Boundary Migration Rate Research Articles

Saturation of Grain Fragmentation upon Severe Plastic Deformation: Fact or Fiction?

There has been general agreement that grain refinement upon severe plastic deformation (SPD) saturates at equivalent strains of 10–20, as a dynamic equilibrium between refinement and coarsening is established. Meanwhile, few reports question such steady state, but suggest another strain hardening regime might be entered for strains >1000. So far, neither an in‐depth analysis nor a general theory for such ultra‐SPD strain hardening has been established. The present work provides clear evidence for additional strain hardening at ultra‐severe strains. Although at this stage the strain hardening rate is awfully weak (≈0.03 MPa), it manifests in noticeable grain refinement and hardness increase. Texture and the existence of subgrains still support dislocation‐based plasticity. Specimens deformed to ultra‐severe strains possess improved thermal stability. Although an unambiguous conclusion regarding the origin of the ultra‐SPD strain hardening is currently not possible, the potential mechanisms are being discussed. While continuous impurity uptake from the anvils could explain the hardening and improved thermal stability, estimation of grain boundary migration rates suggests that a slight but continuous net refinement is also plausible. Together with structural transformations of grain boundaries, this offers an alternative, intrinsic source for ultra‐SPD hardening. It is hoped that this thought‐provoking conclusion stimulates further research into this subject.

Effect of Ti content on abnormal grain growth of Fe–Mn–Al–Ni–Ti shape memory alloy

In this paper, the effects of Ti content on the solvus temperature of γ-phase and abnormal grain growth (AGG) in Fe43.5−x Mn34Al15Ni7.5Ti x (x = 0, 0.5, 1 and 1.5) shape memory alloys (SMAs) were investigated. It is found that, the increase of Ti content leads to a significant reduction of the solvus temperature of γ-phase, a significant refinement of γ-phase, and a decrease of subgrain size. After 3 times cyclic heat treatments, the average grain size of Fe42Mn34Al15Ni7.5Ti1.5 SMA reaches about 9.0 mm, which is about twice of that for Fe42.5Mn34Al15Ni7.5Ti1 SMA. This is attributed to the small subgrains can provide a higher subgrain boundary energy (ΔG s) and grain boundary (GB) migration rate. The subgrain size of Fe42Mn34Al15Ni7.5Ti1.5 SMA (9.7 μm) is significantly smaller than that of Fe42.5Mn34Al15Ni7.5Ti1 SMA (21.3 μm). Thereby, the ΔG s (15.3 × 10−2 J mol−1) and GB migration rate (11.3 × 10−6 m s−1) of Fe42Mn34Al15Ni7.5Ti1.5 SMA are significantly higher than those of Fe42.5Mn34Al15Ni7.5Ti1 SMA (7.1 × 10−2 J mol−1, 6.3 × 10−6 m s−1). In addition, when the applied strain was up to 10%, the maximum superelastic strain of Fe42Mn34Al15Ni7.5Ti1.5 and Fe42.5Mn34Al15Ni7.5Ti1 were 5.5% and 5.1%, respectively. In summary, the addition of 1.5 at.% Ti in Fe–Mn–Al–Ni–Ti SMA can promote the AGG with relatively small loss in superelasticity.

Microstructure evolution and EBSD analysis of multi-principal element alloy coatings after high-temperature oxidation

Multi-principal element alloy coatings (MPEACs) have great application potential for high-temperature diffusion barrier coatings. The sluggish diffusion and lattice distortion of this new coating material due to its own atomic size effect give the coating excellent resistance to high temperature oxidation and grain coarsening. In this study, the high-temperature oxidation properties, and microstructure evolution of quaternary, quinary, senary, and denary alloy coatings (whose lattice distortion magnitude is linearly increased) at 1100 °C were studied. The evolution of grain orientation, size, and grain boundaries of alloy coatings after high temperature exposure were analyzed by EBSD. The results showed that from quaternary to denary alloy coatings, with the rise in lattice distortion magnitude of the alloy system, the sluggish diffusion ability of the coating at high temperatures was more obvious. Specifically, the oxidative activation energy of quaternary to denary alloy coatings is 355.41, 480.96, 478.76, and 654.03 kJ/mol, respectively. A high distortion magnitude is beneficial to increase the activation energy of the alloy coating, as a result, grain growth of alloy coatings with high activation energy is significantly inhibited, and under the combined action of storage strain energy and solute dragging, the grain boundary migration rate of the coatings also declines. Finally, the denary alloy coating shows good resistance to high temperature softening. This study provides a positive reference for promoting the application of multi-principal alloys in diffusion barrier coatings.

A novel low thermal conductivity refractory aggregate for high-temperature applications: Lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4)

This study aimed to develop lightweight refractory aggregates with high closed porosity and low thermal conductivity. We report the preparation of a series of lightweight microporous alumina-rich spinel (Mg0.4Al2.4O4) aggregates with low open porosity, high closed porosity, small intragranular pore size, and low thermal conductivity. The sintering activity of the nano-zirconia sol increased the grain boundary migration rate under the action of surface tension during sintering; therefore, the pores were closed before they were eliminated. Simultaneously, Zr4+ replaced Mg2+ and Al3+, which increased the number of cation vacancies in the crystal and accelerated the ion migration, resulting in a faster formation of Mg0.4Al2.4O4. When 0.75 wt% nano-zirconia sol was added, it increased the diffusion rate of ions and the migration rate of grain boundary, which improved the sintering reactivity. The pores were mainly small-sized intragranular pores (median size: 1.89 μm), which reduced the thermal conductivity (2.5 W•m−1K−1 at 1000 °C).

On the basic mechanism of grain growth: the role of texture on grain boundary migration rate

In this paper we will review some of the many achievements of Statistical Theory of Grain Growth (which is 40 years old, this year!), based on the classic law of grain growth. For instance, we will show that grain growth in presence of texture (which is the usual feature in a real microstructure) is not exclusively depending on the curvature of the grain boundary, as in this case the effect of curvature at the vertexes (in 2-D case) contributes as well (which, depending on the texture pattern, sometimes may be more relevant than curvature along the boundary). Such phenomenon requires a reformulation of Von Neuman’s equation in 2-D by involving, beside the different boundary mobilities, the different energies of the “third” grain boundary, which is pulling the vertex and inducing an extra curvature which is not balanced along the grain perimeter (for simplicity’s sake we are referring to a 2-D case, but the analysis can also be done in 3-D). We can then observe that boundary movement is not only depending on a couple of grains in contact (with the boundary curvature acting like in a bicrystal) but also includes the network of “third” grains operating at the vertexes. The overall consequence is that the growth kinetics and the grain size distribution shape are unique and variable for each texture component which contradicts classic growth laws as well as the expected results for the whole grain size distribution.

Impact of sintering thermal history on MgO‐doped alumina: Reduction of grain growth constant through high heating rates

AbstractThis work examines the impact of sintering thermal history on kinetics of grain growth for 1264 ppm (0.1 wt%) MgO‐doped alumina. Results suggest that sintering with a high heating rate in the order of ∼103°C min−1 retards grain growth rate. By contrast, a slow heating rate of 5°C min−1 resulted in an increase in grain growth constant by a factor of ∼4. The segregation of Mg to grain boundaries, favored by short sintering duration, is considered to be one of the possible factors contributing to the retarded grain growth rate for the rapidly heated compacts. Furthermore, it is proposed that the simultaneous combination of retarded grain boundary migration rate and limited surface‐diffusion‐driven grain coarsening, promoted by rapid heating, enables pressure‐less fabrication of translucent alumina without a need for vacuum or hydrogen atmosphere. In fact, it is demonstrated that weakly translucent alumina can be produced at relatively low temperatures of 1475–1550°C by sintering with a high heating rate (herein, ∼103°C min−1) in ambient atmosphere.

Martensitic transformation dominated tensile plastic deformation of nanograins in a gradient nanostructured 316L stainless steel

We systematically investigated the tensile deformation mechanisms and mechanical properties of a gradient nanograined AISI 316 L stainless steel prepared by means of surface mechanical grinding treatment. The gradient nanograined samples exhibit a good synergy of high strength and remarkable tensile ductility at room temperature. Microstructural observations revealed that martensitic transformation co-operating with grain boundary migration accommodates the tensile plastic deformation of nanograins in the topmost layer of gradient nanograined 316 L SS, which is distinct from the traditional partial dislocation associated deformation twinning in homogeneous coarse-grained counterpart under tension loading. Accompanied by the decrease in grain boundary migration rate, the martensite content increases significantly with increasing tensile strain, reaching ∼50% at a true strain of 50%. The newly formed martensites as strengthening phase not only provide dynamic work hardening, but also effectively suppress the strain localization, resulting in the substantial tensile ductility of the gradient nanograins.

Dissolution mechanism of magnesium aluminate spinel in copper smelting slag based on the shrinking core reaction models of corrosion

MgAl2O4 spinel is a critical raw material in spinel refractory production. However, the dissolution behavior of the spinel and its interactions with Cu smelting slag have not been previously studied in detail in terms of thermodynamics and kinetics. Therefore, to investigate the interactions of MgAl2O4 spinel in Cu smelting slag, MgAl2O4 spinel was simulated in Cu smelting slag using FactSage. Additionally, the dissolution mechanism of MgAl2O4 spinel in Cu smelting slag was studied via a slag immersion method. It was found that the Fe ions of the Cu smelting slag diffused mutually with the Mg2+ and Al3+ in the spinel, and the final products of the spinel reaction with the slag were ASpinel (Mg1-xFexAl2-yFeyO4) solid solution and liquid slag phases. The dissolution mechanism was consistent with the shrinking core reaction models. In the early stage of dissolution, ASpinel (Mg1-xFexAl2-yFeyO4) was generated at the interface, and the dissolution mechanism was controlled by the rate of solid solution phase generation, with an activation energy of E1 = 153.17 kJ/mol. At the later stage of dissolution, the Fe ions continued to diffuse into the spinel through the solid solution phase, while the spinel dissolved into the slag, and the dissolution mechanism was controlled by the rate of boundary migration of the solid solution phase, with an activation energy of E2 = 110.84 kJ/mol. Meanwhile, this interdiffusion behavior led to an increased liquid viscosity compared to that of the original slag that could further slow slag infiltration. The spinel could absorb Fe ions, rendering it a promising candidate for use in the Cu industry in the field of chromium-free refractories.

Effect of grain boundary state and grain size on the microstructure and mechanical properties of alumina obtained by SPS: A case of the amorphous layer on particle surface

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al2O3 particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al2O3 powder, (ii) submicron α-Al2O3 powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al2O3 powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (Vh) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al2O3 particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles’ surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers.

Sub-grain Boundary Mobility of 6XXX Alloys during Annealing

In this paper, 6061 aluminum alloy and a new type of dispersion-strengthened 6XXX aluminum alloy (named WQ) were compressed to a true strain of 1.2 at a strain rate of 0.1/s at 300/400/500°C and then annealed at different temperatures. The microstructure evolution of both heat treated alloys were characterized by EBSD. The results showed that during the annealing process of the two deformed alloys, the sub-grain size did not increase significantly. Due to the pinning effect of the dispersoids in WQ alloy, the sub-grain size of WQ was smaller than that of 6061 alloy. Under the same deformation conditions and annealing process, the deformation stored energy of WQ alloy was higher than that of 6061 alloy. The deformation stored energy of 6061 compressed alloy was more sensitive to the annealing temperature. Combining theoretical formulas and EBSD data, the sub-grain boundary migration rate curves of 6061 and WQ deformed alloys during the annealing process were obtained, which could provide a theoretical basis for the future study of the sub-grain growth process of multi-element aluminum alloys.

Microstructure and Texture Evolution during Severe Plastic Deformation at Cryogenic Temperatures in an Al-0.1Mg Alloy

The deformation structures formed in an Al-0.1Mg single-phase aluminium alloy have been studied during plane strain compression (PSC) down to liquid nitrogen temperature, following prior equal channel angular extrusion (ECAE) to a strain of ten. Under constant deformation conditions a steady state was approached irrespective of the temperature, where the rate of grain refinement stagnated and a minimum grain size was reached which could not be further reduced. A 98% reduction at 77 K (−196 °C) only transformed the ECAE processed submicron grain structure into a microstructure with thin ribbon grains, where a nanoscale high angle boundary (HAB) spacing was only approached in the sheet normal direction. It is shown that the minimum grain size achievable in severe deformation processing is controlled by a balance between the rate of compression of the HAB structure and dynamic recovery. The required boundary migration rate to maintain a constant boundary spacing is found far higher than can be justified from conventional diffusion-controlled grain growth and at low temperatures, a constant boundary spacing can only be maintained by invoking an athermal mechanism and is considered to be dominated by the operation of grain boundary dislocations.

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.

Topological model of type II deformation twinning in 10M Ni-Mn-Ga

The structure of type II twins in 10M Ni-Mn-Ga is modeled using the topological method. This method predicts the same twinning parameters as the kinematic model of Bevis and Crocker. Furthermore, topological modeling provides mechanistic insight into boundary migration rates, the twinning stresses and their temperature dependence. A type II twin is envisaged to form from a precursor, which is its type I conjugate. Disconnections on the precursor k1 plane align into a tilt wall, which, after the relaxation of the rotational distortions, forms the type II boundary parallel on average to the k2 plane. The component defects may align into a sharp wall or relax by kinking into a less orderly configuration. Both interfaces can host additional glissile disconnections whose motion along a boundary produces combined migration and shear. The ease of motion of these defects increases with their core width, and this, in turn, decreases with increasing sharpness of the boundary. Some experimental evidence in other materials suggests that type II twins can reduce their interfacial energy by adopting a configuration of low-index facets, which reduces twin boundary mobility. Topological modeling suggests that such a coherently faceted structure is unlikely in 10M Ni-Mn-Ga, in agreement with the high mobility of type II twin boundaries.

Modeling Marsh‐Forest Boundary Transgression in Response to Storms and Sea‐Level Rise

AbstractThe lateral extent and vertical stability of salt marshes experiencing rising sea levels depend on interacting drivers and feedbacks with potential for nonlinear behaviors. A two‐dimensional transect model was developed to examine changes in marsh and upland forest lateral extent and to explore controls on marsh inland transgression. Model behavior demonstrates limited and abrupt forest retreat with long‐term upland boundary migration rates controlled by slope, sea‐level rise (SLR), high water events, and biotic‐abiotic interactions. For low to moderate upland slopes the landward marsh edge is controlled by the interaction of these inundation events and forest recovery resulting in punctuated transgressive events. As SLR rates increase, the importance of the timing and frequency of water‐level deviations diminishes, and migration rates revert back to a slope‐SLR‐dominated process.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression > 15% compression > 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression > 15% compression > 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

Misorientation dependence of the grain boundary migration rate: role of elastic anisotropy

ABSTRACT In this work, numerical computations of image forces and stored energy during the growth of a strain free grain within a recovered matrix containing an array of identical dislocations are performed. A decrease of the stored energy is observed when image forces are attractive and an increase when image forces are repulsive. The variation of elastic energy depends closely on the material elastic anisotropy, the number of dislocations in the array and its initial distance to grain boundary. For an array of 20 edge dislocations and the motion of a planar grain boundary over approximately 180 atomic spacing, the magnitude of the energy variation is on the order of one dislocation line energy for Al and six for Cu, which is significant in both cases. Furthermore, by sweeping the whole orientation space for FCC crystals, it is shown that grain boundaries near a misorientation are among those that can display the highest attractive forces on edge dislocations. Since grain boundary migration is driven by the reduction of stored energy, the results are analysed in light of experimental recrystallization data in FCC metals which show that the fastest moving boundaries are often characterised by misorientation axis around and misorientation angle close to .

Enhancement of the densification and thermal properties of Ca2Mg2Al28O46 ceramic by MnO addition

Ca2Mg2Al28O46 consists of platelet or platelet-like structural features derived from CaAl12O19. However, its application in dense structural materials is limited due to its poor sinterability and loose structure. In this work, Ca2Mg2Al28O46 ceramic was fabricated by solid-state reaction sintering, and the effects of MnO addition on the densification and thermal properties were investigated at 1600-1750 °C. The results showed that the Mn2+ solid dissolved into the Ca2Mg2Al28O46 lattice by the isovalent substitution of Mg2+ at 1650 °C, causing the grain morphology to change from hexagonal to equiaxed and resulting in a distorted magnetoplumbite structure. At 1700 °C, the solid solution reaction accelerated the densification, with the apparent porosity decreasing from 32.5% to 8.4% when 3 wt% MnO is added, promoted grain growth and pore discharge by increasing the number of lattice defects, grain boundary migration rate and in-situ stress. The enhanced thermal expansion coefficient and thermal conduction were mainly due to lattice expansion and grain coarsening. Moreover, the formation of trace amount of the secondary phase Mg1-xMnxAl2O4 also contributed to the improvement in the thermal properties.

Phase field crystal study of grain boundary structure and annihilation mechanism in low-angle grain boundary

Using the advantage of phase field crystal model in microstructure evolution, we simulated the details of two-dimensional triangular phase on atomic scale and diffusion time scale. The results show that the annihilation process of grain boundaries can be divided into three stages. The energy changes of the system at different stages are accurately calculated and the process is quantitatively analyzed. By comparing the atomic density distributions at different temperatures and orientation angles, it is shown that the grain boundary is sensitive to temperature. Within the range of low-angle, the increase of orientation angle inhibits the occurrence of annihilation and decreases the grain boundary migration rate. Under the applied stress, the orientation angle has a certain response relationship with the yield strength of the material. This work has some practical significance in the influence of microstructure on the mechanical properties of materials.

Flow softening behavior and microstructure evolution of aluminum alloy 6061 due to dynamic recovery

Hot compression tests for Al–Mg–Si aluminum alloy were performed on Gleeble-1500 at the deformation temperatures of 300 ∼ 500 °C and strain rates of 0.01–10 s−1. It was found that the flow stress curves exhibit continuous flow softening after reaching the peak value. Microstructure observations indicate that dynamic recovery is the dominant restoration mechanism during the deformation. Continuous flow softening is considered to arise mainly due to thermal softening by deformation heating and microstructural softening by dynamic grain boundary migration. The effect of deformation heating on flow stresses was corrected with the measured instantaneous sample temperature. The dynamic grain boundary migration rate was calculated based on the measured grain thickness. A modified Laasraoui-Jonas model was developed through introducing the flow softening item induced by dynamic grain boundary migration. The predicted flow stresses overall agree well with the experimental data.