전기 침전법으로 제조한 나노 니켈에서 결정립의 다중 비정상 성장 현상

김재중,Suresh, S.,김낙준 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.9

Here we show multiple abnormal grain growth phenomena in nanocrystalline pure Ni with temperature change, based on the microstructural analysis. Abnormally large grains are observed in the nanocrystalline Ni thermally exposed in air for 1 hr at 200℃, 500℃, and 800℃, respectively. Such large grains formed at 500℃ for 1 hr having facet grain boundary structure, show kinetically fast growth rate rather than others exposed to 200℃ and 800℃ for the same time period. The facet grain boundary appears to be related to the abnormal grain growth rate, suppressing the fine matrix grain growth. The mechanism underlying abnormal grain growth is examined for each temperature range.

고장력강 용접부에서의 국부취화영역 연구 (2) : 파괴기구와 파괴모델 Fracture Mechanism and Fracture model

김병천,이두영,이성학,김낙준 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.6

This study is concerned with a correlation between microstructure and plane strain fracture toughness in heat affected zones of high-strength low alloy (HSLA) steels, i.e., a normalized steel and a TMCP steel. The micromechanism of fracture processes involved in void and microcrack formation is identified and quantified. The fracture toughness results were also interpreted using simple fracture initiation models based on the basic assumption that crack initiates at a certain critical strain or stress developed over some microstructurally significant distance. The calculated K_IC values are found to scale roughly with the square root of the spacing of the stringer type martensite islands, confirming that martensite islands play an important role in reducing the toughness of the coarse grained HAZ. These foundings suggest that the LBZ in the HSLA steels studied can be explained quantitatively using different fracture models.

SiC 휘스커 보강 Al 복합재료의 미세조직과 기계적 특성에 관한 연구

김영환,이두영,이성학,김낙준 대한금속재료학회(대한금속학회) 1990 대한금속·재료학회지 Vol.28 No.7

This study aims at establishing the correlation of microstructure and mechanical properties of 2124-T6 Al composites reinforced with 7, 15, 20 v/o SiC whiskers. Tensile tests along with Charpy impact tests were conducted over the test temperature range from -196℃ to 100℃. Notched round tensile tests and subsequent metallographic observations as well as in-situ SEM observation of crack opening processes were performed for the purpose of identifying the micro-mechanical processes involved in void initiation. The experimental results indicated that the impact energy was found to increase monotonically with decreasing the amount of SiC whiskers and with inceasing the test temperature. Microvoids were observed to nucleate at the interface between the aluminum matrix and SiC whikskers, confirming that the formation of voids at the interfaces is the main metallurgical factor which contributes to the reduction of fracture toughness. In addition, the Mn-containing intermetallic particles strongly affect the fracture processes in these composites.

정밀주조된 A356 Al 합금의 미세조직과 기계적 성질과 미치는 HIPping의 영향

김낙준,이민하,김재중,김경훈,이성학,이의휘 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.2

The effects of HIPping on high-cycle fatigue properties of investment cast A356 Al alloys were investigated in this study. Tensile and high-cycle fatigue tests were conducted for cast alloys, two of which were HIPped, and then the test data were analyzed in relation with microstructures, tensile and fracture properties, and fatigue fracture mode. Eutectic Si particles were homogeneously dispersed in the matrix of the cast A356 Al alloys, but there were many large pores formed as cast defects. The high-cycle fatigue results indicated that fatigue strength of the HIPped alloys was higher than that of the non-HIPped alloys because of the significant reduction in volume fraction of pores by HiPping. In the non-HIPped specimens, fatigue cracks initiated at large pores adjacent to the specimen surface and then propagated down to several hundreds micrometers depth while coalescing with other large pores. On the other hand, the HIPped specimens, where pores did not affect the fatigue substantially, fatigue cracks propagated along eutectic Si particles formed at solidification cell boundaries, thereby showing improved fatigue strength by 40∼50% over the non-HIPped specimens.

주조 A356.0 Al-SiC 복합재료의 기계적 성질과 파괴과정에 대한 공정 Si 입자의 영향

김영환,이두영,이성학,김낙준 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.2

In this study an attempt has been made to improve mechanical properties of a cast aluminum matrix composite. Detailed microstructural analyses of a cast A356.0 Al-SiC composite indicated that SiC particulates and eutectic Si particles were segregated to the intercellular regions. The presence of these eutectic Si particles resulted in cleavage fracture along the intercellular regions, which deteriorated mechanical properties of the cast composite. In-situ SEM fracture observations clearly showed that eutectic Si particles were cracked first before the matrix-SiC decohesion or the SiC breakage occurred. In order to modify the distribution of eutectic Si particles the cast composite was re-cast with adding small amount of Sr. Tensile properties of the re-cast composite with 0.03wt.% Sr were significantly improved. The improvement of mechanical properties is attributed to a more homogeneous distribution of SiC and eutectic Si particles due to the addition of Sr and recasting.

용탕단조법으로 제조한 AISI 304 와이어 강화 AM60 Mg 복합재의 특성 연구 (2) : 와이어의 다중 네킹 현상 및 기구 Multiple Necking Phenomena and Mechanism of Reinforcement Wires

김재중,김낙준 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.6

The multiple-necking phenomenon and mechanism of reinforcement wires in AM60 Mg composites reinforced with AISI 304 wires has been investigated in the present study. AM60 Mg composite reinforced with AISI 304 wires exhibits ductility greater than the ductility of wire itself due to the occurrence of multiple-necking along the length of wire. As the volume fraction of AISI 304 wires decreases, the number of wire necking and the ductility of composites increase. It has been shown that the multiple-necking phenomenon is closely related to wires necking, interface reaction, and volume fraction of reinforcement. This phenomenon is also related to diameter of wires, strain rate, structure of metal, and interface strength. The multiple-necking results from local strain hardening of the matrix surrounding the neck, enabling the matrix to decrease neck growth by attraction force. Because multiple-necking plays an important role in the ductility and deformation behavior of composite, further works is needed to characterize the causes of this phenomenon in other metal/metal composites system.

초고장력강에서의 템퍼드 마르텐사이트 취성파괴기구

김병천,이두영,이성학,김낙준 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.2

An attempt to explain the fracture mechanism of tempered martensite embrittlement (TME) was made by investigating microstructural changes and impurity segregation during tempering of an AISI 4340 steel Particular emphasis was placed on studying microfracture processes at crack tips using an in-situ SEM technique. At the relatively low stress levels, shear bands were formed preferentially along the arrays of interlath cementite particles and microcracking occurred at the prior austenite grain boundaries. The failure eventually took place along localized shear bands in a ductile manner, although the crack path was occasionally deviated from the shear band only when the main crack met the discrete microcracks associated with the prior austenite grain boundaries. Examination of the fracture surfaces clearly revealed a transgranular ductile fracture mode without any evidence of intergranular fracture. Thus, the direct observations of the crack initiation and the fractographic results strongly suggest that the presence of carbide particles is the most important metallurgical factor leading to the tempered martensite embrittlement.

급냉응고 Al-Li-X (Zr, Cu, Mg) 합금에서 첨가 합금원소가 미세조직 및 기계적 성질에 미치는 영향

김영환,이두영,이성학,김낙준 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.1

A study was made of the effects of alloying elements on the structure and mechanical properties of high Li($gt;3wt%) containing RSP Al-Li-X(X=Zr, Cu, Mg) alloys. Particular emphasis was placed on the role of Mg and Cu, which are the major alloying elements in ingot metallurgy (IM) Al-Li alloys. RSP Al-Li-X alloys with several alloy compositions were subjected to various thermal treatments without any pre-aging cold working such as stretching. It has been found that, in the case of plain Al-Li-Zr alloy, the deformation behavior and the resultant mechanical properties are mainly controlled by the presence of uniformly distributed composite Al₃Li/Al₃Zr precipitates. Composite precipitates are also the major strengthening precipitates which control the deformation behavior of the alloys containing Cu and Mg. Addition of Cu and Mg to Al-Li-Zr alloys, when subjected to thermal treatment without any mechanical working, do not promote the precipitation of other strengthening phases such as T₁ and S' as expected ; however, they have a very important effect on increasing the ultimate tensile strength of Al-Li-Zr alloys due to their solid solution strengthening effects. Comparison of the present results with those of IM Al-Li alloys indicates that optimum mechanical properties can be achieved in high Li and Zr containing Al-Li alloys by simple thermal treatment without stretching, and this is due to the presence of uniformly distributed composite precipitates.

연주크랙 저감을 위한 API X-50 강의 고온연성에 대한 미세조직의 영향

金成圭,金俊星,金洛준 대한금속재료학회 2002 대한금속·재료학회지 Vol.40 No.2

The rapid cooling in continuous casting process produces large thermal stress in the slabs, due to steep thermal gradient between the interior and the exterior of the slabs and a mechanically induced stress caused by friction in the mold, roll pressure, etc. There is also an occurrence of tensile stress at the upper region and compressive stress at the lower region of the slabs at the straightening zone at around 700℃-900℃ in the case of vertical continuous casters. In the case of steels with poor hot ductility, the combination of the stresses mentioned above quite often results in the formation of cracks. Among various types of steels, it is known that steels containing strong carbide formers such as Nb are particularly susceptible to cracking during continuous casting. The present study is concerned with microstructural analyses of the Nb containing steel (A steel) and Nb+B containing steels (B steel) to reduce the transverse corner crack at the straightening zone, with emphasis on elucidating the mechanism of hot ductility. There are differences in the microstructures between the A steel and the B steel. The B steel has a smaller amount of ferrite films along the prior austenite grain boundaries than the A steel. Also, the B steel contains proeutectoid ferrite within the prior austenite grains and 5-㎛-sized coarse Fe_23(B, C)_6 along the prior austenite grain boundaries as well as within the prior austenite grains. The ductility trough is virtually non-existent in the B steel, although variation of values of reduction of area (RA) with temperature shows that the A steel has a trough of RA between 700℃ to 850℃. It is believed that the presence of intraganular ferrite improves the homogeneity of deformation and accordingly the hot ductility of the B steel.

고인성 API X70 라인파이프강의 파괴특성 : Ⅲ. Drop-Weight Tear Test 시 발생되는 역파면에 대한 연구 Ⅲ. Inverse Fracture Occurring during Drop-Weight Tear Test

황병철,김양곤,이성학,김낙준,유장용 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.4

This study is concerned with the effects of microstructure on inverse fracture in the hammer-impacted region during drop-weight tear test(DWTT) of three high-toughness API X70 pipeline steels. Pressed notch or Chevron notch DWTT and Charpy V-notch impact test were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from tensile and Charpy impact tests of pre-strained specimens. In the hammer-impacted region of the DWTT specimens, cleavage-type inverse fracture appeared, and its fracture area was correlated well with upper shelf energy (USE) obtained from the Charpy impact test and with microstructure. The steel specimens having higher USE or having coarse polygonal ferrite had the larger inverse fracture area than those having lower USE or having fine acicular ferrite because of the larger compressive pre-strain induced by work hardening of the hammer-impacted region. These results were confirmed by relating to the Charpy impact test data of pre-strained steel specimens. (Received January 19, 2004)