Papers by Author: Seng Ho Yu

Abstract: To study the effect of grain size on texture and tensile properties of equal channel angular pressed commercially pure Ti, specimens were made to have the grain size ranging from 4 m to 60 m with the severe plastic deformation followed by recrystallization annealing. In this initial condition all the specimens exhibited a strong bimodal split basal texture. During subsequent repressing at 350°C, the texture pattern became randomized through crystal rotations, the phenomenon being more pronounced in coarse-grained specimens. The microstructure of deformed specimens, as examined by electron back scattered diffraction, showed formation of mechanical twins and microstructural inhomogeneity in the coarse-grained specimens. The room temperature tensile properties of the re-pressed specimens showed that the yield strength was remarkably enhanced regardless of the grain size whereas the elongation was reduced as compared to the initial condition, particularly in coarse-grained specimen. It was concluded that microstructural refinement during the severe plastic deformation was the main cause of the improved yield strength while the twinning and microstructural inhomogeneity were responsible for the texture randomization and the impaired ductility.

Abstract: Zr702 and commercial purity Ti were grain refined through severe plastic deformation and their microstructure and texture characteristics were studied by optical microscopy, transmission electron microscopy, X-ray diffraction and electron back-scattered diffraction. The equal channel angular pressing was conducted at 350°C using a 90°/20° die. Up to 8 passes of pressing were performed via three different routes, A, C and BC, which resulted in reduction of the grain size down to 0.2-0.5.m. The two materials showed similar evolution characteristics of microstructure and texture. While the specimens pressed via route A showed lamellar grain shapes those via route C or BC exhibited equiaxed grains. A split basal texture was developed in the microstructure obtained by the route A pressing, which was similar to the cold rolling texture, whereas an asymmetric texture was produced in the specimens pressed via route BC. In the case of route C pressing, the texture characteristics were closer to those of the route BC case.

Abstract: A detailed X-ray study of Zr rods, subjected to ECAP at 350oC by routes C and BC, was conducted by the new X-ray method of Generalized Pole Figures, combining texture measurement with registration of X-ray line profiles. The data analysis is based on conceptions of the texture formation theory, connecting features of grain reorientation with activated deformation mechanisms. A degree of reproduction of the same distinctive texture by successive ECAP passes with antecedent rotation of the rod reflects attendant structure changes in material.

Abstract: The commercial purity zirconium (Zr702) has been deformed by Equal Channel Angular Press (ECAP) up to eight passes using route A and BC. The deformation microstructure and the bulk texture have been characterized by Electron Backscattered Diffraction (EBSD) in SEM and X-rays diffraction (XRD), respectively. The homogeneously lamellar deformation structure in route A and the equiaxed deformation structure with local heterogeneity in route BC have both been determined. In both cases the subgrain size flattened at about 0.33 ìm after 2 passes but average misorientation increased with increasing strain. After 8 passes high angle fraction is over 70% both in route A and route BC. The bimodal {0002} pole texture (high strain rolling texture) in route A and the strong unimodal {0002} pole texture with weak high strain rolling texture in route BC have been identified. The boundary texture has been analysed both in sample frame and crystallographic frame. It was found in sample frame that the boundary texture of high angle boundaries developed gradually in route A and more strongly than that in route BC, implying that the boundary characters evolved from the mixed characters (tilted and twist) to tilted boundary in route A. Based on the deformation structures and the bulk textures, the deformation mechanism of large local orientation rotation and the stability of the dislocation structures have been proposed to interpret the grain refinement and the texture development of Zr702 during ECAE processing.