Jeonghyeon Do

Design of cost-effective Fe-based amorphous coating alloys having high amorphous forming ability by thermodynamic calculation

In this study, new cost-effective Fe-based amorphous coating alloys having high amorphous forming ability were developed by varying the Fe content, while their microstructure, hardness, and corrosion resistance were also evaluated. Chemical compositions that have the lowest driving force of formation of crystalline phases such as Fe3P, Fe3C, and α-Fe were obtained from thermodynamically calculated phase diagrams of the representative FexAl2(P10.83C7.47B1.7)98-x alloy system at a crystallization temperature of 443 °C. Considering the intersections of driving force curves of Fe3P and Fe3C, Fe3P and α-Fe, and Fe3C and α-Fe, the Fe contents were found to be 77.8, 76.2, and 75.8 at.%, respectively. The microstructural analysis results of 1.5-mm-diameter suction-cast Fe-based alloys indicated that the Fe76.5Al2(P10.83C7.47B1.7)21.5 alloy had a fully amorphous microstructure, whereas crystalline phases were formed in other alloys. This alloy showed a better hardness and corrosion resistance than conventional thermal spray coating alloys, and its production cost could also be reduced by using less expensive alloying elements, which could provide a good way to practically apply this alloy to Fe-based amorphous thermal spray coatings.

Effect of Ti content on creep properties of Ni-base single crystal superalloys

The effect of Ti content on the creep properties and microstructures of experimental Ni-base single crystal superalloys has been investigated. The experimental alloys were designed to provide better high temperature properties than the commercial single crystal alloy CMSX-4. The creep properties of the experimental alloys, Alloy 2 and Alloy 3, were superior to those of CMSX-4. Alloy 3 showed a longer creep life than Alloy 2 at 900 °C and 950 °C, while it has similar creep life with Alloy 2 at 982 °C. Transmission electron microscopy micrographs of the experimental alloys after the creep test showed distinct deformation features as a function of temperature and Ti content. The dissociation of dislocations into partial dislocations with stacking faults in Alloy 3 was found to improve resistance to creep deformation at 950 °C. The effect of Ti on the creep deformation mechanism was not evident at 982 °C, which resulted in similar creep properties in both experimental alloys. The transition of the γ′ cutting mechanism from dislocations coupled with stacking faults to anti-phase boundary coupled pairs occurred both in Alloy 2 and Alloy 3. However, the transition temperature was higher in Alloy 3 than in Alloy 2 because of the difference in Ti contents.

A Study on Microstructures and Cryogenic Mechanical Properties of Electron Beam Welds between Cast and Forged Inconel 718 Superalloys for Liquid Rocket Combustion Head

Abstract Characterization of microstructures and cryogenic mechanical properties of electro beam (EB) welds between cast and forged Inconel 718 superalloys has been investigated. Optimal EBW condition was found in the beam current range of 36~39 mA with the constant travel speed of 12 mm/s and arc voltage of 120 kV for 10 mm-thick specimens. Electron beam current lower than 25 mA caused to occur the liquationmicrofissuring in cast-side heat affected zone (HAZ) of EB welds. The HAZ liquation microfissure was found on the liquated grain boundaries with resolidified γ/Laves and γ/NbC eutectic constituents. EBW produced welds showing a fine dendritic structure with relatively discrete Laves phase due to fast cooling rate. After post weld aging treatment, blocky Laves phase and formation of γ΄+ γ΄΄ strengtheners were observed. Presence of primary strengthener and coarse Laves particles in PWHT weld may cause to reduce micro-plastic zone ahead of a crack, leading to a significant decrease in Charpy impact toughness at -196℃. Fracture initiation and propagation induced by Charpy impact testing were discussed in terms of the dislocation structures ahead of crack arisen from the fractured Laves phase.Key Words : Superalloy, Electron beam welding, Cryogenic, Microfissure, Fracture toughness, Laves, Dislocation