is coupled with shear via disconnection gliding. The disconnection-gliding dynamics and the collective motions of ambient particles are resolved. We also observed that grain rotation can violate the conventional relation [Formula: see text] (R is the grain radius, and θ is the misorientation angle between two grains) by emission and annihilation of dislocations across the grain, resulting in a step-by-step rotation. Besides grain growth, we discover a result in shear-induced melting: The melting volume fraction varies sinusoidally on the angle mismatch between the triangular lattice orientation of the grain and the shear direction. These discoveries hold potential to inform microstructure engineering of polycrystalline materials.

Growth kinetics of σ-phase precipitates and underlying diffusion processes in CrMnFeCoNi high-entropy alloys

Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e., aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[(-300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 5.0 × 10-3 exp[(-205 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[(-145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys.

Simulating Thermodynamic Stabilization in Nanocrystalline Binary Alloys Using a Novel Cellular Automaton Model

Predictive models for grain growth of nanocrystalline binary alloys are designed to select appropriate solutes and assess thermodynamic stabilization in nanoscale alloy systems. The available models incorporate concepts of free energy, solute segregation, size-misfit elastic strain energy, grain boundary energy and phase field framework. Besides the above factors, the present work proposes a novel cellular automaton model by considering normal grain boundary (GB) diffusion, triple junction GB kinetics and grain misorientation. The experimental data for two kinds of binary alloy system were used to validate the reasonability of the proposed model. For nanocrystalline Fe-4% Zr alloy with large atomic size mismatch and negative mixing enthalpy, compared with the available models, the proposed model shows a better fit to the experimental results for grain size as a function of annealing temperatures. For another binary alloy system with small atomic size mismatch and positive mixing enthalpy, the proposed model also captures well the measurements for grain size of nanocrystalline W-20% Ti alloy. The comparisons reveal that the proposed model has a wide application in addressing the thermal stabilization of nanocrystalline grain size.

A process-microstructure finite element simulation framework for predicting phase transformations and microhardness for directed energy deposition of Ti6Al4V

This paper presents a process-microstructure finite element modeling framework for predicting the evolution of volumetric phase fractions and microhardness during laser directed energy deposition (DED) additive manufacturing of Ti6Al4V. Based on recent experimental observations, the present microstructure evolution model is formulated to combine the formation and dissolution kinetics of grain boundary, Widmanstätten colony/basketweave, massive/martensitic alpha and beta phases of Ti6Al4V. The microstructure evolution algorithm is verified and embedded into a three-dimensional finite element process simulation model to simulate thermally driven phase transformations during DED processing of a Ti6Al4V thin-walled rectangular sample. The microhardness values of different locations of the part, which experience different thermal histories, are computed based on the simulated fractions and hardness values of different phases in the final microstructure. The simulated volumetric phase fractions and related microhardness distribution agree reasonably well with experimental measurements performed on the sample. Thus the proposed simulation model could be useful for designers to understand and control process-microstructure-property relationships in a DED-processed part.

Growth Kinetics of Σ-Phase Precipitates and Underlying Diffusion Processes in Crmnfeconi High-Entropy Alloys

Key mechanisms and elementary diffusion processes that control the growth kinetics of σ precipitates in high-entropy alloys were investigated in the present study. For this purpose, an off-equiatomic Cr26Mn20Fe20Co20Ni14 alloy with an initially single-phase FCC structure was subjected to isothermal heat treatments, which are known to promote the formation of σ phase, i.e. aging between 600 °C and 1000 °C for times ranging from 0.1 h to 1000 h. The growth kinetics of σ precipitates at grain boundaries of the FCC matrix and those located within the interior of the grains were analyzed separately. The latter precipitates are found to grow through direct substitutional diffusion of Cr-solutes towards and Mn, Fe, Co, and Ni away from them and the growth rate of the allotriomorphs can be rationalized by the collector plate mechanism of interfacial diffusion-aided growth. From the growth-kinetics data obtained in the present study, lattice interdiffusion coefficients as well as diffusivities along crystalline defects were obtained. Above 800 °C, the growth kinetics are dominated by lattice interdiffusion of Cr in the FCC matrix described by DL = 9.8 × 10-4 exp[( 300 kJ/mol)/(RT)] m2/s. At lower temperatures, the growth kinetics are enhanced by fast interdiffusion along dislocation pipes, which temperature dependence is given by DD = 2.5 × 10-3 exp[( 210 kJ/mol)/(RT)] m2/s. The Cr-diffusivity along σ/FCC interphase boundaries deduced from the thickening kinetics of grain boundary precipitates can be represented by the Arrhenius relationship DI = 0.5 × 10-4 exp[( 145 kJ/mol)/(RT)] m2/s, which is similar to that found for grain boundary interdiffusion in metals and alloys.

Dislocation-controlled formation and kinetics of grain boundary loops in two-dimensional crystals

The formation and kinetics of grain boundaries are closely related to the topological constraints imposed on their complex dislocation structure. Loop-shaped grain boundaries are unique structures to establish such a link because their overall topological "charge" is zero due to their null net Burgers vector. Here, we observe that a local rotational deformation of a 2D colloidal crystal with an optical vortex results in a grain boundary loop only if the product of its radius and misorientation exceeds a critical value. Above this value, the deformation is plastic and the grain boundary loop spontaneously shrinks at a rate that solely depends on this product, while otherwise, the deformation is elastically restored. We show that this elastic-to-plastic crossover is a direct consequence of the unique dislocation structure of grain boundary loops. At the critical value, the loop is structurally equivalent to the so-called "flower defect" and the shrinkage rate diverges. Our results thus reveal a general limit on the formation of grain boundary loops in 2D crystals and elucidate the central role of defects in both the onset of plasticity and the kinetics of grain boundaries.

Effect of oxygen partial pressure on grain-boundary transport in alumina

The oxidation of the bond coat in a thermal barrier coating results in the formation of a protective alumina scale. Such scales experience large gradients in oxygen partial pressure (PO2) during formation, but it is unclear how PO2 affects grain-boundary (GB) transport in alumina. To study the effect of PO2 on GB kinetics in alumina, the rate of oxygen penetration was measured at 1400 °C over a wide range of PO2. This was accomplished using a novel reduction experiment using nickel aluminate spinel particles. The results indicated that the dependence of the boundary transport on PO2 is mainly due to the driving force, namely the gradient in PO2 across the alumina. Point-defect models were then invoked to interpret the measurements, and it was found that transport in an alumina scale is controlled by extrinsic defects.

Mechanistic Approach to Predict the Combined Effects of Additives and Surface Templates on Calcium Carbonate Mineralization

The biomineralization of calcium carbonate is masterfully directed by organic macromolecules present in many organisms. In this biomimetic study, absorbance measurements accompanied by microscopy and infrared spectroscopy are used to evaluate the superposed effect of organic additives and surface chemistries on the kinetics of surface-directed nucleation and growth of calcium carbonate. Polyelectrolyte films with carboxylic and/or amine groups serve as the organic template for mineralization, and amino acids and monosaccharides as the selected solution additives. A grain-boundary kinetics model describes surface precipitation of calcium carbonate to provide a mechanistic insight into the precipitation pathway via two parameters, the near-surface supersaturation and the crystal number density. While an organic matrix rich in ternary amines strongly promotes vaterite nucleation, the selected carboxylic-enriched polyelectrolyte film significantly stabilizes ACC in the near-surface region, while it equally promotes vaterite and calcite nucleation. The combined effect of organic additive and surface template determines the near-surface supersaturation. Soluble additives can also be directly involved in surface nucleation if they strongly interact with the organic interface. Our mechanistic approach reveals two different precipitation pathways that result from the synergy between surface template and organic additives.

Analysis of Local and Global Segregation Occurring in Grain Boundary Diffusion

It is generally well recognized that in the course of a grain boundary (GB) diffusion experiment the diffusion of solute atoms in grain boundaries must exhibit a strong time-dependent segregation. But there has been no clear understanding of exactly how this time dependence develops. In this chapter, we review and analyse transient solute GB diffusion by means of the computer simulation technique of Lattice Monte Carlo (LMC). This technique has been successfully used on numerous occasions for the purposes of systematically studying the GB transition regimes that occur between the principal well-defined Harrison GB kinetics regimes (A, B and C-Types). Recently, the analysis using LMC has been extended to the case of solute GB diffusion when the segregation factor is independent of time. In the present paper, we analyse two cases of solute segregation in GB diffusion: first, where the solute atoms are homogeneously distributed along the tracer source plane but their mobility is not high at this plane; and the second, where the mobility of the solute atoms along the tracer source plane is comparable to their mobility along the GB. It is shown that the time dependence of the segregation can contribute significantly into the resulting values of the triple-product that is usually obtained experimentally in the Harrison Type-B kinetics regime.

Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films.

In this study, the contribution of grain boundaries to the oxygen reduction and diffusion kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. (18)O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen diffusion but also fast oxygen exchange kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.

Role of the mesoscale in migration kinetics of flat grain boundaries

Role of the mesoscale in migration kinetics of flat grain boundaries

Thermodynamic and Kinetic Properties of Grain Boundary Junctions

The results of recent experimental, theoretical and computer simulation studies of the thermodynamics and kinetics of grain boundaries and grain boundary junctions are presented and discussed.

Effect of Fe-Rich Particles on the Formation of Die Streaks on Anodized Aluminum Extrusions

Fe-rich intermetallic particles are one of the important constituents in the microstructure of 6xxx series aluminum extrusions. The size, amount, and distribution of these particles can be different in different locations of the extrudate due to inhomogeneous deformation during the extrusion process. Furthermore, these intermetallic particles change the dissolution kinetics of grain interiors and grain boundaries during etching, influencing the formation and severity of grain boundary grooves. An inhomogeneous distribution of Fe-rich particles results in a nonuniform distribution of grain boundary grooves and occurrence of die streaks.

Superplasticity governed by effective grain size and its distribution in fine-grained aluminum alloys

► Different grain-sized Al–Mg–Sc alloys were produced by friction stir processing. ► Medium-grained sample exhibited the highest elongation and strain rate. ► Superplasticity was governed by the effective grain size and its distribution. ► A modified constitutive equation for superplasticity was developed. ► Enhanced deformation kinetics is associated to high ratio of high-angle boundaries. Although many efforts have been made to improve superplasticity in aluminum alloys by reducing grain size, the present study indicated that just reducing grain size was not sufficient. Three different grain-sized Al–Mg–Sc alloys with a similar misorientation distribution were fabricated by friction stir processing (FSP) and subjected to superplastic investigation. It was revealed that the medium-grained sample exhibited the largest elongation at the highest strain rate due to the optimum combination of proper fine grain and thermal stability. In-depth analyses showed that superplasticity was governed by the effective grain size ( d eff ) and its distribution just before tension rather than the initial grain size ( d ini ). The analyses on the superplastic data indicated that the large variation range in the dimensionless constant for superplastic deformation of FSP aluminum alloys was associated with the use of d ini rather than d eff . A modified constitutive equation was developed to elucidate the d eff dependencies of superplastic flow stress which contributed to obtaining the true dimensionless constant and supporting the suggestion that the enhanced kinetics of grain boundary sliding was associated with high fraction of high-angle grain boundaries.

Grain Boundary Diffusion of Fe in High-Purity Copper

Grain boundary (GB) diffusion of 59Fe in high purity polycrystalline copper was measured using the radiotracer technique and precision parallel sectioning in an extended temperature interval. The results of diffusion measurements below 900 K are consistent with Harrison's C kinetics and yield the GB diffusion coefficient of Fe in Cu with the pre-exponential factor of 5.610-6 m2/s and the activation enthalpy of 121 kJ/mol. Unexpectedly strong GB segregation of Fe in Cu hindered reliable determination of the Fe diffusivity in the B kinetics. Additionally, unconventional penetration profiles were measured for GB diffusion of 59Fe in Cu at high temperatures. Molecular dynamics simulation with the literature Finnis-Sinclair type interatomic potentials was performed to shed light into the observed features. A strong effect of the Fe coverage on GB structure and kinetics in pure Cu is predicted at increased temperatures above 900 K.