Papers by Author: Dong Nyung Lee

Abstract: Flow behavior of the surface and center layers of solution-treated, peak-aged, or reversion-treated 2090 Al-Li alloy specimens has been reviewed and discussed in terms of microstructures and textures.

Abstract: Asymmetric rolling is a novel technique for giving rise to an intense plastic shear strain through the sheet thickness. The shear strain also develops shear deformation textures close to the {001}<110> and <111>//ND orientations, among which the latter is the most wanted component for the deep drawability, and give rise to the grain refinement. Previously we analyzed various rolling variables influencing the texture development and grain refinement in aluminum sheets obtained by asymmetric rolling with different roll-radius ratios at the same rotation rate and varied reduction per pass. In this study, AA1050 Al alloy sheets were asymmetrically rolled with a two-high mill of which two rolls had the same diameter, but rotated at different rotation rates, with emphasis on effects of combinations of shear directions in several passes. Textures and microstructures of the rolled sheets were investigated by x-ray diffraction and electron backscattered diffraction analyses.

Abstract: Park et al. measured the deposition and annealing textures of nanocrystalline Ni and Ni- 20 % Fe electrodeposits. They found that the deposition texture of major <100> + minor <111> changed to the texture characterized by major <111> + minor <100> after annealing. They also found that the lattice constants of the <100> oriented grains in the as-deposited state were larger than those of <111> oriented grains. In this paper, a model has been advanced to explain the unusual results of lattice constants, and the texture transition has been discussed.

Abstract: The drawing textures of aluminum, copper, gold, silver, and Cu-7.3% Al bronze wires are approximated by major <111>+minor <100>, except silver wire, which can have the <100> texture at extremely high reductions. The <111> component in the drawing textures of aluminum, copper, gold, and silver transform to the <100> component after recrystallization. On the other hand, the <111> deformation texture of the Cu-7.3% Al bronze wire, which has very low stackingfault- energy, remains unchanged after recrystallization. The Brass component {110}<112> in rolling textures of high stacking-fault-energy metals such as aluminum and copper alloys changes to the Goss orientation {110}<001> after recrystallization. However, the Brass orientation in rolling textures of low stacking-fault-energy fcc metals such as brass appears to change to the {236}<385> orientation after recrystallization. These results seem to be related to the stability of dislocations during annealing.

Abstract: Tensile specimens cut from the surface layer to the center layer of a 12.7 mm thick 2090 Al-Li alloy plate were solution treated at 550°C for 30 min and subsequently peak-aged at 190°C for 18 h. They were tensile tested along the rolling direction at 25°C at various strain rates. The solution-treated specimens gave rise to serrated flows at a strain rate of 2×10-4 s-1. On the other hand, for the peak-aged alloy, the surface-layer and subsurface-layer specimens underwent complex, serrated flows (fine and coarse types superimposed each other), whereas the center-layer and near-center-layer specimens were devoid of serrated flows. The textures of the surface-layer and subsurface-layer specimens were approximated by the {001}<110> orientation, while those of the center-layer and near-center-layer specimens were approximated by the {011}<211> orientation. The different flow behaviors were discussed based on the crystallographic textures, microstructures and the strain rates.

Abstract: Vapor-, electro-, and electroless-deposits have usually strong fiber textures. When annealed, the deposits undergo recrystallization or abnormal grain growth to reduce their energy stored during deposition. The driving force for recrystallization is mainly caused by dislocations, whereas that for abnormal grain growth is due to the grain boundary, surface, interface, and strain energies. During recrystallization and abnormal grain growth, the texture change can take place. The recrystallization and abnormal grain growth textures are in general of fiber type. However, copper interconnects are subjected to non-planar stress state due to geometric constraints during room temperature and/or elevated temperature annealing. The annealing textures of the thin films and copper interconnects are discussed in terms of the minimization of the surface, interface, and strain energies, the grain boundary energy and mobility, and the strain-energy-release maximization.

Abstract: The properties of deposited metal films and interconnect structures at submicrometer scale are sensitive functions of microstructural features. Therefore, understanding of the factors which control microstructural evolution is necessary for the development and design of reliable, manufacturable interconnect structures, especially in copper damascene interconnects. The annealing textures of copper interconnects depend on their deposition textures and geometries. The copper interconnects are subjected to stresses even at room temperature, which in turn give rise to strain energies. The stress distributions in interconnects are not homogeneous, which in turn give rise to non-fiber type textures after annealing. The self-annealing textures of interconnects is discussed based on the strain-energy release maximization model, in which grains whose Young’s modulus direction is parallel to the absolute, maximum stress direction grow in preference to others.

Abstract: Asymmetric rolling, in which the circumferential velocities of the upper and lower rolls are different, can give rise to intense plastic shear strains and in turn shear deformation textures through the sheet thickness. The ideal shear deformation texture of fcc metals can be approximated by the <111> // ND and {001}<110> orientations, among which the former improves the deep drawability. The ideal shear deformation texture for bcc metals can be approximated by the Goss {110}<001> and {112}<111> orientations, among which the former improves the magnetic permeability along the <100> directions and is the prime orientation in grain oriented silicon steels. The intense shear strains can result in the grain refinement and hence improve mechanical properties. Steel sheets, especially ferritic stainless steel sheets, and aluminum alloy sheets may exhibit an undesirable surface roughening known as ridging or roping, when elongated along RD and TD, respectively. The ridging or roping is caused by differently oriented colonies, which are resulted from the <100> oriented columnar structure in ingots or billets, especially for ferritic stainless steels, that is not easily destroyed by the conventional rolling. The breakdown of columnar structure and the grain refinement can be achieved by asymmetric rolling, resulting in a decrease in the ridging problem.