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나노인덴테이션과 유한요소법을 결합한 철 분말의 기계적 물성 취득
정혁재,이동준,윤은유,이언식,김낙준,김형섭,Jeong, Hyeok Jae,Lee, Dong Jun,Yoon, Eun Yoo,Lee, Eon Sik,Kim, Nack Joon,Kim, Hyeong Seop 한국분말야금학회 2013 한국분말재료학회지 (KPMI) Vol.20 No.4
Stress-strain curves are fundamental properties to study characteristics of materials. Flow stress curves of the powder materials are obtained by indirect testing methods, such as tensile test with the bulk materials and powder compaction test, because it is hard to measure the stress-strain curves of the powder materials using conventional uniaxial tensile test due to the limitation of the size and shape of the specimen. Instrumented nanoindentation can measure mechanical properties of very small region from several nanometers to several micrometers, so nanoindentation technique is suitable to obtain the stress-strain curve of the powder materials. In this study, a novel technique to obtain the stress-strain curves using the combination of instrumented nanoindentation and finite element method was introduced and the flow stress curves of Fe powder were measured. Then obtained stress-strain curves were verified by the comparison of the experimental results and the FEA results for powder compaction test.
Ca, P 의 첨가가 용탕단조 Mg-Al-Zn-Si 합금의 미세조직에 미치는 영향
박성진,김낙준,김도향,김재중,신철수 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.12
Effects of alloying elements such as Ca and P on the microstructure of squeeze cast Mg-Al-Zn-Si alloys have been investigated in the present study. Addition of Si in Mg-Al-Zn alloys results in the formation of Mg₂Si in two types of morphologies; polygonal type and chinese script type. With increasing Al content the formation of polygonal type Mg₂Si is pronounced. Addition of Ca results in the homogeneous distribution of fine polygonal Mg₂Si particles. Ca is segregated mainly around the secondary phase such as Mg_(17)Al_(12), P provides the nucleation sites for Mg₂Si, results in the homogeneous distribution of the fine polygonal Mg₂Si particles. Accordingly, Ca and P containing Mg-Al-Zn-Si alloys have the much finer microstructure than those with no Ca and P since the fine polygonal Mg₂Si particles restrict the grain growth of Mg alloys during squeeze casting and subsequent heat treatment.
Fe-(12-17) %Cr-(0.19-0.32) %Cr 스테인레스 강으로 육성된 융접층의 미세조직과 기계적 특성에 미치는 C 및 Cr 함량의 영향
안상호,이성학,김낙준,추성훈,백응율,정재영 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.10
Fe-Cr-C hardfacing stainless steels were deposited on S45C carbon steel using a submerged arc welding process. The effects of C and Cr contents on the as-welded structures of Fe-(12-17)%Cr-(0.19-0.32)%C hardfacing stainless steels have been studied by microstructural observation and mechanical testing. The variation of as-welded structure as a function of C and Cr contents was compared with the microstructure predicted by the Schaeffler diagram. In Fe-12%Cr-(0.19-0.32)%C stainless steels, carbon solute atoms were primarily segregated in the interdendritic regions. And Cr-rich carbides tends to precipitate along the interdendritic regions in the stainless steels having C contents higher than 0.2%C. This explains why that the abrupt decreases of mechanical properties and the cleavage fracture surfaces appear in the stainless steels having C contents higher than 0.20%. The sliding wear mechanism varied from the adhesion to tribochemical reaction with increasing hardness of weld metal. However, the wear resistance tends to be affected by the introduction of abrasion. wear mode due to the hard carbide particles. This can be confirmed by the fact that its wear resistance increases with the increase of matrix hardness of weld metal.
원소분말압출법으로 제조된 TiAl-계 합금의 크리프 성질
남수우,황선근,김낙준,조한서 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.10
Creep properties of the lamellar structured Ti-46.6Al-1.4Mn-2Mo(at%) alloy, which is made by hot extrusion of a blended elemental powder mixture, are investigated in air environment over the temperature range from 1023K to 1173K at constant stress levels ranging from 100MPa to 250MPa. The average activation energy for creep within the temperature range from 1048K∼1173K at stress level 150MPa and 200MPa is found to tie 384kJ/㏖. The value of the stress exponent n in the range of 3.8∼4.3 was obtained within the stress range from 150MPa to 250MPa and 1.3 between 100MPa and 150MPa at 1073K. Microstructural studies conducted on creep fractured specimen showed the secondary cracks along the lamellar grain boundaries and the secondary cracks are assumed to be formed by pore nucleation, growth and coalescence during the tertiary stage.
급냉응고/분말야금법으로 제조된 Al-Fe-V-Si 합금의 기계적 성질 (2) : 파괴기구에 관한 연구 Micromechanism of Fracture
이종철,이두영,이성학,김낙준 대한금속재료학회(대한금속학회) 1990 대한금속·재료학회지 Vol.28 No.4
This study is concerned with a correlation between the microstructure and the micromechanism of fracture processes in an RSP/PM Al-8.5Fe-1.3V-1.7Si alloy. The specimens were exposed to the temperatures ranging from 150℃ to 480℃ for 100 hours, followed by room temperature tensile tests, fracture toughness tests and void initiation tests. This RSP/PM Al alloy did not show any significant change in room temperature tensile strength, tensile elongation and fracture toughness even after exposure up to 425℃ for 100 hours. However, tensile elongation and fracture toughness decreased significantly after exposed to 480℃ for 100 hours. Detailed microstructructural analyses showed that the alloy ahs a band structure due to non-uniform distribution of the silicide dispersoids. After exposure to 480℃, the formation of θ-Al₃Fe phases occurs at band structure boundaries by the dissolution of coarse silicide dispersoids, resulting in a loss of ductility and fracture toughness. The micromechanical processes involved in void and microcrack formation were identified and quantified. The results are interpreted using a simplified ductile fracture initiation model based on the basic assumption that crack extention starts to occur at a certain critical strain over a microsturcturally significant critical distance. This model enables us to correlate fracture toughness and microstructure, confirming that the formation of θ-Al₃Fe phases is the main metallurgical factor which contributes to the embrittlement phenomenon after exposure to 480℃ for 100 hours.