Hyun Uk Hong

Significant decrease in interfacial energy of grain boundary through serrated grain boundary transition

The mechanism of serrated grain boundary formation and its effect on liquation behaviour have been studied in a wrought nickel-based superalloy – Alloy 263. It was newly discovered that grain boundaries are considerably serrated in the absence of γ ′-phase or M23C6 at the grain boundaries. An electron energy-loss spectroscopy study suggests that serration is triggered by the discontinuous segregation of C and Cr atoms at grain boundaries for the purpose of relieving the excessive elastic strain energy. The grain boundaries serrate to have specific segments approaching one {111} low-index plane at a boundary so that the interfacial free energy of the grain boundary can be decreased, which may be responsible for the driving force of the serration. The serrated grain boundaries effectively suppress grain coarsening and are highly resistant to liquation due to their lower wettability resulting from a lower interfacial energy of the grain boundary.

Investigation on the Secondary Reaction Zone Formation of Metallic Bond Coating Layer Produced by High Velocity Oxygen Fuel Deposition in a Ni-Based Single Crystal Superalloy

The formation of the secondary reaction zone (SRZ) at the interface between the CoNiCrAlY coating and experimental Ni-based single crystal superalloy substrate was investigated. Transmission elec- tron microscope (TEM) observation revealed that the SRZ was composed of recrystallized γ΄ grains with a high density of fine topologically close-packed (TCP) precipitates. The TCP precipitate was identified as the faulted and twinned rhombohedral μ phase. TCP precipitation appears to be analogous to the discontinuous precipitation. The γ΄ grains nucleated epitaxially with grains in the coated layer. Preferential formation of the SRZ was observed along the direction. This anisotropy of SRZ formation may be essentially related to the preferential dislocation gliding direction where recrystallization would be most active due to high strain energy and the supply of fast interdiffusion pathways. Based on the suggested SRZ formation mechanism, the intermediate annealing is proposed to suppress SRZ formation in terms of mitigation of strain energy and interdiffusion. †(Received September 26, 2013)

Heat treatment response of TiC-reinforced steel matrix composite

A particulate TiC-reinforced SKD11 steel matrix composite is fabricated by using a pressure infiltration casting, achieving a homogeneous distribution of the particles with 60 vol%. The retained austenite fraction in the composite matrix is approximately 19% after quenching from the austenitization temperature of 1010 °C, which is larger than 13% in as-quenched condition of unreinforced SKD11. A combined analysis on the austenite lattice parameter using XRD profiles and first-principle calculation suggests the increase of carbon content in the steel matrix possibly by partial dissolution of TiC during casting. The change of carbon content and prior austenite grain size reasonably accounts for the increase of retained austenite fraction in the composite matrix. In the austenitizing temperatures ranging from 950 °C to 1040 °C, the retained austenite fraction in the composite matrix in as-quenched condition increases more rapidly than that of unreinforced SKD11 with the increase of austenitization temperature, while the hardness of the composite is less sensitive to the austenitization temperature. This suggests that it is advantageous to conduct the austenitization at a temperature below 1010 °C, which is typical practice of austenitization of the unreinforced SKD11, because the retention of austenite is effectively suppressed while minimizing the loss of hardness.

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.