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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.
김지식,장영원,이종수,김낙준 국립경상대학교 공과대학 부설 첨단소재연구소 1995 尖端素材 Vol.5 No.-
Superplastic forming (SPF) has become an attractive manufacturing technique in the aerospace industry due to its advantages of weight and cost savings for producing the complicated airframe structures. Combined with the diffusion bonding (DB), the design flexibility to produce complex multisheet structures can be further enhanced. In this paper, a brief review has been made as to the application of superplasticity to the fabrication of aircraft components, the various forming methods, the analytical and numerical simulation methods for predicting and optimizing both the design and production of superplastically formed parts. Finally, current status and future directions for SPF/DB technology in Korea have also been descibed.
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.
조성 변화에 따른 2상 (α2+γ) TiAl 합금의 미세조직 및 기계적 성질의 변화
김낙준,장영원,표성규 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.7
The purpose of this study is to investigate the effect of composition on the microstructure and mechanical properties of TiAl alloys. The composition ranges studied were Ti-xAl(x=43.7, 45.6, 47)-yMn(y=0.3). The mechanical properties have been characterized and correlated with microstructure, tetragonality, and deformation behavior. Two phase (α₂+γ) alloys have single γ grains and lamellar grains, which consist of twin-related L1。γ Phase and α₂phase. Addition of Mn promotes the formation of twin-related structure and the refinement of lameller structure and grain size. Increase in Ti/Al ratio promotes the refinement of grain size, but increases lamellar spacing. The modification of microstructure directly influences the compression properties and deformation mode of TiAl alloys. It has been found that Mn addition and increase of Ti/Al ratio enhance the plasticity of two phase (α₂+γ) alloys. The ductilization effect of Mn is partly due to its ability to promote the formation of deformation twins and dislocation generation at the twin intersections.