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Conghui Hu,Jianlei Zhang,Yunhu Zhang,Ke Han,Changjiang Song,Qijie Zhai 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.2
High-entropy alloys (HEAs) are novel multi-element alloys based on five or more constituent elements in a range of 5–35 at%. Here we present a method to improve strength of a body-centered cubic (bcc) matrix HEA without loss of ductility. Theimprovement was achieved by phase modulation combined other strengthening effect of interstitial carbon addition. Carbonaddition can enhance strength and retain good ductility in some steels because carbon increases the volume fraction offace-centered cubic (fcc) phase. We used the same principle to design and fabricate a set of Al8(FeCuCrMn)92Cx (x = 0, 1,2, 3, 4 at%) HEAs under near-rapid solidification. Our results showed that carbon addition modulated constituent phases byincreasing the volume fraction of fcc phase and carbides. As a result, addition of carbon increased yield strength of this bccmatrix HEA. But the ductility decreased, especially when carbon content was higher than 3 at%, which was ascribed to unevendistribution of Cu-rich fcc phase and carbides precipitated in bcc phase region. After annealing at 1173 K for 2 h, additionof 1 at% carbon improved yield strength without compressive fracture. It demonstrated that a proper carbon content additionwith annealing can enhance the yield strength without loss of ductility for this bcc matrix HEA. Thus, interstitial carbon additionis a meaningful method to improve the mechanical properties by phase modulation combined other strengthening effect.
Nb 활용 탄질소 스테인리스강의 부동태 및 대기산화 피막 보호성 연구
이창근,하헌영,이태호,조경목 대한금속·재료학회 2017 대한금속·재료학회지 Vol.55 No.12
The effects of Nb on grain refinement and the pitting corrosion resistance of Febalance18Cr10 Mn0.3C0.3NxNb (x = 0.1 and 0.2 wt%) high interstitial alloys were investigated through microstructure analysis and potentiodynamic polarization tests. The resistance to pitting corrosion of the two alloys, covered with a passive film and an air-formed oxide films, was also examined. As the Nb content increased from 0.1 to 0.2 wt%, the grain size decreased, the volume fraction of Nb(C,N) particles increased, and the pitting potential was elevated. In addition, the two alloys covered with the passive film exhibited higher pitting potentials than the alloys with the air-formed oxide films. The physico-chemical and electronic properties of the passive film formed in an aqueous environment, and air-formed oxide of the two alloys were examined using X-ray photoelectron spectroscopy and Mott-Schottky analysis. As a result, it was revealed that the defect density level in the passive film was the dominant factor in determining the pitting corrosion resistance of the specimens. Nb addition was found to promote the formation of a protective passive film with low defect density, which resulted in the increase in the pitting potential. Moreover, the fact that the point defect density of the passive film was smaller than the air-formed oxide films was observed in both alloys, which can explain the higher pitting corrosion resistance of the passivated specimen than the specimen covered with an air-formed oxide film.
침입형 원소 첨가 스테인리스강 용접부 미세조직 변화 및 부식특성에 관한 연구
문준오(Joonoh Moon),하헌영(Heon-Young Ha),이태호(Tae-Ho Lee) 대한용접·접합학회 2017 대한용접·접합학회지 Vol.35 No.4
Microstructure evolution and corrosion behavior in the simulated HAZ and real welds of high interstitial alloyed austenitic stainless steels were investigated. Three Fe-18Cr-10Mn-N-C alloys with different N and C contents were fabricated using a commercial pressurized vacuum-induction melting furnace. First, the HAZ samples with three different peak temperatures were experimentally simulated using Gleeble simulator under welding condition of 30kJ/cm heat input. In the HAZs, δ-ferrite was formed and its fraction increased with increase in the peak temperature. The electrochemical tests indicated that pitting corrosion resistance significantly decreased by the formation of δ-ferrite, however did not changed with increasing δ-ferrite fraction. Next, the samples for real welds evaluation were prepared by autogeneous welding. The corrosion behavior in real welds was good agreement with that in Gleeble-simulated HAZ. Cross sectional microstructure of real welds indicated that δ-ferrite was formed in the HAZ and thus pitting corrosion selectively occurred at δ-ferrite/austenite phase boundaries in the HAZ.
CoCrFeMnNi 고엔트로피 합금에서 탄소 첨가 및 재결정열처리가 미세구조 및 기계적 특성에 미치는 영향
고준영,송재숙,홍순익 대한금속·재료학회 2018 대한금속·재료학회지 Vol.56 No.1
The effect of carbon addition on the cast and rolled microstructures of CoCrFeMnNi high entropy alloys were studied. Both as-cast CoCrFeMnNi and CoCrFeMnNiC0.1 alloys have a dendritic microstructure. Small carbide particles were observed at the inter-dendritic region in the as-cast CoCrFeMnNiC0.1 and elongated-grained structure with aligned carbides were developed at grain boundaries in CoCrFeMnNiC0.1 after homogenization. The grain size decreased from 9.2 μm to 5.8 μm with the addition of 0.1 wt% carbon. The decrease in grain size is due to its high stored cold work energy during deformation processing, resulting in more active recrystallization in CoCrFeMnNiC0.1 and the pinning effect of carbides. The strengthening in CoCrFeMnNiC0.1 is most likely attributed to solid solution strengthening and nanoscale carbide strengthening, because grain size strengthening was found to be not as effective.
Effects of Nb on Pitting Corrosion Resistance of Ni-Free FeCrMnCN-Based Stainless Steels
Lee, Chang-Geun,Ha, Heon-Young,Lee, Tae-Ho,Cho, Kyung-Mox The Electrochemical Society 2017 Journal of the Electrochemical Society Vol.164 No.9
<P>The effects of Nb on grain refining and pitting corrosion behavior of Fe(balance)18Cr10Mn0.3C0.3NxNb (x = 0-0.5 wt%) alloys were investigated through microstructure analysis, potentiodynamic polarization tests, and passive film analyses. As Nb content increased, the grain size decreased and the volume fraction of Nb(C,N) particles increased. The pitting potentials of the alloys increased as Nb content increased up to 0.2 wt%, but further addition of Nb caused a decrease in the pitting potential. On the grain-refined alloy with the addition of 0.2 wt% Nb, a thick and less defective passive film was formed resulting in the rise of the pitting potential. On the other hand, the decrease in the pitting potentials of the alloys with the addition of Nb more than 0.3 wt% Nb was primarily attributed to the increased volume fraction of coarse Nb(C,N) which provided initiation sites for pitting corrosion. (C) 2017 The Electrochemical Society. All rights reserved.</P>
Thermodynamic calculation of the stacking fault energy in Fe-Cr-Mn-C-N steels
Lee, Seung-Joon,Fujii, Hidetoshi,Ushioda, Kohsaku Elsevier 2018 Journal of Alloys and Compounds Vol.749 No.-
<P><B>Abstract</B></P> <P>To determine the thermodynamic parameters for the calculation of the accurate stacking fault energy (SFE) in Fe-Cr-Mn-C-N steels using the sublattice model, the comparison between the calculated and experimental SFE values was conducted. It was realized that the relationship between SFE and alloying elements in Fe-Cr-Mn-C-N steels was different from that of the conventional Fe-Cr-Ni stainless steels. The SFE increased with the increasing Cr concentration up to a critical value, then decreased again with further increased Cr concentration. The critical value decreased with the addition of Mn, C and N. In contrast to Cr, the addition of Mn continuously increased the SFE, regardless of the additions of C and N. Regarding the C and N, they also increased the SFE linearly and the impact of N on the SFE was only slightly effective relative to that of C. Accordingly, we realized that the thermodynamic calculation using the suggested combination of thermodynamic parameters should be considered for more accurate SFE calculation in Fe-Cr-Mn-C-N steels.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Stacking fault energy in Fe-Cr-Mn-C-N steels was thermodynamically calculated. </LI> <LI> For accurate stacking fault energy, measured and predicted energies were compared. </LI> <LI> Cr increases stacking fault energy, then decreases it again over a critical value. </LI> <LI> Mn, C and N continuously increase stacking fault energy. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>