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The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm2 for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO3 has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased. [ABSTRACT FROM AUTHOR]

The recrystallization (RX) kinetics of commercially pure Al alloy is studied under the scope of annealing temperature, time, and degree of deformation. To examine the distribution of recrystallization, Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory is employed, where the path of microstructural transformation from the deformed state to the fully recovered one is studied as a function of the volume fraction of recrystallized grains ( X V ) and annealing time. The drop in hardness is recorded for the samples at various stages of annealing with a corresponding decrease in stored energy as the annealing time increases. The stored energy obtained from the hardness results and Orientation Imaging Microscopy (OIM)-based method is found to be in good agreement with each other, proving the efficiency of both techniques. To determine the volume fraction of the recrystallized microstructure, data obtained from Vickers hardness measurements are used. Various parameters associated with recrystallization statistics such as the critical radius of nuclei, the incubation period, and the mobility of High-Angle Grain Boundaries (HAGB) were derived from the experimental evidence. The experimental data also suggest a sharp drop in the velocity of HAGB as the RX transformation process approaches its completion, which is found to be a direct result of a drop in stored energy. A softening window between 42 s and 55 s is identified for our experimental data where the hardness, stored energy, and velocity of HAGB drops very sharply, and the maximum fraction of deformed grains is expected to be converted to the recrystallized ones. Along with experimental observations, an analytical model was developed, which helps to approximate the kinetics of RX and corresponding parameters for various annealing temperatures and strains while revealing the characteristic feature of Avrami exponent n. Both experimental evidence and model data reveal a very strong dependency of recrystallization behavior on the stored energy. [ABSTRACT FROM AUTHOR]

The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm2 for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO3 has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased.

Different lightweight steels alloys have been treated by different isothermal treatments in order to understand the basics of precipitation kinetics of κ-carbide for such class of new steel grades featured by high concentration of Mn and Al. The κ-carbide precipitation plays a significant role to induce the strengthening of these steel grades that maintain a significant ductility up to the fracture as a consequence of their duplex ferrite-austenite structure that exploits the twinning mechanism of austenite during the plastic deformation. In this paper the results about isothermal transformations involving the κ-carbide precipitation have been discussed.In order to deepen the comprehension of the mechanisms involved in precipitation of κ-carbides, the study has been performed by isothermal (Temperature Transformation Treatment) experiments applying different temperatures and holding times. The observed transformations have been measured and it has been possible to apply the JMAK model to fit such results. In addition, it has been observed a relationship between the steel composition and the activation energy for the transformation.

The recrystallization (RX) kinetics of commercially pure Al alloy is studied under the scope of annealing temperature, time, and degree of deformation. To examine the distribution of recrystallization, Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory is employed, where the path of microstructural transformation from the deformed state to the fully recovered one is studied as a function of the volume fraction of recrystallized grains (XV) and annealing time. The drop in hardness is recorded for the samples at various stages of annealing with a corresponding decrease in stored energy as the annealing time increases. The stored energy obtained from the hardness results and Orientation Imaging Microscopy (OIM)-based method is found to be in good agreement with each other, proving the efficiency of both techniques. To determine the volume fraction of the recrystallized microstructure, data obtained from Vickers hardness measurements are used. Various parameters associated with recrystallization statistics such as the critical radius of nuclei, the incubation period, and the mobility of High-Angle Grain Boundaries (HAGB) were derived from the experimental evidence. The experimental data also suggest a sharp drop in the velocity of HAGB as the RX transformation process approaches its completion, which is found to be a direct result of a drop in stored energy. A softening window between 42 s and 55 s is identified for our experimental data where the hardness, stored energy, and velocity of HAGB drops very sharply, and the maximum fraction of deformed grains is expected to be converted to the recrystallized ones. Along with experimental observations, an analytical model was developed, which helps to approximate the kinetics of RX and corresponding parameters for various annealing temperatures and strains while revealing the characteristic feature of Avrami exponent n. Both experimental evidence and model data reveal a very strong dependency of recrystallization behavior on the stored energy.

Abstract Controlling the recrystallization is an important way to reach grain size refinement and outstanding strength and toughness on alloy metals. This study sets out the application and investigation of mathematical microstructure modeling of a newly designed bainitic steel for hot forging industrial applications. The macro-scale model was used to observe and predict the austenitic grain size behavior during the controlled forging of a gear. Arrhenius grain growth kinetic and recrystallization model for a new class of bainitic steel was established for the given strain rate ranges and temperatures. This model was calibrated through microscopic analysis and used to simulate the unpublished constants of low alloyed bainitic forging steel DIN 18MnCrSiMo6-4 microstructure module using DEFORM® commercial finite element code. The increased temperature due to the adiabatic effect was investigated by numerical analysis, demonstrating its influence on grain coarsening. Local tensile test and Charpy-V notch were compared at different industrial hot forging temperatures and local plastic strain. Changes in yield strength and ductility have demonstrated the grain size influence on the processing parameters. The employed numerical model was an efficient tool to predict and present an alternative path to develop robust industrial forging using semi-empirical models.

This article investigates the influence of temperature and strain on second‐phase transformation strengthening and the resulting mechanical properties in a lean AISI 301LN austenitic stainless steel within a temperature range of −60 to 180 °C. The volume fraction of martensite evolved is determined using nondestructive magnetic Ferritescope measurements that are adjusted by using a calibration factor of 1.7, which is established using the saturation magnetization measurements, X‐ray, and neutron diffraction measurements. The kinetics of strain‐induced martensite transformation (SIMT) as a function of strain and temperature is accurately described by a set of modified constitutive Boltzmann sigmoidal equations at temperatures below 75 °C. For this steel, the Md (30/50) temperature is determined as 61 °C. The absolute Md temperature is established as ≈109 °C, and no athermal transformation to martensite is observed upon cooling to −270 °C using cryogenic neutron diffraction facilities. Extended JMAK analysis of the transformation is used to shed light on the mechanism of martensitic transformation. It is found that the transformation‐induced plasticity (TRIP) effect due to SIMT is at a maximum at 75 °C, which is the maximum elongation temperature (MET) and calculations are performed regarding alloy development which will reduce the MET to room temperature. [ABSTRACT FROM AUTHOR]

The recrystallization, texture, and mechanical properties of hot-rolled and hot-rolled + annealed Mg-10Ho (wt %) are investigated. Mg-10Ho binary alloy is first solution treated followed by multipass rolling at 420 °C for up to 50 % reduction and subsequently annealed at 400 °C for different times for up to 60 min. The microstructure of deformed and annealed samples was characterized using a field-emission secondary electron microscope (FESEM) and electron back-scattered diffraction (EBSD) and the mechanical properties were examined using hardness and tensile tests. The as-rolled microstructure contains fine dynamic recrystallized grains and different types of deformation twins. With increasing annealing time, a decrease in the hardness was observed, suggesting initiation of recovery and static recrystallization. After 10 min of annealing, an equiaxed, nearly fully recrystallized microstructure with a grain size of 14 µm and weaker basal texture was obtained. This annealed sample exhibits an ultimate tensile strength of 213 MPa, yield strength of 145 MPa, and ductility of ∼7 %. Such an optimum combination of strength and ductility is attributed to the combined effect of solid solution strengthening, grain refinement, and texture weakening. Additionally, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to understand the recrystallization kinetics of Mg-10Ho and compared it with that of Mg-10Y. Results indicate that the solute drag effect plays a dominant role in the faster recrystallization kinetics of Mg-10Ho alloy. Finally, the texture variation in Mg-10Ho and Mg-10Y is also explained based on the solute drag effect. • Influence of Ho addition on recrystallization, texture, and mechanical properties of Mg-Ho binary alloy has been discerned. • Solute drag effect plays a dominant role in the faster recrystallization kinetics of Mg-10Ho alloy. • Solid solution strengthening, grain refinement and texture weakening gives optimum strength and ductility in Mg-10Ho alloy. • The role of Ho addition on recrystallization mechanism has been evaluated. [ABSTRACT FROM AUTHOR]

Level-set method is used to simulate phase transformations with anisotropic kinetics where the transforming interface is faceted. The method overcomes previous limitation of this simulation methodology in tracking dynamic evolution of a large number of growing grains. The method is then used to simulate multi-grain phase transformations where the facets are three low-index ([1 0 0], [1 1 1], [1 1 0]) planes that yield morphologies including cube, octahedron and rhombic dodecahedron. The microstructure evolves under site-saturated nucleation and constant nucleation rate. The cube morphology undergoes fastest transformation followed by octahedron, rhombic dodecahedron and sphere. It is also shown that the Johnson-Mehl-Avrami-Kolmorgorov theory can be used to describe the kinetics of the faceted phase transformation. The resulting microstructure shows non-convex grain shapes with highly corrugated surfaces. The structures are also characterized using the average grain length along certain low index crystallographic directions, the coherent length. This measurement shows that for a cubic morphology, there is a significant difference in the coherent length for low index directions, while there is no meaningful difference in the coherent length for kinetic Wulff shapes of other anisotropies examined. [ABSTRACT FROM AUTHOR]

This work presents a kinetic study of the sigma phase formation in hyper duplex stainless steel filler metal. Two sigma phase precipitation kinetics models were developed and compared. Initially, experimental sigma phase precipitation was built using isothermal heat treatments with durations from 30 s to 600 s, and temperatures between 600 °C and 1100 °C performed using a physical simulator. In these experiments, up to 70% of the equilibrium volumetric fraction of the sigma phase was achieved in 600 s. A CALPHAD-based kinetic model was developed using the experimental transformation data. Constant cooling rate conditions were calculated using the CALPHAD-based model revealing a minimum cooling rate of 4 °C/s as the threshold for the sigma phase to form. The microstructure evolution of the sigma phase precipitation follows the known eutectoid decomposition mechanism of ferrite transformation to sigma phase and secondary austenite (α → σ + γ 2), which evolved at the latter stages of the precipitation times, the lamellar σ/γ 2 morphology results from the eutectoid reaction, which is diffusion controlled. Finally, we applied the JMAK kinetic law to model the sigma phase formation on both datasets, the experimental and the CALPHAD-based TTTs. In the JMAK linearized plots, a kinetic mechanism change was found, switching from an eutectoid decomposition stage to a diffusion-controlled growth stage. While the JMAK calculations provided good agreement with the experimental data, the CALPHAD-based data only agreed near the maximum kinetics temperatures between 900 °C and 925 °C. Nevertheless, the sigma phase transformation kinetics modeled using JMAK equations properly described the experimental data describing its double kinetics behavior and reproduced the CALPHAD-based TTT at the maximum kinetics temperature range. • A controlled precipitation experiment and quantitative metallography produced experimental sigma phase Kinetics TTT maps. • CALPHAD-based kinetics model adjusted as a function of interfacial energy and nucleation sites to match the experimental data. • JMAK double-stage Kinetics for sigma phase formation: 1- eutectoid decomposition and 2 – diffusion-controlled growths. • It was found that the lamellar σ + γ 2 colonies occurred more consistently at lower temperatures and longer times. • JMAK on the CALPHAD-based TTT provided a similar average Avrami's exponent n. [ABSTRACT FROM AUTHOR]

This research investigates the effect of inter-critical annealing parameters on ferrite recrystallization and austenite formation during processing of a dual phase microstructure from a cold rolled low carbon steel. The main effort was to determine optimum annealing parameters for producing a desired ferrite-martensite dual phase microstructure in the steel for improved strength–ductility combination. A 57% cold rolled steel sheet was subjected to inter-critical annealing under different temperature–time conditions. Annealing temperatures were determined using Thermo-Calc. After annealing experiments, the resulting microstructures and corresponding hardness values were evaluated to determine ferrite recrystallization and austenite fraction under different conditions. The activation energy for ferrite recrystallization was 235.6 kJ/mol using standard Johnson–Mehl–Avrami–Kolmogorov analysis. Experiments showed that inter-critical annealing parameters affect the phenomenon of ferrite recrystallization and austenite formation. It was observed that both the rate of ferrite recrystallization and austenite formation increase with an increase in annealing temperature. Finally, steel was annealed under conditions similar to industrial processing in an annealing simulator with the selected annealing parameters to obtain improved strength–percentage elongation combinations. The steel under these conditions showed significant improvements in strength–ductility combination (610 MPa–26%; 680 MPa–15%) with an ideal yield strength to an ultimate tensile strength ratio of 0.5. [ABSTRACT FROM PUBLISHER]

In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4 ± 0.3 and 4.0 ± 0.2 nm/s were detected during isochronal heating with 0.5 °C/min. The mean interphase boundary velocity was ranging between 5.0 ± 0.2 and 6.4 ± 0.2 nm/s for austenite-ferrite transformation with a cooling rate of 1 °C/min. Ledge growth at isothermal conditions, resulted in velocity of 23 nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found. [ABSTRACT FROM AUTHOR]

In thermal simulation tests， the austenite with the same grain size but different solute Nb% was obtained by holding at austenite region with various times. The dilation curve was recorded when the sample was cooled to room temperature with the cooling rate of 1~5K/s. Originating from Rios' theoretical deduction， a modeling approach based on the JMAK (Johnson-Mehl-Avrami-Kolmogorov) equation and additivity rule was developed. The kinetic model of austenite to ferrite phase transformation under different solute Nb% was established. The results suggest that the exponential n is not affected by the solute Nb%， whereas the kinetic parameter k decreases with increasing solute Nb% that is an enhanced solute drag effect. [ABSTRACT FROM AUTHOR]

The aim of this study is focused on the analysis of phase transformations of 12 wt.% Cr steels. The analysis has been performed through a coupling of the results coming from dilatometry, optical microscopy and hardness measurements. The work has been focused on the α' → γ transformation in a normalized martensitic stainless steel. The phase transformation temperatures have been well defined by isochronal dilatometry that allows to investigate the kinetics of the studied transformations, it permits also to measure the activation energy for the investigated transformations and to estimate JMAK parameters. By coupling the information obtained through the dilatometric techniques and the metallographic investigation the microstructural transformation mechanisms have been enlightened and discussed. [ABSTRACT FROM AUTHOR]

The present study investigates the potential of minor La addition in modifying the texture and recrystallization behavior of cold rolled Mg-10Dy (wt.%) alloy. The samples were rolled and then subsequently annealed at 350 °C for varying time intervals. La addition was found to be significantly effective in enhancing the contraction and double twins. This eventually results in faster recrystallization kinetics, finer grain size and weaker texture upon annealing in a Mg-Dy-La alloy as compared to Mg–Dy alloy. A Johnson-Mehl-Avrami-Kolmogorov (JMAK) model has been employed to capture the evolution of recrystallization in both alloys. A good agreement between model predictions and experimental results has been obtained. • Minor La addition was found sufficient in altering deformation mechanisms in Mg. • Potency of grain refinement in La-added alloy was higher than the non-La alloy. • The La-added alloy showed faster recrystallization kinetics than the non-La alloy. • JMAK model was found appropriate in capturing the recrystallization evolution. • The La-added alloy showed weaker texture in cold rolled and annealed conditions. [ABSTRACT FROM AUTHOR]

In this paper, Fe 76 Ga 5 Ge 5 B 13 Cu 1 ribbon was prepared by melt spinning method and its thermal stability and non-isothermal crystallization kinetics were analyzed. It is found that the primary crystallization phase is α-Fe(Ga,Ge), following by the precipitation of Fe-B compounds. Besides, Kissinger and Ozawa methods were utilized to calculate the initial activation energy E x and apparent activation energy E a. The results show that E x 1 > E a 1 , which is due to that the diffusion of Ga and Ge is difficult during the nucleation process but left a channel for the subsequent diffusion of atoms. In addition, Avrami index n was calculated by Johnson-Mehl-Avrami-Kolmogorov (JMAK) model and the crystallization mechanism of Fe 76 Ga 5 Ge 5 B 13 Cu 1 alloy was analyzed. For the alloy with grains or clusters embedded in the amorphous matrix, it is concluded that the annealing condition with a higher heating rate is more conducive to obtain homogeneous nanocrystalline and improve soft magnetic properties. [ABSTRACT FROM AUTHOR]

A hierarchically coupled cellular automata (CA) model, crystal plasticity finite element method (CPFEM), and thermal finite element (FE) model is developed to predict the softening kinetics of the bulged steel tube during non-isothermal annealing. Through the developed model, the kinetics of softening mechanisms including static recovery (SRV) and static recrystallization (SRX), as well as the recrystallization texture are predicted. Later, the Johnson-Mehl-Avrami-Kohnogorov (JMAK) model based on the predicted SRX data is developed to interpret the recrystallization behavior of the material. To perform this study, diverse experimental tests including tube hydroforming (THF), annealing, uniaxial tensile test, hardness test, as well as microstructure observations through optical microscopy and Electron Backscatter Diffraction (EBSD) tests on steel tube are performed. The obtained experimental data are utilized to calibrate and verify the implemented CPFEM model for simulation of THF process, thermal FE model for prediction of the local temperature over annealing time, and CA algorithm for modeling of the softening kinetics and texture evolution throughout the annealing process. The study shows that the predicted deformation characteristics, softening kinetics, recrystallization texture and temperature profile during non-isothermal annealing are in good agreement with experimental data. During the annealing process, a total of four stages for the kinetics of softening mechanisms is observed: No softening; SRV only; SRV dominant; and SRX dominant. During the progress of SRX, the behavior of recrystallization is abruptly changed, confirming that two different mechanisms are controlling the kinetics of transformation. [ABSTRACT FROM AUTHOR]

Procedures for synthesizing digital polycrystalline microstructures are illustrated, from either 2D statistical data or 3D deterministic data. Finite element meshes representing the digital microstructures are generated using anisotropic and adaptive mesh refinement close to the grain boundaries. Digital mechanical testing based on crystal plasticity theory provides an estimate of the spatial distribution of strain energy within the polycrystalline aggregate. The latter quantity is used as an input for modelling subsequent static recrystallization, grain boundary motion being described within a level set framework. The kinetic law for interface motion accounts for both the stored strain energy and the grain boundary energy. The possibility to include nucleation events within the level set framework is illustrated, as well as the evolving topology of the grain boundary network. The recrystallization model is tested in different configurations and compared to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the microstructure evolutions in the Ti17 near titanium alloy during heat treatments. The phase transformation has first been studied experimentally by combining X-ray diffraction analysis, electrical resistivity and microscopy observations. From a series of isothermal treatments, a IT diagram has been determined, which takes into account the different morphologies. Then, a Johnson–Mehl–Avrami–Kolmogorov (JMAK) model has been successfully used to describe the phase transformation kinetics during either isothermal or cooling treatments. Finally, the coupling of the JMAK model to the finite element software ZeBuLoN allowed us to investigate the evolution of the spatial distribution of the different morphologies during the cooling of an aircraft engine shaft disk after forging. [Copyright &y& Elsevier]

The evolution in description and understanding of recrystallisation kinetics from the late 1930s to today is reviewed. Focus is on the advances in experimental possibilities but also the accompanying modelling improvements are covered. The approaches are divided into two groups where the second group is differentiated from the first by introducing crystallographic aspects into the experimental analysis and modelling. Special attention is given to the newly developed three-dimensional X-ray diffraction (3DXRD) microscope, which allows non-destructive in-situ bulk investigations including crystallographic aspects. Its potentials for advancing the understanding and interpretation of recrystallisation kinetics are discussed. [ABSTRACT FROM AUTHOR]

Microstructure quantification is an essential component of materials science studies, yet, there are no widely applicable, standard methodologies, for image data representation in complex microstructures. Recently, machine learning methods have demonstrated success in image recognition tasks across disciplines, including materials science. In this work, we develop an approach for microstructure quantification for the purpose of kinetic modeling of a discontinuous precipitation reaction. We develop our approach in a case study on a U-Mo alloy which experiences this phase transformation during sub-eutectoid annealing. Prediction of material processing history based on image data (classification), calculation of area fraction of phases present in the micrographs (segmentation), and kinetic modeling from segmentation results were performed as part of this study. Results indicate that features extracted using a convolutional neural network (CNN) represent microstructure image data well, and segmentation via k-means clustering agree well with manually annotated images. Classification accuracy of original and segmented images is both 94% for a 5-class classification problem. Kinetic modeling results are consistent with previously reported data that employed manual thresholding. The image quantification and kinetic modeling approach developed and presented here aims to reduce researcher bias introduced into the characterization process, and allows for efficiently leveraging information in limited, unbalanced image data sets. • A pre-trained, fine-tuned CNN represents micrographs well for multi-class classification. • Original image data and segmented images were classified according to processing history with 94% accuracy. • Machine learning methods were leveraged for understanding trends in microstructure evolution. [ABSTRACT FROM AUTHOR]