Dong W. Kum

Bio: Dong W. Kum is an academic researcher from Korea Institute of Science and Technology. The author has contributed to research in topics: Flow stress & Superplasticity. The author has an hindex of 1, co-authored 1 publications receiving 15 citations.

New aspects on the superplasticity of fine-grained 7475 aluminum alloys

Abstract: Thermomechanical processes were developed which give fine grain sizes of 6 and 8 μm in the 7475 Al alloy. Superplastic properties of this material were evaluated in the temperature range of 400 °C to 545 °C over the strain-rate range of 2.8 x 10-4 to 2.8 X 10-2 s-1. The maximum ductility exhibited by the alloy was approximately 2000 pct, and optimum superplasticity was achieved at a strain rate of 2.8 X 10-3 s-1 which is higher by an order of magnitude than other 7475 Al alloys. This result is attributed to the presence of fine dispersoids which maintain the fine grain size at high homologous temperatures. The flow stress and strain-rate sensitivity strongly depend on the grain size. The superplastic 7475 Al alloy has strain-rate sensitivities of 0.67 (6 μm) and 0.5 (13 μm) and an activation energy which is similar to the one for grain boundary diffusion of aluminum. Microstructural investigation after superplastic tests revealed zones free of dispersoid particles at grain boundaries primarily normal to the tensile direction. These dispersoidfree zones (DFZs) appear even after 100 pct elongation and are occasionally as large as 5 μm across. This result demonstrates the importance of diffusional flow in superplastic deformation of the fine-grained 7475 Al alloy especially at low elongations.

Denuded zones, diffusional creep, and grain boundary sliding

Abstract: The appearance of denuded zones following low stress creep in particle-containing crystalline materials is both a microstructural prediction and observation often cited as irrefutable evidence for the Nabarro-Herring (N-H) mechanism of diffusional creep. The denuded zones are predicted to be at grain boundaries that are orthogonal to the direction of the applied stress. Furthermore, their dimensions should account for the accumulated plastic flow.

Comparison between superplastic deformation mechanisms at primary and steady stages of the fine grain AA7475 aluminium alloy

Abstract: Evolution of the deformation behaviour, surface and bulk structures at superplastic flow of the АА7475 aluminium-based alloy were studied by scanning and transmission electron microscopes and a focused ion beam technique. Differences between deformation behaviour at a primary stage with strains below 0.69 and a steady stage with strains above 0.69 were discussed. The research showed the grain growth and grain elongation to the stress direction at a primary stage of deformation. Stabilisation of both mean grain size and grain aspect ratio was found at a steady stage of deformation. Grain neighbour switching, grain rotation, dispersoid free zones and some insignificant intragranular strain were observed at both stages. Appearing and disappearing of the grains on the sample surface, with increased dislocation activity occurred at the steady stage of deformation. The current results highlighted the importance of diffusion creep as a dominant mechanism at the beginning of superplastic deformation and as an accommodation mechanism of the grain boundary sliding at the steady stage of deformation. The dislocation creep as an additional accommodation mechanism of the grain boundary sliding at the steady stage of superplastic deformation is suggested.

Variations in microstructure and texture during high temperature deformation of Al–Mg alloy

Abstract: The effect of strain rate on the superplastic deformation of a fine-grained aluminium–magnesium alloy has been investigated. Microstructural transformations were quantified by crystallographic texture, the study of the size of the grains, their morphology and their spatial distribution. At low strain rate a progressive reduction of the initial texture is observed, resulting from the predominance of grain boundary sliding. But even for large strains the texture randomisation is not complete, which confirms the role played by dislocation creep in superplasticity of the alloy. At high strain rate textural components of plasticity develop and the grains become elongated along the tensile direction. Nevertheless, transmission electron microscopy observations and a calculation of the expected variation with strain of the shape of the grains, suggest that continuous recrystallisation is likely to take place during deformation.

Refutation of the relationship between denuded zones and diffusional creep

Abstract: The first direct microstructural observation used as evidence for diffusional creep is from the work of Squires, Weiner, and Phillips (S-W-P). They observed denuded zones at grain boundaries after creep in a Mg-0.5 wt.% Zr alloy that contained inert precipitates. It is the purpose of this paper to show that the reported creep data for the Mg-0.5 wt.%Zr alloy was, in fact, obtained in a region where dislocation creep, rather than diffusional creep, was the dominant deformation mechanism. Details of the denuded zone formation will also be presented which question the relation of denuded zones to diffusional creep. Therefore, it is contended that the microstructurally-observed denuded zones cannot be a result of diffusional creep. This contention supports previously-published views by the present authors that conclusive proof for diffusional creep in polycrystalline materials remains elusive.