Papers by Author: Dong Hyuk Shin

Abstract: Cavitation behavior during superplastic flow of ultra-fine grained (UFG) Ti-6Al-4V alloy was established with the variation of grain size and misorientation. After imposing an effective strainup to 8 via equal-channel angular pressing (ECAP) at 873 K, alpha-phase grains were markedly refined from 11 μm to ≈ 0.3 μm, and misorientation angle was increased. Uniaxial-tension tests were conducted for initial coarse grained (CG) and two UFG alloys (ε = 4 and 8) at temperature of 973 K and strain rate of 10-4 s-1. Quantitative measurements of cavitation evidenced that both the average size and the area fraction of cavities significantly decreased with decreasing grain size and/or increasing misorientation. It was also found that, when compared to CG alloy, cavitation as well as diffused necking was less prevalent in UFG alloys, which was presumably due to the higher value of strain-rate sensitivity. Based on the several theoretical models describing the cavity growth behavior, the cavity growth mechanism in UFG alloys was suggested.

Abstract: High-temperature deformation behavior and microstructural evolution process of ELI Ti-6Al-4V alloy having martensite microstructure were investigated with the variation of strain, strain rate and temperature. A series of hot compression tests was carried out isothermally for martensite microstructure at the true strain range of 0.6 to 1.4, strain rate range of 10-3 s-1 to 1 s-1 and temperature range of 700 oC to 950 oC. The processing maps for martensite microstructures were constructed on the basis of dynamic materials model (DMM). At the strain rate higher than 10-2 s-1 and the temperature lower than 750 oC regions of flow instability such as adiabatic shear band and micro-cracking were observed. Also, after imposing an effective strain of ≈ 1.4, deformed microstructure showed the significant kinking/bending behavior of lamellae resulting in the dynamic globularization associated with the fragmentation of beta-phase. The effects of strain, strain rate and temperature for dynamic globularization were discussed based on the microstructure and efficiency of power dissipation.

Abstract: The dual phase steel, which consists of hard martensite islands embedded in a ductile ferrite matrix, is known to possess high strength, toughness, and superior wear resistance. However, the detailed wear mechanism of the steel has not yet been understood thoroughly. In the present study, dry sliding friction and wear characteristics of an ultra-fine grained ferrite-martensite dual phase steel has been investigated at room temperature. Wear tests of the steel were carried out using a pin-on-disk wear tester against an AISI 52100 bearing steel ball at loads ranging from 1N to 10N. Normalizing heat treatment was also performed on the steel to produce a ferrite-pearlite microstructure, and the wear characteristics of the normalized specimen were compared with that of the dual phase steel. The dual phase steel exhibited lower wear rates than the normalized steel, but the steady-state friction coefficients of the two steels were similar. The wear of the dual phase steel proceeded with a tribochemical reaction on the wearing surface accompanied with subsurface strain hardening, which explained the lower wear rate of the steel.

Abstract: Ultrafine grained materials fabricated by severe plastic deformation exhibit both superior and inferior mechanical properties, as the prominent structural materials, compared to coarse grained counterparts. The superior mechanical properties are ultrahigh strength and exceptional ductility at high temperatures (i.e., superplasticity). The inferior mechanical properties are lack of strain hardenability and room temperature ductility. In this study, the relationship between microstructure and mechanical properties of ultrafine grained materials fabricated by severe plastic deformation is investigated in order to provide insight broadening their future applicability.

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 cross-ARB (C-ARB) process, which adopts cross rolling of the two stacked plates, has been performed up to seven cycles on a commercial purity 1050 aluminum alloy to obtain ultrafine grains with an average grain size of 0.7μm. Microstructural evolution of the C-ARB processed aluminum alloy was examined by a transmission electron microscopy as a function of process cycle number (accumulated plastic strain). Tensile property of the severely deformed Al alloy was also explored. Grain size of grains of the C-ARB processed alloy varied across thickness of the rolled plate. The size of grains at the top and bottom of the rolled plate converged to 0.65μm, while that of grains at the center of the plate increased with the number of ARB cycles. Tensile strength of the CARB processed 1050 Al alloy increased from 100MPa (as-received) to 160MPa. Tensile elongation varied with the number of cycles, but 15% of failure strain was measured from the 6-cycle C-ARB processed specimen. The variation of the elongation with the cycle number coincided exactly with the variation of grain size at the center of the processed plate.

Abstract: Ultrafine grained (UFG) ferrite-martensite dual phase steels were fabricated by equal channel angular pressing and subsequent intercritical annealing. Their room temperature tensile properties were examined and compared to those of coarse grained counterpart. The formation of UFG martensite islands of ~ 1 μm was not confined to the former pearlite colonies but they were uniformly distributed throughout UFG matrix. The strength of UFG dual phase steels was much higher than that of coarse grained counterpart but uniform and total elongation were not degraded. More importantly, unlike most UFG metals showing negligible strain hardening, the present UFG dual phase steels exhibited extensive rapid strain hardening.

Abstract: Superplastic behavior of an ultrafine grained (UFG) 5154 Al alloy processed by ECAP and cold rolling (ECAP+CR sample) was investigated and compared with that of the alloy processed by only ECAP without rolling (ECAP sample) in the strain rate range of 10-4~5×10-1 s-1 at 723 K. Processing of the ECAP+CR sample consisted of ECAP of 4 passes, which was less than that showing the optimum microstructure for high strain rate superplasticity of UFG Al alloys (i.e. 8 passes), with route Bc and subsequent cold rolling (70% thickness reduction). The superplastic elongation was remarkably enhanced by post-rolling. An analysis of the mechanical data revealed that deformation of the ECAP+CR sample was dominated by grain boundary sliding, but dislocation viscous glide was the main deformation mechanism for the ECAP sample. In addition, cavitation in the ECAP+CR sample was insignificant up to ∼300% elongation.