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Fe-0.1C-(V, Nb) 제어압연강의 미세조직 및 기계적 특성
조경목 ( Kyung Mox Cho ),김수영 ( Su Young Kim ),박인애 ( In Ae Park ),이영중 ( Young Jung Lee ),강남현 ( Nam Hyun Kang ) 대한금속재료학회 ( 구 대한금속학회 ) 2007 대한금속·재료학회지 Vol.45 No.5
Following the trend of industrial high strength and toughness structural steel manufacturing, it is evolving and increasing needs of basic research and development efforts. One of the examples is, due to the high cost of vanadium, the replacement of Fe-V steel to Fe-Nb steel. However, there still exist difficulties for commercial production of Fe-Nb steel because of poor impact toughness of this steel comparing with Fe-V steel. In this study, a fundamental investigation was carried out to analyze the microstructure, tensile properties and impact toughness of Fe-Nb steel as a function of the rolling conditions with no post heat treatment. The steel samples were prepared with three different compositions, i.e., Fe-V steel (Fe-0.05V-0.001Nb), Fe-V-Nb steel (Fe-0.014V-0.03Nb), Fe-Nb steel(Fe-0.003V-0.033Nb). The steels were deformed with controlled rolling conditions, namely start rolling temperature(SRT) 1150℃ for the all experiment, but two different finish rolling temperature(FRT); 950℃ and 860℃. Ferrite grain size decreased and thus impact toughness was measured higher at FRT of 860℃ than at FRT 950℃ treated steels. Even though the impact toughness of Fe-Nb steel was quite low due to inhomogeneous distribution of ferrite grain size, Fe-V-Nb steel exhibited impact toughness improved with controlled rolling at FRT 860℃. Application of the controlled rolling process showed one of the possibilities to replace Fe-V steel with Fe-V-Nb and/or Fe-Nb steel.
변형구배 결정소성 유한요소해석법을 이용한 니켈기 다결정 합금의 Hall-Petch 관계 모델링
최윤석,조경목,남대근,최일동,Choi, Yoon Suk,Cho, Kyung-Mox,Nam, Dae-Geun,Choi, Il-Dong 한국재료학회 2015 한국재료학회지 Vol.25 No.2
A strain-gradient crystal plasticity constitutive model was developed in order to predict the Hall-Petch behavior of a Ni-base polycrystalline superalloy. The constitutive model involves statistically stored dislocation and geometrically necessary dislocation densities, which were incorporated into the Bailey-Hirsch type flow stress equation with six strength interaction coefficients. A strain-gradient term (called slip-system lattice incompatibility) developed by Acharya was used to calculate the geometrically necessary dislocation density. The description of Kocks-Argon-Ashby type thermally activated strain rate was also used to represent the shear rate of an individual slip system. The constitutive model was implemented in a user material subroutine for crystal plasticity finite element method simulations. The grain size dependence of the flow stress (viz., the Hall-Petch behavior) was predicted for a Ni-base polycrystalline superalloy NIMONIC PE16. Simulation results showed that the present constitutive model fairly reasonably predicts 0.2%-offset yield stresses in a limited range of the grain size.
단결정 압축 변형 거동의 변형구배 결정소성 유한요소해석
정재호,조경목,최윤석,Jung, Jae-Ho,Cho, Kyung-Mox,Choi, Yoon Suk 한국재료학회 2017 한국재료학회지 Vol.27 No.12
A strain-gradient crystal plasticity finite element method(SGCP-FEM) was utilized to simulate the compressive deformation behaviors of single-slip, (111)[$10{\bar{1}}$], oriented FCC single-crystal micro-pillars with two different slip-plane inclination angles, $36.3^{\circ}$ and $48.7^{\circ}$, and the simulation results were compared with those from conventional crystal plasticity finite element method(CP-FEM) simulations. For the low slip-plane inclination angle, a macroscopic diagonal shear band formed along the primary slip direction in both the CP- and SGCP-FEM simulations. However, this shear deformation was limited in the SGCP-FEM, mainly due to the increased slip resistance caused by local strain gradients, which also resulted in strain hardening in the simulated flow curves. The development of a secondly active slip system was altered in the SGCP-FEM, compared to the CP-FEM, for the low slip-plane inclination angle. The shear deformation controlled by the SGCP-FEM reduced the overall crystal rotation of the micro-pillar and limited the evolution of the primary slip system, even at 10 % compression.
용탕단조한 AC4A Al / Al2O3+SiCp 하이브리드 금속복합재료의 미세조직과 기계적 성질
김민수,조경목,박익민 ( Min Soo Kim,Kyung Mox Cho,Ik Min Park ) 한국주조공학회 1994 한국주조공학회지 Vol.14 No.3
N/A AC4A Al/Al₂O₃+SiC_P hybrid composites were fabricated by the squeeze infiltration technique. Effect of applied pressure, volume fraction of reinforcement(AL₂O₃and SiC) and SiC particle size(4.5㎛, 6.5㎛ and 9.3㎛) on the solidification microstructure of the hybrid composites were examined. Mechanical properties were estimated preliminarly by fractographic observation, hardness measurement and wear test. Results show that the microstructure of the hybrid composites were quite satisfactory, namely revealing relatively uniform distribution of reinforcements and refined matrix. Some aggregation of SiC particle caused by particle pushing was observed especially in the hybrid composites containg in fine particle(4.5㎛). Refined matrix was attributed to applied pressure and increased nucleation sites with addition of reinforcements. Fractured facet also revealed finer for the hybrid composites possibly due to refined matrix. Hardness and wear resistance increased with volume fraction of reinforcements. For hybrid composites with 9.3㎛ SiC, hardness was somewhat lower and wear resistance higher than other composites.
Sn-Ag-Cu-X 무연솔더로 솔더링 된 접합부의 진동파괴 거동
진상훈,강남현,조경목,이창우,홍원식,Jin, Sang-Hun,Kang, Nam-Hyun,Cho, Kyung-Mox,Lee, Chang-Woo,Hong, Won-Sik 대한용접접합학회 2012 대한용접·접합학회지 Vol.30 No.2
Environmental and health concerns over the lead have led to investigation of the alternative Pb-free solders to replace commonly used Pb-Sn solders in microelectronic packaging application. The leading candidates for lead-free solder alloys are presently the near eutectic Sn-Ag-Cu alloys. Therefore, extensive studies on reliability related with the composition have been reported. However, the insufficient drop property of the near eutectic Sn-Ag-Cu alloys has demanded solder compositions of low Ag content. In addition, the solder interconnections in automobile applications like a smart box require significantly improved vibration resistance. Therefore, this study investigated the effect of alloying elements (Ag, Bi, In) on the vibration fatigue strength. The vibration fatigue was conducted in 10~1000Hz frequency and 20Grms. The interface of the as-soldered cross section close to the Cu pad indicated the intermetallic compound ($Cu_6Sn_5$) regardless of solder composition. The type and thickness of IMC was not significantly changed after the vibration test. It indicates that no thermal activities occurred significantly during vibration. Furthermore, as a function of alloying composition, the vibration crack path was investigated with a focus on the IMCs. Vibration crack was initiated from the fillet surface of the heel for QFP parts and from the plating layer of chip parts. Regardless of the solder composition, the crack during a vibration test was propagated as same as that during a thermal fatigue test.