Grain Growth Behavior Research Articles

Efficacy of Stationary Shoulder Friction Stir Welding to Minimize the Abnormal Behavior of Grain Growth in 7075 Aluminum Alloy

Efficacy of Stationary Shoulder Friction Stir Welding to Minimize the Abnormal Behavior of Grain Growth in 7075 Aluminum Alloy

Growth Behavior of Iron Grains During Reduction Roasting of Fayalite

Growth Behavior of Iron Grains During Reduction Roasting of Fayalite

In Situ Microstructural Evolution and Precipitate Analysis of High-Nickel Shipbuilding Steel Using High-Temperature Confocal Laser-Scanning Microscopy

This study investigates the microstructural evolution and mechanical properties of high-nickel shipbuilding steel during thermal processing using high-temperature confocal laser-scanning microscopy (HTCLSM). An in situ observation of the heating and holding processes reveals critical insights into phase transformations, grain-growth behavior, and the formation of precipitates. The experimental results demonstrate that austenitization begins at approximately 700 °C, with significant grain-boundary nucleation. At 900 °C, the formation of black precipitates was observed, and their persistence up to temperatures exceeding 1000 °C was confirmed. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analyses identified these precipitates as chromium carbides (Cr7C3), which significantly contribute to the material’s strength. A comprehensive analysis using transmission electron microscopy (TEM) confirmed the presence and distribution of Cr7C3 within the grains and along grain boundaries. These findings provide a deeper understanding of the microstructural dynamics in high-nickel steels, guiding the optimization of heat-treatment processes to enhance mechanical properties for maritime applications.

Abnormal sintering behaviors of chromium carbide under high pressure and high temperature

Controlling grain behavior during high pressure and high temperature (HPHT) sintering is crucial for synthesizing high-performance bulk ceramic materials. Chromium carbide (Cr3C2), a representative transition metal carbide ceramic, finds extensive industrial applications owing to its exceptional combination of properties. In this study, the intricate grain behaviors in Cr3C2 under 15 GPa/25–1700 °C are revealed by HPHT methods. It is found that the mechanical and elastic properties of Cr3C2 exhibit a significant dependence on temperature-induced grain behavior. Particularly, the mechanical properties of Cr3C2 exhibit abnormal softening at 15 GPa/1200 °C and 15 GPa/1500 °C. Through characterization and analysis of the microstructure, it is confirmed that the significant change in mechanical properties stems from the abnormal sintering behaviors of distinctive competitive behavior of grain refinement and growth. The findings highlight the significant influence of the competitive behavior on microstructural evolution, which exhibits strong correlations with the mechanical properties.

Influence of Microstructure on the Material Properties of LLZO Ceramics Derived by Impedance Spectroscopy and Brick Layer Model Analysis.

Variants of garnet-type Li7La3Zr2O12 are being intensively studied as separator materials in solid-state battery research. The material-specific transport properties, such as bulk and grain boundary conductivity, are of prime interest and are mostly investigated by impedance spectroscopy. Data evaluation is usually based on the one-dimensional (1D) brick layer model, which assumes a homogeneous microstructure of identical grains. Real samples show microstructural inhomogeneities in grain size and porosity due to the complex behavior of grain growth in garnets that is very sensitive to the sintering protocol. However, the true microstructure is often omitted in impedance data analysis, hindering the interlaboratory reproducibility and comparability of results reported in the literature. Here, we use a combinatorial approach of structural analysis and three-dimensional (3D) transport modeling to explore the effects of microstructure on the derived material-specific properties of garnet-type ceramics. For this purpose, Al-doped Li7La3Zr2O12 pellets with different microstructures are fabricated and electrochemically characterized. A machine learning-assisted image segmentation approach is used for statistical analysis and quantification of the microstructural changes during sintering. A detailed analysis of transport through statistically modeled twin microstructures demonstrates that the transport parameters derived from a 1D brick layer model approach show uncertainties up to 150%, only due to variations in grain size. These uncertainties can be even larger in the presence of porosity. This study helps to better understand the role of the microstructure of polycrystalline electroceramics and its influence on experimental results.

The oriented growth behavior of α-Al2O3 grains in alumina-mullite biphasic fibers

Understanding the mechanism of the oriented growth in multi-component alumina-based fibers and inhibiting its progression is crucial for enhancing their thermal stability. In this study, a novel diphasic fiber composed of lamellar α-Al2O3 and mosaic-like mullite grains was prepared. We discussed the oriented growth behavior of lamellar α-Al2O3 grains and the diffusion phenomenon of the Al element in SiO2 substrate under the combined addition of iron sol and La2O3, proposing a new mechanism for oriented growth in diphasic fibers. The diffusion of the Si element to the tips of alumina grains contributed to a significant increase in the mobility of non-basal planes, resulting in (0001) surface growth of α-Al2O3 grains. Furthermore, the La segregation at grain boundaries significantly impeded the oriented growth of α-Al2O3 grains, which was critical for improving the thermal stability of multiphase alumina-based fibers.

Recrystallization and Grain Growth in Cu-Cu Joints under Electromigration at Low Temperatures.

The behavior of recrystallization and grain growth was examined in Cu-Cu joints during electromigration at 150 °C. Recrystallization and grain growth were observed in all the joints after electromigration for 9000 h. Voiding was formed in Cu current-feeding lines and in bonding interfaces, and resistance increased with time due to the void formation. However, instead of rising abruptly, the resistance of certain Cu joints dropped after 7000 h. Microstructural analysis revealed that a large grain growth occurred in these joints at 150 °C, and the bonding interface was eliminated. Therefore, the electromigration lifetime can be prolonged for these joints.

Recrystallization and grain growth of Zr-Nb-Sn alloy in 400–500 °C and effect on hydride embrittlement

The recrystallization and grain growth of a reactor-grade Zr-Nb-Sn alloy were experimentally investigated in the temperature range of 400–500 °C. Electron backscatter diffraction (EBSD) analyses were conducted to characterize grain growth and texture change caused by annealing. Hardness measurements were conducted to obtain the recrystallization fractions and were compared with the machine learning-based recrystallization analyses provided by AZtecCrystal. With good agreement with the hardness-based recrystallization fraction, the machine-learning-based method provides classification of deformed (recrystallized and grown) and undeformed (non-recrystallized) grains, thereby revealing the local behavior of dynamic grain growth. Recrystallization occurs in 4 h at 450 °C, with distinct grain growth after 24 h. Mechanisms of intergranular hydride connectivity decrease with grain growth are elucidated. The reduced hydride connectivity with grain growth primarily occurs by the formation of intra-granular hydrides and homogenization of grain boundary energies. The reduced hydride connectivity significantly increases cladding ductility. For cladding with a significant hydrogen content (i.e., ∼600 wppm), the increase in cladding ductility (i.e., strain energy density (SED)) owing to the reduced hydride connectivity supersedes the background matrix ductility enhancement associated with grain growth.

Microstructures and mechanical behavior of non-equiatomic Co29Cr29Fe29Ni13-xVx high-entropy alloys at room and cryogenic temperatures

A series of non-equiatomic Co29Cr29Fe29Ni13-xVx (x = 0, 0.5, 3, and 5) high-entropy alloys (HEAs) with fully recrystallized microstructures were prepared by rolling and annealing. The microstructures were characterized and the mechanical properties were obtained at different temperatures. The recrystallization and grain-growth behavior of the alloys are retarded owing to the addition of V. The lattice friction stress increases drastically with the addition of a minor of V. Ultrahigh yield strengths of 1.8 GPa and 1.4 GPa, and considerable tensile ductility values of ∼30% and ∼50% were developed at a cryogenic temperature (77 K) for the 700 °C-annealed and 800 °C-annealed alloys. The strengthening mechanism and deformation behavior at cryogenic temperatures were investigated. The comprehensive mechanical properties of the alloy at low temperatures are better than those of most reported HEAs. The alloy is easy to process and prepare and therefore, has considerable potential for industrial applications.

Microstructure evolution and interface characteristics of diffusion bonded dissimilar titanium alloys

Microstructure evolution and interface characteristics of dissimilar TC4/TB8 titanium alloys subjected to diffusion bonding and aging treatment are investigated. TC4/TB8 alloys plates can be metallurgically bonded at 830°C for 2h under 18MPa. The microstructure of TC4 alloy is composed of fine intergranular β grains and equiaxed α grains due to recrystallization. The α grains characterized by lath-like, granular, and continuous layer morphology are precipitated from β matrix in the TB8 alloy during furnace cooling and aging treatment. Element distribution mapping and line-profile data indicate that Mo atoms diffuse from the enrichment area (TB8) to the depleted area (TC4), while a small amount of Al and V atoms diffuse along an opposite direction. In addition, the behavior of grain growth across the bonded interface is not obvious under the current parameters.

The heterogeneous nature of mechanically accelerated grain growth

AbstractWhile grain growth is traditionally viewed as a purely thermally driven process, nanocrystalline metals can undergo grain growth under mechanical loads, even at room temperature. We performed a detailed atomistic study of the heterogeneous nature of mechanically accelerated grain growth in a polycrystalline Pt nanowire. Using molecular dynamics simulations, we compared the grain-growth behavior of individual grains during tensile and shear cyclic loading, for three different equivalent strain levels, and at two temperatures. Pure thermal grain growth with no mechanical loading provided a baseline reference case. On average, grains that were already susceptible to thermal grain growth were stimulated to grow faster with mechanical loading, as expected. However, when analyzed on a grain-by-grain basis, the results were far more complex: grains that grew fastest under one stimuli were less accelerated under other stimuli. Even when the magnitude of loading changed, the relative growth of individual grains was distorted. We interpret this complexity from the perspective of superimposed growth mechanisms.

Non-Arrhenius behavior of grain growth in (K, Na)NbO3-based ceramics and its effect on piezoelectric properties

A study was performed of the grain growth behavior of ordinary (K, Na)NbO3–based ceramics, including (K, Na)NbO3–Bi0.5Na0.5TiO3 and (K, Na)NbO3–Bi0.5Na0·5ZrO3 systems. These ceramics were sintered at a series of temperatures. Via the observation of surface microstructures and the analysis of Arrhenius graphs, the grain growth of both ceramics was found to experience three stages with increasing sintering temperature. In the first stage, which was in a relatively low temperature range, increasing the sintering temperature resulted in a gradual increase in grain size, complying with the characteristics of thermal activation. As the sintering temperature rose to a higher temperature range, the grain growth entered the second stage, where increasing the sintering temperature caused the grain size to decrease, showing an anti-thermal grain growth behavior. In the third stage, the ceramics resumed the behavior of thermally activated grain growth with the further increase of sintering temperature. The reasons were discussed for the non-Arrhenius grain growth behavior of (K, Na)NbO3–based ceramics. In addition, the effect of non-Arrhenius grain growth behavior on piezoelectric properties was studied as well.

Selective laser melting of dense and crack-free AlCoCrFeNi2.1 eutectic high entropy alloy: Synergizing strength and ductility

• Dense and crack-free EHEA part was fabricated successfully with SLM. • The yield strength of the as-SLM part was almost doubled to 982 MPa compared to the as-cast sample. • The width and grain morphology in the transition zone were depended on the misorientation between the newly formed and the previous layer. Additively manufactured high-entropy alloys generally suffer from low strength and/or poor ductility. In this work, by leveraging the good castability of eutectic high entropy alloys and high cooling rate of selective laser melting (SLM), we report a nearly fully dense and crack-free as-SLM AlCoCrFeNi 2.1 eutectic high entropy alloy with an exceptional strength-ductility synergy, showing an ultrahigh yield strength of 982.1 ± 35.2 MPa and an ultimate tensile strength of 1322.8 ± 54.9 MPa together with an elongation to fracture of 12.3 ± 0.5%. Such strength-ductility enhancement is owing to the heterogeneous eutectic microstructure consisting of the columnar, equiaxed, and “L-shape” cells with much refined sizes down to nanoscales. The morphology of cells in the transition zone is related to the misorientation between the growth direction of adjacent layers. This heterogeneous eutectic microstructure is the result of the grain-growth behavior dominated by the mechanisms of the epitaxial growth and growth of heterogeneous nuclei in SLM. Our current results provide a new methodology for the future design of ultrahigh-strength and ductile SLM-fabricated metallic materials including HEAs, and other printable alloys for various structural applications.

Modeling Dynamic Recrystallization Behavior in a Novel HIPed P/M Superalloy during High-Temperature Deformation.

The dynamic recrystallization (DRX) features and the evolution of the microstructure of a new hot isostatic pressed (HIPed) powder metallurgy (P/M) superalloy are investigated by hot-compression tests. The sensitivity of grain dimension and DRX behavior to deformation parameters is analyzed. The results reveal that the DRX features and grain-growth behavior are significantly affected by deformation conditions. The DRX process is promoted with a raised temperature/true strain or a reduced strain rate. However, the grains grow up rapidly at relatively high temperatures. At strain rates of o.1 s−1 and 1 s−1, a uniform microstructure and small grains are obtained. Due to the obvious differences in the DRX rate at various temperatures, the piecewise DRX kinetics equations are proposed to predict the DRX behavior. At the same time, a mathematical model for predicting the grain dimension and the grain growth behavior is established. To further analyze the DRX behavior and the changes in grain dimension, the hot deformation process is simulated. The developed grain-growth equation as well as the piecewise DRX kinetics equations are integrated into DEFORM software. The simulated DRX features are consistent with the test results, indicating that the proposed DRX kinetics equations and the established grain-growth model can be well used for describing the microstructure evolution. So, they are very useful for the practical hot forming of P/M superalloy parts.

Effect of niobium alloying on the austenite grain growth and mechanical properties of ultrahigh-strength stainless steel

In the temperature range of 1000 °C–1150 °C and the holding time range of 30–150 min, the effect of niobium (Nb) on the behavior of grain growth and the evolution pattern of the mechanical properties of a martensitic stainless steel was studied. This study found that the addition of Nb allowed a large amount of undissolved NbC phase to be present in the steel, that the dragging effect of the solute atoms such as solute Nb and Mo reduced the migration rate of the grain boundary , and the pinning effect of NbC hindered the growth of grains, and that the growth rate of grains in 0.11Nb steel was slow in the temperature range of 1000 °C–1080 °C and increased significantly at the temperature range of 1080 °C–1150 °C. Next, the kinetic equations of the grain growth of 0.002Nb steel and 0.11Nb steel were constructed. The second phase strengthening of NbC and the fine grain strengthening jointly increased the yield strength of the steel but reduced the plasticity and ultimate tensile strength (UTS) of the steel. The addition of Nb had a minor effect on the content of retained austenite in the steel, but its refining effect on the hierarchical martensite microstructure increased the number of nucleation sites of retained austenite, reduced their sizes, made their distribution more dispersed, and more effectively hindered crack propagation, thus improving the toughness of the steel.

Efficient and Composition‐Tolerant Kesterite Cu2ZnSn(S, Se)4 Solar Cells Derived From an In Situ Formed Multifunctional Carbon Framework

AbstractBroadening the processing window is of high importance for developing efficient Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Herein, a high efficiency (active‐area: 13.5%) of CZTSSe solar cells achieved from thioglycolic acid (TGA)‐ammonia based water solution method is first reported. The cell performance exhibits the high tolerance to the element composition variations of the CZTSSe film. Film crystallization and chemical mechanism studies reveal that this high composition‐tolerance mainly benefits from an existence of a conductive carbon framework layer with high element accommodating ability and a phase‐segregation growth behavior of CZTSSe grains driven by thermodynamics properties of the heterogeneous film system. It is shown that Sn–TGA coordination capable to induce large metal–organic molecule clusters plays a critical role in forming the mesoscopic carbon layer. These results and related material mechanism provide new routes to regulate crystal growth of the CZTSSe film for further improving cell performance.

Phase field simulation of abnormal grain growth mediated by initial particle size distribution

The uniform initial particle size distribution was considered to be an important condition for the preparation of dense ceramics. With the rapid development of templated grain growth method, it is necessary to add large template particles into small matrix to promote the epitaxial growth of grains. However, abnormal grain growth will inevitably occur in this process. In this work, a phase field method was employed to study the growth behavior of the large grains into the small matrix. The effect of initial particle size distribution on the grain growth kinetics was investigated. Our simulations revealed that the growth rate of large grains slowed down as the initial grain size distribution widened. Regulating the initial grain size distribution will be an effective way to control abnormal growth of grain. It was found that the growth rate of a large grain was proportional to the number of surrounding grains. These results have implications for the design of the functional ceramics with a target grain microstructure.

Microstructural evolution of Mg-10Gd-3Y-1Zn-0.4Zr (wt%) alloy prepared by strain-induced melt activation process

The semisolid billets of Mg-10Gd-3Y-1Zn-0.4Zr (wt%) alloy were prepared by the strain-induced melt activation (SIMA) route, and the effects of isothermal heating parameters on the morphology and coarsening kinetics of primary solid grains were studied. It was found that the average grain size increased with the increase of isothermal time while the grain size showed different changing laws with the temperature at different isothermal times. The growth behavior of the primary solid grains was predominantly governed by the Ostwald ripening mechanism, although there is coalescence mechanism in some cases. As the isothermal temperature increased from 550 °C to 590 °C, the coarsening rate constant increased continuously and then declined when the temperature was further increased to 610 °C. Compared with other SIMA processed magnesium alloys, the present alloy showed a significantly lower grain coarsening rate constant due to the inhibiting effect of rare earth (RE) elements on coarsening. This research has provided an insight into the microstructural evolution of Mg-RE alloy under different heating conditions, and will provide valuable information for potential industrial applications.

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.

Multi-physics multi-scale simulation of the solidification process in the molten pool during laser welding of aluminum alloys

The solidification microstructures directly determine the performance of laser welds. However, the present understanding of the dynamic solidification behavior within the melt pool during laser welding is still limited. This paper developed a multi-physics multi-scale model to investigate the complex solidification process during laser welding of 5083 aluminum alloy. The model combines the macroscale model for heat and mass transfer, the microscale model for polycrystalline alloy solidification, and the continuous Gaussian nucleation distribution model for heterogeneous nucleation. The model is validated by comparing with experimental results from three aspects, namely, the fusion profile, the grain structure and the dendritic structure, and good agreements are achieved. The whole solidification process, from the planar growth to cellular growth, columnar dendritic growth and equiaxed dendritic growth, in the laser weld pool is studied through analyzing the evolution of the temperature, solute concentration and constitutional undercooling fields. In particular, the distribution of constitutional undercooling in the liquid ahead of the solidification front is predicted quantitatively. The relationship between the constitutional undercooling distribution and the nucleation/growth of equiaxed grains is demonstrated, which reveals the selective growth behavior of the columnar grains and equiaxed grains at different locations in the laser weld.