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In this study, we propose a new methodology for designing preform shapes for turbofan blisk and disk forgings of IN718 superalloy using a CNN (Convolutional Neural Network). The objective function of the preform design model is set to minimize grain size and reduce forging load. Various preform shape candidates were generated using NURBS (Non-Uniform Rational B-Splines) curves, defined by points on the target shape outline, and the grain size was calculated through FE (Finite Element) analysis based on the JMAK (Johnson-Mehl-Avrami-Kolmogorov) model to train the CNN model. The preform shapes derived from the proposed load and grain size-aware CNN model were compared and validated against those from previous studies regarding grain size and load reduction. An objective function f, which is capable of quantitatively expressing the load and grain size improvement, was developed for comparison and validation. The results showed an improvement of 19.64% for the blisk and 18.96% for the disk compared to previous studies. This research is expected to significantly contribute to the forging of aerospace components.

We investigated the impact of heat treatment on preform deduction during the hot forging of In718 turbine disks. Proposed preform design method leverages a Convolutional Neural Network (CNN) to aim for minimized damage and uniform grain size in the final forged products. For damage calculation, we employ the Lemaitre damage model, while the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is utilized for grain size calculation. The CNN, trained on datasets of NURBS-generated preform shapes and final forged disks from them, showed superior performance in terms of damage and grain size of forged disk compared with other existing approaches. Moreover, the results indicate that preforms deduced with heat treatment considerations can lead to significant improvements in forging results, including up to a 17% reduction in average grain size and a 16% decrease in standard deviation compared to preforms without heat treatment consideration. These results underscore the importance of considering heat treatment in preform design, offering valuable insights for industries where the integrity of forged products, like aerospace, is critical.

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Micro endmilling is considered as a preferred precision micromachining technique to produce complex 3D micro features due to the high material removal rate (MRR) and flexibility. In micromachining, feed/tooth is comparable to tool edge radius and material grain size. The scaling issues, material microstructure, flow stress and cutting temperature has a significant influence on the cutting mechanism and machined surface quality. This work proposes a mathematical model to predict the exit burr height during micro end milling by considering size effect, material microstructure, oblique cutting principle, cutting temperature, chip formation and flow stress. The model is based on the continuity principle and burr formation geometry. The proposed exit burr height model was validated by conducting micro endmilling experiments on Inconel 718. It was found that the exit burr height model based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) grain evolution model gives a good matching with experimental results and the proposed model could predict the exit burr height with a maximum percentage error of 6.4%. Size effect in exit burr height was observed. The exit burr height was observed to be higher at feed/tooth (f/t) much below cutting edge radius due to high ploughing on the workpiece material. The exit burr height at f/t above edge radius increases with f/t similar to conventional milling. The minimum exit burr height was observed in the ploughing to shearing transition region. The proposed model can be utilized for the optimization of machining parameters to control and minimize the exit burr size. [Display omitted] • An exit burr height model was developed for micro endmilling process. • Burr model considers grain evolution, size effect, oblique cutting, cutting temperature, chip formation, and flow stress. • The exit burr model was successfully validated with experiments on Inconel 718 with a maximum percentage error of 6.4%. • Minimum exit burr height was observed at the transition region. Below size effect region, large exit burrs were found. [ABSTRACT FROM AUTHOR]

The phase transformation model is the subject of this article capable of evaluating the hardenability of various bainitic steels. The objective is to provide a tool, which will help to design the chemical composition of steels with enhanced bainitic hardenability. Three general indexes, which are based on the relation between bainite volume fraction and cooling rate, and which characterize the hardenability, are proposed. Calculations of these hardenability indexes are presented for low‐carbon steels with different chemical compositions. Correlation between the chemistry and the hardenability factors is evaluated. Following this, an industrial process of cooling of wire rods is simulated for the steel with hardenability indexes adjusted to the cooling capabilities of Stelmor system. The cooling conditions, which give a favorable multiphase microstructure of wire rod for cold heading applications, are designed. [ABSTRACT FROM AUTHOR]

Geopolymers are miscellaneous materials with superior mechanical strength properties. Geopolymeric materials offer the advantage of simultaneously improving both environmental and engineering performance compared to cement-based materials used in the construction industry. This study presents the geopolymerization kinetics of fly ash-based geopolymer foams by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The experiments were designed using several curing temperatures and alkali concentrations. The degree of reaction (α) of the fly ash was used as a measure of the changes that occurred during the geopolymerization reaction. The compressive strengths of the prepared samples were estimated by using the JMAK method as well as through real-time compression strength analysis, and the results were compared. The values of n were in the range of 0.0931–0.2321, while the values of k were in the range of 0.366–0.671. Regardless of alkali concentration and curing temperature, the degree of reaction continued to increase in all samples until the 28th day, after which the degree of reaction remained almost stable. Although alkalinity and curing temperature had a positive effect on strength gain, low alkalinity had no effect on strength even when the curing temperature was increased. The increase in alkalinity, curing temperature, and curing time had an increasing effect on strength in both measured and calculated values. The strength values obtained on the 1st, 7th, 28th, and 90th days for the samples were compared with the JMAK results, and it was concluded that the values calculated with JMAK were half of the measured strength values. [ABSTRACT FROM AUTHOR]

Controlling titanium dioxide crystallinity is crucial in view of the possible oxide applications. In this study, we examined the effects of calcium doping on the anatase-rutile phase transition via in-situ high-temperature X-ray diffraction. The phase transition temperature of calcium-doped TiO 2 was approximately 900 °C, specifically 900 °C, 875 °C, and 900 °C for the 2%, 4%, and 6% calcium-doped TiO 2 , respectively. The larger radius of the calcium ion compared to that of the titanium ion hindered the calcium incorporation into the TiO 2 lattice. Moreover, the hampered titanium-oxygen bond rotation during the phase transition led to an increase in the phase-transformation temperature. Furthermore, at higher calcium concentrations, the corresponding activation energies increased first and then decreased. This may have been controlled by the decreasing nucleation rate during the reaction. [ABSTRACT FROM AUTHOR]

The influence of plastic anisotropy on the static softening kinetics after cold forming in the AA2024–T3 aluminum alloy sheet was investigated. The uniaxial tensile tests were carried out at room temperature in three directions 0°, 45° and 90° in relation to the rolling direction, at 8% strain level. The samples were then annealed at 350 °C for various holding times. The softening data were analyzed using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) method, and the Avrami's exponent n and rate constant k for each testing condition. The grain growth kinetics followed a power law behavior with an exponent coefficient n g and a Hall–Petch type relationship was found between the grain size and microhardness evolution. The results showed that the conventional JMAK equation could not accurately model the static softening kinetics of AA2024–T3. Moreover, the Avrami's exponent n and rate constant k determined from the modified JMAK equation containing an impingement parameter, the n g parameter from the grain growth power law and the Hall–Petch parameters were found to be different with the three directions. To validate the influence of plastic anisotropy on static softening, microstructure and microtexture characterization was carried out using a combination of Vickers microhardness tests, laser confocal microscopy, scanning electron microscopy, and electron backscatter diffraction before and after annealing. [ABSTRACT FROM AUTHOR]

The paper demonstrates the capability of the developed phase transformation model to design the cooling conditions in the Stelmor process allowing for obtaining different types of microstructures in wire rod of 32CrB4 steel. The model based on modified JMAK equation was developed using the results of the tests conducted in a DIL 805 A/D/T dilatometer. The model is composed of sub-models of ferritic, pearlitic, bainitic and martensitic transformations. Its predictive capability was confirmed in industrial conditions by performing trials with different settings of fans involved in the cooling process on the Stelmor line of CMC Poland. Excellent performance of the model was achieved through the modification of commonly used equations which allows accurate predictions of the phase transformations start and finish temperatures, as well as volume fraction of microstructural constituents, in a wide range of cooling rates. It was demonstrated that the model can effectively be applied to design the cooling conditions in the Stelmor process, which will result in the required microstructural composition of the wire rod. [ABSTRACT FROM AUTHOR]

Catalyst doping is a critical approach to improve the kinetics of MgH 2. However, reported activation energy (E a) values of catalyzed MgH 2 systems show a large discrepancy. This study investigates the effect of the driving force on E a for (de)hydrogenation reactions. The kinetic rate constant is calculated using Johnson-Mehl-Avrami-Kolomogorov (JMAK) model. E a is obtained according to a series of kinetic measurements under a nearly constant driving force. Under a low driving force ( P I n / P e q = 1.5) , the E a for dehydrogenation and hydrogenation are calculated to be 102.3 ± 5.0 and 41.6 ± 7.5 kJ/mol H 2 , respectively. Under a high driving force ( P I n / P e q = 10) , the E a for dehydrogenation and hydrogenation are calculated to be 101.4 ± 25.8 and 57.4 ± 5.0 kJ/mol H 2 , respectively. The result shows that the high driving force leads to higher hydrogenation E a over the E a obtained the low driving force, while dehydrogenation E a remains unchanged under different driving forces. [Display omitted] • Kinetics of VTiCr-catalyzed MgH 2 under different driving forces were measured. • The driving force affected the calculated activation energies of the hydrogenation. • The driving force shows no impact on the calculated activation energies of the dehydrogenation. • The increase of driving force may lead to the change of rate-limiting step of the hydrogenation. [ABSTRACT FROM AUTHOR]

Many of the significant mechanical properties of stainless steel, including ultimate tensile strength, yield strength and hardness, the ductile–brittle transition temperature, and susceptibility to environmental embrittlement, can be improved by grain size refinement. Hall-Petch relation identifies that the improvement can be quantified in a constitutive relation. In this study, a new mathematical model to calculate the number of grains and their average size inside a single printed layer via the laser melting deposition (LMD) process is proposed. The printed layer's thermal history concerning the moving laser beam and co-axial addition of powder debits was analyzed and used to calculate the thermal stress and strain rate. The average grain size within the printed layer was calculated using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The mechanical properties, including ultimate tensile strength, yield strength, and hardness, were estimated using the average grain size. For single depositions of AISI 304 stainless steel powder debits on a steel substrate, dedicated experiments were performed to verify the model’s trustworthiness. Scanning electron microscopy was used to quantify the number and size of grains. Vickers hardness tests were conducted to confirm the mechanical performances predicted by the developed model. It was found that the primary operating parameters strongly influence the grain type, resulting in three varieties of grains: (a) quasi-continuous circular, (b) long lath-shaped, and (c) a combination of the two mentioned above. The deposited layer's thermal history influences the thermal stresses and controls the growing grains' average size. A strong correlation between experimental and computational results, within the range of 10–15 %, and 8–10%, was obtained for the average grain size and Vickers hardness test. The laser scanning speed and laser power were in an inverse relation with the average grain size, while a direct relationship was noticed between the powder feed rate and average grain size. The mechanical computational and experimental results show that the highest yield strength (=208 MPa), ultimate tensile strength (=722 MPa), and hardness (=278 HV) were obtained for the finest grain structure.

Featured Application: The supposed united approach of hot deformation modelling allows us to significantly decrease the development and optimization of the industrial production processes of steels, multiprincipal element alloys, and titanium alloys. Hot deformation is one of the main technological stages of products made from metallic materials. It is strictly required to decrease the costs of developing optimized technologies at this stage without a significant decrease in the products' quality. The present investigation offers an algorithm to unite three different models to predict the hot deformation behavior, fracture, and microstructure evolution. The hot compression and tension tests of the AISI 316Ti steel were conducted using the thermomechanical simulator Gleeble 3800 for the models' construction. The strain-compensated constitutive model and the Johnson–Mehl–Avrami–Kolmogorov (JMAK)-type model of the grain structure evolution show a satisfactory accuracy of 4.38% and 6.9%, respectively. The critical values of the modified Rice and Tracy fracture criteria were determined using the experimental values of the relative cross-section reduction and finite element calculation of the stress triaxiality. The developed models were approved for the stainless AISI 316Ti steel by the hot torsion with tension test. [ABSTRACT FROM AUTHOR]

The ytterbium aluminum garnet composition YbAG (62.5 mol.% Al2O3, 37.5 mol.% Yb2O3) was prepared in the form of glass microspheres by flame synthesis. Precursor powder for flame synthesis with high homogeneity was prepared by modified sol–gel Pechini method. XRD pattern of prepared glass microspheres indicated predominantly amorphous nature of the sample. Detailed study of morphology of the microspheres by scanning electron microscopy revealed the presence of a small fraction of partially or fully crystallized microspheres. The high-temperature X-ray powder diffraction analysis (HT XRD) was carried out in the temperature interval 750–1450 °C: The temperature dependence of phase composition was determined. Crystallization of Yb3Al5O12—ytterbium aluminum garnet phase—was observed in the temperature range 900–1200 °C. The DSC analysis with heating rates 2, 4, 6, 8, 10 °C min−1 in temperature interval 25–1200 °C was performed in N2 atmosphere to study thermal behavior and crystallization kinetics of prepared glass microspheres. The two exothermic effects at 918 and 939 °C were observed, which were attributed to Yb3Al5O12 crystallization. The crystallization kinetics of prepared sample was examined with the use of JMAK model, and the kinetic triplet—frequency factor A = (1.8 ± 2.2) 10+28 min−1 (for the first peak), A = (1.2 ± 1.6) 10+55 min−1 (for the second peak), apparent activation energy Eapp = (6.4 ± 0.1) 10+02 kJ mol−1 (for the first peak), Eapp = (1.3 ± 0.1) 10+03 kJ mol−1 (for the second peak) and the Avrami coefficient m = 3 (for the first peak) and m = 2 (for the second peak)—was determined using RSS, Radj2, AIC and WAIC criteria. [ABSTRACT FROM AUTHOR]

• It was found for the first time that the phonon localization model can be applied to ultra-small sized amorphous Ge clusters. • The dependence between the saturation time of amorphous Ge clusters formation and the annealing temperature was found. • The activation energy of the disproportionation reaction for amorphous GeO x film on Si substrate is defined as 0.9 ± 0.1 eV. In this paper, based on the analysis of Raman spectroscopy data, the kinetics of the disproportionation reaction process (GeO x → (1− x /2)Ge + (x /2)GeO 2) of an amorphous GeO x film during furnace annealing were studied. An approximation of the experimental kinetics of the disproportionation reaction to the theoretical Kolmogorov–Johnson–Mel–Avrami dependence has been carried out. By analyzing the temperature dependence of the formation time of amorphous germanium clusters, the activation energy of the formation process was obtained, which amounted to 0.9 ± 0.1 eV. In addition, it was found that the position of the Raman peak from amorphous germanium nanoclusters depends on their size. Thus, the phonon localization model can be applied not only to germanium nanocrystals but also to amorphous germanium clusters in the case of their ultra-small sizes (less than 1.5 nm), which is less than the phonon correlation length in amorphous germanium. [ABSTRACT FROM AUTHOR]

Copper sulfide has received increasing attention as an electrode material in past decades. In this study, we report the synthesis of copper sulfide with layered copper hydroxide salt (LHS) (Cu 2 (OH) 3 NO 3) precursors using different protocols. X-ray diffraction suggests the presence of numerous non-stoichiometric phases (Cu 1-x S) and not a pure covellite phase and SEM images show particles with flower-like shape but different in size. The solid-state kinetic parameters of the reaction refined by the JMAK method indicate a pseudomorphic transformation controlled by 1D diffusion, different in term of precursors phase, reagents and protocol applied. The microwave method needs less energy to achieve the transformation than the amine digestion (AD) method and the morphology of particles is also different. Tested in sodium acetate electrolyte, CuS provides a maximum capacity of 67 mAh/g for AD, which is much higher than for the commercial grade CuS. This is explained by the difference in nanostructuration of the flower-like shape particles obtained from the layered template. Finally, CuS is used as both a positive and negative electrode material in a complete aqueous battery system but its redox process, which is strongly diffusion limited especially in the lower potential domain, prevents the whole system from operating at high power. Transformation of layered copper hydroxynitrate to copper sulfide, different synthesis and electrochemical behavior as a symmetric battery device. [Display omitted] • Copper sulfide was synthesized by using template-assisted method from layered copper hydroxynitrate. • Pseudo-morphic kinetic transformations depending on synthesis ways was investigated using JMAK model. • A symmetric aqueous battery device was built using copper sulfide and a capacity of 66 mAh/g was raised. [ABSTRACT FROM AUTHOR]

The 45Ni-35Cr-Nb alloy, commonly known as ET45 micro, produced in the form of centrifugally cast tubes, was studied by means of optical microscopy after aging treatments at 1073 and 1173 K for different times. A description of M23C6 secondary carbides precipitation phenomenon was made as a function of time. The analysis after using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model showed that the secondary carbide precipitation occurs in a single stage. It was found that this phenomenon, which is assisted by diffusion, has an activation energy of 196 kJ/mol.

Pancake forging of Inconel 718 was performed using a hydraulic press at temperature of 1273 and 1323 K. The effect of forging temperature on the microstructure of the pancakes was studied using optical microscopy. Variation in the microstructure was observed at different locations of the pancakes with fully recrystallized grains, partially recrystallized grains and necklace structure. Strain, strain rate and temperature across the pancake were determined through FEM analysis, and Johnson–Mehl–Avrami–Kolmogorov (JMAK) type model was employed to estimate the recrystallized grain size and volume fraction. There was close match between the predicted values and actual measurements. [ABSTRACT FROM AUTHOR]

Hot deformation is one of the main technological stages of products made from metallic materials. It is strictly required to decrease the costs of developing optimized technologies at this stage without a significant decrease in the products’ quality. The present investigation offers an algorithm to unite three different models to predict the hot deformation behavior, fracture, and microstructure evolution. The hot compression and tension tests of the AISI 316Ti steel were conducted using the thermomechanical simulator Gleeble 3800 for the models’ construction. The strain-compensated constitutive model and the Johnson–Mehl–Avrami–Kolmogorov (JMAK)-type model of the grain structure evolution show a satisfactory accuracy of 4.38% and 6.9%, respectively. The critical values of the modified Rice and Tracy fracture criteria were determined using the experimental values of the relative cross-section reduction and finite element calculation of the stress triaxiality. The developed models were approved for the stainless AISI 316Ti steel by the hot torsion with tension test.

• 3D ultrasonic vibration-assisted turning (3D-UVAT) method driven by two actuators is proposed. • A modified J-C model is used to simulate the turning process. • Microstructural evolution in heat-assisted 3D-UVAT is compared for simulation and experiment. • The effect of different turning conditions on microstructure and hardness is presented. Medium carbon steel is an excellent carbon structural steel, and is one of the most common materials for metal cutting. Little research has been done on the microstructural changes induced by thermal-force coupling. In this paper, a finite element simulation method based on the improved J-C model is used to predict the grain size, microstructure change depth and surface hardness of medium carbon steel surface induced by heat-assisted 3D-UVAT are studied. The numerical simulation results are compared with the experimental results, and the significant influence of turning conditions on them is analyzed. The results show that heat-assisted 3D-UVAT lowered the grain size of machined induced deformation zone. Numerical model foresees this case with a mean error of 9.4%. Microstructure and hardness measurements under different turning conditions show that the turning speed and feed rate contribute significantly to grain size and grain refinement layer depth in the area being machined. [ABSTRACT FROM AUTHOR]

Abstract The ALaE glass with eutectic composition (76.2 mol% Al 2 O 3 , 23.8 mol% La 2 O 3) and the ALaP glass with LaAlO 3 (lanthanum-aluminium perovskite) composition (50 mol% Al 2 O 3 , 50 mol% La 2 O 3) were prepared by combination of sol-gel Pechini method and flame synthesis in the form of glass microspheres (diameter ≈ 10 μm). The prepared glasses were characterized by OM, SEM, XRD and DSC. The prepared samples contained only completely re-melted spherical particles. However, more detailed study by SEM revealed small fraction of crystalline microspheres. Traces of crystalline LaAlO 3 were detected in X-ray powder diffraction patterns of both glasses. DSC analysis in the temperature range 35–1200 °C with heating rates 2, 4, 6, 8 and 10 °C/min was performed in nitrogen atmosphere to study the thermal behaviour and crystallization kinetics of prepared glasses. In the DSC curve of the eutectic ALaE glass recorded at 10 °C/min two exothermic effects were observed at 920 and 936°. Only one exothermic effect at 868 °C was present in the DSC curve of the ALaP sample with perovskite composition. The high-temperature X-ray powder diffraction analysis was carried out in the temperature interval 25–1200 °C. For both compositions formation of LaAlO 3 as the main crystalline phase was observed in the whole measured temperature range. In order to study the crystallization kinetics, the DSC curves were transformed into dependence of fractional extent of crystallization, α on temperature. The JMAK model was found suitable for description of crystallization kinetics of both studied systems. The kinetic parameters (A, E app and m) were calculated for this model and crystallization mechanisms have been proposed. [ABSTRACT FROM AUTHOR]

The crystallization mechanisms for Li 2 O-Al 2 O 3 -SiO 2 -B 2 O 3 -BaO glass-ceramics were studied by differential scanning calorimetry (DSC) under non-isothermal conduction. The activation energy and Avrami exponent of the glass-ceramics had been calculated by Kissinger method and Kissinger-Akahira-Sunose (KAS) method. Comparison of the measured DSC data with calculated curves at different heating rates using Sestak-Berggren (SB) model and Johnson-Mehl-Avrami-Kolmogorov (JMAK) model revealed the commonly used JMAK was not a suitable kinetic model for this glass-ceramic. However, the empirical SB model was found to be acceptable for analyzing the kinetics of crystallization for the studied glass composition. This model matched well with the DSC curves, with slight deviations at the tails of the peaks. Furthermore, the crystalline phases and microstructure of samples sintered at 730 °C for 2 h were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Energy dispersive spectrometer (EDS). The LiAlSi 2 O 6 and LiAlSi 3 O 8 constituted as the predominant phases in the studied glass-ceramic. [ABSTRACT FROM AUTHOR]

In the literature, a previous work has validated the application of the generalized JMAK equation to model the crystal dissolution kinetics in silicate melts for T > T liq . Because this dissolution can happen at temperatures below T liq , the purpose of this work was to test this model for T < T liq . To do so, the dissolution kinetics of rare earth (RE) silicate in a borosilicate melt were studied by following (with image analysis) the evolution of the crystal fraction as a function of time and temperature. The application of the model enables the mechanism limiting the dissolution to be known, i.e. the diffusion and the activation energy of the dissolution (475 kJ/mol). These conclusions are the same as those resulting from the application for T > T liq . This shows the continuity of the dissolution around T liq which is, in addition, confirmed by the chemical profile study performed by microprobe analysis. Moreover, it validates the application of the JMAK model for crystal dissolution whatever the temperature. Furthermore, the study of the crystal morphologies during dissolution indicates that a mechanism called “crystal attack/splitting” occurs for T > T liq and leads to a change in value of the Avrami exponent. [ABSTRACT FROM AUTHOR]

Rare-earth nickelates (R NiO 3) are an interesting oxide family because of their remarkable and reversible properties related to their structural changes. However, until recently, nickelates were difficult to synthesize without severe or sophisticated conditions. Consequently, a deep understanding of the nucleation and growth process for these versatile perovskites still lacks to date. Here, by correlation of the theory and the experimental data, is presented a clarification of the crystallization mechanism involved for SmNiO 3-δ thin films synthesized by a simple route that combines reactive magnetron sputtering and air-annealing. A thermo-kinetic approach to the amorphous-to-crystalline phase transformation is developed after following the evolution over time at 475, 500, and 525 °C through in situ high-temperature X-ray diffraction. Then, the kinetic parameters, the optimal temperature, and the necessary activation energy of transformation are determined from the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. Notably, the emergence of contrasting grains created on the free surfaces compared to the nanocrystallites formed from the bulk is revealed from the detailed study at 500 °C by Transmission electron microscopy. As classical nucleation theory outlines, such a growth difference is associated with heterogeneous and homogeneous processes. Furthermore, the progression with the annealing time of the crucial stabilization of the Ni3+ and the electronic structure is analyzed by Electron energy-loss spectroscopy and X-ray photoelectron spectroscopy. Finally, the optical properties measurements demonstrate a metal-insulator transition (MIT) at 125 °C, a thermochromic performance of 32%, and mainly, the crystallization significance to achieve functional nickelates thin films, which pave the way as promising candidates for solar thermal applications. [Display omitted] • The crystallization of SmNiO 3 films in air is feasible through a soft-synthesis route. • In situ HTXRD follows the crystallization evolution and the kinetic parameters. • A thermo-kinetic approach describes the nucleation and crystallization mechanisms. • The stabilization of the Ni3+ oxidation degree is achieved with the annealing time. • Highly crystallized SmNiO 3 films show a thermochromic effect at the MIT temperature. [ABSTRACT FROM AUTHOR]

This paper discusses the role of grain size distribution on the recrystallisation rates and Avrami values for a Fe-30 wt. % Ni steel, which was used as a model alloy retaining an austenitic structure to room temperature. Cold deformation was used to provide uniform macroscopic strain distributions (strains of 0.2 and 0.3), followed by recrystallisation during annealing at 850–950 °C. It was shown that the Avrami parameter was directly related to the grain size distribution, with a lower Avrami exponent being seen for a larger average and wider grain size distribution. A method to predict the Avrami exponent from the grain size distribution was proposed. In situ heating in an SEM with EBSD showed the recrystallisation kinetics to be affected by differences in stored energy and nucleation in the different grain sizes supporting the proposed relationship.

The novelty of this work is the comparison of two methods to determine the dissolution kinetics in order to evaluate the applicability of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model for the crystals dissolution in silicate melts. For this, this work focuses on the dissolution of rare earth (RE) silicates (Ca 2 RE 8 (SiO 4 ) 6 O 2 where RE = Nd) with an apatite structure in a sodium-borosilicate glass melt. The first approach consists in the characterization of the dissolution kinetics by following the crystalline area fraction by image analysis as a function of time and temperature. Then, a model based on the generalized JMAK equation is proposed to fit the experimental data. This model enables to determine both the mechanism limiting the dissolution (i.e. the diffusion) and the activation energy of crystals dissolution in the studied glass system (496 kJ/mol). To support these results, a second and most common approach is the measure of the chemical profiles at the crystal/melt interfaces by microprobe. The conclusions obtained by this last method are in agreement with the conclusions based on JMAK model. All these results allowed to confirm that the JMAK model is well suited to model crystals dissolution in silicate melts. [ABSTRACT FROM AUTHOR]

In this study, the phase transformation and recrystallization behavior of hot-forged Fe-Mn-Al-Si-C alloyed duplex steel has been investigated in wide ranges of annealing temperatures (973-1273 K) and durations (15–150 min). The recrystallization kinetics has been evaluated employing Johnson-Mehl-Avrami-Kolmogorov model. A limited extent of phase transformation (i.e. duplex to single phase) has been found to take place during annealing in the range of 973-1173 K. However, during annealing at 1273 K, not only the transformation from duplex to ferritic microstructure accelerates, but also recrystallization is initiated in these ferrite grains. The changes in the free energy of the austenite with the annealing temperature play a pivotal role in dictating the stabilization of the corresponding phase. Moreover, the disappearance of the austenitic phase at 1273 K owing to higher free energy is compensated by the formation of Mn-Al-Si rich carbide precipitates. The phase evolution triggered by annealing at 1273 K affects the recrystallization kinetics of each of the phase constituents. A sharp rise in the Avrami exponent (n) value is noticed following annealing at 1273 K, which clearly denotes that the disappearing austenite-ferrite interfaces facilitate the migration of the ferrite boundaries that stimulate recrystallization. In spite of the increasing values of n with the annealing temperature, its maximum value (at 1273 K) remains close to 1. This is a clear indication of the sluggish nature of the recrystallization process in the developed alloy. [ABSTRACT FROM AUTHOR]

Geopolymers are versatile materials possessing excellent mechanical properties and resistance against aggressive environments, these materials present a benefit of improving simultaneously both the environmental and engineering performance as compared to classical conventional materials. This paper determines the geopolymerization kinetics of fly ash based geopolymers using Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The experiments were designed using Taguchi method by varying four factors (Si/Al ratio, Na/Al ratio, W/S ratio, and curing temperature). The degree of reaction of fly ash (α) was used as a measure of the changes occurring during geopolymerization reaction. The characterization of the cured geopolymers was also carried out. The values of n were in the range of 0.0931–0.2321 while the values of k were in the range of 0.366–0.671. According to the JMAK model results, geopolymerization of fly ash based geopolymers is a one dimensional diffusion controlled reaction and its growth follows the mechanism of thickening of large product layers. The mechanism of geopolymerization consists of initial dissolution which is a first order chemical reaction, and further reactions including dissolution, gelation, and polycondensation are the diffusion controlled reactions. The asymmetric stretching band of Si-O-T shifted to 992 cm −1 and increased in intensity indicating the formation of geopolymer. Microstructural analysis showed the heterogeneous nature of geopolymers consisting of geopolymer, unreacted fly ash, and different types of needle like structures while one sample showed plate like morphology consistent with the JMAK model results. The geopolymer was found to be an amorphous material with only few peaks due to unreacted crystalline fly ash. [ABSTRACT FROM AUTHOR]

Laser powder bed fused (L-PBF) Ti–6Al–2Sn–4Zr–2Mo-0.08Si (L-PBF-Ti-6242) and two grades of L-PBF-Ti-6Al–4V (L-PBF-Ti-64) alloys are studied using differential scanning calorimetry (DSC), to determine the kinetics of α to β phase transformation to design and develop post-process heat treatments. Both non-isothermal and isothermal kinetics models are developed using the DSC results and employing the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The mechanism of non-isothermal phase transformation is discussed based on the local Avrami parameters. The transformation process is divided into three stages showing a high rate of transformation in the initial stage (dominated by nucleation) followed by a steady-state rate (dominated by growth) and again a rapid increase in the transformation rate in the final stage. The effect of the β stabilizing elements on α to β phase transformation kinetics is discussed for all three alloys in both non-isothermal and isothermal transformations. L-PBF-Ti-6242 needs higher activation energy to initiate the transformation and the process occurs at a comparatively higher temperature range since this is a near-α Ti alloy. The oxygen homogeneity in the L-PBF-Ti-64 shows a clear effect on the transformation kinetics. The initial microstructure also affects the kinetics of α to β phase transformation, as the as-built L-PBF microstructure results in the highest transformation kinetics. Any heat treatment that alters the as-built microstructure results in slower transformation kinetics. The results of the isothermal kinetics models are employed to design a two-step heat treatment for L-PBF-Ti-6242 to enhance the ductility with minimal strength loss. The properties are then correlated to the microstructure. This study provides great insight into the importance of considering the effect of the material system, alloying elements, and initial microstructure on the kinetics of phase transformation in L-PBF titanium alloys and the role of phase transformation kinetics in designing heat treatments to achieve the desired microstructure and properties. [ABSTRACT FROM AUTHOR]

A non-isothermal phase transformation kinetics model obtained by modifying the well-known JMAK approach is proposed for application to a low carbon boron steel (22MnB5) sheet. In the modified kinetics model, the parameters are functions of both temperature and cooling rate, and can be identified by a numerical optimization method. Moreover, in this approach the transformation start and finish temperatures are variable instead of the constants that depend on chemical composition. These variable reference temperatures are determined from the measured CCT diagram using dilatation experiments. The kinetics model developed in this work captures the complex transformation behavior of the boron steel sheet sample accurately. In particular, the predicted hardness and phase fractions in the specimens subjected to a wide range of cooling rates were validated by experiments. [ABSTRACT FROM AUTHOR]

Formation of gold nanoparticles (GNPs) is a physicochemical process with two paralleling aspects, i.e., the redox process and the crystallization. Herein the classical JMAK model which was originally developed for phase transformation process was applied for formation kinetics of GNPs prepared through both chemical and biogenic approaches; the relationship between JMAK model parameters, the crystallization behaviors and the dimensionality of the GNPs were clarified. It reveals that the rate parameter k mainly depends on the nucleation rate, and the Avrami exponent n could reflect the nucleation mechanism and characterize the geometric dimension of GNPs. Also, the crystallization is the rate-determining process that controls the dynamic regime of GNPs formation. Mechanistic differences between the chemical and bio synthetic routes, as well as those between spherical and flaky GNPs are also clarified via interpretation of the JMAK model parameters. [ABSTRACT FROM AUTHOR]

Abstract: The presence of low quantities of water vapour can seriously affect the kinetics of reduction of iron oxides when they are used as catalyst or to store and/or purify hydrogen from streams in the steam-iron process. Only 5% (v) of steam should be enough to inhibit the complete reduction of the solids. Since steam is a product of the reduction reaction, small amounts of water present in the reactive atmosphere can slow down the reduction itself. To account for the effect of the steam pressure during the reduction stage of the steam-iron process, two approaches have been considered and the resulting models, i.e. ‘competitive model’ and ‘inhibitive model’ have been tested against experimental measurements. Both models are based on the known Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. The ‘competitive model’, accounts for the discretization of groups of moles of iron oxide/iron reducing and oxidizing with their own reaction rates. By using the kinetic parameters obtained from independent reduction and oxidation processes, this model is not capable of predicting properly the behaviour of the solid subjected to successive reductive and oxidative cycles. On the contrary, the ‘inhibitive model’, which takes into account the hydrogen and water vapour partial pressures in a Langmuir–Hinshelwood type kinetic constant dependency, seems to be very appropriate to predict correctly the effect of the presence of water in the reducing atmosphere. [Copyright &y& Elsevier]

• Modelling of precipitation effect on magnetic coercive field and iron losses. • Using JMAK law to assess the evolution of magnetic properties during ageing. • Integration of JMAK law into an iron loss model to predict magnetic ageing effect. This article deals with the modelling of iron losses due to the magnetic ageing of electrical steels used in energy conversion devices. This phenomenon is the consequence of irreversible mechanisms in the material which can be triggered by the operating temperature of electrical devices. At first, an experimental study is carried out at 180° C in order to assess the effect of isothermal ageing on electromagnetic properties of a magnetic steel sample. The results show that, during the thermal ageing, the hysteresis losses and the coercive field increase. These experimental observations, mainly caused by the formation of precipitates at the material microstructural scale, are then discussed. Considering the link between the effect of magnetic ageing on macroscopic magnetic properties (effect) and the microscopic precipitation (cause), the Johnson – Mehl – Avrami – Kolmogorov (JMAK) law describing the kinetics of precipitation was applied to model the time evolution of magnetic ageing. Once this approach was validated, it is proposed to integrate the JMAK law in the Steinmetz iron loss model. [ABSTRACT FROM AUTHOR]

Abstract: The behaviour of an iron oxide sample (Fe2O3) during successive reduction–oxidation cycles (steam–iron process) has been investigated by means of a thermobalance system acting as a differential reactor. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, based on nucleation and growth mechanisms, has been used to describe the individual reoxidation processes as well as to predict the stability of the oxide after a high number of redox cycles. The effect of temperature, steam partial pressure and oxidation length has been discussed. The experimental results, as well as the parameters for the JMAK model, show that there is no significant effect of the temperature and the steam partial pressure used in the oxidation stages on the behaviour of the subsequent cycles (taking apart the first one). This conclusion applies also to the length of the oxidation period, if for such variable a value above a given threshold is used, which could be theoretically foreseen. [Copyright &y& Elsevier]

• Using conventional DSC and flash DSC to investigate the isothermal crystallization behavior of an industrial-grade bulk metallic glass. This industrial-grade bulk metallic glass in powder form has been extensively used as feedstock for selective laser melting (SLM). • The experimentally determined time-temperature-transformation (TTT) diagrams cover the complete temperature range from glass transition to liquidus temperature. • The influence of oxygen content on crystallization behavior is investigated, and the underlying mechanisms are revealed with the aid of classical nucleation theory and Johnson-Mehl-Avrami-Kolmogorov (JMAK) modeling. Bulk metallic glasses (BMGs), due to their amorphous structure, exhibit remarkable mechanical properties, and there is an increasing interest in their commercialization. For the industrial fabrication of BMG, knowledge about the isothermal crystallization kinetics of industrial-grade BMG is required. Previous investigations on isothermal crystallization kinetics are mainly based on high-purity samples with very good glass forming ability and/or mainly limited to the low temperature regime. In the present study, a systematic investigation on the isothermal crystallization kinetics of an industrial-grade Zr-based BMG (Zr 59.3 Cu 28.8 Al 10.4 Nb 1.5 at.%, trade name: AMZ4) has been performed using conventional and flash differential scanning calorimetry. We report the time-temperature-transformation (TTT) diagrams of the AMZ4 with two different oxygen levels. The diagrams cover the temperature range from glass transition temperature up to liquidus temperature, that have the typical "C-shaped" noses. Faster crystallization of the higher oxygen level AMZ4 was observed, and the underlying mechanisms were investigated. The universal isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was employed to model the isothermal crystallization kinetics. Satisfactory match was achieved between the experimental facts and the JMAK model, and the interfacial energies between the crystalline phase and liquid were determined as ∼ 0.04 J/ m 2 for the industrial-grade AMZ4. The crystallization fraction dependence of Avrami index and activation energy is studied and found to be neglectable in the JMAK modeling. The critical casting thicknesses were estimated based on the TTT diagrams. [ABSTRACT FROM AUTHOR]

• A new mathematical model is proposed to compute the average grain size in a single layer printed by the laser-melting deposition (LMD) process. • Layer's thermal history was proposed based on the moving laser beam and the co-axial powder addition. • Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, in correspondence with thermal stress and strain rate, was adopted to calculate average grain size. • The mechanical properties, including ultimate tensile strength, yield strength, and hardness, were estimated using the average grain size. • Single-track depositions of AISI 304 stainless steel powder debits on an AISI 304 stainless steel substrate were carried out to verify the model's trustworthiness. • Scanning electron microscopy and image processing were performed to quantify the average number and size of grains. • Vickers hardness tests were conducted to confirm the developed model's mechanical performances. Many of the significant mechanical properties of stainless steel, including ultimate tensile strength, yield strength and hardness, the ductile–brittle transition temperature, and susceptibility to environmental embrittlement, can be improved by grain size refinement. Hall-Petch relation identifies that the improvement can be quantified in a constitutive relation. In this study, a new mathematical model to calculate the number of grains and their average size inside a single printed layer via the laser melting deposition (LMD) process is proposed. The printed layer's thermal history concerning the moving laser beam and co-axial addition of powder debits was analyzed and used to calculate the thermal stress and strain rate. The average grain size within the printed layer was calculated using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The mechanical properties, including ultimate tensile strength, yield strength, and hardness, were estimated using the average grain size. For single depositions of AISI 304 stainless steel powder debits on a steel substrate, dedicated experiments were performed to verify the model's trustworthiness. Scanning electron microscopy was used to quantify the number and size of grains. Vickers hardness tests were conducted to confirm the mechanical performances predicted by the developed model. It was found that the primary operating parameters strongly influence the grain type, resulting in three varieties of grains: (a) quasi-continuous circular, (b) long lath-shaped, and (c) a combination of the two mentioned above. The deposited layer's thermal history influences the thermal stresses and controls the growing grains' average size. A strong correlation between experimental and computational results, within the range of 10–15 %, and 8–10%, was obtained for the average grain size and Vickers hardness test. The laser scanning speed and laser power were in an inverse relation with the average grain size, while a direct relationship was noticed between the powder feed rate and average grain size. The mechanical computational and experimental results show that the highest yield strength (=208 MPa), ultimate tensile strength (=722 MPa), and hardness (=278 HV) were obtained for the finest grain structure. [ABSTRACT FROM AUTHOR]

• Proposed a fast and efficient way to predict the grain size of metal additive manufacturing parts based on the physical mechanism. • Obtained relationships among manufacturing parameters, temperature distribution, and grain size of build part. • More reliable method to acquire a specified microstructure of build part. • To give a support for process control and process design of metal additive manufacturing in industry. Metal additive manufacturing technology not only has advantages on geometry complicated components small batch production, but also has the ability to produce the components with specific requirements in properties. However, the properties of the components mainly depend on its microstructure, such as grain size. In this paper, an analytical model for post-printing grain size prediction in metal additive manufacturing is presented. First, the solution of a moving point heat source for the convection-diffusion equation, which is utilized to describe the thermal equilibrium during the additive manufacturing process, is deduced to calculate the temperature distribution in the build part. The thermal stresses due to non-uniform temperature distribution are predicted by the Green's function. Second, the grain size of the build part is obtained from nucleation and growth rate during the additive manufacturing process. The grain growth due to heating is established based on the recrystallized volume fraction of the material during the heating process, which is estimated through Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. Subsequently, the grain refinement due to cooling during additive manufacturing process is established based upon the transformation kinetics, which is predicted by the negative exponential equation. Finally, the proposed model is verified by the literature data of selective laser melting of Ti-45Al-2Cr-5Nb alloy experiment. The predicted temperature distributions in the build parts under different conditions are analyzed. Comparison results show that the predictions are in good agreement with the experimental data under various conditions of laser power 200 W, scan speeds 500 mm/s, 600 mm/s, 700 mm/s, and 800 mm/s. Based on the proposed model, the sensitivity analysis of grain size of the build part with respect to laser power and scan speed is discussed. Results indicate that the temperature at top surface of the build part is not affected by the scan speed, while both the affected zone and peak value of the temperature beneath the surface of the build part increase with the increase of the laser power. The average grain size decreases with the increase of the laser power, while the average grain size decreases with the increase of the scan speed and then increases due to the incomplete melting effect. [ABSTRACT FROM AUTHOR]

This paper discusses the role of grain size distribution on the recrystallisation rates and Avrami values for a Fe-30 wt. % Ni steel, which was used as a model alloy retaining an austenitic structure to room temperature. Cold deformation was used to provide uniform macroscopic strain distributions (strains of 0.2 and 0.3), followed by recrystallisation during annealing at 850–950 °C. It was shown that the Avrami parameter was directly related to the grain size distribution, with a lower Avrami exponent being seen for a larger average and wider grain size distribution. A method to predict the Avrami exponent from the grain size distribution was proposed. In situ heating in an SEM with EBSD showed the recrystallisation kinetics to be affected by differences in stored energy and nucleation in the different grain sizes supporting the proposed relationship. [ABSTRACT FROM AUTHOR]