Papers by Author: Sun Ig Hong

Abstract: Stress-strain responses and microstructure of multi-phase CoCrCuMnNi and CoCrMnFeCu alloys in which Fe or Ni was replaced by Cu from Cantor alloy were studied. The deformation mechanisms of CoCrCuMnNi and CoCrMnFeCu were observed to be influenced by the presence of brittle sigma phase and the separated Cu-rich and the matrix phase. CoCrCuMnNi exhibited the relatively lower strength and excellent deformability, while CrMnFeCoCu alloy exhibited higher strength and lower ductility. The higher strength and the lower ductility of CoCrCuMnNi is associated with the presence more frequent and coarser sigma phase than those in CoCrCuMnNi.

Abstract: Hot deformability and fracture of as-cast Fe-Cr-Mn-Ni stainless steel ingots with high nitrogen and high carbon contents were studied. Effective stress-strain curve indicates a decrease of the UTS from 510MPa to 90MPa with increase of temperatures from 600°C to 1200°C. Effective stress-strain curves exhibited typical work hardening until the final stage of fracture from 600°C up to 900°C. The fracture elongation decreased from 600°C with the increase of temperature up 850°C, but started to increase appreciably as the work softening becomes dominant in the stress-strain curves above 950°C, suggesting the increase of ductility above 950°C is associated with dynamic recrystallization. The high temperature deformability was enhanced above 1000°C by homogenization of the as-cast alloy due to the dissolution and redistribution of segregated particles, providing the homogeneous distribution of the nucleation site of dynamic recrystallization. . The presence of fatal crack at 1,250°C and the loss of hot ductility can be attributed to the partial melting in austenite grain boundaries at high temperatures.

Abstract: Microstructure and mechanical properties of cast and cold-rolled equitomic CoCrFeCuNi alloy in which Mn was substituted by Cu from Cantor alloy was studied. The separation into two solid solutions (Cr-Co-Fe rich and Cu-rich phases) were observed in CoCrFeCuNi. Coarsening and widening of interdendritic Cu-rich phase after homogenization was observed after homogenization, suggesting Cu-rich phase is thermodynamically stable. The compressive stress-strain curves of homogenized cast CoCrFeCuNi alloy exhibited the reasonably high strength and excellent deformability for the cast alloy. The yield strength increased up to 960MPa after cold rolling from 265MPa of the homogenized cast alloy. The significant increase of yield strength is thought to be associated with the alignment of Cu-rich second phase in addition to cold work dislocation storage after cold rolling.

Abstract: Microstructure and mechanical properties of equiatomic CrMnCoNiCu alloy in which Fe was substituted by Cu from Cantor alloy was studied. The separation of solid solution phase into two solid solutions (Cr-Co rich and Cu-rich phases) were observed in CrMnCoNiCu. The coarsening and widening of interdendritic Cu-rich phase after homogenization was observed and supported by the increase of XRD peak height from Cu-rich phase compared to that from Cr-Co rich phase after homogenization. The increase of the peak from Cu-rich phase can be attributed to the thermodynamic stability of Cu due to positive mixing enthalpy of adding Cu. The stress-strain curves of CrMnCoNiCu alloy exhibited the reasonably high strength and excellent deformability for the cast alloy. The yield stress of CrMnCoNiCu was observed to be 390MPa and it could be deformed without crack formation up to the true strain 0.85 to reach the flow stress as high as 662Mpa.

Abstract: In this study, electro nickel coating was applied and its effect on the electrical conductivity and mechanical properties was investigated in Cu/Ni/Al/Ni/Cu clad composite after annealing at various temperature. After annealing at 300^OC for 3hrs, the interface reaction layer at the Cu/Al interface was observed to be 3μm and no reaction layer was observed at the Ni/Al interface. After annealing above 400^OC for 3hrs, the interface reaction layer at the Cu/Al interface in the absence of Ni layer was observed to grow more rapidly with increase of annealing temperature compared to that at the Ni/Al interface. The electrical conductivity increased after annealing up to 300^OC for 3 hrs. possibly because of enhanced interface bonding and the recovery in the matrix. However, it was deteriorated after annealing above 400^OC for 3 hrs. because of the formation of interface intermetallic. The more localized bending in the as-roll-bonded clad composite and that annealed at 200°C can be attributed to the near-zero and negative hardening rate in bending over the whole displacement. In this case, once bending occurs, bending continue to occur in the localized region because the work hardening due to the localized bending is negligible, leading to the localized fracture.

Abstract: In this study, the effect of carbon addition the cast and rolled microstructures of Cantor alloy type FeCoCrNiMn high entropy alloys. Both as-cast FeCoCrNiMn and FeCoCrNiMnC_0.1 alloys have dendritic microstructure. Small particles, which may be associated carbon addition exist in the dendrite arms in FeCoCrNiMnC_0.1 alloy. After homogenization treatment at 1327K for 24 hrs., dendritic structure was completely eliminated after annealing. Dendritic structure was converted to the structure with elongated grains, especially for carbon added FeCoCrNiMnC_0.1. The development of elongated grains is associated with the direction of the primary arms in the dendritic structure. Carbides are segregated at the grain boundaries in FeCoCrNiMnC_0.1 alloy. It also appears that growth of grains is impeded by the segregation of carbides. It is apparent that the grain boundary precipitates are Cr-rich. Both the strength and ductility of FeCoCrNiMnC_0.1 increased over FeCoCrNiMn with the addition of 0.1 wt. % carbon. The increase of ductility in FeCoCrNiMnC_0.1 may be caused by the rapid hardening in FeCoCrNiMnC_0.1 due to dislocation-solute interaction.

Abstract: In this study, creep properties and fracture behavior of CrMnFeCoNi high entropy alloy (HEA) were studied at intermediate temperatures. The invert-type transient primary creep behaviors were observed in CrMnFeCoNi high entropy alloy. Creep behaviors of HEA are similar to those of class I solid solution alloys. The transient creep curves upon increase of stress by 5MPa in the steady state creep region did not change much except the sudden strain increase. And, no decrease of creep rate was observed upon increase of stress. Instead, the slightly invert transient creep or almost straight creep curves were observed, supporting the high friction stress. CrMnFeCoNi high entropy alloy has a stress exponent of 3.75 and the creep activation energy was calculated to be 278KJ/mole. The fracture strain increased from 1.3 to 1.6 with the decrease of stress from 96 MPa to 48MPa. The lower stress exponent along with the invert type primary creep curves strongly suggest that the creep of CrMnFeCoNi high entropy alloy at 600°C~650°C occurs by a glide controlled process.

Abstract: One of the important issue in the production of the industrial scale Cu/Al/Cu clad plate is the evolution of residual stress/strain during rolling the stack of thick slabs into thin wide and long plates. In this study, the effect of residual stress/strain and annealing on the bend forming behaviors of Cu/Al/Cu clad composite were studied. Tensile strength of separated convex side Cu layer from the bent Cu/Al/Cu clad plate was observed 350MPa, smaller than that of concave side Cu-layer (380MPa). The lower strength and higher fracture strain in the convex side Cu layer is attributed to the presence of the compressive residual strain in the convex side. After heat-treatment at 380°C no pronounced interface debonding and separation between Cu and Al layers were noted, supporting a reliable interface bonding between Cu and Al. The ductility of Cu/Al/Cu clad plate was observed to be greater than that of Al, which can be explained by the co-deformation induced the interaction and constraint between bonded Al and Cu layers. The bend forming behavior of the bent Cu-Al-Cu clad composite was not greatly influenced by the residual stress/strain developed during rolling because of the thick Al layers with negligible residual stress between Cu.

Abstract: In this study, electro nickel coating was applied and its effect on the interfacial stability and cracking behaviors were investigated in the multi-layered Ni-plated-Cu/Al/Ni-plated-Cu clad composite. Ni plating with 5μm thickness on the Cu sheet before cladding does not form a continuous layer between Cu and Al because of its low ductility. Ni layer covers the part of the Cu/Al interface. In the interface region without Ni layer, CuAl₂, CuAl and Cu₉Al₄ were found to be formed after annealing whereas Al₃Ni₂ and Al₃Ni were found to be formed in the interface region with Ni layer. After bending, interface crack developed initially in the interface region with plated Ni layer. On the other hand, the interface region with no Ni layer, no interface cracks were found to formed, suggesting that the bonding between Al and Ni is not strong enough. After interface crack formation, cracks developed in the Al layer, which may lead to the fatal fracture.

Abstract: In the present study, the microstructural stability and mechanical properties of a MnFeCoNiCu alloy in which Cr was replaced by Cu from Cantor composition (CoCrFeMnNi) was studied. In the as-cast alloy, the dendrite arms are enriched with Cu and Mn and matrix between dendrite arms is enriched with Fe and Co. Ni was richer in the matrix, but also observed in the dendrite arms. Cu and Mn tend to segregate and solidify initially because the melting temperatures of Cu and Mn are lower than Fe and Co, resulting in the growth of Cu-Mn dendrite. After homogenization, the dendrites structure disappeared and grain boundaries are visible, indicating the segregated elements in the dendrite structure were homogenized. The presence of single phase FCC structure was confirmed after homogenization. The tensile strength of 1220 MPa with the ductility of 6 % was obtained in MnFeCoNiCu alloy.