Papers by Author: Yong Suk Kim

Abstract: Sliding wear mechanisms of the solution treated 18Cr-18Mn-2Mo-0.9N high nitrogen steel (HNS) were studied. Room-temperature dry sliding wear tests of the steel were carried out at various applied loads to explore the mechanism as a function of the load. The wear rate of the steel increased with the increase of the load; however, the increase rate was not constant. The rate increased slowly at low loads, rapidly at intermediate loads, and finally, the increase-rate became low again at high loads. Worn surfaces, their cross sections, and wear debris were examined, and phases of the heat-treated HNS as well as the wear debris were identified to find out the mechanism. The wear of the steel was found to be controlled by the tribo-oxidation, strain-induced phase transformation, and reverse transformation due to temperature rise on a wearing surface. The influence of each mechanism on the rate varied depending on the magnitude of the applied load.

Abstract: Tensile deformation behavior of the high-nitrogen austenitic Fe-18Cr-14Mn-4Ni-3MoxN steel with various nitrogen contents has been studied. The nitrogen content of the steel varied from 0.28 to 0.88 wt. %. Nitrogen atoms in high nitrogen steel (HNS) make an interstitial solid solution by being scattered in the steel constituting a short-range order. They strengthen the austenite matrix without deteriorating ductility of the steel. The present investigation was carried out to elucidate the hardening and plasticizing role of the nitrogen in the HNS by analyzing tensile deformation behavior of the steel containing various nitrogen contents. Tensile tests of the steel specimens were performed at room temperature with a constant strain rate of 5x10-5/sec. Microstructure of the tested specimens was analyzed to explore the deformation mechanism of the HNS as a function of nitrogen contents. The flow stress of the steel increased with the increase of the nitrogen content; however, the specimen with the highest nitrogen content (0.88 wt. %) showed saturated strength and reduced ductility. The superior mechanical property of the HNS was explained by the low stacking fault energy and the twin-induced plasticity provoked by the nitrogen.

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: Room-temperature dry sliding wear behavior of hot-pressure sintered monolithic Co, Co- 20 wt.% CuSn and Co-20 wt.% WC composites were investigated. Wear tests of the materials were carried out using a pin-on-disk wear tester at various loads of 10N-100N under a constant sliding speed condition of 0.38m/s against glass (83% SiO2) beads. Sliding distances were varied with a range of 100m-600m. A scanning electron microscopy was used to examine worn surfaces, cross sections, and wear debris. X-ray diffraction (XRD) was utilized to identify phases of the specimen and wear debris. All specimens exhibited low friction coefficients ranging from 0.12 to 0.4. The sintered Co exhibited distinctive wear that was characterized by shallow dug canals on worn surface, a very thin detaching surface layer, and fine debris. Thermal transformation of the Co specimen from ε (hcp) phase to α (fcc) phase occurred during the wear of the Co, which was inferred from XRD analysis of the wear debris. The transformation was suggested to cause the thin detaching surface layer and the fine wear debris of the sintered Co. The wear of the Co-CuSn composite proceeded by shear deformation of the CuSn particles, while WC particles of the Co-WC composite sustained most of the applied load, which resulted in the low wear rate with fine wear debris of the Co-WC composite.

Abstract: Effect of phase transformation and grain-size variation
of hot-pressed cobalt on its dry sliding wear was investigated. The sintered cobalt specimens were heat treated under different conditions and their tribological characteristics were examined. The sliding wear test was carried out against glass (83% SiO2) beads at 100N load using a pin-on-disk wear tester. A constant sliding speed of 0.38m/s and sliding distance of 600m were adapted. Worn surfaces, cross sections, and wear debris were examined by a scanning electron microscopy (SEM). X-ray diffraction (XRD) was utilized to identify phases of the specimen and wear debris. The cobalt specimens exhibited low friction coefficients of around 0.2. Thermal transformation of the cobalt from the hcp ε phase to the γ (fcc) phase during the wear was detected, which was deduced as a wear mechanism of the sintered cobalt. Typical wear characteristics of the cobalt including a thin detaching surface layer and fine wear debris were explained by the transformation. A correlationship between the grain size and the transformation was also explored.

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.

Abstract: Coarse grains of commercial 5052 Al and 5083 Al alloys were refined by the accumulative roll bonding (ARB) process. Average grain size of the refined microstructure was 200 nm. The 5083 Al alloy that has higher Mg content required more deformation for the refinement. Dry sliding wear behavior of the ultra-fine grained (UFG) Al alloys was investigated using a pin-on-disk wear tester at room temperature. The UFG microstructure of the processed alloys hardly increased the wear resistance of the Al alloys in spite of the increased strength and hardness. Wear rate of the UFG Al alloys was higher than that of the non processed coarse-grained starting alloys. The SEM observation of worn surfaces revealed that surface deformation controlled the wear. The low wear resistance of the UFG Al alloys was attributed to non-equilibrium and unstable grain boundaries and low strain hardening capability of the alloys.