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1.
Sangho Kim Young-Roc Im Sunghak Lee Hu-Chul Lee Sung-Joon Kim Jun Hwa Hong 《Metallurgical and Materials Transactions A》2004,35(7):2027-2037
This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of
base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied
from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment,
and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and
increased Mo and Ni content, the number of fine M2C carbides was greatly increased and the number of coarse M3C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle
martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture
toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient
impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation
of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to
precipitate fine M2C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base
metals. 相似文献
2.
Sunghak Lee Dongwoo Suh Dongil Kwon 《Metallurgical and Materials Transactions A》1996,27(12):3893-3901
A microstructural analysis of local microfracture of cast A356 Al-SiC
p
composites fabricated by permanent mold re-casting and squeeze-casting methods was made. Notch fracture toughness tests were
conducted on these composites to identify critical fracture parameters using a stress-modified critical-strain criterion.
The composite microstructure shows continuous networks of densely populated SiC and eutectic Si particles along the intercellular
regions. Squeeze casting produces a more homogeneous structure and larger spacing of brittle particles and increases the tensile
ductility and fracture toughness, while strength levels are almost identical to the re-casting case. The calculated values
of the microstructurally characteristic distancel* for the re-cast and squeeze-cast composites are about 40 μm, which is comparable to the average sizes of the intercellular
network. However, the reference critical strain
for squeeze casting is larger than that for re-casting, showing a trend to higher ductility and fracture toughness. 相似文献
3.
Dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing 总被引:1,自引:0,他引:1
Byoungchul Hwang Yang Gon Kim Han Sang Lee Sunghak Lee Byoung Doo Ahn Dong Hyuk Shin Chang Gil Lee 《Metallurgical and Materials Transactions A》2005,36(2):389-397
The dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing (ECAP)
was investigated in this study. Dynamic torsional tests, using a torsional Kolsky bar, were conducted on four steel specimens,
two of which were annealed at 480 °C after ECAP, and then the test data were compared in terms of microstructures, tensile
properties, and adiabatic shear-band formation. The equal-channel angular pressed specimen consisted of very fine, equiaxed
grains of 0.2 to 0.3 μm in size, which were slightly coarsened after annealing. The dynamic torsional test results indicated that maximum shear
stress decreased with increasing annealing time, whereas fracture shear strain increased. Some adiabatic shear bands were
observed at the gage center of the dynamically deformed torsional specimen. Their width was smaller in the equal-channel angular
pressed specimen than in the 1-hour-annealed specimen, but they were not found in the 24-hour-annealed specimen. Ultrafine,
equiaxed grains of 0.05 to 0.2 μm in size were formed inside the adiabatic shear band, and their boundaries had characteristics of high-angle grain boundaries.
These phenomena were explained by dynamic recrystallization due to a highly localized plastic strain and temperature rise
during dynamic deformation. 相似文献
4.
Dong-Geun Lee You Hwan Lee Sunghak Lee Chong Soo Lee Sun-Moo Hur 《Metallurgical and Materials Transactions A》2004,35(10):3103-3112
Effects of microstructural morphology on dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy plates
were investigated in this study. Dynamic torsional and 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 and
tensile properties. According to the dynamic torsional test data, maximum shear stress and fracture shear strain of the bimodal
microstructure were higher than those of the equiaxed microstructure, and the possibility of the adiabatic shear band formation
was more likely in the equiaxed microstructure than in the bimodal microstructure. In the ballistically 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 flow lines or shear bands. In the case of the bimodal microstructure,
shear bands were found in limited areas near the penetrated surface without occurring delamination, and their number was smaller
than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of
the equiaxed microstructure, which was consistent with the dynamic torsional test results. 相似文献
5.
Jinkeun Oh W. C. Lee Sung Gyu Pyo Woojin Park Sunghak Lee Nack J. Kim 《Metallurgical and Materials Transactions A》2002,33(12):3649-3659
This study is concerned with the microstructural analysis of multilayered or bulk Ti aluminide sheets fabricated by the self-propagating
high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. Multilayered Ti/Al sheets were prepared by
stacking thin Ti and Al sheets alternately, and a good Ti/Al interfacial bonding was achieved after rolling at 500 °C. When
these sheets were held at 1000 °C, spheroidal TiAl3 phases were formed by the SHS reaction at Ti/Al interfaces and inside Al layers. Microstructural analysis on the hot-rolled,
multilayered Ti/TiAl3 sheets revealed that intermetallic phases such as TiAl2, TiAl, and Ti3Al were formed at Ti/TiAl3 interfaces due to interaction between Ti and TiAl3 and that pores formed in the TiAl3 layer were significantly reduced during hot rolling. When multilayered Ti/Ti aluminide sheets were heat treated at 1000 °C,
Ti3Al, TiAl, and TiAl2 were grown as Ti and TiAl3 were consumed. As the heat treatment proceeded, TiAl grew further, eventually leading to the fabrication of multilayered
sheets composed of Ti3Al and TiAl. Bulk Ti aluminide sheets, having a lamellar structure of Ti3Al and TiAl, instead of multilayered sheets, were also fabricated successfully by heat treatment at 1400 °C. This fabrication
method of the bulk sheets had several advantages over the method by hot forging or rolling of conventional cast Ti aluminides.
From these findings, an idea to fabricate multilayered or bulk Ti aluminide sheets by hot rolling and heat treatment is suggested
as an economical and continuous fabrication method, and the formation and growth mechanisms of interfacial phases are elucidated
in this study. 相似文献
6.
Seung Youb Han Sang Yong Shin Hyuk-Joong Lee Byeong-Joo Lee Sunghak Lee Nack J. Kim Jai-Hyun Kwak 《Metallurgical and Materials Transactions A》2012,43(3):843-853
An investigation was conducted into the effects of annealing temperature on microstructure and tensile properties of ferritic
lightweight steels. Two steels were fabricated by varying the C content, and were annealed at 573 K to 1173 K (300 °C to 900 °C)
for 1 hour. According to the microstructural analysis results, κ-carbides were formed at about 973 K (700 °C), which was confirmed by equilibrium phase diagrams calculated from a THERMO-CALC
program. In the steel containing low carbon content, needle-shaped κ-carbides were homogeneously dispersed in the ferrite matrix, whereas bulky band-shaped martensites were distributed in the
steel containing high carbon content. In the 973 K (700 °C)-annealed specimen of the steel containing high carbon content,
deformation bands were formed throughout the specimen, while fine carbides were sufficiently deformed inside the deformation
bands, thereby resulting in the greatest level of strength and ductility. These results indicated that the appropriate annealing
treatment of steel containing high carbon content was useful for the improvement of both strength and ductility over steel
containing low carbon content. 相似文献
7.
8.
Effects of plasma spraying conditions on wear resistance of nanostructured Al2O3-8 wt.%TiO2 coatings plasma-sprayed with nanopowders were investigated in this study. Five kinds of nanostructured coatings were plasma-sprayed on a low-carbon steel substrate by varying critical plasma spray parameter (CPSP) and spray distance. The coatings consisted of fully melted region of γ-Al2O3 and partially melted region, and the fraction of the partially melted regions and pores decreased with increasing CPSP or decreasing spray distance. The hardness and wear test results revealed that the hardness of the coatings increased with increasing CPSP or decreasing spray distance, and that the hardness increase generally led to the increase in wear resistance, although the hardness and wear resistance were not correlated in the coating fabricated with the low CPSP. The main wear mechanism was a delamination one in the coatings, but an abrasive wear mode also appeared in the coating fabricated with the low CPSP. According to these wear mechanisms, the improvement of wear resistance in the coating fabricated with the low CPSP could be explained because the improved resistance to fracture due to the presence of partially melted regions might compensate a deleterious effect of the hardness decrease. 相似文献
9.
Sang Yong Shin Hackcheol Lee Seung Youb Han Chang-Hyo Seo Kayoung Choi Sunghak Lee Nack J. Kim Jai-Hyun Kwak Kwang-Geun Chin 《Metallurgical and Materials Transactions A》2010,41(1):138-148
An investigation was conducted into the correlation of microstructure and the cracking phenomenon that often occurred in hot-rolled
lightweight steel plates. Two kinds of steels were fabricated with varying Mn and Al contents, and their microstructures,
tensile properties, and high-temperature transformation behavior were investigated. In the two steels, banded structures containing
ferrite grains and κ-carbides were well developed along the rolling direction. Detailed microstructural analyses showed that cracks initiated
at film-type κ-carbides continuously formed interfaces between bands, while the band populated with κ-carbides did not play an important role in initiating cracks. Thus, the formation of band structures and film-type interfacial
κ-carbides must be minimized to prevent the cracking. The decreased content of hardenability elements, including aluminum,
higher finish-rolling temperature, reduced central segregation during the slabmaking process, and decreased material variation
during hot rolling, were suggested as practical methods for preventing the cracking. 相似文献
10.
Seungmun Jung Jeonghyeon Do Dong-Geun Lee Byeong-Joo Lee Gil-up Cha Sunghak Lee 《Metals and Materials International》2014,20(4):577-583
In this study, new cost-effective Fe-based amorphous coating alloys having high amorphous forming ability were developed by varying the Fe content, while their microstructure, hardness, and corrosion resistance were also evaluated. Chemical compositions that have the lowest driving force of formation of crystalline phases such as Fe3P, Fe3C, and α-Fe were obtained from thermodynamically calculated phase diagrams of the representative FexAl2(P10.83C7.47B1.7)98-x alloy system at a crystallization temperature of 443 °C. Considering the intersections of driving force curves of Fe3P and Fe3C, Fe3P and α-Fe, and Fe3C and α-Fe, the Fe contents were found to be 77.8, 76.2, and 75.8 at.%, respectively. The microstructural analysis results of 1.5-mm-diameter suction-cast Fe-based alloys indicated that the Fe76.5Al2(P10.83C7.47B1.7)21.5 alloy had a fully amorphous microstructure, whereas crystalline phases were formed in other alloys. This alloy showed a better hardness and corrosion resistance than conventional thermal spray coating alloys, and its production cost could also be reduced by using less expensive alloying elements, which could provide a good way to practically apply this alloy to Fe-based amorphous thermal spray coatings. 相似文献