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급냉응고된 Al-Fe-V-Si 합금의 미세조직 형성 및 상변태과정
김낙준,박우진,백응건 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.2
Evolution of microstructure in a rapidly solidified Al-Fe-V-Si alloy has been investigated by TEM. The as-cast, microstructure varies depending on casting conditions and also through the thickness of melt-spun ribbon. Microstructure of the completely zone A ribbon consistis of the microcellular precipitation of the mocroquasicrystalline phase, with the globular particles of clustered mcroquasicrystalline phase near the air side of the ribbon. For the zone A and zone B mixed ribbon, on the other hand, intercellular particle is composed of silicide phase, but the formation of globular particles of microquasicrystalline phase(somewhat decomposed) is again observed near the air side of the ribbon. During heating, as observed by annealing studies and by in-situ hot-stage TEM studies, intercellular microquasicrystalline particles are quite nustable and decompose at a very fast rate, resulting in a coarse distribution of silicide particles. This decomposition of the intercellular microquasicrystalline particles assists the decomposition of glebular partioles of clustered microquasicrystalline phase in completely zone A ribbon, resulting in a very coarse distribution of silicide particles after annealing. For the zone A and zone B mixed ribbon, decomposition of globular particles of clustered microquasicrystalline phase results in the precipitation of silicide particles along the cell boundaries.
열처리에 의한 급냉응고 Ng-Al-Zn-Nd 합금의 미세조직 변화
김낙준,김도향,김상혁 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.4
Microstructural changes in rapidly solidified Mg-Al-Zn-Nd during heat treatment at 100℃, 250℃, 350℃ for 1hr and 24hrs have been investigated by using TEM, EDS, DSC, and X-ray diffraction analysis and hardness measurements. As-extruded Mg-Al-Zn-Nd consists of 1∼2㎛ size fine grains with the homogeneous distribution of 10∼50㎚ size Al₂Nd phase particles and 0.1∼0.3㎛ size quasicrystal-related particles. No microstructural changes occur during heat treatment at 100℃ and 250℃ for 1hr and 24hrs. After heat treatment at 350℃, the Al₂Nd phase particles are still stable, but the quasicrystal-related phase particles are replaced with Al_(11)Nd₃ phase particles. The amount of hardness decrease after heat treatment at 350℃ for 24hrs corresponds to only 10% of the hardness of as-extruded material, which suggests high thermal stability of rapidly solidified Mg-Al-Zn-Nd due to the relatively fine grain size and, particularly, the presence of thermally stable 50㎚ size Al₂Nd phase particles.
Ti3al-Nb 합금의 피로균열전파에 미치는 α2 상의 영향
김낙준,정현호 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.1
Fatigue crack propagation behavior of a two-phase (α₂+β) Ti₃Al-Nb alloy has been investigated. Various heat treatments were used to vary the volume fraction and the morphology of α₂ phase. Results show that the volume fraction and morphology of α₂ phase have large influence on the fatigue crack propagation behavior. Fine colony structure shows no variation in the fatigue crack propagation rate as α₂ volume fraction changes. On the other hand, equiaxed structure and basket-weave structure shows the increased crack growth resistance as α₂ volume fraction decreases. Such increased crack growth resistance is attributed primarily to a tortuous morphology of crack path which rsuslts in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanism. Quantitative metallography and experimental crack closure mesurements are presented to substantiate such interpretations.
정밀주조된 A356 Al 합금의 미세조직과 기계적 성질과 미치는 HIPping의 영향
김낙준,이민하,김재중,김경훈,이성학,이의휘 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.2
The effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys were investigated in this study. Tensile and high-cycle fatigue tests were conducted for cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the cast A356 Al alloys, but there were many large pores formed as cast defects. The high-cycle fatigue results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HiPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue substantially, fatigue cracks propagated along eutectic Si particles formed at solidification cell boundaries, thereby showing improved fatigue strength by 40∼50% over the non-HIPped specimens.