Luis A. Barrales-Mora

Texture Evolution of a Cold-Rolled Fe-28Mn-0.28C TWIP Steel during Recrystallization

Texture evolution during static primary recrystallization of an austenitic Fe-28Mn-0.28C TWIP steel was analyzed. The cold-rolled material, which showed a Brass-type texture at medium (30% and 50%), and additionally a γ-fiber at high (80%) deformation degrees, was subjected to isothermal annealing at 700°C. The influence of rolling degree/starting texture on the development of particular texture components was studied. After recrystallization a weakened, retained rolling texture was observed for the examined reduction levels. In addition to the deformation components, Brass and Goss, further α-fiber components were formed mainly by annealing twinning leading to the development of this fiber.

The Effect of Grain Boundary Junctions on Grain Microstructure Evolution: 3D Vertex Simulation

A 3D Vertex Model has been successfully implemented to investigate the evolution of a special grain assembly during grain growth. The model considers the mean curvature as driving force for the motion of the vertices and allows the consideration of all parameters affecting the motion of the system, i.e., grain boundary energy and line tension of the triple lines, as well as grain boundary (GB), triple line (TL) and quadruple point (QP) mobility as well. The used special configuration makes it possible to study the influence of all structural elements of a grain boundary network on the evolution of the system by allowing the steady-state motion of the boundaries of a shrinking grain. In the present work the different mobilities have been systematically varied and the evolution of the grain size with time has been studied as a function of TL and QP mobility. The results of the simulations are finally linked to the different kinetic regimes reached by the system.

A massively parallel cellular automaton for the simulation of recrystallization

A new implementation of a cellular automaton for the simulation of primary recrystallization in 3D space is presented. In this new approach, a parallel computer architecture is utilized to partition the simulation domain into multiple computational subdomains that can be treated as coupled, gradually coupled or decoupled entities. This enabled us to identify the characteristic growth length associated with the space repartitioning during nucleus growth. In doing so, several communication strategies between the simulation domains were implemented and tested for accuracy and parallel performance. Specifically, the model was applied to investigate the effect of a gradual spatial decoupling on microstructure evolution during oriented growth of random texture components into a deformed Al single crystal. For a domain discretized into one billion cells, it was found that a particular decoupling strategy resulted in faster executions of about two orders of magnitude and highly accurate simulations. Further partition of the domain into isolated entities systematically and negatively impacts microstructure evolution. We investigated this effect quantitatively by geometrical considerations.

Application of Texture Analysis for Optimizing Thermo-Mechanical Treatment of a High Mn TWIP Steel

A recently introduced processing route consisting of cold rolling and recovery annealing allows the production of TWIP steels with high yield strength along with appreciable uniform elongation due to the thermal stability of mechanically induced nanoscale twins. A wide range of strength-ductility combinations was obtained using recovery and recrystallization annealing of 30%, 40%, and 50% cold-rolled Fe-23Mn-1.5Al-0.3C TWIP steel. Texture measurement during cold rolling and annealing was proven to be a suitable tool to determine the optimal deformation degree and annealing time for this processing method. As a consequence, texture analysis can be used to predict the final materials properties.

Effect of finite boundary junction mobility on the growth rate of grains in 3D polycrystals

With decreasing grain size, grain boundary junctions become increasingly important for microstructure evolution. We show that the effect of a limited mobility of triple junctions on the growth rate of polycrystals can be implemented in theories of three-dimensional (3D) grain growth. Respective analytical relations are derived on the basis of the average n-hedra approach introduced by Glicksman to describe the volume rate of change of 3D grains in a polycrystalline aggregate under the impact of a limited triple junction mobility. The theoretical predictions were compared to network-model computer simulations, and good agreement was obtained.

Mesoscopic Simulations of Recrystallization and Grain Growth in a Fe-0.374%C-21.64%Mn Alloy

A cellular automaton and a vertex model were used, respectively, for the simulation of recrystallization and grain growth in a Fe-0.374%C-21.64%Mn alloy. The results of the recrystallization simulations revealed that the preferential nucleation during the annealing of the rolled sheet occurs at shear bands, which is corroborated by experimental observations. Subsequently, grain growth simulations were carried out with a 2D vertex model. The model used experimental data as input for its validation in this specific steel. The simulations showed a good agreement with the experimental results.

Effect of Grain Boundary Geometry on Grain Rotation during Curvature-Driven Grain Shrinkage

Molecular dynamics simulations were performed to analyze the curvature-driven shrinkage of individual cylindrical grains with geometrically different boundaries in Al. Grains with <100> tilt and mixed tilt-twist boundaries with the misorientations 5.5°, 16.3°, and 22.6° were simulated. The results revealed that the shrinking grains with tilt boundaries concurrently rotate increasing the misorientation angles, whereas grains with the mixed boundaries did not rotate during their shrinkage. Apparently, the grain boundary geometry/structure has a crucial impact on the observed rotational behavior of the computed grains. The grains with tilt boundaries rotate due to the lack of effectively operating mechanisms for annihilation of edge dislocations, which compose such boundaries. In contrast, for the mixed boundaries composed of edge-screw dislocations the sufficiently fast operating mechanisms of dislocation elimination are available, which facilitates grain shrinkage without rotation.

Grain boundary motion and grain rotation in aluminum bicrystals: recent experiments and simulations

The results of experimental and computational efforts over recent years to study the motion of geometrically different grain boundaries and grain rotation under various driving forces are briefly reviewed. Novel in-situ measuring techniques based on orientation contrast imaging and applied simulation techniques are described. The experimental results obtained on specially grown aluminum bicrystals are presented and discussed. Particularly, the faceting and migration behavior of low angle grain boundaries under the curvature force is addressed. In contrast to the pure tilt boundaries, which remained flat/faceted and immobile during annealing at elevated temperatures, mixed tilt-twist boundaries readily assumed a curved shape and steadily moved under the capillary force. Computational analysis revealed that this behavior is due to the inclinational anisotropy of grain boundary energy, which in turn depends on boundary geometry. The shape evolution and shrinkage kinetics of cylindrical grains with different tilt and mixed boundaries were studied by molecular dynamics simulations. The mobility of low angle boundaries with misorientation angles higher than 10°, obtained by both the experiments and simulations, was found not to differ from that of the high angle boundaries, but decreases essentially with further decrease of misorientation. The shape evolution of the embedded grains in simulations was found to relate directly to results of the energy computations. Further simulation results revealed that the shrinkage of grains with pure tilt boundaries is accompanied by grain rotation. In contrast, grains with the tilt-twist boundaries composed of dislocations with the mixed edge-screw character do not rotate during their shrinkage. Stress driven boundary migration in aluminium bicrystals was observed to be coupled to a tangential translation of the grains. The activation enthalpy of high angle boundary migration was found to vary non-monotonically with misorientation angle, whereas for low angle boundaries the migration activation enthalpy was virtually the same. The motion of the mixed tilt-twist boundaries under stress was observed to be accompanied by both the translation of adjacent grains parallel to the boundary plane and their rotation around the boundary plane normal.

Microstructure and Texture Evolution during Recrystallization of a Fe-Mn-C Alloy

Microstructure and texture evolution of a 60% cold-rolled Fe-22.3Mn-0.3C alloy during annealing at 550°C were studied. Shear bands, triple junctions and grain boundaries were found to be the preferential nucleation sites. The orientations of the nuclei from these sites were found to be mainly random, but also partly located in α-(//ND) and γ-fibers (//ND). After annealing, fine recrystallized grains formed with abundant annealing twins which had a strong impact on the texture development. The final texture was of low intensity and revealed a weak α-fiber.

Recent Advances in the Simulation of Recrystallization and Grain Growth

Modeling and simulation of recrystallization, grain growth, and related phenomena are important tools for the fundamental understanding of microstructural evolution and prediction of engineering properties. In particular for ultra fine grained and nanocrystalline materials proper account of microstructural evolution is essential for the optimal processing of these materials. It is shown that for modeling of softening phenomena it is important to discriminate between discontinuous primary recrystallization and discontinuous grain growth owing to their quite different underlying physics. Recent developments in recrystallization modeling and simulation of grain growth are addressed, in particular nucleation of recrystallization and junction effects in grain growth. Major progress is also expected from atomistic modeling and quantum-mechanical computations for making available specific material properties.