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Park, Nokeun,Lee, Byeong-Joo,Tsuji, Nobuhiro ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.719 No.-
<P><B>Abstract</B></P> <P>A comparative study about the phase stability of equiatomic CoCrFeMnNi alloy of thermodynamic calculation (Thermo-Calc) and experimental result is shown. The alloy was processed by through a high-pressure torsion (HPT) process and annealing treatment at temperatures ranging from 900 to 600 °C. Phase identification using X-ray diffraction and energy dispersive X-ray spectroscopy indicated that the Cr-rich sigma phase formed during annealing at temperatures lower than 800 °C after the HPT process, and fraction of sigma phase increased as decreasing annealing temperature as predicted by Thermo-Calc. The thermodynamic calculation based on the TCFE database was found to be suitable for predicting the possible phases in the CoCrFeMnNi alloy.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Second phase in CoCrFeMnNi was identified as <I>sigma phase</I> although kinetics of sigma phase is slow. </LI> <LI> A number of defects generated via HPT <I>accelerated formation of sigma phase.</I> </LI> <LI> <I>Thermo-Calc successfully predicted</I> the formation of meta-stable sigma phase. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Sanghyeon Park,Hyunbin Nam,Yoona Lee,Nokeun Park,Sunig Hong,Youngsang Na,Cheolho Park,Namhyun Kang 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.8
This study investigated the gas tungsten arc (GTA) weldability of a base metal (BM) Fe60Co15Ni15Cr10medium-entropyalloy (MEA) by applying CoCrFeMnNi and CrFeMnNiCu fillers. Sound welds without macro defects, such as internal poresor cracks, were obtained. The CoCrFeMnNi-based weld consisted of a face-centered cubic (FCC) structure, whereas theCrFeMnNiCu-based weld comprised (Co, Cr, Fe)-rich phase of FCC1 and a Cu-rich phase of FCC2 with the same FCC structureformed by phase separation. In the CrFeMnNiCu-based weld with a Cu-rich phase, the weld metal (WM) was strongerthan that in the CoCrFeMnNi-based weld. The tensile fracture of all specimens occurred in the coarse-grained heat-affectedzone (CGHAZ) at 298 K, while the tensile properties of the CrFeMnNiCu-based weld were improved compared to thoseof the CoCrFeMnNi-based weld, probably because the Cu-rich phase present in WM blocks the movement of dislocations. The WM of all tensile specimens at 77 K exhibited deformation twins, and deformation-induced martensitic transformationoccurred in the BM and CGHAZ, showing improved strength and ductility compared to those at 298 K.
Desrilia Nursyifaulkhair,Nokeun Park,Eung Ryul Baek 대한용접·접합학회 2021 대한용접·접합학회지 Vol.39 No.5
This study investigated the effect of microstructural decomposition on the mechanical properties of additively manufactured Ti-6Al-4V by directed energy deposition. The formation of α’ martensite and α massive phase (αm) was observed in the deposited layers. The α’ and αm in the lastly deposited layer appeared as a needle-shaped and a sub-lamellar structure, respectively. However, the morphology of α’ and αm was decomposed in the lower layers due to the intrinsic heat treatment. Moreover, The heat conduction rate calculation showed that the lower powder feed rate generates more heat conduction. Therefore, the microstructure was further decomposed for the specimen with a lower powder feed rate. These phenomena consequently affected the mechanical properties and fracture behavior of the Ti-6Al-4V alloy.
Desrilia Nursyifaulkhair,Nokeun Park,Eung Ryul Baek,Jong-bong Lee 대한용접·접합학회 2019 대한용접·접합학회지 Vol.37 No.6
The effect of different process parameters on the formation of lack of fusion (LOF) in Ti-6Al-4V alloy fabricated using DED was studied. The specific energy was calculated to evaluate the minimal amount of required energy to avoid LOF. The results showed that a specific energy smaller than 2700 J.g<SUP>-1</SUP> led to the formation of LOF; however, a higher specific energy was able to successfully prevent the defect. A smaller amount of specific energy resulted in inadequate heat, which was too low to completely melt the metal powder, and resulted in insufficient penetration depth. Subsequently, LOF was observed between the interface of the deposited layers. Furthermore, a higher powder feed rate had a more significant effect on the formation of LOF than higher laser power.
Precipitate behavior in nitrogen-containing CoCrNi medium-entropy alloys
Jodi, Dennis Edgard,Park, Joohyun,Park, Nokeun Elsevier 2019 Materials characterization Vol.157 No.-
<P><B>Abstract</B></P> <P>The effect of nitrogen addition toward nitride precipitate formation and the corresponding behavior in a medium-entropy alloy system of N-CoCrNi was investigated. It was observed that the addition of N induced the formation of a Cr- and N-rich precipitate of Cr<SUB>2</SUB>N. Cr<SUB>2</SUB>N and the face-centered cubic (FCC) matrix in the N-CoCrNi system exhibited a Kikuchi orientation relation in the {111}<I>γ</I>//{0001}Cr<SUB>2</SUB>N plane and 〈110〉<I>γ</I>//〈 1 1 ¯ 00 〉Cr<SUB>2</SUB>N direction, where Cr<SUB>2</SUB>N was observed to be semi-coherent with the FCC matrix. The presence of Cr<SUB>2</SUB>N was also observed to contribute in the hindering of grain coarsening via Zener pinning pressure. The distribution of finer and higher fraction of Cr<SUB>2</SUB>N in the lower annealing temperatures of N-CoCrNi contributed more toward higher Zener pinning pressure compared to the coarser and lower fraction of Cr<SUB>2</SUB>N in the higher annealing temperatures, which subsequently affected grain growth during the annealing treatment.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The effect of nitrogen addition in CoCrNi medium-entropy alloy was investigated. </LI> <LI> Cr<SUB>2</SUB>N precipitate was formed along with the face-centered cubic matrix. </LI> <LI> A Kikuchi crystal orientation relation was observed between the Cr<SUB>2</SUB>N and the matrix. </LI> <LI> Cr<SUB>2</SUB>N precipitate contributed in Zener pinning pressure. </LI> <LI> These caused the formation of significantly finer grain size in N-CoCrNi than CoCrNi. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>