Development Of Abnormal Grain Growth Research Articles

Secondary Recrystallization Goss Texture Development in a Binary Fe81Ga19 Sheet Induced by Inherent Grain Boundary Mobility

Secondary recrystallization Goss texture was efficiently achieved in rolled, binary Fe81Ga19 alloy sheets without the traditional dependence on inhibitors and the surface energy effect. The development of abnormal grain growth (AGG) of Goss grains was analyzed by quasi-situ electron backscatter diffraction (EBSD). The special primary recrystallization texture with strong {112}–{111}<110> and weak Goss texture provides the inherent pinning effect for normal grain growth by a large number of low angle grain boundaries (<15°) and very high angle grain boundaries (>45°) according to the calculation of misorientation angle distribution. The evolution of grain orientation and grain boundary characteristic indicates that the higher fraction of high energy grain boundaries (20–45°) around primary Goss grains supplies a relative advantage in grain boundary mobility from 950 °C to 1000 °C. The secondary recrystallization in binary Fe81Ga19 alloy is realized in terms of the controllable grain boundary mobility difference between Goss and matrix grains, coupled with the orientation and misorientation angle distribution of adjacent matrix grains.

Strong NbC particle pinning for promoting abnormal growth of Goss grain in Fe82Ga4.5Al13.5 rolled sheets

A great number of fine and dispersive NbC precipitates were obtained in Fe82Ga4.5Al13.5 sheets by rolling process consisting of hot rolling at 900°C, warm rolling at 500°C and intermediate annealing at 650°C for 15min. The mean size of precipitates in primarily recrystallized sheets is ∼45nm. Grain growth in final sheets was investigated to study the influence of fine precipitates on the promotion of abnormal grain growth (AGG). Fully developed AGG of Goss grain was observed in samples with the heating rate during continuous process of 0.5°C/min, 1°C/min and 2°C/min. In contrast, AGG in samples of 0.25°C/min and 4°C/min were partially developed. The influence of heating rate on the development of AGG was examined by the investigation of precipitates and grain configurations during continuous heating process. With the fine precipitates, sharp Goss orientation was developed in Fe82Ga4.5Al13.5 sheets annealed at Ar atmosphere. A maximum saturation magnetostriction of 170ppm was obtained in sample of 2°C/min.

Development of Abnormal Grain Growth in Cold Rolled and Recrystallized AA 5182 Sheet

Grain growth in the cold rolled and subsequently recrystallized AA 5182 sheets was investigated by means of microstructure observations and texture measurements. Grain growth behavior strongly depends on the annealing temperatures. Grain growth hardly took place at temperatures lower than 470°C, which is attributed to a low mobility of grain boundaries. Abnormal grain growth occurred at temperatures ranging from 480 to 530°C. Annealing above 560°C gave rise to the dissolution of inhibitor precipitates, which led to normal grain growth.

Abnormal Grain Growth: Computer Simulation

The data on nucleation and necessary conditions for abnormal grain growth obtained by computer modelling are presented. Microstructure in the model description of grain growth is characterized by the grain size distribution and the distribution of the grain boundary properties. In a polycrystal with random grain orientation a pinning force promoting a completed abnormal grain growth should decrease in two stages i.e., first a slow one associated with particle coarsening or particle dissolution and then a rapid one caused by particle dissolution. In a polycrystal with weak texture a slightly increased boundary mobility of some large grains can lead to their rapid growth which results in the beginning of abnormal grain growth as well as in the appearance of a strong texture provided the pinning force is sufficient for matrix stabilization. In a polycrystal with a strong single-component texture, abnormal grain growth can evolve at both an intermediate scatter and a high intensity of the main component. An increased mobility of the boundaries with a thin liquid layer cannot alone provide the development of abnormal grain growth in oxide ceramics.

Abnormal grain growth development from uniform grain size distributions

The development of abnormal grain growth from uniform grain size distributions pinned by particles is examined. The main assumption is that a locally lower pinning force adjacent to a large grain can cause this grain to grow abnormally. The case of a matrix pinned by unstable particles is considered in detail. A quantitative criterion is derived which shows the interplay among the variables involved. From this it is shown how both deterministic and probabilistic factors contribute to abnormal grain growth. Practical implications are discussed with particular emphasis on why abnormal grain growth can be so difficult to control. The present treatment offers a new and plausible rationale for the development of abnormal grain growth from uniform grain size distributions containing unstable particles.

On the theory of normal and abnormal grain growth

A growth equation for individual grains in single-phase materials is suggested. It is used to calculate a rate equation for normal grain growth and the size distribution in the material. It predicts a maximum size of twice the average size. The theory is modified to take into account the effect of second-phase particles. In an alternative treatment the array of grains is described in terms of a kind of defects introduced into a perfect array. The defects move through the array during grain growth. The rate of grain growth is calculated from the number of defects and their mobility. The defect concentration is predicted by comparing the two treatments. The defect-model predicts two grain size limits due to second-phase particles. Normal grain growth takes place below the lower limit. Abnormal grain growth can take place between the two limits if the material contains at least one very large grain. No grain growth can take place above the higher limit. Several possible mechanisms for the development of abnormal grain growth are examined. An explanation is offered for the observation that most of the well-known cases occur as the second-phase is dissolving.