Phase Transformation Model Research Articles

Strength mismatch effect on residual stress of high strength steel butt-welded joints

This study was mainly concerned with the residual stresses of 10CrNi3MoV high strength steel multipass butt welded joints by experimental measurement and numerical calculation considering the weld metal strength mismatch effect. Firstly, the Solid-State Phase Transformation (SSPT) processing of weldment was described by the thermo-metallurgical-mechanical simulation model. The residual stress distribution on the basis of the established procedure. Phase transformation modeling of diffusionless transformation kinetics based on the Koistinen-Marburger formula was performed by a user subroutine implemented in ABAQUS. Additionally, strain transformation calculation due to volumetric and heating change during the SSPT was completed by the user-defined subroutine. The results show that more accurate predictions of welding residual stress are obtained when the SSPT effect is incorporated in the simulation procedure. For evenmatched welded joints, the compressive residual stresses were distributed in the top and back weld layer due to the involving of SSPT behavior. The proposed finite element model can predict residual stress of high strength steel multipass welded joints and provide a theoretical basis to conduct the structural integrity assessment.

Approach to oriented grain growth accounting during aluminum alloys recrystallization simulation

The study focuses on the new approach to orientated growth modeling for aluminum alloys. Avrami equation for JMAK phase transformation model was applied to derive the relationship for texture volume fractions change during recrystallization. Two phases, recrystallized and non-recrystallized, are considered for Avrami equation derivation. Two recrystallized volume types were addressed: first, containing cube-oriented nuclei, growing with β-fiber orientations, second, containing the remaining nuclei, including cube-oriented nuclei without the boundary with β-fiber texture orientation material. The linear rate of cube-oriented volume nuclei migration k times exceeded the growth rate of the volume, identified as G and containing the other grains. The obtained relationships were integrated in the previously developed unique model of texture evolution during aluminum alloys thermomechanical treatment. It was used to simulate 3104 alloy commercial rolling in the continuous rolling mill and subsequent self-annealing in a coil. Modeling demonstrated, that accounting for grain orientated growth enabled improving the accuracy of texture composition calculation, its cube constituent, in particular.

A phenomenological model of deformation twinning kinetics

Abstract Inspired by the classical Olson and Cohen model for martensitic phase transformations, a simple phenomenological model of the deformation twinning kinetics is proposed with account for twin-twin interactions. The model was validated for different materials using experimental data abundantly available in the literature for the evolution of the twin volume fraction with strain.

Coupled heat transfer and phase transformations of dual-phase steel in coil cooling

Dual-phase steels are generally used in the car industry due to high tensile strength and good formability, which are obtained by a mixture of bainite and ferrite phases. This microstructure is achieved through slow rate coil cooling. However, the manufacturing of dual-phase steels introduces various challenges such as the instability of the cold rolling process. An important factor affecting this is the non-uniform coil cooling of a hot rolled strip. In coil cooling the cooling rates are not controlled and there are different thermal contacts during coil conveyance causing unequal cooling of the steel coil. Unequal cooling rates lead to non-uniform coil cooling, producing irregular phase transformations on different sides of the coil, which causes periodical variations of the phase fractions in the steel strip. Varying phase fractions cause thickness deviations in the strip during the cold rolling process. A three-dimensional transient heat transfer finite element model was developed and used for modeling the complete coil conveyance chain and coil field cooling of the coil on an industrial scale. A coupled phase transformation model is implemented as a subroutine into the finite element model for calculating the resulting phase fractions. It was found that the different thermal contacts during the coil conveyance produce uneven cooling rates causing length- and widthwise variations in the phase fractions. The heat transfer model is validated by comparing temperature profiles between the simulated and measured coil edges. The phase transformation model is fitted into experimental data and verification is carried out in industrial conditions by comparing the modeled phase fractions and test samples from a cooled and unwound steel coil.

Understanding the anatase-rutile stability in flame-made TiO2

The relative stability of anatase and rutile in stagnation flame synthesis with stoichiometric mixtures is investigated experimentally. The measurements reveal a high sensitivity of anatase-rutile composition to the flame dilution. It is demonstrated that anatase formation is favoured in more dilute (colder) flames while rutile is favoured in less dilute (hotter) flames. A particle model with a detailed description of aggregate morphology and crystal phase composition is applied to investigate the anatase-rutile stability. A phase transformation model is implemented in which rutile is formed for particles larger than a “crossover” size while anatase is formed for those smaller. Two formation mechanisms/pathways are discussed and evaluated. In the first pathway, the nascent particles are assumed to be stoichiometric and the crossover size is determined solely by the surface free energy. This hypothesis captures the general trend in the measured anatase-rutile composition but fails to explain the sensitivity. In the second pathway, non-stoichiometric TiO 2 − x oxide intermediates are assumed and the crossover size is hypothesised to be composition-dependent. This shows an excellent agreement with the experimental data. However, this hypothesis is found to be strongly influenced by assumptions about the initial particle growth stages. This study demonstrates the importance of a better description of the high-temperature chemistry and initial clustering mechanism in order to understand the crystal phase formation.

Physical and Numerical Simulations of Closed Die Hot Forging and Heat Treatment of Forged Parts.

The paper describes physical and numerical simulations of a manufacturing process composed of hot forging and controlled cooling, which replace the conventional heat treatment technology. The objective was to investigate possibilities and limitations of the heat treatment with the use of the heat of forging. Three steels used to manufacture automotive parts were investigated. Experiments were composed of two sets of tests. The first were isothermal (TTT) and constant cooling rate (CCT) dilatometric tests, which supplied data for the identification of the numerical phase transformation model. The second was a physical simulation of the sequence forging-cooling on Gleeble 3800, which supplied data for the validation of the models. In the numerical part, a finite element (FE) thermal-mechanical code was combined with metallurgical models describing recrystallization and grain growth during forging and phase transformations during cooling. The FE model predicted distributions of the temperature and the austenite grain size in the forging, which were input data for further simulations of phase transformations during cooling. A modified JMAK equation was used to calculate the kinetics of transformation and volume fraction of microstructural constituents after cooling. Since the dilatometric tests were performed for various austenitization temperatures before cooling, it was possible to include austenite grain size as a variable in the model. An inverse algorithm developed by the authors was applied in the identification procedure. The model with optimal material parameters was used for simulations of hot forging and controlled cooling in one of the forging shops in Poland. Distributions of microstructural constituents in the forging after cooling were calculated. As a consequence, various cooling sequences during heat treatment could be analyzed and compared.

Prediction of Mechanical Behaviors of the Q & P980 Steel Under Different Temperatures and Strain Rates Using Crystal Plasticity Method

The martensitic transformation in quenching and partitioning (Q&P) steels is greatly influenced by loading conditions such as environmental temperature and loading speed, and thus impacts the macroscopic mechanical properties during the deformation process. Within this work, an elastic-visco-plastic self-consistent (EVPSC) framework coupling with phase transformation model is used to simulate the stress strain responses as well as the microstructure evolution of the multi-phases Q& P980 steel under uniaxial tension process. A temperature and strain rate dependent transformation kinetics is incorporated into the model and phase transformation behaviors of the Q& P980 steel under different temperatures (25°C∼100 °C) and strain rates (0.0002s-1∼2s-1) are successfully characterized. The corresponding stress strain responses under different loading conditions are predicted and compared with the experimental data.

The Features of Bainite Transformation in Alloyed Steels

The main factors affecting the kinetics of isothermal bainite transformation in alloyed steels were determined using the algorithms of the computer modelling of phase transformations in the solid state. The experimental and calculated transformation kinetics was described and compared using well known Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation and Austin-Rickett (AR) equation. The effect of the initial configuration and the nucleation rate of the new phase nuclei on the parameters of the bainite transformation was analysed. The calculated kinetics of phase transformation at different nucleation rates was compared with the experimentally observed for HY-TUF and D6AC steels. The mechanism of the bainite transformation in alloyed steels was explained.

Experimental modeling of phase transformations in a weakly ordered carbon substance under impact treatment

The results of experimental modeling of impact transformation of weakly ordered carbon substance by short-pulse laser radiation on glassy carbon are presented. The experiments yielded extremely high temperatures of ~14500 K and pressures of ~300 GPa, which are comparable with temperatures and pressure of the formation of large Earth’s meteorite craters. The analysis of the transformation products of a target substance showed melting of glassy carbon, its further solidifcation, partial crystallization upon cooling and formation of polyphase composites, which contain hexagonal nanocrystalline graphite and hollow onion-like and one- and two-layer fullerene-like structures. The synthetic products, including high-pressure carbon polymers and hollow onionlike multi-layer fullerene-like structures are of interest as carbon materials, which form at ultrahigh pressures and temperatures. The results of experimental modeling can also be used for the comparison with natural products to explain the formation of natural high-pressure carbon composites after non-graphite precursor. Figures 7. Tables 1. References 64.

In situ neutron diffraction and crystal plasticity analysis on Q&P1180 steel during plastic deformation

Q&P1180 is a typical third generation of advanced high strength steel. In this paper, the microstructure evolution of the Q&P1180 steel during uniaxial tension is investigated comprehensively by using in situ neutron diffraction technique, electron backscatter diffraction (EBSD), scanning electron microscope (SEM) and the elastic-visco-plastic self-consistent with phase transformation (EVPSC-PT) model. Tempered martensite (TM) is determined as the matrix based on SEM and EBSD analysis. Tiny blocky retained austenite (RA) islands are distributed in the matrix, and transformed into martensite during deformation. Effects of phase transformation on the stress-strain response, lattice strains, diffraction intensities and texture evolution of the Q&P1180 are discussed based on experiments and the EVPSC-PT model. The progressive phase transformation from austenite to martensite is orientation-dependent, and the γ{200} orientation has the lowest stability, followed by γ{220} and γ{311} orientations. The flow stress of each phase with straining shows that the strength of RA in Q&P1180 is higher than that of TM, due to its high carbon content.

Predicting strain-induced martensite in austenitic steels by combining physical modelling and machine learning

Computational materials design has made significant progress lately. However, one underexploited opportunity lies in the combination of physically based modelling and machine learning (ML). In the present work we exploit this combination for modelling of strain-induced martensitic phase transformation (SIMT) in austenitic steels. A fully predictive model for SIMT, responsible for the TRIP effect in many steels, is devised. An experimental dataset correlating SIMT with composition, temperature and strain is collected from the open literature firstly. Secondly, the Olson-Cohen model is applied to make physically based predictions on temperature and strain dependence of SIMT in order to expand the database to the final size of 16,500 entries relating the features and the target. Thirdly, ensemble ML methods are applied to model the data and the final model is validated on a holdout dataset, including also dual-phase alloys. The final model provides accurate predictions of SIMT in a temperature range from −196 to 100 °C and from 0 to 1 in strain. The model can readily be extended to consider further factors such as strain rate and stress state. Moreover, it can be used together with thermodynamic and kinetic calculations, or thermomechanical simulations, for the design of steels and components, respectively.

Coupled Temperature-Microstructure Model for Predicting Temperature Distribution and Phase Transformation in Steel for Arbitrary Cooling Curves

Abstract This study aims at developing a numerical model that can be employed for simulating the thermomechanical treatment to develop the advanced high strength steels. The developed numerical method is used to calculate the heat transfer coefficient of the quenching medium during the continuous cooling of the steel using the inverse heat transfer model for predefined cooling paths. Further, the phase transformation models are used to predict the final microstructure of the steel plate. The cooling rate, plate thickness, and rolling speed are varied to evaluate the temperature and microstructure distribution in the steel plate. It is found that on increasing the quenching time, the transformation fraction from austenite to ferrite and bainite phases increases and the corresponding martensite fraction decreases. The temperature variation in the plate is significant due to the change in plate thickness and rolling speed for a given quenching time. The present model will be useful for designing process parameters to obtain desired microstructures in third-generation advanced high strength steels.

Numerical modeling of diffusion-controlled phase transformation using the Darken method: Application to the dissolution/precipitation processes in materials

Many material phase transformations are controlled by mass transport induced by diffusion. To better understand such transformations, numerous modeling strategies at the scale of the moving interfaces exist, with their strengths and weaknesses. Phase-field approaches are based on diffuse interfaces that do not require any interface tracking, as opposed to those based on fixed-grid sharp interface tracking. In the case of binary two-phase systems, in this paper we address the key point of the mass balance equation at the interface involving a concentration jump, which determines the interface moving velocity. We propose a unique diffusion equation for both phases and their interface, based on the component’s chemical potentials which are continuous through the interface and a smooth volume-of-fluid phase representation. This model is achieved in the framework of the Darken method, which involves intrinsic diffusion of components and a drift velocity to which all compounds are subjected. This drift velocity is shown to be that of the interface displacement as well. This methodology is verified for 1D and 3D dissolution/precipitation problems and has a first-order spatial convergence. The 3D simulations of precipitation and dissolution processes of more complex microstructures clearly show a bifurcation of the particle morphology from the initial spherical shape when the diffusion edges of each particle interact with each other. An extension of the diffusion potential to mechanical driving forces should make it possible to deal with mechano-chemical coupling of mass transport.

A computational phase transformation model for selective laser melting processes

Selective laser melting (SLM) has gained large interest due to advanced manufacturing possibilities. However, the growing potential also necessitates reliable predictions of structures in particular regarding their long-term behaviour. The constitutive and structural response is thereby challenging to reproduce, due to the complex material behaviour. This motivates the aims of this contribution: To establish a material model that accounts for the behaviour of the different phases occurring during SLM but that still allows the use of (basic) process simulations. In particular, the present modelling framework explicitly takes into account the mass fractions of the different phases, their mass densities, and specific inelastic strain contributions. The thermomechanically fully coupled framework is implemented into the software Abaqus. The numerical examples emphasise the capabilities of the framework to predict, e.g., the residual stresses occurring in the final part. Furthermore, a postprocessing of averaged inelastic strains is presented yielding a micromechanics-based motivation for inherent strains.

Effects of phase transformation on the ultraviolet optical properties of alumina clusters in aircraft plumes.

Alumina particles experience phase transition as an undercooling process along the plume, during which the liquid alumina clusters transform into multiphase, and then into α phase. The phase transformation model was built by an improved diffusion limited aggregation (DLA) algorithm with monomers of stratified structure. The effects of phase transformation on the ultraviolet optical characteristics of alumina clusters were studied using the superposition T-matrix method (STMM). We found that the alumina clusters in phase transition had completely different optical properties compared with the fixed phase ones. Forward scattering, absorption efficiency and asymmetry parameter gradually decreased, whereas backward scattering, scattering efficiency, and single-scattering albedo gradually increased during the phase transformation process. Besides, multiphase alumina clusters were compared with the other two equivalent models, including the sphere model approximated by equivalent volume sphere (EVS) and the equivalent surface sphere (ESS) approaches and single-phase cluster model approximated by Maxwell-Garnett (MG) and Bruggeman (BR) approaches. Generally speaking, the optical properties of the single-phase cluster approximated by MG and BR approaches were relatively close to those of the real multiphase alumina cluster. Whereas the spheres approximated by EVS and ESS had great deviations, especially when the number of monomers in the cluster was 20, the relative error of scattering efficiency calculated by ESS was up to 52%. Therefore, approximate approaches for multiphase clusters should be chosen cautiously. Our results give further the understanding of the optical properties of alumina clusters. As the phase states are usually closely related to the plume radiation and burning process, these kinds of researches will be helpful to aircraft detection, identification, and other related fields.

Finite-strain scale-free phase-field approach to multivariant martensitic phase transformations with stress-dependent effective thresholds

A scale-free phase-field model for martensitic phase transformations (PTs) at finite strains is developed as an essential generalization of small-strain models in Levitas et al. (2004) and Idesman et al. (2005). The theory includes finite elastic and transformational strains and rotations as well as anisotropic and different elastic properties of phases. The gradient energy term is excluded, and the model is applicable for any scale greater than 100 nm. The model tracks finite-width interfaces between austenite and the mixture of martensitic variants only; volume fractions of martensitic variants are the internal variables rather than order parameters. The concept of the effective threshold for the driving force is introduced, which can be either positive (e.g., due to interface friction) or negative (e.g., due to defects and stress concentrators promoting PTs). To reproduce PT conditions obtained from experiments or atomistic simulations under general stress tensor, the effective threshold depends on the stress tensor components. Material parameters are calibrated for martensitic PT between single crystalline cubic Si I and tetragonal Si II phases, which has large transformation strains (ɛt1=0.1753;ɛt2=0.1753;ɛt3=−0.447). Finite element algorithms and numerical procedures are implemented in the code deal.II. Multiple 3D problems are solved to study the effect of mesh size, holding time during quasi-static loading, and strain rate on the multivariant microstructure evolution in Si I → Si II PT under uniaxial and hydrostatic loadings. The solution exhibits significant lattice rotations both in Si I and Si II, reproducing the appearance of diffuse grain boundaries in Si I and Si II and transforming them in polycrystals, which corresponds to known experiments. While finer mesh can produce a more detailed microstructure, the solution becomes practically mesh-independent after the mesh size is 80 times smaller than the sample size. When approaching the stationary solution, rough mesh leads to convergence to the correct microstructure faster than the fine mesh, because it neglects fine details in the microstructure. In some regions, reverse PT occurs at continuous compression, despite large transformation hysteresis. For most stationary interfaces, local thermodynamic equilibrium conditions (thermodynamic driving force for the interface motion equal to the effective threshold) are satisfied.

Modeling precipitation process of Al-Cu alloy in electron beam selective melting with a 3D cellular automaton model

In additive manufacturing (AM), the microstructure evolution is rather complex and plays a critical role in the mechanical performance. To predict the microstructure, many numerical models have been proposed, but solid-state phase transformations are usually not incorporated. In this work, we develop a Cellular Automaton (CA) model to simulate the solid-state phase transformation with alloy element partition. In this model, a mixed-mode phase transformation model is used, and both the growth and decomposition of precipitates are included. With the temperature profile from the validation experiment, the model well reproduces the precipitation process in the solidified Al 2024 alloy during Electron Beam Selective Melting (EBSM). The simulation results are in good agreement with the experimental results and illustrate the causes of some critical morphological features. The impact of temperature profile on the precipitation process is studied and the results indicate: (1) the rising of the powder-bed temperature during the fabrication process might cause precipitate decomposition; (2) the precipitates are evenly distributed and their sizes are uniform when the powder-bed temperature is low.

Abnormal orientation relation between fcc and hcp structures revealed in a deformed high manganese steel

The crystallography of phase transformation is an important issue for studying metallic alloys. Here, we demonstrate an abnormal orientation relation between face-centered cubic (fcc) and hexagonal close-packed (hcp) phases due to the intersection of two εhcp-martensite variants in a high manganese steel. The corresponding crystallographic characteristics, including invariant line, habit plane and atomic steps, have been characterized by transmission electron microscopy and the quasi-O-line model. In addition, the models of phase transformation about the intersection are proposed based on transmission mechanisms of dislocations. Our findings enrich the theories of phase transformation and implicate the possibility to fabricate stronger and tougher steel.

Modelling of phase transformations in structural steels

Modelling of phase transformations in structural steels

Phase Transformation Prediction Considering Crystallographic Orientation in Microgrinding Multiphase Material

AbstractThis investigation proposes a physics-based model to predict the solid-state phase transformation of maraging steel subjected to microgrinding. In microgrinding, the effect of crystallography is significant on the grinding phase transformation in light of the fact that the depth of cut is on the same order of magnitude as the grain size. This paper proposes a predictive model of phase transformation considering crystallographic orientation (CO) with respect to the grinding direction based on the Taylor factor model. In addition, the flow stress model is modified by adding a CO sensitive term and incorporating the mechanical-thermal loadings. Furthermore, the temperature, temperature rate, strain rate, and Taylor factor are also combined in the model of phase transition. The kinetics parameters of the models are obtained by a regression analysis against experimental data. Finally, the modified models are validated with experiments data and compared with the previous prediction.