Papers by Author: Kyung Tae Park

Abstract: The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.

Abstract: Ultrafine grained (UFG) 5083 Al and 5154 Al alloys were prepared by equal channel angular pressing (ECAP) with an effective strain of ~ 4 or ~ 8. This investigation was aimed at examining the effect of the ECAP strain and post-rolling inducing different microstructure in these alloys on the deformation mechanisms at low temperature superplastic (LTS) and high strain superplastic (HSRS) regimes. The sample after 4 passes (a strain of ∼ 4) did not exhibit LTS, but superplastic elongations were obtained in the sample after 8 passes (a strain of ∼ 8). An analysis of the mechanical data in light of the standard deformation mechanisms revealed that deformation of the sample after 4 passes was governed by dislocation climb while grain boundary sliding attributed to LTS of the sample after 8 passes. In addition, the 5154 Al alloy processed by ECAP and postrolling was capable of enhancing HSRS elongation significantly. An analysis revealed that the deformation mode was changed from dislocation viscous glide to grain boundary sliding by additional ECAP strain and post-rolling.

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 activation energy for recovery and recrystallization was calculated using DSC data. The annealing below 250°C resulted in the bimodal grain size distribution, while that above 300°C resulted in the uniform distribution of coarse grains. The formation of a bimodal microstructure would be responsible for the good combination of uniform elongation and tensile strength. Additionally, the little variation of hardness for different annealing time at 300°C also indicated that mechanical properties of deformed and annealed 5052 Al alloy were significantly influenced by the volume fraction of recrystallized grains rather than the coarsening of recrystallized grains.

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: Two commercial Al alloys having different second phase particle distributions were subjected to severe plastic deformation (SPD) via equal channel angular pressing with or without subsequent cold rolling, and the effect of such SPD on the particle size distribution of the alloys was investigated. The particles larger than ∼ 3 μm were fragmented into several smaller ones by SPD. Contrarily, those smaller than ∼ 3 μm were hardly broken up by SPD but their distribution became more uniform. Along with these findings and the theoretical models for cavity nucleation at second phase particles, the cavitation behavior of ultrafine grained Al alloys during low temperature or high strain rate superplastic deformation was discussed.

Abstract: The effects of annealing temperature and annealing time on mechanical properties of cold drawn pearlitic steel wires containing 0.84wt% of silicon were investigated. Annealing treatment was performed on cold drawn steel wires for the temperature range of 200°C to 450°C with the different annealing time of 30sec, 1min, 15min and 1hr. The increase of tensile strength at the low annealing temperatures would be related with strain ageing behavior, while the decrease of tensile strength at the high annealing temperature is due to the spheroidization of cementite plates and the occurrence of recovery of the lamellar ferrite in the pearlite.

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: An ultrafine grained structure was obtained in the two grades of a 5083 Al alloy with or without scandium by using equal channel angular pressing and its superplastic behavior was characterized. For the alloy without scandium, low temperature superplasticity was obtained but high strain rate superplasticity was unlikely to occur. By contrast, the alloy containing a small amount of scandium exhibited high strain rate superplasticity. It was found that, in both cases, the strain rate region showing superplasticity was very limited, i.e. one order of magnitude. From the mechanical data, the deformation mechanisms were examined.