Grain Growth Evolution Research Articles

A cellular automata modelling approach for grain growth topological evolution process of ternary cathode materials combined with deep neural networks

Ternary cathode materials are pivotal in high-performance battery technologies, with grain size influencing their electrochemical performance. However, the absence of real-time grain size inspection during material preparation poses challenges in maintaining the consistent quality of ternary cathode materials. To address this, this paper proposes a novel method employing cell automata to model the topological evolution of grain growth in these materials, integrated with deep neural networks (DNN). The grain growth process is divided into two stages: heating and constant temperature. In the heating stage, varying heating rates and plateau temperatures serve as DNN inputs, yielding the primary grain distribution for the cell automata model in the constant temperature stage. Based on cell automata, the grain growth model links the grain growth rate to the grain size distribution in the constant temperature stage. A surface energy constraint rule, based on local curvature and grain boundary surface tension, governs growth rates. The grain boundary growth ratio is also used to create a grain ID transition variable, dictating grain ID conversion in the model. This approach accurately simulates the dynamic evolution of polycrystalline grain size and morphology. Simulation results show that this method effectively models grain growth in ternary cathode materials, offering insights for optimising the sintering process and improving material quality.

A comparison of heat treatment effect on structural and tribological properties of tungsten rich Ni-W and Ni-W-P alloy coatings

In this paper, thick, metallic bright electrodeposited tungsten rich Ni–W–P alloy coatings with two different compositions Ni-36.3wt%W-3.3wt%P (designated as Ni–W–LP) and Ni-19.5wt%W-7.7wt%P (designated as Ni–W–HP) were chosen for comparison of heat-treatment dependent evolution of crystal structure, grain growth, nanohardness and tribological properties with respect to Ni-50.7wt%W coating. The systematic temperature dependent evolution of crystal structure and phases from as-deposited amorphous alloys were mapped using in-situ high temperature GIXRD investigation and peak-broadening analysis. Subsequent variation of nanohardness/microhardess was measured. Detailed tribological investigation of both as-deposited and heat-treated coatings was done by ball-on-plate dry sliding experiment and the observed coefficient of friction and wear data were correlated with measured nanohardness, H/E and H3/E2 ratio and a wear mechanism was proposed based on detailed FESEM investigation of wear scars.

Recrystallization of bulk nanostructured magnesium alloy AZ31 after severe plastic deformation: an in situ diffraction study

The magnesium alloy AZ31, which has undergone high-pressure torsion processing, was subjected to in situ annealing microbeam synchrotron high-energy X-ray diffraction and compared to the as-received rolled sheet material that was investigated through in situ neutron diffraction. While the latter only exhibits thermal expansion and minor recovery, the nanostructured specimen displays a complex evolution, including recovery, strong recrystallization, phase transformations, and various regimes of grain growth. Nanometer-scale grain sizes, determined using Williamson–Hall analysis, exhibit seamless growth, aligning with the transition to larger grains, as assessed through the occupancy of single-grain reflections on the diffraction rings. The study uncovers strain anomalies resulting from thermal expansion, segregation of Al atoms, and the kinetics of vacancy creation and annihilation. Notably, a substantial number of excess vacancies were generated through high-pressure torsion and maintained for driving the recrystallization and forming highly activated volumes for diffusion and phase precipitation during heating. The unsystematic scatter observed in the Williamson–Hall plot indicates high dislocation densities following severe plastic deformation, which significantly decrease during recrystallization. Subsequently, dislocations reappear during grain growth, likely in response to torque gradients in larger grains. It is worth noting that the characteristics of unsystematic scatter differ for dislocations created at high and low temperatures, underscoring the strong temperature dependence of slip system activation.Graphical

A unified constitutive relationship with the internal state variables of grain evolution behavior and the application in numerical simulation of cross wedge rolling

Controlling the grain size is an essential issue in the metal plastic working processes. Based on the consensus view that grain boundaries are impassable borders for dislocations and that the number of dislocations within a grain affects the stress builds up among the adjacent grains, altering grain size has a great influence on the post-forming mechanical properties of the deformed workpieces. In the present research, the deformation behaviors of 20CrMoH, a typical hardenability steel, are investigated and the microstructure evolution of grain growth experiencing heat load and grain refinement under deformation load is also uncovered. According to the experimental stress-strain curves and grain size variation data, a set of internal variable constitutive equations are established, in which the material constants are calibrated by a genetic optimization algorithm. Comparing the predicted curves with the experimental values, it can be concluded that the constitutive equations can accurately reproduce the thermal deformation behaviors and grain size variation of the steel. The correlation coefficient of the calculated and experimental values is 0.9861, and the average relative error approximates 4%. The unified constitutive equations are compiled into a commercial finite element software with the help of user-defined subroutine interfaces. The simulation results demonstrate that the maximum accuracy error of a hollow shaft size is about 10%, and the error in the average grain size is 12.1%. It is confirmed that the internal variable constitutive equations describing the evolution of grain size established in this paper are suitable for the numerical simulation of the bulk-forming process.

A novel cyclic thermal treatment for enhanced globularisation kinetics in Ti-6Al-4V alloy: Experimental, constitutive and FE based analyses

Secondary hot-working on dual phase titanium alloys are essential for microstructural modification to tailor mechanical properties, which is typically challenging due to a narrow available processing window, especially during industrial scale manufacturing. Poor workability, strain induced porosity and adiabatic temperature rise in α + β phase region (i.e., sub-transus) are some of the main challenges faced. Cyclic thermal treatment (CTT) is an emerging technology showing potentials for microstructure modification (i.e., globularisation) in Ti‐6Al‐4V with significantly reduced mechanical work in the α + β region. This study summarises the results of CTT investigations conducted on a wrought Ti‐6Al‐4V alloy subjected to various thermo-mechanical conditions to develop different initial microstructures. Samples with uniform strain distributions were extracted from pre-forged samples and subsequently subjected to CTT using both conventional electric furnace (i.e., for slow heating and cooling rates), and induction heating (i.e., for faster heating and cooling rates). CTT of the samples forged at sub-transus temperature in conventional furnace led to maximum (i.e. ~100%) globularisation and significant coarsening of α grains, resulting in an equiaxed bimodal microstructure. On the other hand, CTT with induction heating method has resulted in a maximum of 80% globularisation fraction in samples forged to 60% reduction, and ~35% globularisation fraction in those forged to 20% reduction. The globularisation mechanisms during CTT of the sub-transus forged samples was dominated by the boundary splitting and thermal grooving. A Johnson-Mehl-Avarmi-Kolmogorov (JMAK) based model has been developed to predict the evolution of globularisation and grain growth during CTT. The developed JMAK model was then successfully incorporated into DEFORM® software as post-processing user subroutines. The predicted microstructure evolution by Finite Element (FE) simulations shown a good convergence towards the experimentally measured data following CTT.

A new model of competitive grain growth dominated by the solute field of the Nickel-based superalloys during directional solidification

To accurately predict the microstructure evolution of competitive columnar grain growth, the influence of the potential dominant factors of competition growth such as thermal field, solute field and flow field on the overgrowth behavior of trinary-crystal samples during directional solidification was systematically investigated, with the preferred orientation of the experimental grains orienting parallel and at a limited misorientation angle with respect to the temperature gradient direction. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient, which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the solute field around the dendrite tips. Additionally, with increasing the natural convection, the grain overgrowth rate tended to be promoted at low withdraw speed. These phenomena were attributed to the mechanisms of solute interaction in the converging case and sidebranching events in the diverging case, while the solute field was the dominant factor to govern the overgrowth behavior of the competing grains. Moreover, a new model of competitive grain growth based on the solute field was proposed to predict the microstructure evolution of the Nickel-based superalloys during the directional solidification process. In this new model, the primary spacing of the well-oriented grain in the converging case was smaller than that in the diverging case due to the sidebranching events and dendrite lateral motion, which highlighted a new mechanism of primary spacing evolution in the directional columnar solidification structure.

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.

Phase-field simulation of the interaction between intergranular voids and grain boundaries during radiation in UO2

In the present work, a phase-field model is developed to simulate the interaction between voids and grain boundary (GB) by a set of modified Cahn-Hilliard and Allen-Cahn equations with spatially and temporally evolving field variables and order parameters. Both the evolution of grain growth and the effect of moving GBs on the formation of voids during radiation were captured in the present phase-field model. A flat GB with an artificial driving force was used to analyze the resistive pressure of the void during the interaction of the void and moving GB, and the interaction between GB and void with different radius was also simulated. The effects of moving GB on the void formation during radiation was also discussed in detail. In addition, the grain growth and voids formation during radiation in polycrystalline uranium dioxide were studied based on the modified phase-field model.

Strongly Enhanced Dielectric Response and Structural Investigation of (Sr2+, Ge4+) Co-Doped CCTO Ceramics

A solid–state reaction method was used to prepare (Sr2+, Ge4+) co-doped CaCu3Ti4O12 ceramics. A single-phase of CaCu3Ti4O12 was detected in all the ceramics. An enormous evolution of grain growth i...

Grain size engineering in nanocrystalline Y2Zr2O7: A detailed study on the grain size correlated electrical properties

Abstract A set of nanocrystalline Y2Zr2O7 (YZO) ceramic materials possessing grain sizes ranging from 7 nm to 65 nm was prepared by chemical co-precipitation technique followed by conventional sintering at various temperatures. Evolution of structure and grain growth was investigated by XRD and TEM analysis. Disordered pyrochlore phase of YZO was obtained. Electrical conductivity of YZO has been increased with increasing measuring temperature as well as grain size. The behavior of total electrical conductivity comprising grain interior and grain boundary components was studied. When the grain size was increased, the activation energy for grain boundary and total ionic conduction has been decreased from 1.59 to 1.51 eV whereas the same for bulk grain conductivity remained more or less the same near to 1.43 ± 0.01 eV. The Jonscher’s universal law had explained the behavior of ac conductivity suggesting a long-range hopping of mobile ions-based conduction in YZO. In addition, Maxwell–Wagner model had explained the dielectric behavior of YZO and showed an increasing dielectric constant with increasing grain size. Electric modulus, analysed by Kohlrausch-Williams-Watts function, showed an increasing relaxation frequency at higher grain sizes. A complete electrical characterization of YZO ceramics with good correlation with their grain size is reported.

The curious mechanism of irradiation-induced cryogenic grain growth in tungsten thin films: A pathway to single crystals

With high resistance against interdiffusion, electromigration, creep and fatigue, single-crystal films of refractory metals are highly desired for applications in opto- and micro-electronic devices. Their fabrication is nevertheless very challenging, primarily due to their high melting points. Requiring no external thermal input, ion-irradiation presents an alternative route for single-crystal films by converting from their polycrystalline counterparts through irradiation-induced selective grain growth. The incomplete poly- to single-crystal conversion is however a common problem which limits the application of this method. Here we use refractory W thin films, the element with the highest melting point, as a model system. We systematically investigate the influences of film microstructure (texture and grain size) and irradiation temperature (77 K and room temperature) on the evolution of selective grain growth under 4.5 MeV Au+ ion-irradiation. We find that the driving force for selective grain growth can be increased by narrowing the texture spread of the films, refining the grain size and decreasing the irradiation temperature. Following this guidance, a complete conversion of a polycrystalline W film into a single crystal is achieved. This study therefore provides insights into the mechanism of selective grain growth and proves that it is an effective technique for microstructure engineering in thin film materials. The concepts can be applied to all crystalline metal films.

Evolution of Fe-containing intermetallic phases and abnormal grain growth in 6063 aluminum alloy during homogenization

The 6063 aluminum billet alloy has been widely used as raw materials for aluminum extrusion profiles. A high-quality billet for good extrusion products is provided from a heat treatment process called homogenization. This process can give a homogeneous microstructure by reducing microsegregation and dissolving intermetallic phases. However, homogenization can create a very large grain size (abnormal grains) in 6063 aluminum billets. Fe content is one of the main factors that is strongly related to abnormal grain growth because Fe can form Fe-containing intermetallic phases in 6063 aluminum structure. The morphology and volume fraction of Fe-containing intermetallic phases are subjected to change during homogenization. These results relate to a decrease in the volume fraction of intermetallics, which corresponds to the Zener pinning pressure, grain boundary migration, and abnormal grain growth. Therefore, this study aims to understand the evolution of Fe-containing intermetallic phases on abnormal grain growth in 6063 aluminum billets during homogenization. Ex-situ characterization by energy dispersive spectrometer (EDS) and electron backscattered diffraction (EBSD) was performed on 6063 aluminum alloy to gain an in-depth understanding of abnormal grain growth.

3D microstructural evolution of primary recrystallization and grain growth in cold rolled single-phase aluminum alloys

Modeling the microstructural evolution during recrystallization is a powerful tool for the profound understanding of alloy behavior and for use in optimizing engineering properties through annealing. In particular, the mechanical properties of metallic alloys are highly dependent upon evolved microstructure and texture from the softening process. In the present work, a Monte Carlo (MC) Potts model was used to model the primary recrystallization and grain growth in cold rolled single-phase Al alloy. The microstructural representation of two kinds of dislocation densities, statistically stored dislocations and geometrically necessary dislocations were quantified based on the ViscoPlastic Fast Fourier transform method. This representation was then introduced into the MC Potts model to identify the favorable sites for nucleation where orientation gradients and entanglements of dislocations are high. Additionally, in situ observations of non-isothermal microstructure evolution for single-phase aluminum alloy 1100 were made to validate the simulation. The influence of the texture inhomogeneity is analyzed from a theoretical point of view using an orientation distribution function for deformed and evolved texture.

Multiferroic properties of Ba0.995Fe0.005Ti0.995Mn0.005O3 synthesized by glycine assisted sol gel method

Magnetic and ferroelectric properties in nanoscale are interesting topic to deal with. The primary interest is to understand the effect of simple, fast and cost effective sol gel method on the magnetic and electrical properties of Ba0.995Fe0.005Ti0.995Mn0.005O3 compounds. The study unravels the strategy for realizing multiferrocity of this composition by changing the processing temperature. Structural study confirms the infusion of Fe, Mn into the BaTiO3 lattice and its subsequent effects on the pseudo cubic and tetragonal phases. Morphological results manifest the formation of nanoparticles and evolution of grain growth upon increasing the processing temperature. Incorporation of Fe, Mn and its substantial effects on the various polymorphs of BaTiO3 is clearly portrayed by dielectric results. Room temperature ferroelectric measurements reveal the electric polarization in them. Magnetic measurements show an unusual high coercivity of 4000 Oe and its transformation to a completely paramagnetic behaviour with the subsequent changes in their phase. Thus, the pseudo cubic phase aids to retain the ferroelectric polarization with simultaneous presence of magnetic signature hints their possibility towards fruitful multi-functionality.

Microstructure evolution and EBSD analysis of a graded steel fabricated by laser additive manufacturing

This paper demonstrates the successful fabrication of a graded steel with successive 15 layers by direct laser metal deposition. The phase evolution, microstructure and grain growth of the as-built graded steel were investigated by X-ray diffraction (XRD), optical microscopy (OM), and electron back-scatter diffraction (EBSD), respectively. Experimental results showed that the crystal structures of the graded steel evolved from FCC (Austenite) in the first two grades to BCC (Ferrite) with a small trace of Austenite phase. Columnar dendrites are observed in the first and second grade with the growth orientation horizontal to the direction of the laser deposition. The third grade has a finer microstructure with equiaxed grains. Preferred texture was dominant in the microstructure for the first two grades with low-angle grain boundaries (about 2°). No preferred texture was observed in the third grade. Large-angle grain boundaries are available in the third grade with distribution from 50° to 60°. Our findings suggest that a graded steel with microstructure and grain growth orientation evolution can be obtained by direct laser metal deposition. The evolution of microstructure and grain growth was not changed by different scanning rate.

The onset and evolution of fatigue-induced abnormal grain growth in nanocrystalline Ni–Fe

Conventional structural metals suffer from fatigue-crack initiation through dislocation activity which forms persistent slip bands leading to notch-like extrusions and intrusions. Ultrafine-grained and nanocrystalline metals can potentially exhibit superior fatigue-crack initiation resistance by suppressing these cumulative dislocation activities. Prior studies on these metals have confirmed improved high-cycle fatigue performance. In the case of nano-grained metals, analyses of subsurface crack initiation sites have indicated that the crack nucleation is associated with abnormally large grains. However, these post-mortem analyses have led to only speculation about when abnormal grain growth occurs (e.g., during fatigue, after crack initiation, or during crack growth). In this study, a recently developed synchrotron X-ray diffraction technique was used to detect the onset and progression of abnormal grain growth during stress-controlled fatigue loading. This study provides the first direct evidence that the grain coarsening is cyclically induced and occurs well before final fatigue failure—our results indicate that the first half of the fatigue life was spent prior to the detectable onset of abnormal grain growth, while the second half was spent coarsening the nanocrystalline structure and cyclically deforming the abnormally large grains until crack initiation. Post-mortem fractography, coupled with cycle-dependent diffraction data, provides the first details regarding the kinetics of this abnormal grain growth process during high-cycle fatigue testing. Precession electron diffraction images collected in a transmission electron microscope after the in situ fatigue experiment also confirm the X-ray evidence that the abnormally large grains contain substantial misorientation gradients and sub-grain boundaries.

Grain growth jointly affected by immobile and mobile particles

Grain growth under the influence of randomly distributed particles is studied by numerical simulation using Onsager's principle. It was found that at small initial volume fraction of particles, normal grain growth evolves, with its kinetic law depending on the particle dissolution rate and its rate affected by particle mobility. At large initial volume fraction of particles, abnormal grain growth develops whereas an increase in particle mobility can lead to evolution of normal grain growth instead. Results of the simulations describe the effects behind some experimental data, lacking an explanation so far. They also allow improving existing and develop new technological solutions in the field of grain structure control and design.

Microstructure Prediction of 316LN Stainless Steel for Dynamic Recrystallization Based on Cellular Automata

The established cellular automata model of dynamic recrystallization for 316LN simulated microstructure evolution of recrystallization nucleation and grain growth under different conditions. And on the basis of cellular automata model, the influence of strain, strain rate, deformation temperature on dynamic recrystallization behavior was analyzed. Though the hot compress experiment done on the Gleeble-3500 thermo mechanical simulator, combined with metallographic experiment, the microstructure at deformation temperature of 950 oC, 1050 oC and 1150 oC with strain rate of 0.001 s-1, 0.01 s-1, 0.1 s-1 and 1 s-1 was obtained. Simulation results are compared with metallographic microstructure, the error is small.

Substrates with Programmable Heater Arrays for In-Situ Observation of Microstructural Evolution of Polycrystalline Films: Towards Real Time Control of Grain Growth

ABSTRACTAn integrated experimental – simulation – control theory approach designed to enable adaptive control of microstructural evolution in polycrystalline metals is described. A micro-heater array, containing ten addressable channels, is used to create desired temperature profiles across thin polycrystalline films in situ to a scanning electron microscope (SEM). The goal is that on heating with controlled temperature profiles, the evolution of grain growth within the film can be continuously monitored and compared to Monte Carlo simulations of trajectories towards a desired microstructure. Feed-forward and feedback control strategies are then used to guide the microstructure along the desired trajectory.

The mechanism of grain growth and thermal stability in Ni-1 at.% Pb alloy

The effect of undercooling and subsequent annealing on the evolution of grain growth of Ni-1 at.% Pb alloy was investigated with the help of glass fluxing technique and isothermal annealing process. To investigate the mechanism of grain growth, isothermal annealing processes of the as-quenched samples at different undercoolings were conducted at different annealing temperatures. The whole process of grain size evolution could be rationally categorized as three stages: 1. The slow growth stage; 2. The rapid growth stage; 3. The stable growth stage. With the utilization of grain boundary energy σb model, it is shown that the stability in stage 1 is due to the reduction of σb. As a result of second phase precipitation occurring at 2–3 stages, rapid grain growth occurs.