Effect of Zr Addition on Microstructure and Mechanical Properties of a Cast Cu60Fe40 Alloy

Jongun Moon,Eun Seong Kim,Yeon Taek Choi,Ho‑Joon Moon,Jung‑Wook Cho,김형섭 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.6

Cu–Fe alloys have drawn extensive attraction due to excellent multi-functional properties, including electrical, magnetic,and mechanical properties. However, the immiscible nature of Cu–Fe alloys results in heterogeneous microstructure andunexpected mechanical properties. In this study, a small amount of Zr was added to the cast Cu60Fe40(wt%) alloy to suppressthe liquid–liquid phase separation during solidification of the alloy. The microstructure and mechanical propertiesof the Zr-free and Zr-containing Cu–Fe alloy were investigated. It is found that the addition of Zr successfully inhibits theheterogeneous microstructural evolution caused by liquid–liquid phase separation, while the FeZr-rich precipitates form inthe dual-phase microstructure. The FeZr-rich precipitates affect grain growth of the Zr-containing alloy through the Zenerpinning effect, leading to extra grain boundary strengthening and precipitation hardening. The resulting microstructure ofthe Zr-containing alloy substantially enhances the strength of the alloy with a loss of ductility. The fractured surface revealsthat microcracks are formed near the precipitates, which is associated with the reduced elongation of the Zr-containing alloy.

Mechanical behavior and solid solution strengthening model for face-centered cubic single crystalline and polycrystalline high-entropy alloys

Moon, Jongun,Jang, Min Ji,Bae, Jae Wung,Yim, Dami,Park, Jeong Min,Lee, Jehyun,Kim, Hyoung Seop Elsevier 2018 INTERMETALLICS Vol.98 No.-

<P><B>Abstract</B></P> <P>In the present work, a solid solution strengthening effect in high-entropy alloys (HEAs) was studied by investigating mechanical characteristics of single crystalline CoCrFeMnNi HEA and a corresponding model was developed. (100) and (110) oriented single crystals of the CoCrFeMnNi alloy were grown and their single crystallinity was identified using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) analyses. The mechanical testing of the single crystalline CoCrFeMnNi alloy was performed and the critical resolved shear stress (CRSS) was obtained. A solid solution strengthening modeling based on the lattice friction stress and an intrinsic residual strain of HEAs was performed. The solid solution strengthening effect of HEAs was verified using the model proposed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Mechanical behavior of CoCrFeMnNi single crystals shows a typical face-centered cubic single crystal response. </LI> <LI> The root mean square residual strain of a CoCrFeMnNi alloy system was calculated using a geometrical model. </LI> <LI> A novel model for solid solution strengthening in high-entropy alloys was developed. </LI> <LI> The proposed model predicts the intrinsic yield strength of face-centered cubic high-entropy alloys with remarkable accuracy. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Effects of homogenization temperature on cracking during cold-rolling of Al_0.5CoCrFeMnNi high-entropy alloy

Moon, Jongun,Bae, Jae Wung,Jang, Min Ji,Baek, Seung Mi,Yim, Dami,Lee, Byeong-Joo,Kim, Hyoung Seop Elsevier Sequoia S.A 2018 Materials chemistry and physics Vol.210 No.-

<P><B>Abstract</B></P> <P>In this work, the effects of homogenization annealing temperature of Al<SUB>0.5</SUB>CoCrFeMnNi HEA on cracking during cold-rolling were investigated. AlNi B2 phases were formed by low-temperature homogenization and affected the cracking phenomenon during cold-rolling. X-ray diffraction, microstructure, and composition analyses and thermodynamic calculations were conducted to identify the crystal structure of the alloy after the homogenization annealing treatment. The phase fraction and hardness of the homogenized alloy confirmed that the formation of the AlNi B2 phase induces cracking during cold-rolling of the Al<SUB>0.5</SUB>CoCrFeMnNi alloy. High-temperature annealing for homogenization of the alloy is recommended to prevent cracking during cold-rolling.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The Al<SUB>0.5</SUB>CoCrFeMnNi HEA annealed at 1200 °C was successfully cold-rolled. </LI> <LI> The alloys annealed at low temperatures were cracked during cold-rolling. </LI> <LI> AlNi-rich B2 phases were formed at lower annealing temperatures than 1200 °C. </LI> <LI> The formation of AlNi-rich B2 phases hardened the alloy. </LI> <LI> The method to prevent cracking during the cold-rolling of the alloy was proposed. </LI> </UL> </P>

Superior cryogenic tensile properties of ultrafine-grained CoCrNi medium-entropy alloy produced by high-pressure torsion and annealing

Sathiyamoorthi, Praveen,Moon, Jongun,Bae, Jae Wung,Asghari-Rad, Peyman,Kim, Hyoung Seop Elsevier 2019 Scripta materialia Vol.163 No.-

<P><B>Abstract</B></P> <P>Ultrafine-grained materials with nanotwins are expected to produce a remarkable combination of strength and ductility. In the present study, ultrafine-grained CoCrNi medium-entropy alloy with nanotwins is fabricated by high-pressure torsion followed by annealing; and investigated for cryogenic tensile properties. The alloy exhibits superior cryogenic tensile properties with a tensile strength of ~2 GPa and tensile strain of ~27%. The cryogenic tensile strength of ultrafine-grained sample increased by 67% as compared to the cryogenic tensile strength of coarse-grained sample due to fine grain size, annealing nanotwins, residual dislocation density, and strong temperature dependence of yield strength.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Role of BCC phase on tensile behavior of dual-phase Al_0.5CoCrFeMnNi high-entropy alloy at cryogenic temperature

Park, Jeong Min,Moon, Jongun,Bae, Jae Wung,Kim, Dong Hyuk,Jo, Yong Hee,Lee, Sunghak,Kim, Hyoung Seop Elsevier 2019 Materials science & engineering. properties, micro Vol.746 No.-

<P><B>Abstract</B></P> <P>Dual-phase Al<SUB>0.5</SUB>CoCrFeMnNi high-entropy alloy consisted of face-centered cubic (FCC) and body-centered cubic (BCC) phases exhibited enhancement of both strength and strain hardening ability at 77 K. It resulted from back stress hardening in high work hardening due to large strength difference of two constituent phases with decreasing temperature.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Effect of annealing heat treatment on microstructural evolution and tensile behavior of Al_0.5CoCrFeMnNi high-entropy alloy

Park, Jeong Min,Moon, Jongun,Bae, Jae Wung,Jung, Jaimyun,Lee, Sunghak,Kim, Hyoung Seop Elsevier 2018 Materials science & engineering. properties, micro Vol.728 No.-

<P><B>Abstract</B></P> <P>In this work, the mechanical characteristics and microstructural evolution of Al<SUB>0.5</SUB>CoCrFeMnNi high-entropy alloy (HEA) were studied after annealing at various temperatures (1000, 1100, and 1200 °C). X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses were performed to reveal the phase and microstructural variations. The mechanical properties related to different microstructures of the alloy were characterized using tensile testing with digital image correlation. Annealing at lower temperatures led to a higher fraction of B2 phase and finer grain size of FCC (face-centered cubic) phase. A good combination of strength and ductility in this alloy was attributed to the ductile FCC matrix and hard secondary B2 phase. The alloy showed the active evolution of deformation twinning due to the low stacking fault energy when Al was added to CoCrFeMnNi to make the HEA. However, for alloy annealed at lower temperatures, twinning activity was suppressed by the smaller size of grains and depletion of Al content in the FCC matrix. The correlation between the microstructure and mechanical properties was also explored using a simple composite model.</P>

Thermally activated deformation and the rate controlling mechanism in CoCrFeMnNi high entropy alloy

Hong, Sun Ig,Moon, Jongun,Hong, Soon Ku,Kim, Hyoung Seop Elsevier 2017 Materials science & engineering. properties, micro Vol.682 No.-

<P><B>Abstract</B></P> <P>The nature of obstacles to dislocation motion in CoCrFeMnNi alloy was analyzed using the thermally activated deformation analyses at low temperatures. The strong temperature dependence of yield stress and small activation volume in CoCrFeMnNi favor the dislocation glide over the obstacles with high friction stress. The activation volume of CoCrFeMnNi alloy (10–100 b<SUP>3</SUP>) in this study is much smaller than those of conventional FCC metals (10<SUP>2</SUP>~10<SUP>3</SUP> b<SUP>3</SUP>), but close to those observed in BCC metals (8–100 b<SUP>3</SUP>) and HCP metals (5–100 b<SUP>3</SUP>). The increase of the activation volume with strain supports overcoming the nanoscale inhomogeneity such as co-clusters and/or short range orders as the rate controlling mechanism. The transition of dislocation structure from planar array to cell structure at 20% strain in CoCrFeMnNi reported in the literature can be attributed to the prevalent shearing of nanoscale inhomogeneity with strain.</P>

Laser weldability of cast and rolled high-entropy alloys for cryogenic applications

Nam, Hyunbin,Park, Chulho,Moon, Jongun,Na, Youngsang,Kim, Hyoungseop,Kang, Namhyun Elsevier 2019 Materials science & engineering. properties, micro Vol.742 No.-

<P><B>Abstract</B></P> <P>Laser similar welding of cast and rolled high-entropy alloys (HEAs) was performed using the cantor system (Co<SUB>0.2</SUB>Cr<SUB>0.2</SUB>Fe<SUB>0.2</SUB>Mn<SUB>0.2</SUB>Ni<SUB>0.2</SUB>). As the welding velocity was increased from 6 to 10 m min<SUP>−1</SUP>, the shrinkage voids, primary dendrite arm spacing, and dendrite packet size decreased, thus improving the mechanical properties of the cast and rolled HEA welds. The cast HEA welds showed tensile properties comparable to those of the base metal (BM). In all the specimens fracture occurred near the heat-affected zone and BM at 298 K. However, the rolled HEA welds showed lower tensile strength than the BM, and fracture occurred in the weld metal (WM). This can be attributed to the larger dendrite packet size of the WM than the grain size of the BM. In addition, the tensile properties of the specimens at the cryogenic temperature were superior to those observed at 298 K, regardless of the cast and rolled HEA welds. This is because the formation of deformation twins and dislocations was predominant at 77 K. Therefore, the laser similar welds of cast and rolled HEAs are suitable for cryogenic applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Laser similar welds of the cast and rolled HEAs were obtained with no defects. </LI> <LI> The cast HEA welds and BM showed similar tensile properties. </LI> <LI> The tensile strength of the rolled HEA welds was ~ 90% of that of the BM. </LI> <LI> The tensile properties of all the specimens at 77 K were superior to those at 298 K. </LI> <LI> The formation of deformation twins and dislocations was predominant at 77 K. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Strain rate effects of dynamic compressive deformation on mechanical properties and microstructure of CoCrFeMnNi high-entropy alloy

Park, Jeong Min,Moon, Jongun,Bae, Jae Wung,Jang, Min Ji,Park, Jaeyeong,Lee, Sunghak,Kim, Hyoung Seop Elsevier Sequoia 2018 Materials science & engineering Structural materia Vol.719 No.-

<P><B>Abstract</B></P> <P>In this work, the effects of strain rate on mechanical deformation and microstructural evolution of CoCrFeMnNi high-entropy alloy (HEA) under quasi-static and dynamic compression were investigated. The HEA exhibited high strain-rate sensitivity values (m = 0.028) of yield strength under quasi-static conditions. In particular, due to the viscous drag effect, the variation of yield strength with strain rate under dynamic compression was much larger than that under quasi-static compression. Microstructural analysis using electron backscatter diffraction shows profuse twinning under both conditions. The dynamically deformed specimens exhibited strongly localized deformation regions (i.e., adiabatic shear bands). The process of dynamic compressive behavior in this HEA is competitive between hardening by dislocation and twinning, and thermal softening. To analyze numerically the flow behavior of the HEA under dynamic conditions, the modified Johnson-Cook model considering adiabatic temperature rise was employed. The modified Johnson-Cook model offered good agreement with experimental results regarding dynamic flow curves of this HEA.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Superplasticity of V₁₀Cr₁₅Mn₅Fe₃₅Co₁₀Ni₂₅ high-entropy alloy processed using high-pressure torsion

Thi-Cam Nguyen, Nhung,Moon, Jongun,Sathiyamoorthi, Praveen,Asghari-Rad, Peyman,Kim, Geon Hyeong,Lee, Chong Soo,Kim, Hyoung Seop Elsevier 2019 Materials science & engineering. properties, micro Vol.764 No.-

<P><B>Abstract</B></P> <P>In this study, the superplasticity of nanostructured V<SUB>10</SUB>Cr<SUB>15</SUB>Mn<SUB>5</SUB>Fe<SUB>35</SUB>Co<SUB>10</SUB>Ni<SUB>25</SUB> (at%) high-entropy alloy processed by high-pressure torsion was investigated using high-temperature tensile testing in the temperature range of 873–1073 K and strain rate range of 5.0✕10<SUP>−4</SUP> to 1.0✕10<SUP>−2</SUP> s<SUP>−1</SUP>. The alloy exhibited extreme elongation at these elevated temperatures, with the greatest elongation of 770% at 973 K without any necking or a notable cavity in the fracture area. Other impressive achievements were also recorded (700% elongation at 1073 K and 3.3✕10<SUP>−3</SUP> s<SUP>−1</SUP> and 600% elongation under other conditions). The equiaxed microstructure was maintained in both the deformed and undeformed regions of the tensile specimen, demonstrating that grain-boundary sliding is the dominant mechanism of superplasticity.</P>