Materials Science Forum Vols. 558-559

Abstract: Grain growth may occur in two forms, normal grain growth, characterized by a constant grain size distribution during growth, and abnormal grain growth, where one or more abnormally large grains may form in the microstructure. The presence of abnormally large grains in an otherwise uniform microstructure may be detrimental to the mechanical properties of a polycrystalline structure. Little is understood of the exact cause of abnormal grain growth. The annealing conditions leading to the onset of abnormal grain growth have been investigated via a series of grain growth experiments carried out on an Al-4wt%Cu alloy. The structure of which consisted of equiaxed grains (<8μ) pinned by a fine dispersion of sub-micron second phase particles, which may dissolve upon annealing. Minority texture components may experience accelerated growth due to a higher energy and mobility compared to the surrounding grain structure. The combination of these two events may result in the abnormal growth of some grains. SEM imaging and EBSD data has then made it possible to characterize the influence of particle dissolution and grain boundary misorientation on the onset of abnormal grain growth. The stability of ‘island grains’ found to exist internally in abnormally large grains has also been investigated in relation to the misorientation relationship and localized second phase volume fraction found there. There was only weak evidence of special misorientation relationships between the island grains and the abnormally large grains in which they exist, and although there was evidence of an enhanced fraction of pinning particles at island grain boundaries, this was also true of boundaries in general. The larger size of island grains is their dominant characteristic, and grains which become island grains may have been incipient abnormal grains.

Abstract: The idea that a single subgrain is sufficient to produce a single recrystallised grain is the simplest explanation for the recrystallisation process. Likewise, a single Goss oriented grain arising from the primary recrystallisation process is the simplest unit which can give rise to a secondary Goss oriented grain. More complicated cluster models, for example subgrain coalescence is also considered feasible for primary recrystallisation, clusters of Goss oriented grains might be another mechanism for forming Goss oriented secondary grains. This paper examines the cluster theory using material which is produced by the ARMCO process which requires two stages of rolling. In order to achieve this aim it is necessary to destroy the connectivity between individual Goss oriented grains by using thin foils derived from sheet which gives a strong Goss texture on conventional annealing. The foils were sectioned from the subsurface which had a strong η fibre after primary recrystallisation, and ranged in thickness from 18μm (the average grain size after primary recrystallisation) up to 80μm, which is the approximate thickness of the η textured layer. The central layer, which had the classical {111} primary recrystallised texture, was similarly processed, but this did not produce secondary recrystallisation. The experiment followed the secondary recrystallisation process in the same area using sequential annealing in a vacuum furnace by a combination of EBSD and Channelling contrast microscopy. The data does not support the high energy boundary hypothesis nor the CSL explanation. But it is clear that connectivity is important, because when this is destroyed by the thin foil two dimensional morphology, as it is in the thinnest foil, secondary recrystallisation does not occur.

Abstract: A supersaturated Al-0.3 wt.% Sc alloy was cold deformed by ECAP to an equivalent von Mises strain of 9.2 then pre-aged at 350 °C to generate a fine-grained alloy with an average grain size of 1 μm. The microstructure was highly resistant to grain coarsening at temperatures up to 500 °C with a detailed statistical analysis showing that the initial grain size distribution was very close to lognormal and, throughout annealing, remained lognormal and the normalized frequency distribution was time/temperature invariant despite a moderate broadening of the size distribution. This behaviour is largely similar to subgrain coarsening during recovery and grain growth after recrystallization. The homogeneous evolution of the microstructure during annealing, coupled with no appreciable change in texture, is also consistent with the advanced stages of continuous recrystallization.

Abstract: The microstructural evolution during thermal annealing of a cryogenic rolled 5052 Al alloy was investigated. The activation energy for annealing behavior was calculated using DSC data. For the heating rate of 16°C/min, the precipitation occurred at the annealing temperature of 150~230°C due to Mg self diffusion, recovery occurred at the annealing temperature of 230~260°C, and recrystallization proceeded at a higher temperature up to about 370°C. Both recovery and recrystallization gave rise to non-uniform, bimodal grain-size distributions, which may result from heterogeneous nanostructures. In addition, the activation energy for the precipitation was found as ~115kJ/mol, indicating the process was diffusion-controlled (Mg in Al), and the activation energy for recovery was found to be ~140kJ/mol, representing self-diffusion in pure Al.

Abstract: Ultrafine dispersed copper powders were prepared from copper (I) oxide (Cu2O) slurry by a wet chemical reduction with hydrazine as a reductant. Palladium chloride (PdCl2) was used as a nucleator along with its polymer stabilizer, polyvinylpyrrolidone (PVP). Sodium pyrophosphate (Na4O7P2) was added as a surfactant. It was found that the use of Cu2O and Na4O7P2 exhibit very strong impact on dispersity of the prepared Cu powders. In this study, we indicate the critical conditions at which the particles aggregation still occurs even though the powders produced at high zeta potential resulted from an appropriate addition of Na4O7P2.

Abstract: Recrystallization texture in grain oriented silicon steel at different annealing temperature is investigated. Normalized x-ray intensities of various orientation components observed in Orientation Distribution Function are used for comparison. The computed CSL boundary distributions about Goss component with annealing condition were calculated. The misorientation angle distribution is also measured in order to find the importance of high-energy boundary with misorientation 20~45° range for the secondary recrystallization of Goss grain. From the analysis of CSL boundary distribution and misorienation angle distribution, the distribution of CSL boundaries does not evidently show any preferred difference between Goss and other texture components. Whereas, the misorientation angle analysis shows that the number of 20°~45° misoriented boundaries is higher around the Goss grains than around other texture components.

Abstract: Grain structures in polycrystalline materials are typically three dimensional (3D) structures, but by far the most characterizations of grain structures are done by microscopy and are thus limited to 2D. In the present work 3D grain structures in a well-annealed cylindrical aluminium (AA1050) sample is characterized and analyzed. The characterization is done by 2 methods i) by non-destructive 3-dimensional x-ray diffraction (3DXRD) ii) by serial sectioning and subsequent EBSP mapping of entire circular 2D sample sections; 50 sections are mapped In total 333 grains are reconstructed. It is found that the 3D grain morphologies can be quite complex in particular for the larger grains, the number of neighbours varies significantly and values above 20 are not unusual. When the results from the 2 methods are compared, it is found that the crystallographic agreement is very good and within experimental uncertainties. Slightly more significant differences are found when the reconstructed grain morphologies are compared. Reasons for this are discussed.

Abstract: Dense 0.95(Na0.5K0.5)NbO3-0.05LiTaO3 (NKN-5LT) ceramics, as a candidate for leadfree piezoelectric materials, were developed by conventional sintering process. The effect of additions with A-site ions in perovskite structure on abnormal grain growth and piezoelectric properties in NKN-5LT ceramics was investigated. Sintering temperature was lowered by adding A-site ions as a sintering aid. Abnormal grain growth in NKN-5LT ceramics was observed with varying additions. This grain growth behavior was explained in terms of interface reactioncontrolled nucleation and growth. The electrical properties of NKN-5LT ceramics were investigated as a function of concentration of additions. The electromechanical coupling factor and piezoelectric constant of NKN-5LT ceramics were improved in the samples with A-site ions excess NKN-5LT. These results show that the piezoelectric properties of NKN-5LT samples can be improved by control of the microstructures.

Abstract: In order to save natural resources and to reduce production costs, many industries have conducted studies on new developed steels and manufacturing processes. For instance, the use of high-strength bolts can decrease the number, size and weight of bolts used in a car, thereby decreasing fuel consumption. For this reason, steel makers are developing boron-added steel for high-strength bolts, which eliminates production processes. However, it has been pointed out that this boron-added steel is sensitive to austenite grain coarsening. The austenite grain coarsening does not occur uniformly, hence it can affect mechanical properties such as tensile, fatigue and so on. Therefore, the aim of this study is to investigate the abnormal behavior of grain growth and to determine the optimum amount of alloying elements in boron-added steel for use in making high-strength bolts.