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고향진,김낙준 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.12
Second phases in the melt-overflowed Al-Fe-V-Si alloy have been investigated, and found to be classified into three different types : e.g. icosahedral phase, hexagonal phase, and bcc α-AlFeSi. Crystallographic orientation relationships among the phases were determined by electron diffraction. All the phases have similar atomic arrangements, which are constructed by several (Fe+V) and (Al+Si) icosahedra connected through chains of (Al+Si) octahedra, and thereby providing close orientation relationships among them. Phase decomposition behavior of the second phases has been also investigated by means of hot-stage in-situ TEM technique. During heating icosahedral phase transforms to thermally more stable bcc phase, and such transformation is controlled by Si diffusion. On the other hand, transformation of the hexagonal phase to bcc phase is not controlled by diffusion.
API-X 70 강의 Spiral 파이프 성형시 일어나는 항복강도의 감소현상 연구
이성학,고향진,최철우 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.4
An investigation was conducted into the effects of microstructural parameters on the yield strength drop occurring during spiral piping of two high-strength API-X70 steels processed with titanium and molybdenum additions. Compression-tension tests were conducted as well as tensile tests in order to investigate the Bauschinger effect during piping, and then the test data were compared via microstructures, tensile properties, textures, and yield point phenomenon. The tensile results indicated that the yield strength of the Ti-containing steel dropped, together with yield point phenomenon, after piping, whereas this drop did not occur in the Mo-containing steel. It appeared that competing mechanisms existed in the steels studied herein; the yield strength decrease due to the Bauschinger effect accompanied by the yield strength increase due to the strain hardening effect. In the case of the Ti-containing steel, the yield strength drop was associated with the small strain hardening overrode by a large amount of the Bauschinger effect. For the Mocontaining steel. the large strain hardening, commonly associated with continuous yielding behavior, overrode the Bauschinger effect, and finally caused the increase in yield strength after piping.
박성호,이순기,고향진,김낙준 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.6
The present study is aimed at developing the high strength, high formability low alloy steels having retained austenite as one of the constituents. The main emphasis has been put on understanding the effects of processing variables and alloying elements (Cu, Nb) on the microstructure and mechanical properties of hot-rolled Fe-C-Mn-Si base TRIP (Transformation Induced Plasticity) steels. Experimental results show that microstructure of hot-rolled alloys consists of ferrite, bainite, martensite and retained austenite, whose volume fraction and morphology vary depending on the hot-rolling conditions and alloy compositions. The increase in the quench-finish temperature results in a homogeneous distribution of fine granular particles of retained austenite and hard second phases in a ferrite matrix and accordingly a good combination of strength and ductility. The Cu-containing alloy has been found to improve mechanical properties over the base alloy due to the precipitation of fine ε-Cu in ferrite grains. Nb addition increases the dynamic recrystallization temperature of austenite, resulting in the formation of coarse pan-caked bainite packets. Based on these results, the optimum microstructure for a better combination of strength and ductility is suggested to be the one having granular particles of retained austenite and hard phase (martensite or bainite) in a fine ferrite matrix.
압광 페인트를 이용한 MgO 안정화 지르코니아 소재의 급작스러운 파괴거동에 관한 실시간적 관찰연구
김지식 ( Ji Sik Kim ),손기선 ( Kee Sun Sohn ),고향진 ( Hyang Jin Koh ),이완두 ( Wan Boo Lee ) 대한금속재료학회 ( 구 대한금속학회 ) 2007 대한금속·재료학회지 Vol.45 No.5
The recently devlopoped ML(Mechano-Luminescence) technique enables macro-scale visualization of crack propagation during catastrophic failure, together with transformation zone (crack wake) in both crack front and side areas of Mg-PSZ in realistic time frame. The ML made it possible to precisely detect a relatively fast crack propagating in the speed range from 5 to 15 m/sec, thereby realizing so-called quasi-dynamic R-curve. A systematic observation of growth in wake height, as well as wake length, was made successfully in quasi-dynamic condition. Effective-toughening applied-stress intensity factor increased up to 22 MPa(m)(1/2). The h0 values obtained from the ML observation deviated significantly from that predicted by Evans-McMeeking model(J. Am. Ceram. Soc. 65,242(1982)); rahter it supports Marshall et al.`s results by interference microscopy and Raman spectroscopy (J Am. Ceram. Soc. 73,2659(1990)) in the quasi-static crack propagation. It was also mentioned that conventional R-curves spanning over several millimeters range reported for long-crack systems such as DCB(Double Cantilever Beam), CT(Compact Tension), SENB(Single Edged Notched Bar), and etc. can not be explained by Evans-McMeeking model.