Grain Growth Rate Research Articles

Investigation of microstructural and thermal stability of Ni-Y-Zr ternary nanocrystalline alloy

This investigation focused on the thermal stability of nanocrystalline (NC) binary and ternary Ni-Y-Zr alloys synthesized through ball-milling. The microstructural changes following annealing, conducted up to 1200 °C, were studied using various techniques, including X-ray-line-broadening, micro-hardness, and transmission electron microscopy. The results revealed that the rate of grain growth observed in the Ni-Y-Zr ternary alloy at 600 °C resembled that in pure NC-Ni at 100 °C. Moreover, the Ni-1.4Y-1.1Zr ternary system exhibited a maximum hardness of 753 HV (average) at 600 °C, which was approximately 60 HV higher than the Ni-1.2Y/1.9Y and Ni-1.5Zr/2.7Zr binary alloys and more than double that of pure NC-Ni for similar grain sizes. These characteristics were linked to the formation of nano-sized oxides and nitrides of Y and Zr within the microstructure. In summary, this study emphasizes the notable high-temperature microstructure stability of Ni-based ternary alloys, attributed to the additive effects of Y and Zr. Due to this extended stability, this ternary system could find potential applications at elevated temperatures in areas such as jet engine turbine blades and power plants.

Nucleation-dependent early growth of dendritic grains in Al-Cu alloys: The real-time observations and large-scale phase-field simulations

Formation of a dendritic grain starts from nucleation and then growth propagation occurs. Through the in situ and real-time solidification experiments of Al-Cu alloys observed by synchrotron X-ray imaging, it is found that the early-stage free growth rate of dendritic grains goes far beyond the concept described by the classical crystal growth theory. The rate gradually drops down even though the liquid is continuously cooled down. Quantitative 3D phase-field simulations demonstrate that this abnormal early-stage growth behavior depends strongly on the nucleation. A critical nucleus can grow rapidly to a peak rate driven by the initial nucleation undercooling, and then its rate gradually drops down to a minimum before it approaches the steady-state free growth regime. This strong dependence of the early-stage growth on the nucleation is then supported by an analytical model, and this correlation enables the accurate identification of the nucleation undercooling for each grain in the experiment and thus allows for a large-scale quantitative simulation of the real-time observed polycrystalline growth. This progress provides a comprehensive understanding on the crystal growth kinetics from a critical nucleus to the growth end, and thus will provide a new theoretical framework to design novel technology to control the solidification microstructures.

Influence of crystallite size and impurity density on the grain structure evolution of electroplated copper films during thermal and laser annealing

In this paper, the changes in microstructure and conductivity of the films before and after thermal and laser annealing are observed, which corresponds to various combinations of crystallite sizes and impurity densities. The results show that the impurities promote grain boundary migration at larger initial crystallite sizes, which contradicts previous reports. This is because the rate of grain growth is determined by the driving force and resistance to grain growth, the former provided by the energy of grain boundaries and the impurities outside the nanograin boundaries, and the latter by the impurities enriched within the boundaries. Additionally, laser annealing is more effective than thermal annealing in stimulating impurity diffusion, resulting in a reduced influence of impurities on grain growth.

Fabrication and microstructure refinement of calcium hexaaluminate ceramics with a relative density of over 95 %

Calcium hexaaluminate (CaAl12O19, CA6) has great potential for application in high-temperature industries. However, owing to its intrinsic hexagonal lamellar crystal structure, it is difficult to obtain calcium hexaaluminate materials with high densification. In this study, using a two-stage sintering method and TiO2 sintering aid, the densification of calcium hexaaluminate materials (relative density >95 %) was achieved. The evolution behaviors of the phase composition and microstructure and the intrinsic sintering kinetics were investigated in detail. The results showed that the addition of TiO2 promoted the formation of CA6 grains with a more homogeneous morphology and an increased-density equiaxial morphology. The elevation of the sintering temperature the sintering temperature also facilitated the densification of CA6, as the growth rate of the CA6 grains along the C-axis raised significantly with the sintering temperature. When 1 wt% of TiO2 was added and the sintering temperature was 1750 °C, a CA6 material with a bulk density of 3.61 g/cm3 (relative density over 95 %) could be fabricated.

A high-pressure strategy to refine and multiply nanoprecipitates for improved strength-ductility synergy in a medium-entropy FeCrNiTiAl alloy

Dispersion strengthening is one of the most effective methods to strengthen a metallic material. The strength of an aged alloy is largely affected by the precipitate size and number density of precipitates, which are often controlled by the change in temperature, time, and composition. Here we report a new high-pressure (HP) strategy for regulating the nanoprecipitates (NPs) in a Fe50Cr16Ni27Ti4Al3 medium-entropy alloy. Using a HP of 4 GPa largely refines the size of NPs from 15 to 4nm (due to the suppressed grain-growth rate) and increases the number density of NPs by two orders of magnitude (due to the increased nucleation rate). The decreased size and increased density of NPs lower the spacing distance of high-density dislocation walls and result in the formation of microbands during deformation, causing sustainable strain-hardening in the MEA, thereby improving both strength and ductility.

Modeling and simulation of grain growth for FGH96 superalloy using a developed cellular automaton model

Through heat treatment experiments and numerical simulations, the effects of the heating temperature (1313–1423 K) and holding time (10–240 min) on the grain growth behavior of the extruded FGH96 alloy were investigated. A two-dimensional cellular automata (CA) model that considered the dissolution of the γ′ phase over time and the distribution characteristics with different sizes was developed to explore the grain growth behavior above the γ′ phase over-solution temperature (1423 K) and below the γ′ sub-solution temperature (1383 K), respectively. The results showed that the rate of grain growth of FGH96 alloy was obviously enhanced when the heating temperature exceeded 1363 K, which was mainly related to the dissolution of the γ′ phase, and the grain growth of FGH96 alloy mainly occurred during the initial stage of insulation. The grain growth model of the extruded FHG96 alloy could accurately predict the grain growth behavior, and the simulation results were in good agreement with the experimental results at over-solution temperature or sub-solution temperature. The effects of volume fraction and radius of γ′ phase on the grain growth behavior of FGH96 alloy were studied by simulating the grain growth behavior of FGH96 alloy under different sizes and volume fractions of γ′ phase. The results follow the Zener relation, and the coefficient n in the Zener relation was determined by fitting the simulation results.

An alternate approach for estimating grain-growth kinetics

Rate of grain growth, which aides in achieving desired properties in polycrystalline materials, is conventionally estimated by measuring the size of grains and tracking its change in micrographs reflecting the temporal evolution. Techniques adopting this conventional approach demand an absolute distinction between the grains and the interface separating them to yield an accurate result. Edge-detection, segmentation and other deep-learning algorithms are increasingly adopted to expose the network of boundaries and the associated grains precisely. An alternate approach for measuring grain-growth kinetics, that curtails the need for advanced image-processing treatment, is presented in this work. Grain-growth rate in the current technique is ascertained by counting the number of triple-( and quadruple-) junctions, and monitoring its change during the microstructural evolution. The shifted focus of this junction-based treatment minimises the significance of a well-defined grain-boundary network, and consequently, the involvement of the sophisticated techniques that expose them. A regression-based object-detection algorithm is extended to realise, and count, the number of junctions in polycrystalline microstructures. By examining the change in the number of junctions with time, the growth rate is subsequently determined. Growth kinetics estimated by the present junction-based approach, across a wide-range of multiphase polycrystalline microstructures, agree convincingly with the outcomes of the conventional treatment. Besides offering a novel technique for grain-growth measurement, the analysis accompanying the current work unravels a trend, compatible with the topological events, in the progressive evolution of the triple-junctions count. The present approach, through its underlying algorithm, provides a promising option for monitoring grain-growth during in-situ investigations.

Unraveling grain growth of metallic tungsten: Investigating the nanoscale realm of hydrogen reduction of tungsten oxides

This paper focuses on the experimental observation of tungsten grain evolution during the process of hydrogen reduction of tungsten trioxide. The objective is to gain insights into the grain size distribution (GSD) evolution and morphology changes of metallic tungsten. The study employs an amalgamation of conventional characterization techniques – such as TGA, SEM-EDS, TEM and XRD – alongside cutting-edge techniques like phase contrast nano-tomography from synchrotron sources. Conducted as a quasi in-situ study, this research offers the opportunity to observe the transient evolution of the formed metallic tungsten phase within its precursor WO2 particles. The paper presents and discusses quantitative results concerning the evolution of GSD and morphological changes, encompassing growth rates of both crystallites and grains of the synthesized metallic phase. Additionally, the quasi in-situ study highlights the dependence of grain size on water concentration during the reduction process. The qualitative and quantitative findings contribute to a more nuanced understanding of the kinetics of grain growth of metallic tungsten and offer insights into the intricate interplay between reduction parameters, GSD evolution and morphological changes. Gaining a deeper understanding of the underlying mechanisms driving the changes in GSD establishes a foundation for predictive theory with immediate academic and industrial impact.

Investigate the relationship between densification and grain size of IGZO target by phase‐field simulation

AbstractThe phase‐field method was employed to simulate the effects of pores on grain growth in the indium gallium zinc oxide (IGZO) targets during conventional and two‐stage sintering (TSS). The findings indicated that the TSS method effectively suppressed the rapid growth of grains. With an increase in porosity and a more prominent pore size, the rate of grain growth accelerated, resulting in the formation of coarser grains. Subsequently, IGZO targets were fabricated using the molding‐cold isostatic pressing (CIP) and TSS methods. The densification activation energies of IGZO green compacts were calculated to be 510 and 370 kJ/mol for CIP at 180 and 250 MPa, respectively, utilizing the main sintering curve (MSC). Additionally, by optimizing the TSS process, a highly dense IGZO target with a relative density of 99.53%, a resistivity of 4.3 mΩ/cm, and an average grain size of 8.59 µm was successfully achieved.

Grain Engineering of Sb2 S3 Thin Films to Enable Efficient Planar Solar Cells with High Open-Circuit Voltage.

Sb2 S3 is a promising environmentally friendly semiconductor for high performance solar cells. But, like many other polycrystalline materials, Sb2 S3 is limited by nonradiative recombination and carrier scattering by grain boundaries (GBs). This work shows how the GB density in Sb2 S3 films can be significantly reduced from 1068±40to 327±23nm µm-2 by incorporating an appropriate amount of Ce3+ into the precursor solution for Sb2 S3 deposition. Through extensive characterization of structural, morphological, and optoelectronic properties, complemented with computations, it is revealed that a critical factor is the formation of an ultrathin Ce2 S3 layer at the CdS/Sb2 S3 interface, which can reduce the interfacial energy and increase the adhesion work between Sb2 S3 and the substrate to encourage heterogeneous nucleation of Sb2 S3 , as well as promote lateral grain growth. Through reductions in nonradiative recombination at GBs and/or the CdS/Sb2 S3 heterointerface, as well as improved charge-carrier transport properties at the heterojunction, this work achieves high performance Sb2 S3 solar cells with a power conversion efficiency reaching 7.66%. An impressive open-circuit voltage (VOC ) of 796mV is achieved, which is the highest reported thus far for Sb2 S3 solar cells. This work provides a strategy to simultaneously regulate the nucleation and growth of Sb2 S3 absorber films for enhanced device performance.

Solute drag-controlled grain growth in magnesium investigated by quasi in-situ orientation mapping and level-set simulations

Critical properties of metallic materials, such as the yield stress, corrosion resistance and ductility depend on the microstructure and its grain size and size distribution. Solute atoms that favorably segregate to grain boundaries produce a pinning atmosphere that exerts a drag pressure on the boundary motion, which strongly affects the grain growth behavior during annealing. In the current work, the characteristics of grain growth in an annealed Mg-1 wt.%Mn-1 wt.%Nd magnesium alloy were investigated by advanced experimental and modeling techniques. Systematic quasi in-situ orientation mappings with a scanning electron microscope were performed to track the evolution of local and global microstructural characteristics as a function of annealing time. Solute segregation at targeted grain boundaries was measured using three-dimensional atom probe tomography. Level-set computer simulations were carried with different setups of driving forces to explore their contribution to the microstructure development with and without solute drag. The results showed that the favorable growth advantage for some grains leading to a transient stage of abnormal grain growth is controlled by several drivers with varying importance at different stages of annealing. For longer annealing times, residual dislocation density gradients between large and smaller grains are no longer important, which leads to microstructure stability due to predominant solute drag. Local fluctuations in residual dislocation energy and solute concentration near grain boundaries cause different boundary segments to migrate at different rates, which affects the average growth rate of large grains and their evolved shape.

Effects of external loads on microstructure and properties of P92 steel

During the metal-solid phase transition, the mechanical properties of metal materials are affected by the nucleation rate and growth rate of grain. In this work, external loads (200 N, 400 N, 600 N, and 800 N) were applied to P92 steel during the heat treatment process in an attempt to improve and optimize the microstructure and mechanical properties of P92 steel. The effects of different external loads on the morphology, corrosion resistance, microhardness, and mechanical properties of P92 steel were investigated by using optical microscopy, microhardness testing, electrochemical testing, tensile strength measurements, and semi-automatic impact tester. The results show that the grain growth rate of P92 steel was moderate under an external load of 600 N; the grain distribution was uniform and orderly whereas the microhardness and corrosion resistance of the sample reached the optimum value (528 Hv). The tensile strength, yield strength, and elongation of the sample were 786 MPa, 525 MPa, and 37.8 %, respectively. However, the impact power of the P92 steel under an external load of 200 N was the minimum, and its average value was 132 J. The results depict that the external load can improve the mechanical properties of P92 steel.

Effect of Nb on microstructure and corrosion resistance of X80 pipeline steel

In this study, a series of χNb-X80 pipeline steels were obtained by adding different contents of Nb to X80 pipeline steel and after different cooling rates. The effects of Nb on the microstructure and corrosion resistance of X80 pipeline steel were studied by metallographic microscope, scanning electron microscope, differential scanning calorimeter, electrochemical corrosion, and salt spray corrosion. The results show that the high cooling rate will hinder the phase transformation, reduce the diffusion ability of Nb element at the grain boundary, inhibit the growth rate of ferrite grains, and refine the grains of the sample steel structure. The grain refinement effect is the best when the cooling rate is 40 °C/min. In addition, the reason why Nb improves the corrosion resistance of X80 pipeline steel is discussed from two aspects: the change of corrosion products and the effect of grain refinement. The analysis shows that Nb promotes the formation of a dense corrosion protective film, and this positive effect of the protective film overcomes the negative corrosion effect caused by the grain refinement caused by the precipitated Nb (CN), thereby enhancing the corrosion resistance of the substrate. The corrosion resistance is the best when the Nb content is 0.16 wt %.

3D grain growth in nanocrystalline Al via molecular dynamics: Influence of size, topology and integral mean curvature on grain kinetics

Grain growth in nanosized polycrystalline Al was studied via molecular dynamics. In particular, the volumetric growth rate of grains as a function of size, topology and mean curvature was analyzed. To this aim, an algorithm for the identification of the topological features and calculation of the mean curvature was developed. It was found that in average grains with ≈15 faces have zero integral mean curvature and show no change in volume implying that the critical number of face (Fcr) is ≈15. Furthermore, integral mean curvature of grains was found to be linearly correlated with the difference between the number of faces grains have and the average number of faces of their neighbors, F−<FNN>. That is, a grain with F−<FNN>=0 tends to have zero integral mean curvature and neither shrinks nor grows whereas grains with F−<FNN> greater than 0 grow and those with F−<FNN> less than 0 shrink. This agrees with the theoretical expectations from MacPherson–Srolovitz relationship and topology based descriptions of grain growth. Nonetheless, considering individual grain kinetics, the comparison of theoretical predictions (MacPherson–Srolovitz relationship, F−<FNN> and topological class) with measured growth rates showed more scatter. In addition, the deviation between the models and the measure values occurred for grains having a substantial relative size difference with the average grain size (|Rgrain−<R>|) and grains with relatively high |F−Fcr|. These grains also had average turning angles at triple lines (βTJ), which differed from the average equilibrium value expected in isotropic metals, π/3, but were consistent with grain growth under finite triple line mobility. This seems to confirm the existence of triple junction drag at the nanocrystalline length scale, which is not accounted for in classical theoretical models.

Effect of CaO doping on densification and microstructure control of highly transparent La0.4Gd1.6Zr2O7 ceramics

AbstractCalcium oxide (CaO) as sintering additive was first used to fabricate La0.4Gd1.6Zr2O7 transparent ceramics by a simple solid‐state reaction and one‐step vacuum sintering method. The effects of CaO dopant amount on the densification, as well as sintering behaviors and microstructure evolution of the as‐fabricated La0.4Gd1.6Zr2O7 ceramics, were systematically investigated. Under the different sintering temperatures, the relationships during the sintering process between grain growth and zpore elimination were analyzed as well. It was found that 0.1 wt% CaO doping can effectively control the rate of grain growth and promote densification dominated by surface diffusion. Furthermore, Ca2+ entered the lattice of La0.4Gd1.6Zr2O7 ceramics to accelerate ion diffusion and suppress grain boundary migration. With the introduction of 0.1 wt% CaO doping, the highly transparent La0.4Gd1.6Zr2O7 ceramics (T = 80.4% at 1100 nm) were successfully fabricated at the traditional sintering temperature (1850°C).

Effect of biaxial strain on grain growth in nanocrystalline films: Coupling between grain-boundary energy and strain energy

A primary shortcoming of nanocrystalline materials is that they tend to coarsen at elevated temperatures, which limits their practical application. Nowadays, it is crucial to develop a non-alloying strategy to improve the thermal stability of nanocrystalline materials without changing their chemical composition. This work has demonstrated that varying the biaxial strain in nanocrystalline films can drastically change the grain-growth rate in nanocrystalline Ni films and even enable the tailoring of stable nanograin sizes in nanocrystalline Ni-Mo films. The biaxial strain can be adjusted, in this case, by introducing different thermal strains via using substrates with different coefficients of thermal expansion. Thus the thermal stability of nanocrystalline Ni and Ni-Mo films on Cu substrates is higher than that on W or Ni substrates. The underlying mechanism of this exceptional substrate-dependent thermal stability was revealed by performing thermodynamic model calculations, which provided insights into the effects of coupling between the grain-boundary energy and strain energy on the thermal stability of nanocrystalline films. In addition, the kinetic reasons for the different thermal stabilities of the Ni and Ni-Mo films were assessed, and the effect of the biaxial strain on the kinetic stabilizations was discussed. This work presents a strain-induced stabilization strategy for nanocrystalline materials, which is significant for enhancing the thermal stability of nanocrystalline materials without requiring alloying elements.

Effects of Pressure on Homogeneous Nucleation and Growth during Isothermal Solidification in Pure Al: A Molecular Dynamics Simulation Study

Effects of different pressures on the isothermal-solidification process of pure Al were studied by molecular dynamics (MD) simulation using the embedded-atom method (EAM). Al was first subjected to a rapid-cooling process, and then it was annealed under different pressures conditions. Mean first-passage times (MFPT) method, Johnson-Mehl-Avrami (JMA) law, and X-ray diffraction (XRD) simulation analysis method were used to qualify the solidification- kinetic processing. Nucleation rate, critical-nucleus size, Avrami exponent, growth exponent, and crystallite size were calculated. Results show that the nucleation rate increases as the pressure increases. The change of critical-nucleation size is not obvious as the pressure increases. With the pressure increasing, growth exponent decreases, indicative of decreased grain-growth rate. It was also found that with the pressure increasing, the Avrami exponent decreases, indicating that the increased pressure has an effect on growth modes during solidification, which changes from three-dimensional growth to one-dimensional growth. Results of XRD simulation shows that with pressure increasing, crystallite size decreases.

High-fidelity simulations and data-driven insights on rate-governing phases in duplex and triplex systems during isotropic normal grain growth

Normal grain growth in multiphase polycrystalline systems includes sluggish evolution of minor-phase grains and relatively faster growth of the major-phase grains. Which of these two factors predominantly affects the growth kinetics of the overall microstructure remains an open question. Critical insights to answer this question are offered in this work, through data science techniques, by cumulatively analyzing the influence of the evolution rate of all constituent phase grains on the simulated growth kinetics of duplex and triplex microstructures. Moreover, predictive models relating volume fraction of the constituent phases to the growth kinetics of multiphase systems are developed and are extensively validated. The simulated data set encompassing a large area of interest and feature-rich information, which lends itself to the current analyses, is built through a suitable data-acquisition strategy, and a thermodynamically consistent multiphase-field approach. Statistical analyses of 14 triplex and 5 duplex microstructures, included in the data set, unravel that the grain-growth kinetics in both multiphase systems is primarily governed by the evolution rate of phase(s) occupying larger volume fraction. This effect is observed in spite of the sluggish growth exhibited by the minor-phase grains. Furthermore, the predictive models, developed using regression and feed-forward neural network, indicate a nonlinear, but proportional, relation between the phase fractions and kinetic coefficients that quantify the grain-growth rate of multiphase systems.

Impact of High Night Temperature on Yield and Pasting Properties of Flour in Early and Late-Maturing Wheat Genotypes.

The inexorable process of climate change in terms of the rise in minimum (nighttime) temperature delineates its huge impact on crop plants. It can affect the yield and quality of various crops. We investigated the effect of high night temperature (HNT) (+2.3 °C over ambient) from booting to physiological maturity on the yield parameters, grain growth rate (GGR), starch content, composition, and flour rheological properties in early (HI 1544, HI 1563) and late-maturing (HD 2932) wheat genotypes. The change in yield under HNT was highly correlated with grain number per plant (r = 0.740 ***) and hundred-grain weight (r = 0.628 **), although the reduction in grain weight was not significantly different. This was also reflected as an insignificant change in starch content (except in HI 1544). Under HNT, late-sown genotypes (HI 1563 and HD 2932) maintained high GGR compared to the timely sown (HI 1544) genotype during the early period of grain growth (5 to 10 days after anthesis), which declined during the later phase of grain development. The increased rheological properties under HNT can be attributed to a significant reduction in the amylose to amylopectin (AMY/AMP) ratio in early-maturity genotypes (HI 1544 and HI 1563). The AMY/AMP ratio was positively correlated to flour rheological parameters (except setback from peak) under HNT. Our study reports the HNT-induced change in the amylose/amylopectin ratio in early maturing wheat genotypes, which determines the stability of flour starches for specific end-use products.

A Common Diffusional Mechanism for Creep and Grain Growth in Polymineralic Rocks: Experiments

AbstractWe conducted uniaxial compression and grain growth experiments on fine‐grained forsterite (Mg2SiO4) + 10 vol% periclase (MgO) aggregates. This aggregate is a unique polymineralic system in which all of the constituent elements in the secondary phase constitute the primary mineral phase, except for Si, which is the slowest diffusing species in the primary phase. Grain growth in polymineralic aggregates, where the presence of a secondary phase results in grain boundary pinning of the primary phase, proceeds via Ostwald ripening of the secondary phase. Grain growth and diffusion creep rates were analyzed, and both rates were found to be limited by the same grain‐boundary diffusivity. For periclase ripening, forsterite grains surrounding the periclase grains must be deformed, such that diffusion of Si as well as Mg and O is necessary for periclase ripening, similarly to diffusion creep of the aggregates. Most rocks are polymineralic and usually contain a silicate mineral as their main phase, in which Si is the slowest diffusing species. This leads to the prediction that a single diffusivity, which is Si diffusivity in most cases, determines the rates of grain growth and diffusion creep in much of the Earth's interior. The grain size, which is the result of grain growth, and the time it takes to achieve that size, indicate the diffusivity. Viscosity during diffusion creep is determined by these grain size and diffusivity values. We view these findings as a unique opportunity to estimate the viscosity from the Earth's crust to the lower mantle.