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나영상,김인수,손성만,박노광 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.9
Thermodynamic phase calculation and its experimental confirmation were made on a series of wrought Ni base superalloys. The Gibbs free energy of the constituent phases such as γ, γ', σ, etc can be expressed by applying the Sublattice model. Minimum Gibbs free energy condition was selected as the criteria for determining the equilibrium state. The calculated phase fraction and the transition temperature were experimentally validated by comparing the calculated value with the measured one. Isothermal aging treatment followed by electrochemical extraction and quantitative X-ray diffraction was employed for experimental validation of the calculated phase fraction, and the differential thermal analysis for the phase transition temperature. The calculated values for both phase fraction and transition temperature are well matched with the measured values. The calculated phase diagrams offer lots of useful information such as solution treatment temperature, TCP(Topologically Close Packed) phase formation, γ' phase fraction and driving force for precipitation, etc.
고온 압축변형된 니켈기 초내열 718 합금 결정립 조직의 EBSD 분석
나영상,이만형,염종택,박노광 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.3
In order to characterize the dynamic change of microstructures in commercially-available superalloy 718, a series of compression tests were conducted by varying test temperature and strain rate. Orientation relationship and the misorientations between recrystallized grains as well as unrecrystallized grains were thoroughly investigated with EBSD technique. The dynamically-recrystallized grain size was similar regardless of the compression test condition except for the sample tested at 1066℃-0.005/sec condition, where the dynamically-recrystallized grain size was as large as 20 μm in average diameter. In case of the sample tested at 1066℃, flow stress drops caused by the strain softening were almost same at the strain rates of 0.5/sec and 0.005/sec in spite of the different fraction of dynamic recrystallization. It was concluded from the observation of the misorientation angle distribution that it was due to the active dynamic recovery in the unrecrystallized large grains at high temperature. It was also suggested from the variations of CSL boundary distribution that the characteristic redistribution of the dynamically-recrystallized grain boundary might be taken place during the compression at high temperature and low strain rate.
동적재료모델에 기초한 초내열 718 합금의 고온 압축변형 거동 해석
나영상,염종택,박노광 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.2
In order to understand more clearly the hot deformation mechanism of Alloy 718 and consequently to determine the "safe" process design window, a number of compression tests were carried out in the range of temperature, 927℃-1066℃ and strain rate, 5×10^-4/s-5/s up to the true strain of 0.7. Based on dynamic materials model, the optimal process design windows, where the deformation is stable, were proposed through the analysis of the constitutive relationships among true stress, true strain, strain rate and temperature. The instability of deformation was confirmed by observing the microstructure of the compression tested samples. Some characteristic microstructures of the localized deformation, such as twinning and shear band, were observed in the samples deformed under the unstable deformation conditions.
나영상,김종엽,이종훈,박노광,Na, Yeong-Sang,Kim, Jong-Yeop,Lee, Jong-Hun,Park, No-Gwang 한국기계연구원 2003 硏究論文集 Vol.33 No.-
Alloy 713LC was developed to improve the tensile strength and ductility by reducing the carbon content of Alloy 713C. As Alloy 713LC was designed to minimize the mechanical property change with process conditions, it is generally utilized in the parts which have thick and thin sections simultaneously. In the thick and the thin sections, quite different properties are required. Consequently it is essential to crucially control the local mechanical properties of a parts by optimizing the process condition and heat treatment. In this research, high temperature mechanical properties including creep-rupture and strain-control low cycle fatigue were investigated together with the microstructural variations with heat treatment. Failure mechanism was also analyzed by observing the fracture surface to correlate the variation of mechanical properties with the microstructural change.