Grain Growth Kinetics Research Articles

Cellular automaton simulation of solid-phase grain growth under conditions involving scanning heat sources and temperature gradients

Abstract A cellular automaton simulation of grain growth in a solid phase was conducted, considering the temperature gradient, heat source movement, and conditions favoring prioritized grain growth. The results reveal that, under optimal conditions, cellular grains elongated in the direction of the heat source movement. Detailed simulations illustrate the dynamics of grain growth and effect of mobility and driving forces on the dynamics, providing valuable insights into cellular grain growth in a solid phase.

Excellent magnetic properties obtained in planar flow casting Fe-6.5 %Si steel via low strain cold-rolling and annealing

Planar flow casting (PFC) has been applied to prepare Fe-6.5wt. %Si ultra-thin ribbons with a strong 〈001〉 fiber texture. However, critical issues such as fine grain size and large surface roughness are vital to overcome. In this study, low strain cold-rolling and subsequent annealing are applied to planar flow casting Fe-6.5 %Si ribbons. Cold-rolling reduces surface roughness and introduces heterogeneous deformation in the ribbons. Recrystallization occurs preferentially in high deformed areas during annealing. Recrystallizing grains grow significantly by consuming neighboring deformed grains with fine size and 〈001〉 orientation, leading to greatly reduced iron loss. Thus, the ribbon annealed at 1000 ℃ exhibits the best combination of magnetic properties with B8 of 1.29 T and P2/5000 of 6.95 W/kg. The kinetics of recrystallization and grain growth are described along with the calculations of activation energies, which suggests the cold-rolled ribbon has a faster growth rate than the undeformed one. Two mechanisms, namely strain-induced boundary migration (SIBM) and thermo-induced boundary migration (TIBM), are used to explain the grain growth. SIBM is dominant at low temperatures while TIBM is dominant at high temperatures. This work presents a strategy to tailor the microstructures and properties of PFC-prepared alloys with fine grain.

Microstructure evolution during solidification in a low superheat casting process of the Al-Mg2Si composites having excess Si: A phase field study

A two-dimensional phase field (PF) model of solidification during low superheat casting has been developed to grab insight into the microstructure formation mechanism of the Al-xMg2Si-ySi composite, having extra Si as the grain refining agent. The developed PF model employs a seed undercooling based nucleation model to simulate the formation of primary Mg2Si and α-Al grains, wherein the interfacial energy of the Al-melt interface is taken from literature, and a Molecular Dynamics (MD) model is employed to calculate the interfacial energy of the Mg2Si-melt interface. The simulation study predicts the microstructural parameters such as grain size, and sphericity of the primary Al and primary Mg2Si phases, as well identifies the appropriate numerical parameters such as mobility, anisotropy parameters to study the microstructure evolution in the Al-xMg2Si-ySi composites. The kinetics of grain growth of both α-Al and primary Mg2Si have been studied. Novelty of the study lies in developing a near-accurate PF model capable of optimising the weight fraction of Mg2Si and excess Si in the low superheat cast Al-xMg2Si-ySi composite system, in view of obtaining the lower grain size and higher sphericity of both primary Al and primary Mg2Si grains, which is in line with the experimental observations. The proposed PF model is capable of predicting the faceted growth of Mg2Si particles in the Al-xMg2Si-ySi composite, in presence of extra Si and during low superheat casting, and the dendritic growth of Mg2Si in the binary Al-Mg2Si composite having high Mg2Si weight fraction. Following the Jackson’s model of interface growth, it has been demonstrated that the modification of kinetic coefficient leads to a transformation of morphology of Mg2Si particles from large dendritic to faceted ones.

Suppress Austenite Grain Coarsening by Nb Alloying in High-Temperature-Pseudo-Carburized Bearing Steel.

The effect of Nb alloying on the suppression of austenite grain coarsening behavior during pseudo-carburizing is investigated in high-temperature-carburized SAE4320 bearing steel. To explore the role of the Nb element in the pseudo-carburizing process, the morphology, composition, size, and distribution of NbC precipitates were analyzed. The results show that the fine austenite grain observed in Nb micro-alloyed steel is caused by the pinning effect of NbC precipitates, which hinders the coarsening of austenite grains and changes the growth dynamics of austenite grains. After the SAE4320 carburized bearing steel with the addition of 0.45 wt.% Nb element is kept at 1150 °C for 4 h, the PAG size is still below 20 μm, which indicates the Nb element has obvious advantages in limiting PAG growth at high temperatures and shows great potential for the development of high-temperature carburized bearing steel.

Microstructural Analysis of K-TIG-Welded New Ni-Based Superalloy VDM Alloy 780

The fusion zone microstructures in K-TIG-welded and post-weld solution heat-treated new superalloy VDM Alloy 780 were examined. In addition, the kinetics of the base metal grain growth during solution heat treatments were analyzed. (S)TEM analyses show that major interdendritic microconstituents formed in the fusion zone due to elemental microsegregation are MC carbides and coarse irregularly shaped Laves phase. Additionally, minor secondary interdendritic phases are found to include γ′, γ″, and tiny plate-like Laves particles. To prevent any potential deterioration of mechanical properties caused by the irregular Laves phase, post-weld solution heat treatments (PWSHTs) at 954 °C to 1060 °C/1 hours were performed to remove the Laves phase. PWSHT at 954 °C only partially eliminates the Laves particles while forming an abundance of interdendritic δ/η phase. Laves phase is dissolved entirely without forming δ/η platelets after PWSHT at 1060 °C. It is proven that Laves eutectics in VDM Alloy 780’s fusion zone can be eliminated through PWSHT without significantly coarsening the base metal’s grain size in comparison to Alloy 718 as a result of substantial grain growth inhibition likely caused by solute segregation at grain boundaries.

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.

Grain growth of ice doped with soluble impurities

Abstract. The grain size of polycrystalline ice affects key parameters related to the dynamics of ice masses, such as the rheological and dielectric properties of terrestrial ice as well as the ice shells of icy satellites. To investigate the effect of soluble impurities on the grain-growth kinetics of polycrystalline ice, we conducted annealing experiments on polycrystalline ice samples doped with different concentrations of KCl (10−2, 10−3, 10−4 and 10−5 mol L−1) or MgSO4 (10−2 and 10−5 mol L−1). Samples were annealed for a maximum of 100 h at a hydrostatic confining pressure of 20 MPa (corresponding to a depth of about 2 km) and different constant temperatures of 268, 263, 258 and 253 K (corresponding to −5, −10, −15 and −20 °C, respectively). After each experiment, images of a polished sample surface were obtained using an optical microscope equipped with a cold stage. With grain boundaries detected, grains were reconstructed from the images, and an average grain size was determined for each sample. Normal grain growth occurred in all samples. Grain-size data are interpreted using the following grain-growth model: dn-d0n=kt (d: grain size; d0: starting grain size; n: grain-growth exponent; k: growth constant; t: duration). Values of the best-fit grain-growth exponent, n, for all samples range from 2.6 to 6.2, with an average value of 4.7. Pure ice exhibits 3.1 ⩽n⩽ 4.6, with an average value of 3.8. Above the eutectic point, soluble impurities enhance grain growth, as a melt phase is formed, and it could provide a fast diffusion pathway. Below the eutectic point, soluble impurities impede grain growth probably via the formation of salt hydrates that could pin the grain boundaries. Close to the eutectic point, the grain growth of doped ice is similar to pure ice. Natural ice is impure, often containing air bubbles and soluble impurities, and is usually subjected to a hydrostatic pressure. Our data set will provide new insights into the evolution of grain size within and the dynamics of natural ice masses.

Combining strength and ductility in a precipitation (γ’) hardened Fe-rich compositionally complex alloy via recrystallisation

Fe-rich alloys are the most attractive among the compositionally complex alloys (CCAs) owing to the excellent combinations of mechanical properties combined with low cost. The kinetics of recrystallisation and grain growth of an austenitic Fe-35.5Ni–4Cr-6.3Al-3.2Ti alloy (in at.%) is systematically studied to balance the combination of high strength and good ductility through grain refinement and the precipitation of L12 (γ′) phase. The yield strength of the Fe-35.5Ni–4Cr-6.3Al-3.2Ti alloy can be tuned by thermo-mechanical processing from 294 MPa in the coarse-grained FCC (γ) phase structure to 1142 MPa through inducing a heterogeneous grain structure consisting of fine recrystallised grains, subgrains/deformed grains and precipitates. The fully recrystallised alloy showed a balanced combination of strength (∼1.35 GPa) and uniform elongation (∼24%). The alloy showed good strengthening by grain refinement (Hall-Petch coefficient of 581 MPa μm0.5) and displayed an activation energy of 385 kJ/mol for static recrystallisation. The present Fe-rich CCA exhibited similar mechanical properties offered by Ni and Co-rich CCA compositions with a γ+γ′-phase structure, but at a substantially low alloying cost (<50%).

Study on the Growth Dynamics of Tartary Buckwheat Flowers and Grains, as Well as Material Basis and Physiological Changes of Their Seed-Setting Differences

Tartary buckwheat is a cereal crop that has both medicinal and food origins. However, the underlying factors that contribute to Tartary buckwheat’s flowering and seed-setting characteristics, effective grain formation, and physiological changes are still not well understood. This study aimed to investigate the flowering and seed-setting characteristics of different parts of Tartary buckwheat, as well as the grain-filling characteristics after flowering. To achieve this, Tartary buckwheat cultivars with high (QK3) and low (XQ2) seed-setting rates were selected for pot and field experiments. The study found that Tartary buckwheat undergoes flowering and seed setting simultaneously. Many wilted flowers and grains were observed 45 and 51 d after flowering. Compared to XQ2, QK3 exhibited a higher grain formation rate and seed-setting rate by 7.42% and 26.16%, respectively. Additionally, QK3 had a significantly lower grain abortion rate by 12.03%. The 1000-grain weight and average grain-filling rate of QK3 were 11.10% and 14.81% higher than those of XQ2, respectively. QK3 exhibited a faster maximum grain-filling rate (Rmax), reaching 18.38% faster than XQ2. Additionally, the dry matter average distribution rate in the main stem and branched grains of QK3 was 13.26% and 23.07% higher than that of XQ2, respectively. The sucrose concentration, SS, and SSS enzyme activities of QK3 were all higher than those of XQ2, by 0.29–25.99%, 5.22–11.62%, and 6.64–12.47%, respectively. Correlation analysis revealed a significant positive correlation between sucrose, soluble sugar, and starch concentration during the grain formation process and SS and SSS activities. This suggests that the levels of SS, SSS, soluble sugar, and sucrose in the grain play a crucial role in grain filling.

Crystallographic texture and grain boundary character development of hot-press sintered tungsten heavy alloy: The effect of rhenium concentration

In this study, the effects of rhenium on the evolution of microstructure and texture in hot press sintered tungsten heavy alloy (93W-4.9Ni-2.1Cu) is examined. Rhenium is added (i.e., increased rhenium from 0 to 1 wt % with interval of 0.25 wt %) to a conventional tungsten heavy alloy while maintaining a constant Ni: Cu (7:3) ratio. The samples were prepared using a vacuum hot press sintering method with parameters including 50 MPa pressure, 10 °C/min heating rate, and 1450 °C temperature for a 20-min holding time. The impact of rhenium on densification, crystallographic texture development, and grain boundary features of Re alloyed WHA was studied using electron backscattered diffraction (EBSD). The development of grain boundary texture was the result of a Ʃ3 CSL increase in sintered compact. The base alloy exhibits fibre texture in the (001) plane with the <111> direction and a higher proportion of low-angle grain boundaries. Reduced kinetics of grain growth during sintering led to a higher degree of densification and randomised weak fibre texture in rhenium-alloyed samples. The deterioration of texture in the (001) <111> direction in rhenium-added alloys was caused by recrystallization following recovery during sintering. Metallographic studies reveal that an increase in rhenium in the WHA's stoichiometry resulted in particle refinement during sintering, thereby enhancing the WHA's properties. Maximum micro-hardness (432 HV0.5), electrical conductivity (23.74 % of IACS), and relative density (99.90 %) were observed in a 93W-4.9Ni-2.1Cu (Re 1 wt% alloy).

Effects of additional heat treatment on struc-ture and microhardness of high-strength steel shipbuilding

The paper studies kinetics of austenitic grain growth (when heating) and the peculiarities of phase transformations (when cooling) that depend on the manufacturing technology of high-strength steel. The method of vacuum etching has been applied to reveal former austenitic grain boundaries in steel. It is shown that the necessary homogeneous structure in morphology and dimensions of structural elements in martensitic steels is formed under the influence of additional quenching from furnace heating with tempering after quenching from rolling heating.

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.

Austenite-Grain-Growth Kinetics and Mechanism in Type 347H Alloy Steel for Boiler Tubes

Abstract The research material (type 347H alloy steel) has been characterized using optical microscopy and an EDS/SEM system. Annealing experiments have been conducted at temperatures range of 600–1050°C for 30 min–20 h by using an atmosphere-controlled furnace. Normal grain growth with intermediate grain size has been related to the favouring of creep resistance to recommend the material suitable for boiler tubes at operating temperatures up to 750°C for long duration. The kinetics of grain growth in the 347H has been shown to behave similar to a pure metal in the initial stage of annealing in the range of 0–30 min, beyond which the grain-growth process was found to be suppressed due to second-phase (NbC) particle-pinning and solute drag effects. The grain-growth exponent n is computed to be in the range of 0.117–0.313; the deviation from ideal kinetic behavior (n=0.5) has been scientifically justified. The activation energy for grain growth Qg, for the investigated alloy, has been graphically computed and validated.

Grain growth of natural and synthetic ice at 0 °C

Abstract. Grain growth can modify the microstructure of natural ice, including the grain size and crystallographic preferred orientation (CPO). To better understand grain-growth processes and kinetics, we compared microstructural data from synthetic and natural ice samples of similar starting grain sizes that were annealed at the solidus temperature (0 ∘C) for durations of a few hours to 33 d. The synthetic ice has a homogeneous initial microstructure characterized by polygonal grains, little intragranular distortion, few bubbles, and a near-random CPO. The natural ice samples were subsampled from ice cores acquired from the Priestley Glacier, Antarctica. This natural ice has a heterogeneous microstructure characterized by a considerable number of air bubbles, widespread intragranular distortion, and a CPO. During annealing, the average grain size of the natural ice barely changes, whereas the average grain size of the synthetic ice gradually increases. These observations demonstrate that grain growth in natural ice can be much slower than in synthetic ice and therefore that the grain-growth law derived from synthetic ice cannot be directly applied to estimate the grain-size evolution in natural ice with a different microstructure. The microstructure of natural ice is characterized by many bubbles that pin grain boundaries. Previous studies suggest that bubble pinning provides a resisting force that reduces the effective driving force of grain-boundary migration and is therefore linked to the inhibition of grain growth observed in natural ice. As annealing progresses, the number density (number per unit area) of bubbles on grain boundaries in the natural ice decreases, whilst the number density of bubbles in the grain interiors increases. This observation indicates that some grain boundaries sweep through bubbles, which should weaken the pinning effect and thus reduce the resisting force for grain-boundary migration. Some of the Priestley ice grains become abnormally large during annealing. We speculate that the contrast of dislocation density amongst neighbouring grains, which favours the selected growth of grains with low dislocation densities, and bubble pinning, which inhibits grain growth, are tightly associated with abnormal grain growth. The upper 10 m of the Priestley ice core has a weaker CPO and better-developed second maximum than deeper samples. The similarity of this difference to the changes observed in annealing experiments suggests that abnormal grain growth may have occurred in the upper 10 m of the Priestley Glacier during summer warming.

Phase field investigation on the alumina nanocrystalline growth controlled by mullite-alumina phase transition in alumina-mullite fibers

The overgrowth of alumina nanocrystalline is the most important cause for the degradation of alumina-mullite fibers at high temperatures. A profound understanding on the kinetics and mechanisms of alumina nanocrystalline growth controlled by the mullite-alumina phase transition is of great significance to the development and service of alumina-mullite fibers. Here, by using the phase field simulations coupling to materials thermodynamic data, we systematically investigated the growth of alumina nanocrystalline and the mullite-alumina phase transition in alumina-mullite fiber at different temperatures and concentrations of the mullite matrix. The growth rates and exponents of alumina grains at different conditions are quantified and agree well with the experimental results of Nextel 720 fibers. We found that the growth of polycrystalline alumina grains undergoes two stages: rapid growth in the very early stage and then grains coarsening. The mullite-alumina phase transition dominates the rapid growth of alumina grains and the grain coarsening proceeds mainly through grain annexation and dissolution-precipitation. Our results clarify the influence of mullite-alumina phase transition on alumina grain growth from mullite matrix, and quantify the mechanisms and kinetics of alumina grain growth from mullite matrix in the alumina-mullite system.

Grain Growth during Mechanical Processing of Austenitic Stainless Steel AISI 321

The kinetics of austenite grain growth during thermomechanical treatment of AISI 321 steel with a relatively high content of carbon (0.07 wt. %) and titanium (0.50 wt. %) were studied. Hot deformation was carried out by the uniaxial compression of cylindrical specimens on a Gleeble 3800 thermomechanical simulator. A dependence is obtained for calculating the kinetics of austenite grain growth for a temperature range of 1150–1250 °C. The proposed dependence makes it possible to evaluate grain growth under non-isothermal conditions. The verification of the adequacy of the proposed dependence and the method for calculating the grain size at cooling rates 0.2, 1 and 5 °C/s showed its high convergence. The difference between the calculated and experimental grain size did not exceed 8%. The suppression of grain growth is due to the precipitation of titanium carbides and carbonitrides. Using the developed grain growth model, an analysis was made of the reasons for the formation of large grains in the shell after the elongating in the production process.

Dissolution controlled grain growth in Nickel Alloy UDIMET 720Li

The present paper examines the mechanisms for simultaneous manipulation of grain size and γ′ dissolution leading to the microstructure that balance creep and fatigue properties in UDIMET 720Li. For sub (near) solvus heat treatments, grain growth and γ′ dissolution occur simultaneously. Grain size following heat treatment is determined largely by the primary γ′ particle dispersion, via Zener-Smith pinning. Remarkably, grain growth occurs only as a result of precipitate dissolution. The resulting grain size is dictated by a combination of initial particle dispersion in the as-received material and heat treatment temperature. It is likely to hold so long as the initial grain size is close to the Zener-Smith limit and where the kinetics of grain growth and dissolution share similar rates.

Effects of annealing on particle size and pore size distribution of hydrothermally synthesized BaTiO3 nanopowders and their grain-growth kinetics during sintering

With an increase in the annealing temperature, the hydrothermally synthesized BaTiO3 nanopowders increased in particle sizes from 100 to 260 nm and decreased in pore volume from 7.2 to 2.82 cm3 g−1, while the pore size remained constant at 3.06 nm. Samples with different initial particle sizes were sintered in the temperature range of 1210 °C–1300 °C and for periods of 0.5–48 h at 1270 °C. The kinetic grain-growth exponent of the sintered BaTiO3 samples, n, was proportional to the increase of an initial particle size and the decrease of pore volume, and the grain growth obeyed the Arrhenius equation. The activation energies for the grain growth of the sintered BaTiO3 samples with initial particle size of 100, 155 and 260 nm were 737, 702 and 755 kJ mol−1, respectively, indicating that the activation energy was independent of the initial particle size in the range of 100–300 nm under identical purity conditions, and pore volume was supposed to be attributed to the velocity of grain growth.

Powder Metallurgy Route to Ultrafine-Grained Refractory Metals.

Ultrafine-grained (UFG) refractory metals are promising materials for applications in aerospace, microelectronics, nuclear energy, and many others under extreme environments. Powder metallurgy (PM) allows to produce such materials with well-controlled chemistry and microstructure at multiple length scales and near-net shape manufacturing. However, sintering refractory metals to full density while maintaining a fine microstructure is still challenging due to the high sintering temperature and the difficulty to separate the kinetics of densification versus grain growth. Here an overview of the sintering issues, microstructural design rules, and PM practices towards UFG and nanocrystalline refractory metals are sought to be provided. The previous efforts shall be reviewed to address the processing challenges, including the use of fine/nanopowders, second-phase grain growth inhibitors, and field-assisted sintering techniques. Recently, pressureless two-step sintering has been successfully demonstrated in producing dense UFG refractory metals down to ≈300nm average grain size with a uniform microstructure and this technological breakthrough shall be reviewed. PM progresses in specific materials systems shall be next reviewed, including elementary metals (W and Mo), refractory alloys (W-Re), refractory high-entropy alloys, and their composites. Last, future developments and the endeavor towards UFG and nanocrystalline refractory metals with exceptionally uniform microstructure and improved properties are outlined.