Materials Science Forum Vols. 558-559

Abstract: The distribution of grain boundary plane orientations in polycrystalline Ni has been measured before and after grain boundary engineering. The grain boundary engineered microstructure has a relatively higher concentration of Σ3 grain boundaries and, when compared to the initial structure, more of these boundaries have orientations that are inclined by more than 10° from the (111) orientation of the ideal coherent twin. Although the conventionally measured grain size is not affected by the grain boundary engineering process, the average size of the regions containing only Σ3n grain boundaries increases by nearly a factor of two. The observations indicate that the increase in the relative population of Σ3 grain boundaries results both from the preferential elimination of random grain boundaries and the generation of new Σ3 grain boundaries which do not have (111) grain boundary plane orientations.

Abstract: The magnetic properties of nonoriented electrical steels are influenced by grain size and texture of final products. The key technology in the commercial production of nonoriented electrical steels is to grow grains with {hk0}<001> texture up to the optimum size in the final annealing process. The problems related to grain size control have been extensively investigated, while texture control has received much less attention. Therefore, there is enough room to improve the magnetic properties through the control of texture. In this study, systematic investigations on the texture evolution during both recrystallization and grain growth have been made. The formation of recrystallization texture is explained by oriented nucleation. This is supported by the fact that the area fraction of nuclei or recrystallized grains with specific orientation to all new grains remains almost constant during the progress of recrystallization. Most nuclei have a high misorientation angle of 25∼55° with the surrounding deformed matrices. During the progress of grain growth, the Goss texture component continues to decrease because the Goss grains have a high percentage of low angle, low mobility grain boundaries. The grains of Goss orientation have a smaller grain size than those of random orientation.

Abstract: Theories of abnormal grain growth (AGG) in three dimensions usually approximate an abnormal grain by a sphere. The abnormal grain is then represented by its spherical equivalent grain radius. This study, by contrast, treats AGG in terms of concepts that include both the boundary curvature and the number of faces of the abnormal grain. We treat AGG for the case of pinned matrices, including the phenomena of initiation and growth kinetics. The influence of interfacial energy and mobility of the abnormal grain boundary are also discussed.

Abstract: Three recent investigations in the field of thermodynamics of grain boundaries and grain boundary junctions are presented. 1. The grain boundary excess free volume (BFV) along with the surface tension belongs to the major thermodynamic properties of grain boundaries. A special technique, recently developed, makes it possible to measure the BFV for practically any grain boundary and provides a way of estimating the grain boundary excess free volume for grain boundaries of different classes with rather high accuracy. The experimental values of the BFV measured for different grain boundaries are compared and discussed. 2. A new approach will be presented that makes it possible to correctly measure the grain boundary triple line tension. For this the topography at an equilibrated triple junction was measured by atomic force microscopy. Preliminary results of grain boundary triple line energy measurements are presented. 3. The problem is discussed whether it is possible to achieve an equilibrium grain size during grain growth in single phase alloys. Various approaches to the problem are considered. It is shown that the most realistic possibility to stabilize the grain size in a polycrystal is by impurities with negative grain boundary adsorption.

Abstract: In order to produce monodispersed particles, two requirements are considered; they are a complete separation of nucleation and growth steps for the formation of particles and an avoidance of their aggregation during the nucleation and growth. In this study, a new method was presented for preparing narrowly size distributed copper powders from CuO slurry by wet chemical reduction with hydrazine used as a reductant. The rapid nucleation was first induced by a very fast reduction of Pd2+ resulted from batch process. Subsequently copper species which contribute to the growth on the formed Pdo nuclei was slowly generated by semibatch process. A complete separation between nucleation and growth therefore was obtained, resulting in a narrow size distribution of final Cu powder product. The effect parameters on the growth such as Pd2+ concentration and temperature were also investigated.

Abstract: Hot rolling of seamless tubes is performed in two stages: one at high temperature, ie, above 1000oC and a second part at low temperature range, namely below 900oC. Above 1000oC, dynamic, metadynamic and static recrystallization may happen leading to full softening and grain refinement. Below 900oC, however, some pancaking may occur and ferrite is thus refined during transformation. Nonetheless, in order to refine ferrite grains further, cooling to room temperature and re-austenitization at an intermediate reheating furnace can be a viable alternative to be explored. The present paper examines the implications of re-austenitization followed by phase transformation plus low temperature deformation on ferrite grain refinement. Phase transformation on re-heating and cooling as well as recrystallization play important roles in the process of ferrite grain refinement. These mechanisms are analyzed and discussed. Results indicate that ferrite grain refinement is most effective in the case of processing using a re-austenitization cycle in the intermediate furnace followed by rapid cooling after deformation in the stretch reducing mill.

Abstract: The arduous conditions to which hot section turbine components are subjected in service, dictate the superior physical and mechanical properties demanded of them. The demand for both high temperature and creep resistance, and anisotropic property requirements of the components has lead to developments in alloy composition, component geometry and single, oriented grain structure design. The slim tolerances and high quality standards imposed on such design features, combined with the high production volume in industry means that component non-conformances to the customer specifications occur. The input variables contributing to crystallography non-conformance in single crystal production have been investigated with a view to defining optimum process parameters for the successful manufacture of single crystal investment cast components on an industrial scale.

Abstract: The effects of hot band annealing temperature on the texture of the 1% and 2%Si nonoriented electrical steel were investigated. Slab was hot rolled and then hot band annealed in the temperature range of 900°C~1100°C. The magnetic flux density and the core loss were improved by the hot band annealing because of the texture improvement. As the hot band annealing temperature was increased, the magnetic properties were improved. The microstructure of the hot band was composed of a recrystallized structure at the surface and a deformed structure near the middle plane. These hot bands were completely recrystallized after annealing above 1000°C. The main texture of the hot band was rotated cube and gamma-fibre. After hot band annealing, rotated cube changed to cube texture and gamma-fibre intensity gradually decreased. In the case of specimen without hot band annealing, rotated cube in the middle plane was changed to near {111}<112>texture and Goss texture in the surface to gamma fibre after final annealing. In the case of the hot band annealed at 900°C, Goss texture and cube texture were developed. After final annealing, the {111} and {112} texture was dramatically decreased as the hot band annealing temperature was increased. Although the total {100} texture intensity was not changed, Cube texture, {100}<001>, was strengthened and rotated cube texture, {100}<011>, weakened for 2% Si steel. However, 1% Si steel was opposed to 2% Si steel. The {110} texture was strengthened irrespective of hot band annealing temperature. As the hot band annealing temperature was increased, the Goss texture was strengthened, and this makes the anisotropy of the magnetic flux density bigger.

Abstract: A nanosized cobalt-based alloy containing 20 wt % Fe was synthesis by electroless chemical reduction method using alkaline tartarate bath and sodium hypophosphite as a reducing agent . The powder was investigated by optical microscope, SEM and XRD to identify the powder shape, size and the chemical composition. The prepared powder has a spherical shape with a particle size of about 200 nm. The investigated powder was cold compacted at 600 MPa and then sintered in hydrogen atmosphere at 1050 0C. Metallographic, physical, magnetic and electrical properties investigations were carried out for the prepared powder and its sintered compacts. The prepared powder has 2.5% phosphorus content which was liberated by heating the compacts to the sintering temperature in hydrogen atmosphere. From the results of the density measurements we can observe that the prepared sintered FeCo material had a relative density about 96% to the theoretical. But the results of the electrical properties measurements give an indication of the decreasing in the electrical resistivity than the materials produced by the traditional methods. On the other hand the magnetic measurements, of the FeCo powder has a lower specific saturation induction, Bs, than the sintered one which was due to the presence of the paramagnetic metal phosphides in the powder but after rising the temperature to sintering the, Bs, values is increased due to the conversion of the phosphides to the metallic state and the phosphorus was liberated, but the coercive force was decreased by sintering of the powder compacts by lowering the porosity of the materials with sintering and the formation of the soft magnetic materials Fe-Co solid solution which was investigated by XRD having the highest specific saturation induction value. Also the magnetic permeability of the prepared sintered material was increase with increasing the applied field until 50 Oe which has the highest value and decreased with increasing the field more than 50Oe. From the magneto-resistance measurements, it was shown that the sintered material has a positive magneto-resistance in the field direction but a negative one in the direction perpendicular to the current and the field.