Papers by Keyword: Grain Growth

Abstract: The grain growth behavior and mechanical properties in the friction stir weld zone after post weld heat treatment (PWHT) have been investigated. As PWHT temperature increased, a normal grain growth of as-welded equaxied grains ceased and abnormally grown grains with elongated shape coarsened. Huge elongated grains changed into smaller equaxied grains at 500°C. In case of lower heat input condition, abnormal grain growth initiated faster due to smaller initial grain size. The weld zone with bigger initial grains had advantages to maintain the thermal stability at high temperature. The hardness near the weld zone was almost recovered to the 95% of the unaffected base metal at 500 °C and the weld zone under lower heat input condition resulted in the homogeneous recovery through the whole weld zone.

Abstract: Monte Carlo (MC) and finite difference (FD) hybrid method is applied to numerically model the growth of austenite grains caused by welding thermal cycle in the heat affected zone (HAZ) of steel welds. The grain growth behaviors and pinning effects by dispersed particles are modeled by MC method while steep changes in temperatures with time and over space are by FD method. The connection of the two methods is made possible by correlating MC step to the real time through the grain growth theory. The model results are in a good agreement with experiments as well as previous experimental results published, and it is indicated that the developed model can be used to obtain a better understanding of the austenite grain structure development in HAZ and even to improve the grain refinement technology using dispersed particles.

Abstract: The evolution of the microstructure, macrotexture, microtexture and mesotexture has been studied during the annealing at 760°C after temper rolling (9% thickness reduction) of a non-oriented electrical steel sheet containing 2 wt. % Si. Results showed that the coarse grained microstructure, obtained on annealing, is produced through a recrystallization mechanism that advances from the surface to the interior of the sheet. However, starting of this process is delayed due to the presence of Si. The majority of experiments carried out in this work have been repeated for a low-carbon steel (C = 0.0385; Mn = 0.18%) containing only 0.03% Si and the results obtained were practically identical to those observed in the steel containing 2% Si. The main difference observed between both steels was that the process of formation of the exaggeratedly large grains was slower in the steel containing 2% Si.

Abstract: In Alloy 718 ingot cogging process, dynamic and metadynamic recrystallizations, and static grain growth occur, and also the presence of δ phase plays a key role in controlling the grain size. In this study, the evolution of grain structure in VIM/VAR-processed Alloy 718 ingots during post-cogging heat treatments is dealt with. Compression tests were made on VIM/VAR-processed Alloy 718 ingot at temperatures between 900oC ~ 1150oC. Heat treatments were made on the compression-tested specimens, and the variation of grain size was evaluated. Constitutive equations for the grain growth are established to represent the evolution of microstructures. Special attention is paid to the evolution of grain structure under the condition of dynamic and metadynamic recrystallizations, and grain growth. The grain growth rate depends mainly on the presence of δ-phase below the δ-solvus temperature, and on the difference in the grain boundary characteristics above it.

Abstract: Ultra-low carbon steel (ferritic steel), commercial purity aluminum and high purity copper were heavily deformed by the accumulative roll bonding (ARB) process, and the microstructural evolution during the ARB was analyzed. Significant grain refinement by grain subdivision mechanism was confirmed in all three kinds of materials. On the other hand, microstructure refinement slowed down with increasing strain and the grain size stayed in nearly a constant value in the ultrahigh strain region. The mechanism of the grain size saturation was discussed.

Abstract: Alongside volume fraction, VV, and area per unit of volume, SV, the integral interface curvature per unit of volume, MV, or the average interface curvature, H, are important microstructural descriptors. For grain growth, the grain boundary curvature is of special importance because, in addition to its geometrical significance, it is also the driving force for boundary migration. Notwithstanding its importance, curvature has been seldom measured and utilized in the analysis of polycrystals. Geometrical models were derived for the average curvature of individual grains, of grain boundaries and of grain edges, as a function of the mean intercept length. These models show good agreement with curvature measurements in an Al-1mass%Mn alloy. Furthermore, this work shows how grain boundary curvature measurement can be applied to normal grain growth as well as to the effect of particles on grain boundary pinning.

Abstract: The kinetics and topology of grain growth in three dimensions were simulated using a phase-field model with anisotropic grain-boundary mobilities. In order to perform large scale calculations we applied both modifications of algorithms and parallel coding techniques to the Fan and Chen's phase-field algorithm. Kinetics of abnormal grain growth is presented. It is observed that the grains of a minor component which are at the beginning surrounded preferentially by boundaries of high mobility grow faster than the grains of a major component until the texture reverses completely. Additionally, topological results of grain structures, such as grain size distributions and grain face distributions, are discussed

Abstract: One common point amongst extant theories of abnormal grain growth (AGG) is that they treat this phenomenon in terms of the relative grain size, or grain radius, of the abnormal grains. Topological and metrical quantities of abnormal grains, such as the number of their faces, or their grain boundary curvature, are taken into account only indirectly through the grain size itself. This paper, by contrast, treats AGG in terms of concepts, that include both the boundary curvature and the number of faces of the abnormal grain. Two cases are examined: 1) AGG, in which the matrix grains are fully pinned, so normal grain growth cannot occur; 2) AGG in which the matrix grains are free to evolve, so that normal grain growth ensues simultaneously in the matrix.

Abstract: The interparticle mass transport causes the larger particles to grow at the expense of the smaller particles in the process of sintering. Coarsening during sintering results from surface motion, while grain growth results from grain boundary motion. The three-dimensional simulation was performed to study coarsening and grain growth during sintering by using the Surface Evolver program. The coarsening and grain growth were affected by the ratio of grain boundary energy to surface energy, the ratio of grain boundary mobility to surface mobility, the size of a particle, and its coordination number.

Abstract: High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials. In this study, it is applied for the investigation of an intermetallic γ-TiAl based alloy. Not only the diffraction angles, but also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to approach lattice parameters and grain sizes as well as crystallographic relationships. An in-situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl which exhibits high internal stresses. With increasing temperature the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. The data obtained by high-energy synchrotron X-ray diffraction, extremely rich in information, are interpreted step by step.