• Journal of Welding and Joining
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• v.21 no.2
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• pp.89-96
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• 2003

The change of microstructures in the base metal during transient liquid phase bonding process of directionally Ni base superalloy, GTD-111 was investigated. Bonds were fabricated using a series of holding times(0-7.2ks) at three different temperatures(1403, 1418 and 1453K) under a vacuum of 13.3mPa. In raw material, ${\gamma}$- ${\gamma}$' eutectic phases, platelet η phases, MC carbide and PFZ were seen in interdendritic regions or near grain boundary and size of primary ${\gamma}$' precipitates near interdendritic regions were bigger than core region. The primary ${\gamma}$' precipitates in dendrite core were dissolved early in bonding process, but ${\gamma}$' precipitates near interdendritic regions were dissolved partially and shape changed. The dissolution rate increased with increasing temperature. Phases in interdendritic regions or near pain boundary continually changed with time at the bonding temperature. In the bonding temperature of 1403K, eutectic phases had not significantly changed, but η phases had transformed from platelet shape to needle morphology and PFZ region had widened with time. The interdendritic region and near pain boundary were liquated partially at 1423k and fully at 1453k by reaction of η phases and PFZ. In the bonding temperature of 1453K, interdendritic region and near pain boundary were liquated and then new phases which mixed with η phases, PFZ and MC carbide crystallized during cooling. Crystallized η phases transformed from rod shape to platelet shape with increasing holding time.

• Journal of Korea Foundry Society
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• v.44 no.2
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• pp.31-39
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• 2024

Dendrite boundaries and γ/γ' structure of a second generation single crystalline superalloy CMSX-4 almost disappeared during solution treatment above 1310℃. γ' size in the dendrite core was uniform and fine, however, that in the interdendritic region was coarse and nonuniform. With increasing solutionizing temperature and time, γ' size in the interdednritic region became fine and segregation of alloying elements between dendrite core and interdendritic region diminished. Segregation of solid solution strengthening elements such as W, Co, Re in the dendrite core was not fully removed through the heat treatments, especially that of Re still remained to some extent. Tensile properties at room temperature at which does not cause precipitaion of harmful phases, were improved with increasing solutionizing temperature and time.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1818-1832
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• 1994

Transport process during solidification of an AI-CU alloy in a vertical annular mold of which inner wall is cooled is numerically simulated. A model which can take account of local density dependence on the solute concentration is established and incorperated in the analysis. Results show that thermally and solutally induced convections are developed in sequence, so that there is little interaction between them. Thermal convection effectively removes the initial superheat from the melt and vanishes as solidification proceeds from the cooling wall. On the other hand, solutal convection which is developed later over the meshy and the pure liquid regions leads to large-scale redistribution of the consituents. The degree of the initial superheating hardly affects overall solidification behavior except the early stage of the process, when the cooling rate is kept constant. Macrosegregation is reduced remarkably with increasing cooling rate, because not only the liquidus interface advances so quickly that time available for the solute transport is not enough, but also the interdendritic flow is strongly damped by rapid crystal growth within the mushy region.

• Journal of Korea Foundry Society
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• v.38 no.5
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• pp.103-110
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• 2018

The effects of a heat treatment on the carbide formation behavior and mechanical properties of the cobalt-based superalloy X-45 were investigated here. Coarse primary carbides formed in the interdendritic region in the as-cast specimen, along with the precipitation of fine secondary carbides in the vicinity of the primary carbides. Most of the carbides formed in the interdendritic region were dissolved into the matrix by a solution treatment at $1274^{\circ}C$. Solutionizing at $1150^{\circ}C$ led to the dissolution of some carbides at the grain boundaries, though this also caused the precipitation of fine carbides in the vicinity of coarse primary carbides. A solution treatment followed by an aging treatment at $927^{\circ}C$ led to the precipitation of fine secondary carbides in the interdendritic region. Very fine carbides were precipitated in the dendritic region by an aging heat treatment at $927^{\circ}C$ and $982^{\circ}C$ without a solution treatment. The hardness value of the alloy solutionized at $1150^{\circ}C$ was somewhat higher than that in the as-cast condition; however, various aging treatments did not strongly influence the hardness value. The specimens as-cast and aged at $927^{\circ}C$ showed the highest hardness values, though they were not significantly affected by the aging time. The specimens aged only at $982^{\circ}C$ showed outstanding tensile and creep properties. Thermal exposure at high temperatures for 8000 hours led to the precipitation of carbide at the center of the dendrite region and an improvement of the creep rupture lifetimes.

• Journal of the Korean institute of surface engineering
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• v.35 no.4
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• pp.241-248
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• 2002

Hardfacing layers of Stellite 6 alloy with different molybdenum content are deposited on AISI 1045 carbon steel using plasma transferred arc welding (PTAW). The properties of the hardfacing layer are investigated in order to clarify the effect of molybdenum addition to the cobalt-base alloy. With an increase in molybdenum contents, the size of Cr-rich carbides in the interdendritic region is abruptly refined, but volume fraction of the carbide slightly increases. Also, with an increase of Mo, $M_{6}$ C type carbides are formed instead of Cr-rich $M_{7}$ $C_{3}$ type carbided, and this microstructural change enhanced the mechanical properties of Stellite 6 alloy.

• Korean Journal of Materials Research
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• v.31 no.7
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• pp.409-419
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• 2021

The effect of solidification rate on micro-segregation in investment casting of IN738LC superalloy was studied. In Ni-based superalloys, the micro-segregation of solute atoms is formed due to limited diffusion during cast and solidification. The microstructure of cast Ni-based superalloys is largely divided into dendrite core of initial solidification and interdendrite of final solidification. In particular, mosaic shaped eutectic γ/γ' and carbides are formed in the interdendrite of the final solidification region in some cases. The micro-segregation phenomena formed in regions of dendrite core and interdendrite including eutectic γ/γ' and carbides were analyzed using OM, SEM/EDS and micro Vickers hardness. As a result of analysis, the lack of (Cr, W) and the accumulation of Ti were measured in the eutectic γ/γ', and the accumulation of (Cr, Mo) and the lack of Ti were measured in the interdendrite between dendrite and eutectic. Carbides formed in interdendritic region were composed of (Ti, W, Mo, C). The segregation applied to each microstructure is mainly due to the formation of γ' with Ni₃(Al,Ti) composition. The Ni accumulation accompanied by Cr depletion, and the Ti accumulated in the eutectic region as a γ' forming elements. The Mo tends to diffuse out from the dendrite core to the interdendrite, and the W diffuse out from the interdendrite to the dendrite core. Therefore, the accumulation of Mo in the interdendrite and the deficiency of W occur in the eutectic region located in the interdendrite. Heat treatment makes the degree of the micro-segregation decrease due to the diffusion during solid solution. This study could be applied to the heat treatment technology for the micro-segregation control in cast Ni-based superalloys.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.1
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• pp.162-171
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• 1993

This paper deals with the anisotropy of the mushy region during solidification process of a binary mixture. A theoretical model which specifies a permeability tensor in terms of pricipal values is proposed. Also, the governing equations are modified into convenient forms for the numerical analysis with the existing algorithm. Some test computations are performed for soeidification of aqueous ammonium chloride solution contained in a square cavity. Results show that not only the present model is capable of resolving fundamental characteristics of the tranport phenomena, but also the anisotropy significantly affects the interdendritic flow structure, i.e., double-diffusive convection and macrosegregation patterns.

• Korean Journal of Materials Research
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• v.14 no.3
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• pp.168-174
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• 2004

The aim of this study is to investigate the effect of process conditions on the microstructual changes and mechanical properties of large 7175 aluminum die forgings. The cast billets of 370 and 720 mm in diameter were homogenized and die forged after direct chill casting. The size and volume fraction of second phase particles in 720 mm billet were larger than those of 370 mm billet. The interdendritic sites containing the second phase particles was considered to be a crack initiation region in the process of cold upsetting. The tensile and yield strength of die forged specimens of 720 mm billet in the direction of Land L T were higher than those of 370 mm billet. However, the tensile strength of these specimens were 5 to 10% lower than that of American military specification. The plane strain fracture toughness of die forged specimens of 370 mm cast billet showed almost the same level of 720 mm billet, which was die forged after free forging.

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.1542-1547
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• 2007

Laser surface treatment technologies have been used to improve characteristics of wear and to enhance the fatigue resistance for mold parts. The optical lens with the elliptical profile is designed to obtain a wide surface hardening area with a uniform hardness. The objective of this research work is to investigate the influence of the process parameters, such as power of laser and defocused spot position, on the characteristics of laser surface treatment for the case of SKD61 steel and SCM4 steel. From the results of the experiments, it has been shown that the maximum average hardness is approximatly 700${\sim}$780 Hv when the power, focal position and the travel of laser are 1,095 W, 0mm and 0.3 m/min, respectively. In samples treated with lower scanning speeds, some small carbide particles appear in the interdendritic regions. This region contains fine martensite and carbide in proportions which depend on the local thermal cycle.

• Journal of Powder Materials
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• v.4 no.1
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• pp.18-25
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• 1997

High speed steels with commercial compositions of 10V, Rex20, Rex25, T15, and ASP30 were gas-atomized and then consolidated by hot isostatic pressing (HIPping). The microstructures of gas-atomized powder, as-HiPped billet, and heat-treated billet have been characterized using optical microscope, scanning electron microscope and X-ray diffractometer. In the gas-atomized powders, the solidification structures of 10V and Rex25 alloys show that primary MC carbides embedded within the fine equiaxed dendrites, whereas those of Rex20, T15 and ASP30 alloys exhibited eutectic MC and/or M$_2$C carbides in the interdendritic region. The trace and dendritic morphologies of gas-atomized powder have been retained in as-HiPped billets. The microstructures of as-HiPped billets have been observed to consist of ferrite, $M_6C$ and MC carbides in other alloys with the exception of 10V alloy, which consists of ferrite and MC carbides. The hardness of heat-treated billet makes a favorable comparison with that of as-HIPped billet. This seems mainly to be due to the strengthening by the precipitation of secondary carbides and the change of matrix phase from $\alpha$-ferrite to martensite.