Modeling Of Microstructure Evolution Research Articles

Irradiation-induced chemical disordering in ceramics: The case of SiC

Chemical disordering is one feature of damage that occurs in irradiation environments and is common in ceramic compounds. This paper reviews irradiation-induced chemical disordering in SiC-based materials at elevated temperatures. The results were obtained using advanced analytical tools to characterize atomistic defects and to evaluate the chemistry of defect clusters and homonuclear bond structures. This paper demonstrates that chemical disordering is crucial to understanding microstructural stability and establishing a mechanistic modeling of microstructural evolution of irradiated SiC.

Comprehensive unified model and simulation approach for microstructure evolution

The prediction of microstructure constituents and their morphologies is of great importance for the evaluation of material properties and design of advanced materials. There have been considerable efforts to model and simulate microstructure evolution using a wide spectrum of models and simulation approaches. This paper initially reviews the atomistic and mesoscale simulation approaches for microstructure evolution, emphasizing their advantages and disadvantages. Atomistic approaches, such as molecular dynamics, are restricted by the scale of the studied system because they are computationally expensive. Continuum mesoscale simulation approaches, such as phase field, cellular automata, and Monte Carlo, have inconsistent phenomenological equations, each of which only describes one aspect of microstructure evolution. To provide comprehensive insight into microstructure evolution, a unified model that is capable of equally evaluating the nucleation and growth processes is required. In this paper, a physics-based model is proposed that incorporates statistical mechanics, the energy conservation law, and the force equilibrium concept to include all aspects of microstructure evolution. A compatible simulation approach is also described to simulate microstructure evolution during thermomechanical treatments. Furthermore, the microstructure evolution of AISI 304 austenitic steel during isothermal heat treatment and fusion welding is simulated and discussed. The use of fundamental physical rules instead of phenomenological equations, together with the real spatial and temporal scales of the proposed model, facilitates the comparison of the simulation results with experimental results. To examine the accuracy of the proposed simulation approach, the isothermal heat treatment simulation results are compared with experimental data over a broad region of temperatures and time periods.

Multi-physics field coupling and microstructure numerical simulation of laser cladding for engine crankshaft based on CA-FE method and experimental study

During engine operation, the crankshaft journals are prone to wear, resulting in a decrease in surface matching accuracy and abnormal vibration and noise. Therefore, it is important to strengthen the surface and improve the wear resistance. Laser cladding, which involves multiple disciplines, such as automation control, optics, fluid mechanics, heat transfer, etc., is an effective method. In this paper, based on the finite element method, a numerical model of the laser cladding for the crankshaft journal was established. A disk laser was used to laser clad Fe45 powder on the ASTM1045 substrate. The multi-physics field coupling evolution mechanism of the laser cladding process was quantitatively revealed. The cladding temperature, liquid metal flow rate, and stress are positively correlated with the laser power. Combined with the cellular automata method, the microstructure evolution model during the solidification was established, and the morphology and size distribution of the grains in the cladding layer was obtained. In the middle area of the cladding layer, the grains are mostly columnar and equiaxed crystals. At the same time, the change of material physical parameters with temperature was taken into account in the model, which makes the numerical simulation results more accurate. The numerical simulation results were compared with the experimental results through the microscopic characterization experiment, and the accuracy of the numerical model was verified. Finally, through a friction-wear test, the wear resistance between the cladding layer and the substrate is compared. The average wear amount of the substrate is 1.86mm3 and the average wear amount of the cladding layer is 0.64mm3. The experiment shows that the wear resistance of the cladding layer is higher than that of the substrate, and the wear rate is reduced by 65.59%.

Recursive grain remapping scheme for phase‐field models of additive manufacturing

AbstractThe phase‐field method is an attractive tool in modeling microstructural evolution due to rapid solidification under additive manufacturing conditions, but typical polycrystalline models are prone to grain coalescence. Grain tracking and remapping schemes can eliminate artificial coalescence and increase the number of grains and crystallographic orientations that may be considered in a simulation, but previous tracking and remapping schemes may not efficiently capture the complex grain morphologies that form during additive manufacturing. A new recursive scheme is derived from a binary tree of axis‐aligned bounding boxes that can efficiently represent columnar and irregularly shaped grains. We demonstrate the power of this approach by simulating microstructures with hundreds to thousands of grains and quantify the reduction in the number of order parameters required to represent the microstructure. The new scheme leads to orders of magnitude fewer computational resources as compared to the naïve paradigm of one grain per order parameter, and also offers a substantial improvement over algorithms derived from bounding spheres.

Micro-scale thermodynamic model of microstructure and stress evolution in parts via selective laser melting

Micro-scale thermodynamic model of microstructure and stress evolution in parts via selective laser melting

Modeling of microstructure evolution coupled with molten pool oscillation during electron beam welding of an Al-Cu alloy

The physical understanding of electron beam welding (EBW) and prediction of the weld microstructure are valuable but challenging. In this work, a multi-physics modeling framework is adopted to simulate the microstructure evolution during the EBW process. A molten pool model incorporating multiple driving forces and a dynamic heat source model provides the temperature field as the input for the microstructure simulation. Both the temperature and chemical concentration are considered in the grain growth process. Additionally, an efficient method is adopted to track the second phase in the solidification process, and a practical algorithm is proposed to identify the complex weld shapes. The mechanism of microstructure evolution under the effect of molten pool oscillation is elaborated. Based on the simulated results, the effects of the process parameters on the grain morphology, grain size, segregation and second phase are quantified. These predicted results are compared to experimental observations and measurements. This study could be helpful for both understanding microstructure evolution and optimizing the actual welding process to tailor the microstructure.

Microstructure analysis and hardness prediction based on microstructure evolution theory in multi-pass welded joint

During multi-pass butt welding of high-strength steel (HSS) thick plate, it is difficult to estimate the hardness performance of thick weld since multithermal cycles can drive complex microstructure evolution. In this study, butt-welded joint of 75 mm-thick Q690 HSS was prepared by multi-pass shielded metal arc welding (SMAW) in advance; meanwhile, experiments were carried out to characterize microstructure and measure hardness distribution in butt-welded joint. Then, the welding temperature history of butt-welded joint was numerically examined. Based on the irreversibility of Martensite and Bainite, the microstructure evolution model was modified to predict the complicated microstructure evolution in multi-pass welded joint. Compared with experimental data, the validity of modified microstructure evolution model was verified. With the application of simulated thermal cycles and material chemical compositions as input parameters, volume fraction distribution of each phase was obtained by modified microstructure evolution model and hardness value was calculated by hardenability algorithm. Both experimental and predicted results exhibited that microstructure in weld metal (WM) is mainly composed of granular Bainite with a small amount of Ferrite and Martensite, and heat-affected zone (HAZ) consists of predominantly lath Martensite. Hardness distribution in butt-welded joint obtained by the prediction method is in accordance with the measurement. Average hardness value in WM and BM is 290 VPN and 250 VPN. A sharp decrease in HAZ hardness can be found from the fusion line to base metal (BM). The maximal value in the HAZ close to fusion line and the corresponding prediction deviation is 6%.

Physical and Numerical Simulations for Predicting Distribution of Microstructural Features during Thermomechanical Processing of Steels.

The design of modern construction materials with heterogeneous microstructures requires a numerical model that can predict the distribution of microstructural features instead of average values. The accuracy and reliability of such models depend on the proper identification of the coefficients for a particular material. This work was motivated by the need for advanced experimental data to identify stochastic material models. Extensive experiments were performed to supply data to identify a model of austenite microstructure evolution in steels during hot deformation and during the interpass times between deformations. Two sets of tests were performed. The first set involved hot compressions with a nominal strain of 1. The second set involved hot compressions with lower nominal strains, followed by holding at the deformation temperature for different times. Histograms of austenite grain size after each test were measured and used in the identification procedure. The stochastic model, which was developed elsewhere, was identified. Inverse analysis with the objective function based on the distance between the measured and calculated histograms was applied. Validation of the model was performed for the experiments, which were not used in the identification. The distance between the measured and calculated histograms was determined for each test using the Bhattacharyya metric and very low values were obtained. As a case study, the model with the optimal coefficients was applied to the simulation of the selected industrial hot-forming process.

Mechanical behaviour and micromechanical modelling of medium-Mn steel microstructure evolution

Intense research efforts are focused on the development of advanced medium-manganese steels intended for lighweight automotive components and other applications, where high strength and good formability are crucial. The mechanical and technological properties of such steels depend on the strain-induced martensitic transformation (SIMT) of metastable retained austenite (RA), which is affected by several factors including a deformation temperature. The response of the retained austenite to strain-induced martensitic transformation at various temperatures (-60 °C-200 °C) was studied in the advanced medium-Mn 0.17C-3.3Mn-1.7Al-0.22Si steel via tensile testing. It was found that the efficiency of martensitic transformation decreases with increasing the deformation temperature in a range from -60 °C to 100 °C. At 140 °C and 200 °C, the strain hardening ability and strain-induced martensitic transformation were additionally affected by thermally activated processes. The obtained experimental data was applied into a thermodynamic model in order to predict the temperature-dependant stability of retained austenite. Furthermore, the micromechanical modelling using the finite element method was utilized to perform an analysis at the microscale level of the SIMT of the RA during plastic deformation. The experimental and modelling approaches were found to be complementary methods to comprehensively evaluate the mechanical response of investigated steel as well as microstructure evolution of structural constituents at progressive strain levels.

Effect of Nb and Mo on Austenite Microstructural Evolution During Hot Deformation in Boron High Strength Steels

This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The role of Nb and Mo during the hot deformation of low carbon steels is well known: both mainly retard austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, the design of rolling schedules resulting in properly conditioned microstructures, requires microstructural evolution models that take into account the effect of the different alloying elements. In this specific case, the effect that high levels of molybdenum (0.5 pct) have in the recrystallization delay was evaluated. In that respect, hot torsion tests were performed in this work to investigate the microstructural evolution during hot deformation of four boron steels, with different Nb (0.025 pct) and Mo (0.5 pct) combinations. The retardation in recrystallization kinetics was modeled in all cases and measured kinetics agree with those predicted by equations previously developed for Nb–Mo microalloyed steels with lower Mo concentrations (< 0.3 pct). The strain-induced precipitation in the Nb and Nb–Mo bearing steels was also characterized. Finally, the fractional softening evolution during multipass rolling simulations was compared with MicroSim® model predictions, showing a good agreement with experimental results.

A Numerical Approach to Model Microstructure Evolution for NiTi Shape Memory Alloy in Laser Powder Bed Fusion Additive Manufacturing

A Numerical Approach to Model Microstructure Evolution for NiTi Shape Memory Alloy in Laser Powder Bed Fusion Additive Manufacturing

Thermophysical Properties of Fe-Si and Cu-Pb Melts and Their Effects on Solidification Related Processes

Among thermophysical properties, the surface/interfacial tension, viscosity, and density/molar volume of liquid alloys are the key properties for the modelling of microstructural evolution during solidification. Therefore, only reliable input data can yield accurate predictions preventing the error propagation in numerical simulations of solidification related processes. To this aim, the thermophysical properties of the Fe-Si and Cu-Pb systems were analysed and the connections with the peculiarities of their mixing behaviours are highlighted. Due to experimental difficulties related to reactivity of metallic melts at high temperatures, the measured data are often unreliable or even lacking. The application of containerless processing techniques either leads to a significant improvement of the accuracy or makes the measurement possible at all. On the other side, accurate model predicted property values could be used to compensate for the missing data; otherwise, the experimental data are useful for the validation of theoretical models. The choice of models is particularly important for the surface, transport, and structural properties of liquid alloys representing the two limiting cases of mixing, i.e., ordered and phase separating alloy systems.

Microstructural evolution during multiple scans in laser polishing of Ti6Al4V

Consecutive thermal cycles during laser polishing lead the complex microstructural changes. It is more difficult for modeling microstructural evolution in multiple thermal cycles compared to the single thermal cycle of heat treatment. In the present work, a thermo-microstructural model is established to investigate the temperature history and microstructural evolution during multiple scans in laser polishing of Ti6Al4V (TC4). The temperature history of the present model is validated the temperature data recorded by the thermal camera. The phase transformation involved α phase and β phase, martensitic transformation and decomposition, as well as the reservation of β phase is considered in the microstructural model. The numerical results show that the non-uniform phase distribution is attributed to the reservation of β phase and the multiple thermal cycles.

Numerical simulation of microstructure evolution of high-purity tantalum during rolling and annealing

In this study, a unified model of thermoplastic deformation and the corresponding microstructure evolution for tantalum with high purity was first proposed by introducing the state variable equations of recrystallization fraction, average grain size, and grain growth. The subroutine of microstructure evolution model was programmed for embedding into the finite element simulation of tantalum plates upon rolling and annealing, and the parameters of microstructure evolution were obtained by fitting the experimental data. Our simulation results reveal the correlations among the microstructures, rolling reduction, annealing temperature and time. Based on these findings, a guideline for optimizing the microstructure of tantalum is proposed.

Defect cluster configurations and mobilities in α-Zr: A comparison of the BMD19 and M07 interatomic potentials

Preferred defect cluster configurations and diffusion behavior are calculated with the recently published BMD19 interatomic potential for α-zirconium and compared with results calculated from the M07 potentials. Both potentials predict that small SIA clusters form configurations contained entirely within a single basal plane, while large SIA clusters form perfect dislocation loops on first-order prismatic planes with Burgers vector b→=13<112¯0>. The M07 potentials predict stable void formation for vacancy clusters while the BMD19 potential accurately indicates a preference for planar configurations on either first-order or second-order prismatic planes. Small basal vacancy clusters are found to form faulted pyramidal structures and only transform to faulted c-loops at larger sizes. In evaluating the defect diffusivity, single vacancies and interstitials are both found to exhibit anisotropic diffusion within the basal-plane. Small SIA clusters are found to migrate exclusively in 2-D within a single basal plane with the BMD19 potential but migrate in 3-D with the M07 #3 potential. Small vacancy clusters are either unstable or immobile with the M07 #3 and #2 potentials, while these clusters exhibit appreciable mobility either along the c-axis or in a quasi-isotropic manner with the BMD19 potential. The apparent difference in the diffusional anisotropies of defect clusters, rather than point defects, could be a critical component for improving predictive capabilities for mesoscale modeling of microstructural evolution in α-zirconium.

Characterization and statistical modeling of texture and microstructure evolution in dynamically fractured electron beam melted Ti-6Al-4V

Two different Ti-6Al-4V cylindrical rods, horizontally and vertically built, fabricated through the electron beam melting technique, were underwent compression loadings to failure at the strain rates of 2350s−1 and 1750s−1, respectively. Low-angle grain boundary formation, dislocation array, and dislocation pinning were observed and attributed to the stress-induced dislocation formation in the as-built microstructure. Superior strength at each strain rate in vertically built samples was concluded to be a consequence of its finer microstructure and the presence of martensite α˙. Hardness measurements revealed higher values at the areas close to the fracture surface. Electron microscopic characterization revealed parallel-twin formation resulting from adiabatic temperature rise, increasing short-ranged clustering, and the high stacking fault energy. Dynamic compressive deformation led to the appearance of dislocation structure, cell blocks, and extended dislocation walls formation. Texture analysis showed <c+a> type pyramidal slip systems and contraction twins as the most favorable slip systems. Texture evolution interpretations from the region far from the area close to the fractured surface indicated that mean grain size decreased, and higher dislocation densities were obtained. The more preferred texture tended to rotate by approaching the fracture surface so that the basal plane became parallel to the fracture surface, which is directly related to the facilitation of crack propagation. Moreover, the generalized spherical harmonics were used to apply 2-point statistics on the texture and then statistically compare the texture changes. The results were in good agreement statistically, where principal components (PC) were utilized to explain variances in the database.

Simulation based derivation of changed rim zone properties caused by thermal loadings during EDM process

Electrical discharge machining is a non-conventional manufacturing process with main thermal active principle. The induced thermal energy leads to material ablation and formation of numerous modifications in sub-surface layers. These modifications determine strongly the surface integrity of the manufactured component and thus affect the functional component properties directly. Therefore, predicting these material modifications will allow a detailed conclusion regarding the functionality in advance. In this work, a phase field method based model is developed to simulate the microstructure evolution in the heat affected zone of a steel (1.2343). Due to high local energy density of a single discharge, high temperature gradients occur, which cause different thermal loadings within sub-surface layers. Hence, the results of a heat transfer simulation at different locations of the rim zone served as input for the microstructure evolution models to represent the complete final heat affected zone. As a second step, the rim zone properties are derived as a superposition of the properties of pure phases based on the phase fractions resulting from the microstructure evolution model.

Microstructure Evolution and Constitutive Model for a Ni-Mo-Cr Base Alloy in Double-Stages Hot Compression with Step-Strain Rates

Microstructure Evolution and Constitutive Model for a Ni-Mo-Cr Base Alloy in Double-Stages Hot Compression with Step-Strain Rates

Thermomechanical Processing Model and Abnormal Microstructure Evolution of High-Nitrogen Stainless Steel X30crmon15-1

Thermomechanical Processing Model and Abnormal Microstructure Evolution of High-Nitrogen Stainless Steel X30crmon15-1

Thermal-mechanical finite element simulation of flat bar rolling coupled with a stochastic model of microstructure evolution

Thermal-mechanical finite element simulation of flat bar rolling coupled with a stochastic model of microstructure evolution