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이동근,이유환,이성학,이종수,이충열,허선무 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.10
The effects of microstructural morphology on dynamic deformation and fracture behavior and ballistic performance of a Ti-6Al-4V alloy were investigated in this study. Ballistic impact tests were conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and dynamic torsional test data. In the impacted region of the equiaxed microstructure, a number of adiabatic shear bands and cracks were observed to be formed along plastic flow lines, and delamination occurred because of cracking along the plastic flow lines or shear bands. In the case of the bimodal microstructure, shear bands were found in limited area near the impacted region without occurring delamination, and their number was fewer than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure. The ballistic performance of the two microstructures was also discussed by comparing the possibility of adiabatic shear band formation obtained from dynamic torsional test data.
Lee, Hyungsoo,Choi, Jin Hyeok,Jo, Min Chul,Lee, Donghyun,Shin, Sangmin,Jo, Ilguk,Lee, Sang-Kwan,Lee, Sunghak Elsevier 2018 Materials science & engineering. properties, micro Vol.738 No.-
<P><B>Abstract</B></P> <P>Quasi-static and dynamic compressive properties of 7075-T6 Al matrix composites reinforced with SiC<SUB>p</SUB>s (size; 10 µm, 30 µm, and bimodal (10 + 30) μm), <I>i.e</I>., 10S, 30S, and BS composites, respectively, were investigated in relation with fracture mechanisms. SiC<SUB>p</SUB>s were homogeneously distributed without SiC<SUB>p</SUB> agglomeration, pores, or cracks, and their volume fractions were 49.5 vol%, 54.1 vol%, and 56.5 vol% in the 30S, 10S, and BS specimens, respectively. Compressive properties could be explained by a rule of mixtures based on SiC<SUB>p</SUB> volume fraction and by three major fracture phenomena including deformation of Al matrix, cracking of SiC<SUB>p</SUB>s, and interfacial debonding between SiC<SUB>p</SUB> and Al. As the SiC<SUB>p</SUB> size decreased, crack initiation sites changed from SiC<SUB>p</SUB>s themselves to SiC<SUB>p</SUB>/Al interfaces, which mainly affected the compressive strength. In the BS composite, coarse SiC<SUB>p</SUB>s additionally worked for the strengthening by increasing the total reinforcement fraction, and the three fracture phenomena were well homogenized, thereby leading to the highest strain energy density as well as the best combination of strength and strain.</P>
Lee, Kyuhong,Son, Chang-Young,Lee, Sang-Bok,Lee, Sang-Kwan,Lee, Sunghak Elsevier 2010 Materials science & engineering. properties, micro Vol.527 No.4
<P><B>Abstract</B></P><P>Zr-based amorphous matrix composites reinforced with metallic continuous fibers were fabricated by liquid pressing process, and their fracture property improvement was explained by directly observing microfracture processes. About 60vol.% of metallic fibers were homogeneously distributed inside the amorphous matrix. Apparent fracture toughness of the tungsten-fiber-reinforced composites was lower than that of monolithic amorphous alloy, while that of the tantalum-fiber-reinforced composite was higher. According to the microfracture observation, shear bands or cracks were initiated at the amorphous matrix, and the propagation of the initiated shear bands or cracks was effectively blocked by fibers, thereby resulting in stable crack growth which could be confirmed by the fracture resistance curve (R-curve) behavior. This increase in fracture resistance with increasing crack length improved fracture properties of the fiber-reinforced composites, and could be explained by the formation of multiple shear bands or multiple cracks at the amorphous matrix, blocking of crack or shear band propagation, and multiple necking at metallic fibers.</P>
Stir-casting방법을 이용한 A356 Al-SiC_p 복합재료의 제조
李東根,李昌燮,金基鍾,金石洙,李聖鶴 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.7
The aims of this study were to develop a fabrication process of A356 aluminum alloy composites reinforced with SiC particulates using stir-casting method and to establish processing variables to improve their microstructures and mechanical properties. The water-pool modeling was conducted to simulate and establish the proper stir-casting conditions. Good quality Al-SiC_p composites having relatively homogeneous microstructure and sound Al/SiC interfacial bonding were obtained from the conditions of the stirring temperature of 710℃, the stirring speed of 220∼700 rpm, and the injection of Ar gas+SiC_p mixture, although they contained a small amount of micropores. The microstructures and mechanical properties of the composites were found to be comparable with those of the commercial composites processed by Duralcan because of the homogeneous distribution of SiC particulates. In order to improve the mechanical properties of the A356 Al-SiC_p composites, minimization of micropores, fast cooling rate, homogeneous distribution of SiC and eutectic Si particles, and development of the subsequent processes such as die-casting or squeeze-casting were suggested.
Lee, Seok Gyu,Lee, Dong Ho,Sohn, Seok Su,Kim, Woo Gyeom,Um, Kyung-Keun,Kim, Ki-Seok,Lee, Sunghak Elsevier Sequoia 2017 Materials science & engineering Structural materia Vol.697 No.-
<P><B>Abstract</B></P> <P>In order to understand and improve fracture toughness of heat affected zones (HAZs) of high-strength low alloy (HSLA) steels, complex microstructures including quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent should be identified, quantified, and then correlated with critical crack tip opening displacement (CTOD). In this study, microscopic analysis methods were achieved for identification and quantitation of microstructures in the HAZs of three HSLA steels. The coarse-grained HAZ (CGHAZ) consisted of AF, GB, and BF together with a small amount of MA, while the inter-critically heated HAZ (ICHAZ) consisted of QPF, GB, and MA. In the CGHAZ, Ni promoted the formation of AF, while it prevented the formation of GB, and the addition of Ni resulted in very high critical CTOD. In the CGHAZ, both Ni and Mn promoted the formation of AF and prevented the formation of GB, while Ni was more effective than Mn. Thus, the addition of Ni resulted in very high critical CTOD. In the ICHAZ, both Ni and Mn promoted the formation MA. However, in the high-Ni-containing steel, a number of MAs were densified along Ni-segregated bands, and thus readily provided void initiation sites. This played an important role in reducing the mean free path for coalescence of voids and crack propagation, which easily led to the serious deterioration of critical CTOD.</P>
Widmanstatten 조직을 가지는 Ti-6Al-4V 합금의 준정적 및 동적 변형거동에 미치는 미세조직인자의 영향
李東根,李聖鶴,李鍾洙,許善茂 대한금속재료학회 2001 대한금속·재료학회지 Vol.39 No.12
This study is concerned with the effects of microstructural parameters on the quasi-static tensile and dynamic torsional deformation behaviors in Ti-6Al-4V alloys with Widmansta¨tten structures. Dynamic torsional tests were conducted using a torsional Kolsky bar for five Widmansta¨tten structures, which effective grain sizes such as colony size and α lamellar spacing were varied by heat-treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Under dynamic torsional loading, maximum shear stress showed a large dependence on the colony size as the effective grain size, while shear strain at the maximum shear stress point was largely affected by both the α lamellar spacing and the colony size. Adiabatic shear bands were found in the deformed region of the fractured specimens, and their width was smallest in the specimen having the large α lamellar spacing and colony size. The possibility of the adiabatic shear band formation was quantitatively analyzed by the theoretical critical shear strain (γ_c) condition. It was found to be most likely in the coarse Widmanstatten structure, whereas least likely in the fine Widmanstatten structure.
Lee, Seok Gyu,Sohn, Seok Su,Kim, Bohee,Kim, Woo Gyeom,Um, Kyung-Keun,Lee, Sunghak Elsevier 2018 Materials science & engineering. properties, micro Vol.715 No.-
<P><B>Abstract</B></P> <P>Crack susceptibility of martensite-austenite constituent (MA) in inter-critical heat-affected zone (ICHAZ) is varied with properties of adjacent matrix, hardness of MA, and MA/matrix interfacial characteristics, but reasons why MAs are voided at MA/matrix interfaces or cracked at themselves still remain to be addressed. Effects of microstructural features including MA and matrix composed of granular bainite (GB) and quasi-polygonal ferrite (QPF) on crack initiation and propagation behavior, which affected critical crack tip opening displacement (CTOD), were investigated in this study. In the ICHAZ mainly composed of GB (75.2vol%) and QPF (19.5vol%), together with 5.3vol% of MA, QPF areas were more deformed than GB areas during the interrupted three-point bending tests, while some MAs were voided or cracked as MA areas were also highly deformed. The Nano-indentation test data indicated that the hardness of MA depended on boundary characteristics of nearby matrix, <I>e.g</I>., QPF having high-angle boundaries <I>vs.</I> GB having low-angle boundaries. According to detailed analyses on MA/QPF interface by using an atom probe, C, Mn, Mo, Ni, and Cr atoms were segregated at the MA/QPF interface, whereas Si, Cu, Al, and P were hardly segregated. These results indicated that major alloying elements (C, Mn, Ni, and Cr) were segregated at the MA/QPF interface, which influenced the deterioration of critical CTOD by reducing the mean free path for the crack initiation and propagation.</P>
Lee, Kyuhong,Son, Chang-Young,Kim, Jin Suk,Lee, Sunghak,Kim, Kyoo Young,Kim, Choongnyun Paul Elsevier 2006 Materials science & engineering. properties, micro Vol.441 No.1
<P><B>Abstract</B></P><P>Corrosion resistance of steel-based surface alloyed materials fabricated with Fe-based metamorphic powders by high-energy electron beam irradiation was investigated in this study. Two kinds of metamorphic powders were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these powders to fabricate surface alloyed materials. In the surface alloyed layers, 48–64vol.% of Cr<SUB>2</SUB>B or Cr<SUB>1.65</SUB>Fe<SUB>0.35</SUB>B<SUB>0.96</SUB> borides were precipitated in the Cr<SUB>0.19</SUB>Fe<SUB>0.7</SUB>Ni<SUB>0.11</SUB> or martensite matrix, and thus their hardness was two to three times higher than that of the steel substrate. The overall corrosion resistance of the surface alloyed materials was better than that of an STS304 stainless steel or coatings fabricated by high-velocity oxygen fuel spraying of same powders. The observation results of pits formed on the corroded surface revealed that Cr<SUB>0.19</SUB>Fe<SUB>0.7</SUB>Ni<SUB>0.11</SUB> and martensite phases of the surface alloyed materials and coatings was selectively corroded, while borides were retained inside pits, by forming a galvanic coupling. These findings suggested that the fabricated surface alloyed materials presented good application possibilities as excellent wear- and corrosion-resistant materials.</P>