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In the fourth part of this study, the order-disorder phase transition of Cr2N precipitates during electron irradiation was investigated utilizing transmission electron microscopy (TEM). The superlattice reflections characterizing the ordered Cr2N superstructure gradually disappeared during electron irradiation in regular sequence: (001), (1/3 1/3 0) and (1/3 1/3 1), indicating that the order-disorder phase transition of Cr2N occurred. The convergent beam electron diffraction (CBED) observation revealed that the space group of disordered Cr2N was P63/mmc, and corresponded to the hcp sublattice of metal atoms with random distribution of nitrogen atoms in six octahedral interstices. The redistribution model of nitrogen atoms through the order-disorder phase transition was discussed in terms of the characteristics of superlattice reflections.

keyword : Order-disorder transition, ODT, Electron irradiation, Superlattice reflections, TEM and convergent beam electron diffraction, CBED

The effects of Mn on the crystallization behaviors of Fe-bearing intermetallics in A356 alloy were studied. β-Al5FeSi phase was converted into α-Al(Fe, Mn)Si phase in response to Mn addition. As the Mn content was increased in A356 alloy, crystallization temperature of α-Al(Fe, Mn)Si phase was elevated and α-Al(Fe, Mn)Si phases were coarsened into a rod-like or skeleton-like morphology. In the alloy containing 0.40 wt.%Mn, needle-like α-Al(Fe, Mn)Si phase was newly grown nearby Si particles during solution treatment. Increasing Mn/Fe ratio in α-Al(Fe, Mn)Si phase changed crystal structure of α-Al(Fe, Mn)Si phase from BCC(disorder) into SC(B2-ordered) state.

keyword : A356 alloy, Solidification sequence, α-Al(Fe, Mn)Si phase, B2 ordered crystal structure

Ti-15V-3Cr-3Sn-3Al (Ti-15-3) alloy was cold-deformed by flow forming for manufacturing a seamless tube component. The cold-deformed Ti-15-3 alloy was subjected to heat treatment employing an intermediate recovery annealing at 650℃~750℃ and subsequent aging at 500℃ in order to achieve a good combination of tensile strength and ductility. An increase in the amount of cold deformation led to an increase in tensile strength and a decrease in ductility, because of elongated, severely deformed β-grain structure. Direct aging formed fine aggregates of α-precipitates which had the Burgers orientation relationship with β matrix. Further increase in tensile strength occurred after direct aging only, but it caused a significant decrease in ductility to ~3%. An intermediate recovery annealing at 700℃, followed by aging treatment retained a higher ductility of ~10%, with a moderate increase in tensile strength. Very fine β recrystallized grains and sub-grains were formed by an intermediate recovery annealing, and their boundaries provided a preferential nucleation site of α-precipitates during subsequent aging. The uniformly distributed, randomly oriented a-precipitates promoted the improvement of strength-ductility balance.

keyword : Ti-15V-3Cr-3Sn-3Al, Cold flow forming, Recovery, Aging, Precipitation

This study is concerned with the analysis of fracture toughness in the transition temperature region of API X70 pipeline steels according to ASTM E1921 standard test method. Two kinds of steels were fabricated by varying finish rolling temperatures, and their microstructures and mechanical properties were examined. Elastic-plastic cleavage fracture toughness, Kjc was determined by 3-point bend tests, using precracked Charpy V-notch (PCVN) specimens, and then the measured Kjc values were interpreted by the 3-parameter Weibull distribution. Relationship between Jc, critical distance, reference temperature in the transition region, and plane strain fracture toughness were discussed on the basis of the cleavage fracture behavior in the transition temperature region. Fractographic observation indicated that the critical distance from a precrack tip to a cleavage initiation site linearly increased with increasing the critical J (Jc) value. The steel rolled in the single phase region had lower plane strain fracture toughness and higher reference temperature than that rolled in the two phase region because of the higher volume fraction of martensite-austenite constituents and the larger effective grain size.

keyword : API X70 pipeline steel, ASTM E1921 standard test method, Elastic-plastic cleavage fracture toughness, Weibull distribution

Correlation between microstructure and wear resistance of Al2O3-13 wt.%TiO2 coatings plasm-sprayed using nanopowders was investigated in this study. Four types of nanostructured Al2O3-13 wt.%TiO2 coatings were plasma-sprayed on a low-carbon steel substrate by using different critical plasma spray parameters (CPSP), and were compared with an Al2O3-13 wt.%TiO2 coating plasma-sprayed using conventional powders. Microstructural analysis showed that the nanostructured coatings consisted of completely melted region of γ-Al2O3 phase and partly melted region, while the conventional coating consisted mainly of completely melted region of γ-Al2O3, together with some TiO2-rich areas located along splat boundaries. The wear test results revealed that the wear resistance of the nanostructured coatings was three or four times better than that of the conventional coating because the preferential delamination seriously occurred along the TiO2-rich areas in the conventional coating. In the nanostructured Al2O3-13 wt.%TiO2 coatings, bonding between splats was excellent without the TiO2-rich areas, thereby leading to the enhancement of the wear resistance over the conventional coating.

keyword : Plasma spray coating, Nanopowder, Al2O3-TiO2, Wear resistance

In this study, C-Mn-Si based high strength steels for cold forming bolt with the range of 0.15-0.25%C and 1.5-2.0%Mn were designed to reduce C and Si and the effect of alloying elements on microstructures and mechanical properties have been studied. Microstructural observation, tensile test, Charpy impact test, and fatigue test were conducted. Microsructures were changed from bainite+pearlite to martensite+bainite with increasing carbon and manganese. Strength and yield ratio were increased, elongation, reduction of area and impact energy were decreased with increasing carbon and manganese. The fatigue life of Fe-C-Mn-Si steels was 3~5 times longer than conventional machine structural steel (S45C) tested with mean stress of 50% tensile strength

keyword : Cold forming, Microstructure, Alloying element, Fatigue Life, Mechanical properties

In the development of an advanced spent fuel management process based on the molten salt technology, it is essential to choose the optimum material for the process equipment handling the molten salt. In this study, the corrosion behavior of Inconel alloys and Incoloy 800H in a molten salt of LiCl-Li2O under an oxidation atmosphere has been investigated at 650℃ for 24~312 hours. The order of the corrosion rate was Inconel 600 < Inconel 601 < Incoloy 800H < Incnel 690. Inconel 600 showed the highest corrosion resistance among the examined superalloys and Inconel 690 exhibited the highest corrosion rate. The corrosion products of Inconel 600 and Inconel 601 were Cr2O3 and NiFe2O4, and for Incoloy 800H they were Cr2O3 and FeCr2O4. In the case of Inconel 690, a single layer of Cr2O3 was formed in the early stage of the corrosion and an outer layer of NiFe2O4 and an inner layer of Cr2O3 were formed with an increase of the corrosion time. Most of the outer scales of the superalloys were spalled and the unspalled scale adherent to a substrate is composed of three layers. The outer layer, the middle one and the inner one were Fe, Cr, and Ni-rich oxide, respectively. Inconel 600 showed a localized corrosion behavior and Inconel 601, 690 and Incoloy 800H showed a uniform corrosion behavior. A Ni addition improves the corrosion resistance and high Cr and/ or Fe deteriorates the corrosion resistance.

keyword : Hot molten salt, Molten salt corrosion, Corrosion behavior, Hot corrosion

This paper presents the long-term monitoring results from the water droplet erosion (or moisture erosion) and the analysis of the service life of the last stage rotor blades, particularly the flame-hardened and Stellite-brazed blades, of LP (low pressure) section in Gori, Wolsung, Youngkwang, and Uljin nuclear power plants. The flow of wet steam caused the considerable erosion damage on the leading edge of the last stage rotor blades. Characterization of the erosion process with the operation time has shown that there were three characteristic modes including the first region without showing evident material loss, the second region with a drastic material loss with maximum erosion rate, and the third region with relatively slow erosion rate. Such erosion behavior indicated that the erosive damage on the material was developed by the fatigue fracture induced by repeated impact loads. Based on the simplilled model and calculation, the service life of the last stage rotor blade has been estimated at about 223000 hrs for the flame-hardened blade and 185000 hrs for the Stellite-brazed blade.

keyword : Steam turbine, Rotor blades, 12Cr steel, Stellite, Flame hardening, Moisture erosion, Service Life

The microstructures and mechanical properties of AA5083 alloys processed by warm multiple deformation at different temperatures of 573 K, 623 K, and 673 K were investigated. The Annealed AA5083 plates were compressed at a strain rate of 0.l/sec by two deformation routes, that is, route A and route B. Route A is to rotate the specimen throughout 90˚ around the vertical axis of loading direction at every passes. Route B is to rotate the specimen throughout 90˚ around the parallel axis of loading direction and then rotate it again as route A. The specimen warm-multiple deformed by route B showed finer grain size and more uniform distribution of grain than those processed by route A. When the warm multiple deformation process was repeated up to 8 passes at 673 K, the specimen consisted of ultrafine grained structure with the average grain size less than 4 μm. The superplastic properties can be obtained at the strain rates of 0.01/sec to 0.001/sec at 673 K. The maximum elongation to failure and the corresponding strain rate sensitivity, m, were 450% and about 0.44, respectively.

keyword : Warm multiple deformation, Dynamic recrystallization, Deformation route, Superplasticity, AA5083