Abnormal Coarsening Research Articles

The influence of singular surfaces and morphological changes on coarsening

Abstract When the interface energy between coarsening crystals and an intervening phase is anisotropic, mechanisms that do not affect isotropic systems become important. If there are singular surfaces, then growth and dissolution must occur by the lateral motion of steps, formed at a defect center, or by two-dimensional nucleation. Here, it is shown that two-dimensional nucleation is not plausible under typical experimental conditions and that persistent step-creating defects are required for a singular surface to advance or retract during coarsening. The simultaneous presence of crystals with and without defects leads to two populations that grow at very different rates, and this provides an explanation for abnormal coarsening. The influence of extrinsic morphological changes is also considered. It is assumed that when relatively high-energy, non-equilibrium shapes in the starting materials evolve during coarsening to shapes increasingly bound by lower-energy surfaces, the mean surface energy is reduced....

Evolution mechanisms of texture and microstructure during recrystallization of ferritic steel sheets

The formation of recrystallization microstructure and texture in ferritic steel sheets used in the automotive industry is presented and involved mechanisms are discussed in connection with the substructure formed during the deformation preceding the annealing. Nucleation principally happens in the gamma-fibre grains, which develop smaller deformation cells and higher local misorientations than the alpha-fibre grains. The development of the nuclei is then analyzed by analogy with the abnormal coarsening which sometimes occurs after recrystallization. Finally, some relations between recrystallization textures / microstructures and mechanical properties are shown, like the effect of the rolling level on the anisotropy or the decrease of mechanical properties when under-recrystallisation happens.

Coarsening of Faceted Crystals

The influence of the nucleation energy barrier on the capillary‐driven coarsening of faceted crystals that exchange material by diffusion is quantified. Our calculations are based on the assumption that the transport of material between particles must happen in series with the nucleation of partial layers on flat facets. Using a numerical model based on this idea, we simulate the time evolution of distributions of crystals that are made up of perfect faceted crystals (without step‐producing defects), crystals containing step‐producing defects, and mixtures of the two types. We find that the coarsening of a distribution containing only perfect faceted crystals is arrested at a size where the nucleation energy barrier becomes prohibitive. This critical size ranges from a few nanometers to several hundred nanometers, depending on material parameters and experimental conditions. When a small fraction of the crystals have step‐producing defects (for these crystals the nucleation energy barrier vanishes), they can grow to large sizes at the expense of the perfect crystals and a bimodal grain size distribution is created. Based on these results, we hypothesize that when abnormal coarsening is observed in nature, it results from the presence of a small number of crystals with step‐producing defects.

Effect of ferrite formation on abnormal austenite grain coarsening in low-alloy steels during the hot rolling process

Abnormal coarsening of austenite (γ) grains occurred in low-alloy steels during a seamless pipe hotrolling process. Often, the grains became several hundred micrometers in diameter. This made it difficult to apply direct quenching to produce high-performance pipes. The phenomenon of grain coarsening was successfully reproduced using a thermomechanical simulator, and the factors which affected grain coarsening were clarified. The mechanism was found to be basically strain-induced grain rowth which occurred during reheating at around 930 °C. Furthermore, once a pipe temperature decreased to the dual-phase region after the minimal hot working and prior to the reheating process, the grain coarsening was more pronounced. It was understood that the formation of ferrite along grain boundaries had the role of reducing the migration of grain boundaries into neighboring grains, leaving a strain-free, recrystallized region behind. This abnormal grain coarsening was found to be effectively prevented by an addition of Nb, the content of which varied depending on the C content. The effect of the Nb addition was confirmed by an in-line test.

Abnormal growth of faceted (WC) grains in a (Co) liquid matrix

If the grains dispersed in a liquid matrix are spherical, their surface atomic structure is expected to be rough (diffuse), and their coarsening has been observed to be controlled by diffusion in the matrix. They do not, furthermore, undergo abnormal growth. On the other hand, in some compound material systems, the grains in liquid matrices are faceted and often show abnormal coarsening behavior. Their faceted surface planes are expected to be singular (atomically flat) and therefore grow by a defect-assisted process and two-dimensional (2-D) nucleation. Contrary to the usual coarsening the-ories, their growth velocity is not linearly dependent on the driving force arising from the grain size difference. If the growth of the faceted grains occurs by 2-D nucleation, the rate is expected to increase abruptly at a critical supersaturation, as has been observed in crystal growth in melts and solutions. It is proposed that this growth mechanism leads to the abnormal grain coarsening. The 2-D nucleation theory predicts that there is a threshold initial grain size for the abnormal grain growth (AGG), and the propensity for AGG will increase with the heat-treatment temperature. The AGG behavior will also vary with the defects in the grains. These predictions are qualitatively confirmed in the sintered WC-Co alloy prepared from fine (0.85-Μm) and coarse (5.48-Μm) WC powders and their mixtures. The observed dependence of the AGG behavior on the sintering temperature and the milling of the WC powder is also qualitatively consistent with the predicted behavior.

Development of Microstructure in Mechanically Alloyed NiAl

AbstractNiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmosphere. The mechanically alloyed (MA) powders were consolidated by vacuum hot pressing (VHP), resulting in a material which was fully dense. Analytical transmission electron microscopy has been used to understand the development of microstructure with processing. The dispersoid particles present are mostly A12O3 with small amount of A1N. It has been shown that morphology of the dispersoid particles are not uniform and the larger dispersoid particles are located mostly along the grain boundaries with the dispersoid-free zones near them. Coarsening behavior of the dispersoid particles is different depending on their locations, i.e., within the grain or along the grain boundaries. The accelerated coarsening of dispersoids along the grain boundaries occurs concurrently with that of the grains and the formation of dispersoid-free zones (DFZs) are observed along the grain boundaries. It is suggested that this abnormal dispersoid coarsening and the formation of DFZ are due to the coupled migration of dispersoids and grain boundaries.

Abnormal growth and abnormal coarsening with a quasi-stationary distribution

The behaviour of a large grain or particle in a matrix with a quasi-stationary distribution is examined. Initially very large grains or particles quickly decrease their size relative to the critical radius of the distribution. A comparison between abnormal grain growth and abnormal coarsening is carried out and it is shown that a large particle decreases its size relative to the critical radius of the distribution faster than a large grain.