Grain Microstructure Evolution Research Articles

A cellular automata model for recrystallization annealing of aged GH4169 superalloy and its application

For the forging of GH4169, there are often serious mixed and coarse grains that need to be uniformly refined through a multi-stage annealing process. However, it will need to take a lot of time and cost to explore suitable multi-stage annealing process parameters if we only employ the experimental method. The cellular automata (CA) simulation can reveal the evolution of microstructure during forming and annealing, which can obviously reduce the number of experiments. Therefore, in the study, a CA model to simulate the evolution of grain microstructure, dislocation density and δ phase is established. The results demonstrate that the established CA model can effectively predict the microstructural evolution of GH4169. The correlation coefficient exceeds 0.94. Furthermore, using the CA simulation, an optimized multi-stage annealing parameter of “1000 ℃×3min+1000 ℃-5 min-950 ℃+950 ℃×50min” has been identified, which is also verified by experiments. These findings further verify the accuracy and reliability of the model in simulating microstructural evolution during the annealing process of GH4169 forgings. Eventually, based on the CA simulation, the formation mechanism for the uneven grain growth in the annealed microstructure is revealed. At original deformed grain boundaries, dynamic recrystallization (DRX) grains grow slowly, whereas static recrystallization (SRX) grains at DRX boundaries or δ phase boundaries within original deformed grains grow rapidly.

The Evolution of Grain Microstructure in Friction Stir Welding of Dissimilar Al/Mg Alloys with Ultrasonic Assistance.

The process of grain refinement during welding significantly influences both the final microstructure and performance of the weld joint. In the present work, merits of acoustic addition in the conventional Frictions Stir Welding (FSW) process were evaluated for joining dissimilar Al/Mg alloys. To capture the near "in situ" structure around the exit hole, an "emergency stop" followed by rapid cooling using liquid nitrogen was employed. Electron Backscatter Diffraction analysis was utilized to characterize and examine the evolution of grain microstructure within the aluminum matrix as the material flowed around the exit hole. The findings reveal that two mechanisms, continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX), jointly or alternatively influence the grain evolution process. In conventional FSW, CDRX initially governs grain evolution, transitioning to GDRX as material deformation strain and temperature increase. Subsequently, as material deposition commences, CDRX reasserts dominance. Conversely, in acoustic addition, ultrasonic vibration accelerates GDRX, promoting its predominance by enhancing material flow and dislocation movements. Even during the material deposition, GDRX remains the dominant mechanism.

Microstructural evolution of hydroformed Inconel 625 bellows

Fatigue cycle tests of Inconel 625 superalloy bellows expansion joints were conducted using a Fatigue testing machine at both room and elevated (650 °C) temperatures. Optical Microscope, Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were employed to investigate the microstructure evolution of grains and its boundaries. The intermetallic phases like γ″ were found and carbide precipitates were observed on the grain boundaries at elevated temperature. The recrystallization of the grains and its growth at the elevated temperature is characterized.

Grain boundary junctions, ridges and facets: their kinetics and effects on grain microstructure evolution

The kinetics of grain boundary triple junctions, grain boundary ridges and grain boundary facets are investigated. A theoretical concept and results of experimental observations and computer simulations of the behavior of these interfacial elements are presented, and their effect on the evolution of grain microstructure in 2D and 3D polycrystals is considered.

Phase-field study of competitive dendritic growth of converging grains during directional solidification

The microstructure evolution of grains with different orientations during directional solidification is investigated by the phase-field method. For converging dendrites, in addition to the usually accepted overgrowth pattern wherein the favorably oriented dendrites block the unfavorably oriented ones, the opposite pattern of overgrowth observed in some recent experiments is also found in our simulations. The factors which may induce this unusual overgrowth are analyzed. It is found that in addition to the difference in tip undercooling, the solute interaction of converging dendrites, which has been ignored in the classical theoretical model, also has a significant effect on the nature of the overgrowth at low pulling velocities. Solute interaction can retard the growth of dendrites at the grain boundary (GB) and induce a lag of these dendrites relative to their immediate neighbors, which gives the unfavorably oriented dendrite the possibility to overgrow the favorably oriented one. However, this unusual overgrowth only occurs when the spacing between the favorably oriented GB dendrite and its immediate favorably oriented neighbor decreases to a certain level through lateral motion. These findings can broaden our understanding of the overgrowth mechanism of converging dendrites.

Monte Carlo Simulation of Three-Dimensional Nonisothermal Grain-Microstructure Evolution: Application to LENS Rapid Fabrication

A stochastic three-dimensional (3-D) model for grain-microstructure evolution during transient nonisothermal annealing of metallic materials is developed, validated, and applied to the LENS (Laser-Engineered Net Shaping) advanced rapid fabrication process. The model is based on the assumption that the main driving force for microstructure evolution is the reduction in energy contribution arising from the grain boundaries. A temperature-dependent grain-boundary mobility factor is introduced into the expression for the transition probability in order to account for nonisothermal effects, such as those induced by the rastering laser during LENS-based manufacturing. The grain-boundary mobility factor and its temperature dependence are determined using the available experimental isothermal-annealing data. The simulation of grain growth (under nonisothermal annealing conditions encountered in the LENS process) is carried out by coupling a Monte Carlo method for microstructure evolution with a finite difference-based solution to the three-dimensional (3-D) transient energy equation. In response to the computational challenges of the simulations, a highly efficient interprocessor communications methodology is developed, which greatly reduces the simulation time on parallel computers. The results obtained show that under isothermal annealing conditions, the kinetics of grain growth is governed by a temporal power-law behavior and that, after an initial transition period, the grain-size distribution (normalized with respect to the average grain size) becomes time invariant. Furthermore, the application of the model to the LENS process is found to enable establishment of the relationships between process parameters (the laser power, beam rastering velocity, etc.) and the microstructure (grain size distribution, depth of the heat-affected region, etc.) of the deposited material.

Comparison of Amorphous and Polycrystalline Silicon Films as a Solid Diffusion Source for Advanced VLSI Processes

AbstractThis paper discusses the diffusion behavior of P, B and As from as-deposited amorphous/ polycrystalline silicon into the underlying Si substrate as a function of RTA/ furnace annealing conditions. The evolution of grain microstructure has been studied in as-deposited amorphous/ polycrystalline silicon to determine the inter-dependency of diffusion behavior and grain microstructure. After 60 s RTA at 1100° C for P doping, the microstructure of asdeposited amorphous silicon is drastically changed by rapid grain growth, whereas it is only slightly changed for asdeposited polysilicon. This leads to formation of deeper junctions for as-deposited polysilicon than for as-deposited amorphous Si. For B doping, there is little difference in the final microstructure between as-deposited polysilicon and as-deposited amorphous silicon after anneal. For low annealing temperatures, the junctions for both amorphous and polysilicon diffusion sources are laterally uniform in spite of local inhomogeneities due to grain boundaries. This is believed to be due to the rapid lateral spread of dopants along the interface prior to indiffusion into the substrate. At high annealing temperatures, a number of dark fringes are observed in the substrate along the interface from Crosssectional Transmission Electron Microscopy (XTEM) micrographs, which are more conspicuous for as-deposited amorphous Si. We believe that these fringes arise from local doping inhomogeneities or lattice strain in the substrate. For 30 s RTA at 1150° C, the native interfacial oxide appears to break up, causing epitaxial alignment of the polysilicon layer with respect to the underlying substrate.