Papers by Author: Seong Hee Lee

Abstract: Microstructural evolution of a copper alloy processed by accumulative roll-bonding (ARB) was investigated by EBSD analysis. The grains became thinner and elongated to the rolling direction with increasing the number of ARB cycles. The subdivision of the grains to the rolling direction actively begins to occur after 5 cycles of the ARB, resulting in formation of ultrafine grains with small aspect ratio. After 8 cycles, the ultrafine grained structure with the average grain diameter of 250nm developed in almost whole regions of the sample. In addition, the fraction of high-angle grain boundaries increased with the number of ARB cycles and reached about 0.7 after 8 cycles. The texture development of the ARB processed samples was different depending on the number of ARB cycles and the positions in the thickness.

Abstract: The 3-layers accumulative roll bonding process (ARB) has been attempted to increase the strength of copper alloy (Cu-0.02wt.%P) by refining grain size. The 3-layers accumulative roll bonding was conducted up to 7 cycles at room temperature without lubrication. Microstructural evolution of the copper alloy with the number of the 3-layers ARB cycles was investigated by optical microscopy (OM), transmission electron microscopy (TEM), and electron back scatter diffraction (EBSD). The average grain size has been refined from 20 μm before ARB to 170 nm after 7 cycles of 3-layers ARB. More than 70% of ultrafine grains formed by 3-layers ARB were composed of high angle grain boundaries. The average misorientation angle of ultrafine grains was 30.7 degrees in the center of the specimen. Tensile strength after 7 cycles of 3-layers ARB was 605 MPa, which is about 3.2 times higher than the initial value.

Abstract: The effects of strain rate in rolling on microstructures and mechanical properties of a nano-grained high purity copper processed by accumulative roll bonding (ARB) were studied. The rolling during ARB was conducted with two kinds of strain rates (2.6sec-1 and 37sec-1). The microstructural evolution of the copper with ARB proceeding was somewhat different in both methods. However, the variation of mechanical properties with ARB was very similar to each other.

Abstract: The difference in annealing characteristics of oxygen free copper (OFC) and deoxidized low-phosphorous copper (DLP) processed by ARB was studied. The copper alloys processed by eight cycles of the ARB were annealed for 10 minutes at various temperatures ranging from 100 to 400°C. The variation of microstructure and mechanical properties with annealing was significantly different in both copper alloys. In case of OFC, the ultrafine grained (UFG) structure formed by the ARB still remained up to 200°C, and above 200°C it was completely replaced with a coarse grained structure due to an occurrence of the conventional recrystallization. However, in case of DLP, the recrystallization did not occur even at 350°C. The strength of the OFC also decreased significantly at annealing temperatures above 200°C, while the hardness of the DLP did not decrease so largely up to 350°C. These differences in annealing characteristics in both copper alloys were discussed in terms of purity.

Abstract: The accumulative roll bonding (ARB) process is one of the methods to refine the grain size of metallic materials. The ARB process up to 8 cycles was performed for the pure Cu and Cu- Fe-P (PMC-90) alloy at ambient temperature under no lubricant conditions. In the pure Cu, the nano-sized grains were formed after third cycle with an average grain size of 200nm. Once the 200 nm grains formed, further reduction in the grain size was not observed up to the 8 ARB process cycles. On the other hand, the formation of the stable nano-sized grains in PMC-90 alloy was retarded compared to the pure Cu due to the alloying elements. For both alloys, the tensile strength values increased drastically in the initial stage of ARB process. The tensile strength values of both alloys tended to saturate after the third ARB process cycle. The tensile elongation value greatly decreased by 1 cycle of ARB process due to the strain hardening. After the third cycle of ARB process, each alloy showed a gradual increase in tensile elongation due to the dynamic recovery. For PMC-90 alloy, the strength value is higher than that of OFC due to addition of the alloying elements.

Abstract: The effect of working temperature on microstructure and mechanical properties of ultrafine grained monolithic Al and Al-5vol.%SiCp composite processed by accumulative roll bonding (ARB) was studied. The ARB was performed up to eight cycles (an equivalent strain of ~6.4) without lubricant. The working temperature was varied from ambient temperature to 200
C. The samples processed at temperatures below 100
C exhibited an ultrafine grained structure over almost all regions. However, the samples processed at 200
C showed an inhomogeneous structure in which a few coarse grains due to an occurrence of conventional recrystallization is partially seen. The tensile strength of both the monolithic Al and the composite decreased with increasing the ARB working temperature. The variation of microstructure and mechanical properties of the composite with the working temperature was compared to that of the monolithic aluminum.

Abstract: The bimodal-structured materials composed of Al-5wt%Mg alloy and its composites reinforced with SiC and Al2O3 particles were prepared by ball-milling and subsequent compaction under the pressure of 350MPa and sintering at temperatures ranging from 973K to 1173K for 1h, 3h and 5h. The macro-interface between Al-Mg and Al-Mg/ Al2O3 composite was macroscopically well-bonded compared to that between Al-Mg and Al-Mg/SiC composite under same sintering conditions. The bonding of macro-interface became better as the sintering time and temperature increased, resulting in that the bimodal-structured materials sintered at 1173K for 5h showed the relative density of nearly 100%. In addition, the higher sintering temperature was, the more irregular macro-interface was. The microhardness of macro-interface area was in between the Al- Mg and composites, which was independent of the sintering temperature.

Abstract: Nanocomposite formation of metal-metal oxide systems by mechanical alloying (MA) has been investigated at room temperature. The systems we chose are the Fe3O4-M (M=Al, Ti), where pure metals are used as a reducing agent. It is found that nanocomposite powders in which Al2O3 and TiO2 are dispersed in a α-Fe matrix with nano-sized grains are obtained by MA of Fe3O4 with Al and Ti for 25 and 75 hours, respectively. It is suggested that the shorter MA time for the nanocomposite formation in Fe3O4-Al is due to a large negative heat associated with the chemical reduction of magnetite by aluminum. X-ray diffraction results show that the average grain size of α-Fe in Fe-TiO2 nanocomposite powders is in the range of 30 nm. From magnetic measurement, we can also obtain indirect information about the details of the solid-state reduction process during MA.

Abstract: Nano-structured aluminum was fabricated by accumulative roll-bonding (ARB) process using different rolling methods. One is the ARB using conventional rolling (CR) in which the speed of two rolls (3.0m/min) was equal to each other. The other is the ARB using differential speed rolling (DSR) in which the speed of two rolls is different to each other. The roll peripheral speed of one roll was 2.0m/min and that of another roll was 3.6m/min. The roll speed ratio was kept at 1.8. The ARB was conducted up to 6 cycles at ambient temperature without lubrication. In both cases, the ultrafine grains were developed in the samples. The grains formed by the DSR-ARB were more equiaxed and finer than those produced by the CR-ARB. Tensile strength of the DSR-ARB processed sample was superior to that of the CR-ARB processed one. The elongation was not affected significantly by the number of ARB cycles in both cases. Texture analysis demonstrated that the shear strain, in the case of DSR-ARB, was introduced into the center of thickness. It was concluded that the DSR-ARB process was more effective for grain refinement and strengthening than the CR-ARB process.

Abstract: Anodic aluminum oxide (AAO) was prepared in three types of aqueous solutions with various applied voltage. The mechanical property of AAO prepared in different electrolyte was investigated and hardness was increased on account of the increase of the thickness between pores. The mechanical property and microstructure change of AAO prepared in oxalic acid at 40V was investigated by heat treatment. AAO prepared in oxalic acid at 40V was transformed from amorphous to crystalline phase by heat treatment above 800oC and hardness was increased about 2.6 times with increase of heat treatment temperature.