Johnson Mehl Avrami Research Articles

Effect mechanism of Nb addition on grain refinement and inhibition of discontinuous precipitation of Cu–15Ni–8Sn alloy

Cu–15Ni–8Sn-xNb alloys (x = 0, 0.2, 0.6 wt%) were prepared using a medium-frequency induction melting furnace. The effect of Nb addition on grain refinement and inhibition of discontinuous precipitation (DP) in Cu–15Ni–8Sn alloys was systematically studied to elucidate the mechanism by which microstructural characteristics contribute to strength improvement. The results indicate that the increase the Nb content from 0 to 0.6 wt% reduces the average grain size of the as-cast alloy from approximately 524.8 μm to approximately 81.3 μm, and significantly decreases the lamellar transition structure region (α+γ). During solution treatment, dispersed needle-like NbNi3 phases were observed in the Cu–15Ni–8Sn-0.2Nb alloy. After aging, the DP growth rate in the Cu–15Ni–8Sn-0.2Nb alloy was notably slower than those of the other alloys. This was attributed to NbNi3 phases at the grain boundaries hindering DP nucleation, with intragranular NbNi3 phases inhibiting DP growth. The phase transformation order of the solid solution Cu–15Ni–8Sn-0.2Nb at 673 K aging is: spinodal structure → D022 ordered phase → L12 ordered phase → DP. The hardness and tensile strength of the Cu–15Ni–8Sn-0.2Nb alloy peaked at 338.3 HV and 725.42 MPa, respectively, after aging for 120 min. Using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) and Arrhenius equations, the activation energies of DP in Cu–15Ni–8Sn-xNb (x = 0, 0.2, 0.6 wt%) alloys were calculated to be 93.19, 148.64, and 98.33 kJ/mol, respectively. These values suggest that the diffusion of DP atoms in the Nb-containing alloys is hindered, which effectively inhibits DP formation.

Determining of the composition of the metallic core of historical objects from surface XRF spectrometry data

A method for estimating the composition of the metal core in historical and archaeological artifacts using surface X-ray fluorescence (XRF) data is described. The method is based on the combination of fluorescence data recorded at points of the object with different degrees of corrosion exploiting the general phenomenon of demetallation of the less noble components of the metal alloy. A theoretical approach is provided using the Johnson–Mehl–Avrami–Erofeev-Kolmogorov (JMAEK) formalism to describe demetallation. Experimental data for binary ZnCu brass and Pb- and Sn-containing brass from Hofkirche in Innsbruck agree satisfactorily with the model.

Tracking microstructural evolution and hardening in Al–Zn–Mg–Cu alloys aged artificially via electrical conductivity measurements

Precipitation hardening, a crucial mechanism for strengthening aluminum alloys, involves stages like Guinier–Preston (GP) zone formation, precipitation, peak aging, and precipitate coarsening. This study focuses on the aluminum 7050 alloy, proposing a method to gauge artificial aging through electrical conductivity measurement. The evolving microstructure and time to peak hardness during aging are vital for creating high-strength alloys. The electrical conductivity variation over time is utilized to analyze the diffusion process governing the clustering and growth of specific phases (η′, η, and S) during artificial aging. The paper demonstrates the impact of GP zones, precipitate formation, and grain growth on electrical conductivity, correlating these factors with hardness, microstructure, and tensile strength to determine the hardening stage. Differential electrical conductivity plots, highlighting aging stages, assist in identifying the hardening phase. Tensile strength and hardness plots differentiate the precipitation phases. The Johnson–Mehl–Avrami–Kolmogorov equation models particle growth kinetics, determining growth rates for AA 7050 alloy. The overall activation energy for precipitate growth is 40.77 kJ/mol, with a growth constant ( m) of ∼4, indicating S phase nucleation during η′ and η growth.

Composition design and crystallization behavior of Zr–Cu–Ni–Al bulk metallic glasses

The Zr–Cu–Ni–Al system was explored in this study, and seven new compositions of Zr-based bulk metallic glasses (BMGs) were designed by combining different proportions of binary eutectic phases Zr38Cu62, Zr76Ni24 and Zr51Al49, and rod-like shape samples were prepared with different diameters using the copper mold casting method. Modifying the proportion coefficient of these eutectic units, the width of the supercooled liquid region was increased from 79 to 95 K, and the thermal stability of the prepared metallic glass was improved. Differential scanning calorimetry was used to investigate the non-isothermal and isothermal crystallization behavior of the prepared samples. Noticeably, there was a small low-temperature exothermic peak before the glass transition temperature Tg in the non-isothermal crystallization process, which was related to the structural relaxation. The crystallization transition activation energy fitted by the Kissinger equation had the same variation trend as that obtained by the Arrhenius equation. Among the four metallic glasses studied, the glassy phase of Zr55·5Cu23·25Ni9Al12.25 was more stable, showing higher glass transition activation energy and longer incubation time. In addition, the isothermal crystallization processes of four metallic glasses were studied according to the Johnson-Mehl-Avrami (JMA) model. The calculated average Avrami exponents were >2.5, indicating that the nucleation rate increased during the isothermal crystallization process.

Nucleation and growth rate determination on alkali-activated slag under various sodium hydroxide molarity

Alkali-activated slag has been noted as one of potential alternatives to the ordinary Portland cement due to its properties including high early strength performance and capability of ambient curing. However, there is still limited studies available on elucidating the reaction processes towards producing the excellent properties. This study aims to elucidate the mechanism of alkali activation of slag under different molarities of sodium hydroxide, which is one of the most influential factors on the properties of alkali-activated slag. Heat evolution of alkali-activated slag was used as a real-time monitoring technique. For mix designation, the molarity of sodium hydroxide was varied from 6M to 14 M, with solid-to-liquid ratios of 0.6 and alkali activator ratios of 2.0 remaining constant. The calorimetric data obtained was further used for determination of degree of reaction, nucleation and growth rate mechanism using Johnson–Mehl Avrami Kolmogrov model. According to the findings, it was found that regardless of various molarity of sodium hydroxide applied, the nucleation mechanism and growth is governed by instantaneous heterogeneous nucleation with rod-like growth as the n value is approaching 1 in which is observed from the morphology of the alkali-activated slag at lowest molarity applied (6 M). Furthermore, increasing in molarity of sodium hydroxide was found to decrease the total heat evolved and the lowest was obtained when using 14 M.

Microstructure Prediction of 80MnSi8-6 Steel After Hot Deformation Based on Dynamic Recrystallization Kinetics and FEM Simulation

The microstructure evolution during hot deformation of 80MnSi8-6 nanobainitic steel was investigated through hot compression tests at deformation temperatures of 900–1250°C and strain rates of 0.1–20 s−1. The flow curves revealed strain-hardening behavior at the beginning of deformation followed by softening effects caused by microstructure evolution. A Johnson–Mehl–Avrami–Kolmogorov (JMAK) model for grain growth and dynamic recrystallization was developed, and the kinetics were determined. Critical and peak strains were identified, and coefficients for the microstructure evolution models were determined using linear regression. The analysis of S-curves revealed that decreasing the temperature delays the onset of recrystallization and that the strain rate significantly effects the recrystallization rate at lower temperatures. Constitutive modeling and determination of the Zener–Hollomon parameter allowed the determination of the influence of hot processing conditions on material behavior during deformation. Microstructure analysis showed that, at higher deformation temperatures, grain growth occurs simultaneously with grain refinement. Coefficients for the JMAK model were implemented in QForm software. Simulation results were compared with experimental measurements exhibited good arrangement, which confirms the accuracy of the JMAK model in predicting the microstructure evolution. This study demonstrated how microstructure evolution modeling and FEM simulations combined can be used to predict the grain size of 80MnSi8-6 steel after hot deformation.

Determination of phase transformation kinetics under magnetic fields: Modeling based on magnetization and application in a Fe-1 wt.%Cu alloy

A phase transformation model based on magnetization is proposed in this paper, which accurately tracks the change in the phase transformation volume fraction with time/temperature f-T/t by analyzing phase transformation magnetization curves under a magnetic field. This allows for the determination of kinetic parameters related to the nucleation and growth processes such as the phase transformation rate df/dt-T/t and Avrami exponent n, enabling quantitative analysis of phase transformation kinetics under magnetic field effects. Additionally, the phase transformation magnetization under a magnetic field can be accurately fitted by combining the volume fraction calculation model with the Johnson–Mehl–Avrami equation, thus also obtaining the kinetics parameters. The aforementioned two models are applied to study the isothermal and isokinetic transformations of austenite (γ) to ferrite (α) in Fe-1 wt. %Cu alloys, demonstrating the effects of external conditions through variations in kinetic parameters.

Modeling of Yb:YAG Laser Beam Caustics and Thermal Phenomena in Laser-Arc Hybrid Welding Process with Phase Transformations in the Solid State.

This paper focuses on the mathematical and numerical modeling of the electric arc + laser beam welding (HLAW) process using an innovative model of the Yb:YAG laser heat source. Laser energy distribution is measured experimentally using a UFF100 analyzer. The results of experimental research, including the beam profile and energetic characteristics of an electric arc, are used in the model. The laser beam description is based on geostatistical kriging interpolation, whereas the electric arc is modeled using Goldak's distribution. Hybrid heat source models are used in numerical algorithms to analyze physical phenomena occurring in the laser-arc hybrid welding process. Thermal phenomena with fluid flow in the fusion zone (FZ) are described by continuum conservation equations. The kinetics of phase transformations in the solid state are determined using Johnson-Mehl-Avrami (JMA) and Koistinen-Marburger (KM) equations. A continuous cooling transformation (CCT) diagram is determined using interpolation functions and experimental research. An experimental dilatometric analysis for the chosen cooling rates is performed to define the start and final temperatures as well as the start and final times of phase transformations. Computer simulations of butt-welding of S355 steel are executed to describe temperature and melted material velocity profiles. The predicted FZ and heat-affected zone (HAZ) are compared to cross-sections of hybrid welded joints, performed using different laser beam focusing. The obtained results confirm the significant influence of the power distribution of the heat source and the laser beam focusing point on the temperature distribution and the characteristic zones of the joint.

Microstructure Evolution and Homogenization Kinetics of 15Cr–30Ni–Fe Heat‐Resistant Alloy

The as‐cast microstructure, microsegregation of alloying elements, and microstructure evolution during homogenization of the newly designed 15Cr–30Ni–Fe heat‐resistant alloy are studied. Ti, Nb, Si, and Mo segregate at the interdendritic region. The brittle eutectic Laves phase contains Fe, Ti, Ni, Nb, Si, and Mo, which is identified as the Fe2Ti Laves phase. The activation energy for the Fe2Ti Laves phase dissolution is close to the activation energy for Ti diffusion in the γ matrix based on the Johnson–Mehl–Avrami–Kolmogorov analysis. The back‐diffusion of Ti to the γ matrix is the limiting step for the dissolution of Fe2Ti Laves phase. The kinetic equation of homogenization considering lattice parameters and pre‐exponential factor correction is established. After the double‐stage homogenization at 980 °C/10 h + 1140 °C/4 h, the volume fraction of the Laves phase decreases from 5.11% to 0.26%, and the large eutectic Laves phase is completely dissolved. The reduction in the standard deviation of microhardness at different soaking times effectively reflects the gradual decrease in the degree of microsegregation of alloying elements. The average grain size of the alloy after homogenization is 410 ± 63.1 μm, which is beneficial to control the further refinement of grain during the subsequent hot working.

The impact of precipitation on recrystallization in a severely plastically deformed CoCrFeMnNi multi–principal element alloy

The present study investigated the rate-controlling mechanism for recrystallization kinetics in a severely plastically deformed (SPD) multi–principal element alloy. For this purpose, an equiatomic Cantor alloy (CoCrFeMnNi) was subjected to equal channel angular processing (ECAP) at room temperature and annealed at 773–973 K. The major recovery process was found to be concurrent recrystallization with precipitation. The recrystallization kinetics, investigated in the frame of the Johnson–Mehl–Avrami (JMAK) theory, gives a retarded recrystallization with a high recrystallization temperature of above 0.54Tm and an anomalous low JMAK exponent value of 1.1–1.8. The measured apparent activation energy for recrystallization exhibited a steady increase over the duration, with initial values of 220.8 ± 7.5 kJ/mol rising to 313.9 ± 23.8 kJ/mol towards the end. The systematic analysis leads to the conclusion that the rate-controlling mechanism for recrystallization in ECAP processed CoCrFeMnNi MPEA is the grain boundary migration-determined grain growth instead of nucleation. During this process, the Zener pinning effect exerted by precipitates from the concurrent precipitation with recrystallization plays the dominant role.

The recrystallization behavior of cryo- and cold-rolled AlCoCrFeNiTi high entropy alloy

The present work was conducted on the non-equiatomic Al0·2Co1·5CrFeNi1·5Ti0.3 high entropy alloy (HEA), which was separately subjected to 50% cold- and cryo-rolling. Subsequent annealing was performed within the temperature range of 800–1000 °C to investigate their recrystallization behavior. A partial recrystallization was achieved up to 900 °C for the cold-rolled material, while the cryo-rolled one had shown nearly fully recrystallized microstructure after 800 °C. A significant reduction in grain size from the solution-treated state (∼784 μm) to fully recrystallized state (∼71 μm for cold-rolled and ∼6 μm for cryo-rolled) was observed. A combined XRD and TEM analysis confirms that the cryo-rolled sample exhibits higher dislocation density than the cold-rolled sample, which resulted in faster recrystallization kinetics observed in the former than in the latter. Additionally, the Johnson–Mehl–Avrami–Kolmogorov model was employed to understand the recrystallization behavior.

Comparative study of microstructural evolution in W-3Re alloy under high-temperature conditions: High heat flux loading versus furnace heating

In tokamak devices, tungsten (W), as the main candidate for plasma facing material, will suffer from the simultaneous challenges of high heat flux (HHF) and neutron irradiation. This study involved annealing experiments on W-3Re alloy using two distinct heating methods: furnace heating and electron beam heating. The evolution of hardness and microstrure of tungsten-rhenium (W-3Re) alloy after isochronous annealing and isothermal annealing has been analyzed and compared with pure tungsten (PW). The recrystallization kinetics of W-3Re alloy have been obtained employing the Johnson Mehl-Avrami Kolmogorov (JMAK) equation. Experimental results showed that recrystallization kinetics during electron beam annealing were faster than those in furnace annealing. This phenomenon can be attributed to the rapid heating process of electron beam annealing, which preserved more deformation energy within the material, thus providing more driving force for recrystallization at high temperatures. Additionally, Re appeared to inhibit grain boundary migration in W-3Re alloy, causing the recrystallization temperature to be approximately 100 °C higher than in pure tungsten, and also resulted in a smaller average grain size compared to pure tungsten. This research provides experimental data and reference for predicting the microstructural evolution of tungsten-rhenium alloy in future fusion reactor environments.

Barium Titanate Synthesis in Water Vapor: From Mechanism to Ceramics Properties

A facile and environmentally benign method for single-phase barium titanate synthesis in a water vapor medium was studied to reveal the mechanism of phase transformation of the initial simple oxide mixture and estimate the capability of the product to be used as a raw material for low-frequency dielectric ceramics. The composition and structure of the reactants’ mixture, treated in vapor at 130–150 °C as well as at 230 °C for various time periods, were investigated by means of XRD, SEM, TEM, EDX, and FTIR methods. The kinetics of the occurring phase transformation can be described using the Johnson–Mehl–Avrami–Erofeev equation. The reaction between the initial oxides was considered as a topochemical process with an apparent activation energy of 75–80 kJ mol−1. A crucial role in this process belonged to the water vapor medium, which facilitated the generation of the reaction zone and the spreading inward of the solid particles. The synthesized tetragonal barium titanate powder (mean particle size of 135 nm) was sintered using a conventional technique at 1250 °C to obtain ceramics with grains of about 2 μm. Capacitance measurements identified a permittivity and dielectric loss factor of the ceramics that reached 3879 and 6.7 × 10−3, respectively, at 1 kHz and room temperature.

Effect of Austenite Grain Size on Phase Transformation Structure of Low‐Carbon Microalloyed Steel

The effect of austenite grain on the phase transformation of low‐carbon microalloyed steel during continuous cooling is investigated by using Gleeble thermal simulator. The austenite grain size is changed by altering the holding period at the peak temperature. The findings demonstrate that for a given cooling rate, the longer the holding time, the microstructure becomes finer, and bainite begins to show up in the microstructure. The critical temperature of phase transformation and the diagram of phase transformation reaction rate during continuous cooling are determined by dilatometry. The Johnson–Mehl–Avrami–Kolmogorov equation is used to fit the transformation volume fraction, and the variation of kinetic parameters k and n with prior austenite grain size under nonisothermal condition is determined. The n value is trending downward as the prior austenite grain size increases, while the kinetic parameter k barely changes at all. The activation energy of phase transition is calculated by Kissinger method. The activation energy of fine austenite grain increases from 248.6 to 286.3 kJ mol−1 with the increase of austenite grain size, while the activation energy of coarse austenite grain decreases to 176.5 kJ mol−1 due to the competitive nucleation mechanism.

Mechanism and kinetics study on waste acid leaching from copper smelting dust

AbstractCopper smelters generate large amounts of copper smelting dust (CSD) that has a high recycling value because it contains about 20% copper. Waste acid can be used to leach CSD. Here, the effects of different temperatures, times and liquid–solid ratios on copper, iron and arsenic leaching rates were investigated. The optimal conditions were determined to be a temperature of 95°C, leaching time of 3 h, and liquid–solid ratio of 7:1. Under these condition, the leaching rate of copper reached 96.37%, and the copper ion concentration was 32.71 g/L. The leaching kinetics date revealed that the leaching of copper and iron was controlled by two different reaction models. The leaching of copper conformed to the Johnson–Mehl–Avrami (JMA) model and was diffusion controlled, and the reaction was rapid in the first 60 min. In contrast, leaching of iron was controlled by a chemical reaction. Copper and iron activation energies were 2.579 and 62.791 kJ/mol, respectively. These results show that CSD waste acid is can be used to dispose of arsenic‐containing hazardous waste in a green, sustainable, and ecologically method.

Effect of initial grain size on the recrystallization behavior and recrystallization texture of a Mg–3Gd alloy

The effect of initial grain size on the recrystallization and recrystallization texture of a rolled Mg–3Gd (wt.%) alloy is studied in detail. The results show that the deformation microstructure of an initially coarse-grained (CG) sample has a larger twinned area and a higher density of twin boundaries than a fine-grained (FG) sample. After annealing, the CG sample recrystallizes preferentially in the twinned area, whereas the FG sample adopts the higher density grain boundaries as the nucleation sites. Furthermore, weak recrystallization texture components appear from the grain nucleation stage, regardless of the initial grain size, and are preserved after complete recrystallization due to uniform grain growth. The majority of recrystallization texture is deviated 20°–45° away from normal direction (ND), accounting for more than 50 %. Especially, the recrystallization texture of the FG sample is a “Rare Earth texture”, in contrast to the widely reported texture modification unrelated to grain boundary nucleation. Only a scattered basal texture is observed in the CG sample, which also differs from the reported “Rare Earth texture” originating from shear band nucleation in dilute Mg–Gd alloys. Finally, based on the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, the recrystallization kinetics are calculated, and it is found that the initial grain size mainly affects the nucleation rate, and has limited effect on the grain growth rate.

Photoinduced growth of the crystalline phase of tellurium on a 1T′-MoTe2 matrix

Due to the growing demand for miniaturization and energy efficiency in modern electronic devices, there is a renewed interest for optoelectronic memories and sensors based on 2D materials. In particular, the molybdenum ditelluride (MoTe2) is one of the most promising materials for applications in nonvolatile phase-change memory devices, as its properties can be controlled by visible-light illumination. Among the several ways to synthesize MoTe2, the molybdenum oxide tellurization through isothermal close space sublimation (CSS) annealing in gas atmosphere is a simple and low-cost effective method for large-scale production of devices based on this layered material. Therefore, the understanding of the physical properties of MoTe2 thin films produced by this technique is crucial for future applications. Surprisingly, our results indicate that there is a photoinduced growth of the crystalline phase of tellurium on the 1T′-MoTe2 matrix even when the power density of the laser is low. From Raman spectroscopy investigations, we were able to show that nanometer-sized tellurium crystallites work as seed sites for the photocrystallization of tellurium. By assuming that the overall crystallization process is described by a kinetic approach that is based on the Kolmogorov–Johnson–Mehl–Avrami theory, our results indicate that the process is governed by an anisotropic organization of the tellurium atoms in helical structures during the crystal growth.

Johnson-Mehl-Avrami kinetics as a universal description of crystallization in glasses?

Applicability of the Johnson-Mehl-Avrami (JMA) model beyond the typical range of the negative asymmetries of the crystallization peak was tested by means of theoretical simulations. Using the solid-state kinetic equation with implemented temperature dependences of either activation energy E or pre-exponential factor A, it was shown that the originally monotonically increasing transformation rate constant changes its evolution with temperature to a non-monotonous course, exhibiting a broad maximum. If the crystallization process following the non-isothermal JMA kinetics proceeds near this maximum, the asymmetry of the corresponding kinetic peak can gradually change to significantly positive values. Of particular interest are the cases with the JMA kinetic exponent mJMA ≥ 2, where the asymmetry of the JMA peaks changes with increasing heating rate from the typical negative values to largely positive values – such crystallization behavior was indeed observed for several chemically diverse materials (polymers, small organic molecules, chalcogenide and oxide glasses).

Effect of sigma (σ) phase precipitation on the recrystallization kinetics of AISI 904L superaustenitic stainless steel

Microstructural evolutions, σ-phase precipitation, and recrystallization kinetics of cold-rolled AISI 904L stainless steel during annealing were studied in the present work. The microstructure and hardness evolution were different in the temperature ranges of T < 950 °C (with σ-phase precipitation) and T ≥ 950 °C. The former range was associated with the fall of hardness due to recrystallization softening, followed by hardening due to the concurrent σ-phase precipitation; while the latter range was associated with the continuous fall of hardness. Using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, the recrystallization activation energy in these ranges were respectively determined as 325 and 257 kJ/mol, where the higher activation energy in the former range was correlated to the retardation effect of the σ-phase. Via applying the Kissinger's equation to the differential scanning calorimetry (DSC) results, the activation energy of ∼332 kJ/mol was obtained, which was consistent with the precipitation of the σ-phase.

Softening Behavior and Hardness Prediction of H13 Hot‐Work Die Steel during Isothermal Tempering

In this study, the microstructural evolution of H13 hot‐work die steel during isothermal tempering at 500 ≈ 650 °C is evaluated, and kinetic equations and a neural network are constructed to describe the softening behavior. The results show that the softening behavior appears as a loss of hardness with increasing isothermal tempering temperature and time. The softening mechanism is ascribed to the decomposition of martensitic laths, annihilation and rearrangement of dislocations, formation of sub‐grains, and precipitation and coarsening of secondary carbides. The evolution of secondary carbides includes the coarsening of V‐rich MC nanoprecipitates, and the precipitation and coarsening of secondary phases (Cr‐rich M23C6 and Mo‐rich M6C carbides) during long‐term isothermal tempering. Regarding the applicability of the model in precipitation evolution and hardness prediction, the Johnson–Mehl–Avrami model is considered to be more suitable than the Hollomon–Jaffe, Lifshitz–Slyozov–Wagner, and back‐propagation neural network models.