일방향응고시킨 Al-CuAl2 공정복합재료의 인장성질

홍영환,홍종휘 ( Young Hwan Hong,Jong Hwi Hong ) 한국주조공학회 1990 한국주조공학회지 Vol.10 No.6

N/A The effect of interlamellar spacing on tensile behavior and fracture mode at high temperatures has been studied for unidirectionally solidified Al-CuAl₂eutectic composite. The tensile properties at room temperature in Al-CuAl₂eutectic composite improved as the interlamellar spacing decreased due to the constraint effects of closely spaced lamellae, while the opposite behavior was observed at high temperatures due to the annihilation of the constraint effects by phase boundary sliding. The Al-CuAl₂eutectic composite exhibited brittle fracture mode below the temperature at which the reinforcing phase softened but ductile fracture mode above the temperature.

일방향응고시킨 Al-CuAl2 공정복합재료의 열적안정성에 관한 연구

홍영환,홍종휘 ( Young Hwan Hong,Jong Hwi Hong ) 한국주조공학회 1990 한국주조공학회지 Vol.10 No.5

N/A The effect of thermal cycling and isothermal exposure on the high temperature microstructural stability of unidirectionally solidified Al-CuAl₂eutectic composite has been studied. A coarsening procedures of lamellar eutectic structures were initiated at growth fault region because of diffusion through low angle boundary at this region. It was considered that thermally induced residual stresses produced by thermal cycling were high enough to increase the dislocation density in Al-rich matrix phase. However, it was also considered that dislocations generated by these high thermal stresses were annihilated at high temperature by stress relaxation. Consequently, the thermal cycling up to 1440 cycles between 20 and 520℃ did not affect the microstructural stability.

오스템퍼링 열처리한 구상흑연주철의 충격인성에 미치는 Ni 과 Mo 첨가의 영향

홍영환,홍종휘 ( Young Hwan Hong,Jong Hwi Hong ) 한국주조공학회 1989 한국주조공학회지 Vol.9 No.2

N/A The microstructural characteristics of ductile cast iron austempered for various times are characterized and correlated to Charpy impact toughness. The desired matrix structure of austempered ductile cast iron consists of a two-phase mixture of carbide-free bainitic ferrite and austenite. During austempering, bainitic transformation can be described as a too-stage reaction : 1) austenite decomposition to bainitic ferrite and carbon-enriched austenite, followed by 2) decomposition of carbon-enriched sustenite to further ferrite and carbide. Therefore, excessive austempering produces a sharp drop in impact toughness and a fracture mode transition from ductile tearing to brittle quasi-cleavage fracture by a carbide formation. Delaying the embrittling reaction by addition of Ni and Mo should allow a greater latitude in austempering time. However, the onset of the embrittling reaction cannot be detected metallographically. By the way, interruption of the austempering reaction by quenching prematurely will result in a formation of martensite. This also reduces impact toughness.

일방향응고시킨 Ni 기 (基) 초내열 (超耐熱) 공정복합재료에 관한 연구

이주홍,홍영환,홍종휘 ( Joo Hong Lee,Young Hwan Hong,Jong Hwi Hong ) 한국주조공학회 1988 한국주조공학회지 Vol.8 No.4

N/A The effect of interlamellar spacing on microstructural stability at high temperature was studied for unidirectionally solidified ternary Ni / Ni₃ Al-Ni₃ Nb and binary Ni-Ni₃Nb eutectic composite. The interlamellar spacing of both alloy systems were varied with the growth rate according to λ²R=constant relationship. As a result of isothermal heat treatments at high temperature it was considered that coarsening of lamellar structure was due to concentration gradient between the tip with a relatively small radius of curvature and the side of the thick lamellae with a larger radius of the opposite sign. Fault density was increased as the interlamellar spacing decreased. Therefore it is also considered that the higher coarsening rate of the specimen with the smaller interlamellar spacing was due to higher fault density. And the diference of coarsening rate between Ni / Ni₃Al-Ni₃Nb and Ni-Ni₃Nb eutectic composites was not observed when the interlamellar spacing was similar in size. This means that the presence of γ` in γ/γ` -δ eutectic had no b arrier effect to diffusion through the γ matrix.

Al-CuAl2 공정복합재료의 고온거동에 관한 연구

홍종휘,이현규,김영수,홍영환 대한금속재료학회(대한금속학회) 1988 대한금속·재료학회지 Vol.26 No.7

The effect of interlamellar spacing on microstructural stability, tensile behavior and fracture mode at high temperature has been studied for unidirectionally solidified Al-CuAl₂ eutectic. The results of isothermal heat treatments showed the higher coarsening rate at 520℃ than at 450℃. It was observed that the decrease of interface area(Δ5) was inversely proportional to the square of the interlamellar spacing(λ²). Therefore, it can be seen that the microstructural stability at high temperature improves as the interlamellar spacing increases. The tensile properties at high temperature improved as the interlamellar spacing increased, which is opposite behavior at room temperature. The Al-CuAl₂eutectic exhibited brittle fracture mode below the temperature at which the reinforcing materials softened but ductile fracture mode above the temperature.

용탕교반법에 의한 AC8A/SiCp 복합재료의 제조조건에 따른 기계적 성질

홍종휘,홍영환,한준현 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.6

Microstructure, wettability, tensile strength and wear properties were examined in AC8A/SiC_p composites. Increasing Mg content in matrix improved wettability probably due to decreasing surface tension of the melt. By increasing the stirring time and particle size, the particles were more uniformly distributed in the matrix. And the particle distribution of as-cast state had a significant influence on that of extruded state, that is, particles were rarely aligned in the extrusion direction in the composites stirred for 20 mimutes. Tensile strength was increased as stirring time increased and particle size decreased, but significant increase of high temperature strength due to the presence of particles was not obtained. Also wear resistance did not vary significantly with the particle size. It is clear that SiC particle reinforcement improved both tensile strength and wear resistance.

내부반응법에 의한 Ti 합금기 복합재료의 제조 및 기계적 성질

홍종휘,홍영환,박종일 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.12

In this study powder metallurgy method was used to fabricate the TiB reinforced Ti alloy composite materials, and the matrix Ti alloy composition was basically Ti-5Al-2.5Fe. Especially the reinforcing particle TiB was obtained from TiB₂ particles by the internal reaction during sintering process. Sintering at 1300℃ in argon atmosphere, no TiB formation was observed with any addition of TiB₂. And at 1250℃ in vacuum, TiB formation wasn't also observed. At 1300℃ in vacuum, however, TiB formation was observed for the specimens added 2∼9wt% TiB₂. The TiB particles formed have a shape of acicular structure, and have a size of 10∼30 ㎛. Microhardness test showed that specimens with TiB particles have much higher hardness value than that of unreinforced alloy. Therefore internal reaction process, which also has an advantage of a near-net-shape technology, can be said to be a potential method to produce high strength Ti alloy composites using low-cost additive elements.

용탕교반법에 의한 Al2o3(p)/AC8A 복합재료의 제조 및 기계적 성질

홍종휘,홍영환,김상길 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.2

Among various processes in fabricating metal matrix composites, melt-stirring technology is much more attractive route in industrial application because it is simpler and less expensive compared to squeeze casting or powder metallurgy. In the present study, effects of size and volume fraction of reinforcing particles on tensile strength and thermal expansion coefficients of α-Al₂O₃_(p)/AC8A composite, which was made by melt-stirring process, were investigated. And the effect of Mg addition on the above properties was also investigated. Composites with 3%Mg showed better wettability and distribution of particles than that of composites with no Mg. The addition of Al₂O₃ particles was not effective in increasing the tensile strength of the composite except the composite materials with 3%Mg, which showed an increased strength in the case of 5vol% addition of 16 ㎛ α-Al₂O₃ particles. On the other hand, thermal expansion coefficients of the composites decreased with increasing the volume fraction of reinforcing particles and Mg contents.

질화물강화 Al-Li 합금 복합재료의 제조 및 기계적 성질

이현규,홍종휘,홍영환,이승만 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.1

In this study we have tried to fabricate nitride particle-reinforced composite materials through blowing nitrogen gas into Al-Li alloy melts, and identified the formation of nitride particles such as AlN and Li₃N. The average size of AlN and Li₃N dispersoids formed by nitriding reaction were a few ㎛ and nm order, respectively. The addition of Mg appeared to be effective to facilitate the nitriding reaction and to prevent the loss of Li due to deoxidation in the melt. And the tensile strengths of these N₂ gas-blowed composites were greatly increased compared to that of unblowed Al-Li alloy. Therefore it is clear that the formation of nitride particles had a great effect on the increase of tensile strength of the composites. However, long time blowing of more than 20 minutes resulted in a decrease of tensile strength due to the removal of preformed nitrides by the blowing gas. Ductility, which is a major problem of Al-Li alloys, is also greatly improved by this process compared to that of conventional Al-Li alloys.

Al 합금에서 질화물강화 복합재료에 관한 연구

이현규,홍종휘,홍영환,박정민 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.7

In the present study, we made an attempt to fabricate a nitride-reinforced Al alloy composite by blowing N₂gas into Al alloy melts. The addition of reactive element Mg to Al alloy melt facilitated the nitriding reaction, leading to the formation of TiN in Al-5wt.%Ti-10wt.%Mg alloy, but rarely formed in Al-5wt.%Ti alloy due to the absence of Mg. The former alloy showed higher room temperature tensile strength as compared with the latter alloy, which is mainly due to the presence of TiN and partly due to solid solution strengthening effects of Mg. In addition, it is considered that the presence of nitride may also be responsible for high temperature strengthening, judging from the lower decreasing rate of tensile strength with temperature in Al-5wt.%Ti-10wt.%Mg alloy.