Papers by Author: Dong Nyung Lee

Abstract: A study has been made of the evolution of the microstructures and textures in three kinds of low-carbon steel sheets (MAFE, BH and IF) having well developed <111>//ND texture that were rolled by low reductions and annealed at 780 °C in Ar atmosphere. The steel sheets developed different microstructures and textures, even though their initial textures and thermomechanical treatments were similar. MAFE steel showed an unusual behavior that grains with high Taylor factors survived and grew very rapidly. This unusual behavior and the differences in microstructure and texture have been discussed.

Abstract: The solid phase transformation of a metastable phase into a stable phase needs the activation energy. The energy is usually supplied in the form of thermal energy. When the nucleation takes place, the strain energy may develop in the metastable matrix and the stable nucleus. The strain energy can result from differences in density of the nucleus and matrix and the lattice mismatch between the nucleus and matrix. The stable-metastable interface region has the highest strain-energy density in the maximum Youngs modulus direction of the stable phase. Accordingly, the growth rate of the stable phase is the highest in its highest Youngs modulus directions. As the transformation temperature decreases, the strain energy contribution increases and the growth rate anisotropy is likely to increase. When austenite transforms into ferrite at low temperatures, the directed growth of ferrite is observed as forms of Widmanstätten ferrite plates and acicular ferrite plates. The maximum growth direction of ferrite is along the maximum Youngs modulus direction of ferrite, <111>_α, and the broad interfaces are parallel to the maximum growth direction and formed so that they minimizes the shear strain energy in the interface layer. The directed growth results in the Kurdjumov-Sachs orientation relationship between austenite and ferrite, <111>_α//<110>_γ and {110} _α //{111}_γ.

Abstract: A novel method has been discovered for controlling the crystallographic orientation of pure iron using the γ to α phase transformation. When pure iron with clean metal surfaces undergoes the γ to α phase transformation, it develops a strong cube-on-face texture ({100}<0vw>) with the grain size being larger than the sheet thickness. The mechanism controlling the <100> orientation obtained is associated with the fact that the {100} faces are elastically compliant so that the <100> texture can develop in a manner consistent with minimization of strain energy. However, in commercial steels, although so many texture analyses have been conducted, the cube-on-face texture has been rarely observed. According to thermodynamic analysis, surface oxidation in commercial steels appears to be responsible for the deterioration of the <100> texture. This phenomenon can be explained in terms of the modification of the inherent elastic anisotropy of metal surface by the surface oxidation.

Abstract: A two-step rolling-annealing process has been developed to increase the <111>//ND (γ fiber) component in the recrystallization texture of a copper-bearing bake hardening steel. The two step process comprises the first rolling by a low reduction in thickness and subsequent annealing at 780°C, followed by the second rolling by a high reduction and subsequent annealing at 780°C. The first rolling process aims at seeding the γ fiber oriented grains, so that they can grow at the expense of differently oriented grains developed in the second rolling process. In this way the density of γ fiber component in the recrystallization texture of the bake hardening steel much increases compared with that in the conventional one-step rolling-annealing process.

Abstract: There are four prominent orientation relationships (ORs) between directionally grown precipitates and their parent phases in steel. They are ORs between ferrite precipitate and parent austenite (the Kurdjumov and Sachs OR), between orthorhombic cementite precipitate and parent austenite (the Pitsch OR), between cementite precipitate and parent ferrite (the Bagaryatski OR) and between hexagonal molybdenum carbide precipitate and parent ferrite (the Dyson et al. OR). The directed precipitation occurs at low transformation temperatures. The ORs have been explained by the directed growth model. The solid phase transformation of a metastable phase into a stable phase needs the activation energy. The energy is usually supplied in the form of thermal energy. When the nucleation takes place, the strain energy may develop in the stable nucleus and the metastable matrix. The strain energy can result from a difference in density between the nucleus and matrix and the lattice mismatch along the nucleus:matrix interface. The fundamental concept of the model is that the maximum growth rate of precipitate is along the direction that generates the maximum strain energy and the interface energy is minimized. The four ORs are determined, based on the concept, such that the mismatch along the interface between the minimum shear modulus planes of precipitate and its parent phase that are parallel to the maximum Young’s modulus direction of the precipitate is minimized.

Abstract: The recrystallization (Rex) texture of cross-rolled 3.3% Si steel was similar to the deformation texture approximated by {100}. The deformation texture of cross-rolled 99.99% copper was approximated by major {110} plus minor {001}. Its Rex texture was approximated by major {110} plus minor {001}. The results have been discussed based on the strain-energy-release-maximization model.

Abstract: The Goss {110}<001> orientation, which is not stable with respect to plane strain rolling, rotates toward the {111}<112> orientation forming a strong maximum. The {111}<112> rolling component returns to the Goss orientation after recrystallization (Rex). On the other hand, the {111}<112> Rex texture developed in 65% rolled iron electrodeposit with a weak {111}<112> texture, and the {111}<110> Rex texture developed in 80% cold rolled electrodeposit having a strong {111}<112> texture. That is, the {110}<001>, {111}<112>, and {111}<110> Rex textures developed in bcc steels having the {111}<112> rolling textures. The results have been discussed by the strain-energy-release-maximization model for Rex texture.

Abstract: The oriented-nucleation and oriented-growth for recrystallization (Rex) textures of electrodeposits, vapor deposits, and plastically deformed metallic materials have been discussed based on the strain-energy-release-maximization (SERM) model. When the Rex orientation predicted by the SERM model from major components of the deformation texture is the same as a minor component in the deformation texture that is calculated to be thermally stable by the SERM model, the calculated Rex orientation is sure to become the main component of the Rex texture. This implies that the oriented-nucleation and the oriented-growth affect the evolution of the Rex texture. For polycrystalline materials, the Rex orientation predicted by the SERM model is likely to be measured because heterogeneous nucleation can occur in grain boundaries even when shear bands are absent. In other words, the grain boundaries are unlikely to control the Rex texture and the oriented growth dominates the Rex texture.

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 <100> + <111> recrystallization textures change to the <111> texture after abnormal grain growth. The Brass component {110}<112> in rolling textures of high stacking-fault-energy metals such as aluminum, copper, Cu- 16% Mn, and Cu-1% P 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 and silver appears to change to an orientation approximated by the {236}<385> orientation after annealing. The texture changes are discussed based on the strain-energy-release-maximization model for medium to high stacking-fault-energy metals and on grain growth for low stacking-fault energy metals.

Abstract: Amorphous Si films are generally deposited on glass by physical or chemical vapor deposition. When annealed, they undergo crystallization through nucleation and grain growth. At low annealing temperatures, crystallization starts near the glass substrates for pure Si films and near metals for metal-induced crystallization. In this case, crystallites grow along the <111> directions of c-Si nearly parallel to the film plane, that is, the directed crystallization. The directed crystallization is likely to develop the <110> or <111> orientation, which means the <110> or <111> directions are along the film thickness direction. As the annealing temperature increases, equiaxed crystallization tends to increase, which in turn increases random orientation. When the annealing temperature is further increased, the <111> orientation may be obtained.