Jung Guk Lee

Bio: Jung Guk Lee is an academic researcher from Hanyang University. The author has contributed to research in topics: Nanocrystalline material & Strain hardening exponent. The author has an hindex of 2, co-authored 2 publications receiving 236 citations.

Thermal stability and mechanical properties of ultrafine grained low carbon steel

Abstract: Ultrafine grained low carbon steel manufactured by equal channel angular pressing was annealed at 753 K, where negligible grain growth occurred, up to 72 h and the microstructural change and the mechanical properties were examined. This investigation was aimed at providing the guiding information for the effective use of ultrafine grained low carbon steel manufactured by severe plastic deformation processes. Under the present annealing conditions, the microstructural change was dominated by recovery. The tensile behavior of annealed ultrafine grained steel was characterized by much higher strength and the absence of strain hardening compared with that of large grained steel. In addition, the present ultrafine grained steel became mechanically stable by 24 h annealing treatment although recovery was in progress. The microstructure of the deformed sample of annealed ultrafine grained steel exhibited the elongated grains and dislocations distributed densely in the vicinity of grain boundaries. This finding indicated that dynamic recovery during deformation was associated with the absorption of dislocation by grain boundaries. The mechanical behavior of the present ultrafine grained low carbon steel was discussed in light of the recent development explaining that of nanocrystalline materials, i.e. the dislocation bow-out mechanism for high strength and the spreading kinetics of trapped lattice dislocation into grain boundary for the absence of strain hardening.

Microstructures and Mechanical Properties of Equal Channel Angular Pressed 5083 Al Alloy

Abstract: An ultra-fine grained (UFG) structure was introduced in a commercial 5083 Al alloy with an initial grain size of ∼ 200 \\micron using the equal channel angular pressing (ECAP) technique. ECAP was successfully conducted at 473 K on the same sample up to a total of 8 pressings through the die such that the sample was rotated 180° around its longitudinal axis between pressings. The microstructure was reasonably homogeneous after a single pressing and consisted of parallel bands of elongated substructure having an average width of 0.2 \\micron and an average length of 0.8 \\micron. An equiaxed ultra-fine grained structure of ∼ 0.3 \\micron was obtained in the present alloy after 4 pressings. The ultra-fine grains were thermally stable at 523 K . The yield stress of as-received 5083 Al alloy was 129 MPa, whereas it increased to 249 MPa after a single pressing and finally became 290 MPa after 8 pressings, which was superior to that of a conventional 5083-H321 Al alloy. In addition, in this study, the feasibility of low temperature superplasticity of a UFG 5083 Al alloy was examined. It was found that the 5083 Al alloy with a grain size of ∼ 0.3 \\micron exhibited a superplastic-like behavior with elongation to failure in excess of 200% below 523 K.

Severe Plastic Deformation (SPD) Processes for Metals

Abstract: Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown Moreover, the combined processes developed recently are also explained Finally, the applications of the ultra-fine grained (UFG) metals are discussed

Overview of processing, microstructure and mechanical properties of ultrafine grained bcc steels

Abstract: Ultrafine grained steels with grain sizes below about 1 μm offer the prospect of high strength and high toughness with traditional steel compositions. These materials are currently the subject of extensive research efforts worldwide. Ultrafine grained steels can be produced either by advanced thermomechanical processes or by severe plastic deformation strategies. Both approaches are suited to produce submicron grain structures with attractive mechanical properties. This overview describes the various techniques to fabricate ultrafine grained bcc steels, the corresponding microstructures, and the resulting spectrum of mechanical properties.