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The effect of heavy deformation by ECAP on reverse transformation from ferrite and pearlite to austenite during heating has been investigated in a plain carbon-manganese steel using a dilatometer and optical and electron microscopy. Pearlite first transformed to austenite up to about 790℃ and then ferrite changed to austenite in both ECAPed and unECAPed specimens. Austenite preferentially formed at the interface between pearlite and proeutectoid ferrite and at the pearlite nodule boundaries. The recrystallization of proeutectoid ferrite and pearlitic ferrite occurred in advance before the reverse transformation in the case of ECAPed specimen. Consequently, the reverse transformation kinetics of ferrite to austenite is not greatly different in ECAPed and unECAPed specimens. However, the reverse transformation of pearlite to austenite in the ECAPed sample was faster than that in unECAPed one because of enhanced decomposition of pearlitic cementite by ECAP.

keyword : ECAP, Low carbon-manganese steel, Reverse transformation, Recrystallization, Austenite

This paper describes an ab initio study on interface energies, misfit strain energies, and electron structures at coherent interfaces Fe(bcc structure)/MCs(NaCl structure, M = V, Nb, Ta). The interface energies at relaxed interfaces Fe/VC, Fe/NbC, and Fe/TaC were -0.120, -0.169, and -0.158 J/㎡, respectively. It was understood that the dependence of interface energy on the type of carbide was closely related to changes of the binding energies between Fe, M, and C atoms before and after formation of the interfaces Fe/MCs with the help of the NLP/NNBB (Discrete Lattice Plane/ Nearest Neighbour Broken Bond) model and data of the electron structures. The misfit strain energies in Fe/VC, Fe/NbC, and Fe/TaC systems were 0.086, 0.891, and 0.827 eV per 16 atoms(Fe; 8 atoms and MC; 8 atoms). More misfit energy was generated as difference of lattice parameters between the bulk Fe and the bulk MCs increased.

keyword : Interfacial energy, Misfit strain energy, Transition metal carbides, bcc iron, Ab initio calculation

The effects of pre-oxide and pre-hydride on the behavior of the Loss of the Coolant Accident (LOCA) event after extended reactor operation were investigated in this study. High temperature ballooning and quench test were performed at the Zircaloy-4 claddings which have previously been treated up to 50 μm thick oxide and 1000 ppm of hydrides, respectively. In the high temperature ballooning test, initially pressurized claddings were heated until rupture. Compression test was performed on the quenched claddings after high temperature oxidation. The results showed that both pre-oxide and pre-hydride affected the high temperature deformation due to the constraint of α phase by the pre-oxide as well as the expansion of β phase by the pre-hydride. In the quench test, pre-oxide did not affect the high temperature oxidation whereas ductility of the pre-hydrided claddings greatly decreased because pre-hydride increased the oxygen contents in the residual β phase.

keyword : Loss of Coolant Accident, LOCA, zirconium cladding, high burnup, oxide, hydride

The mechanical properties and micro-structure of 0.1%C-1.3%Si-1.5%Mn TRIP steel were investigated. The specimens were annealed with the heat cycle of Continuous Annealing Line (CAL). Experimental results revealed that soaking temperature should be higher than 760℃ and Over-Aging temperature should be 350~400℃ in order to obtain high austenite volume fraction and good formability of TRIP steel. The evolution of austenite phase through the each annealing section was revealed by analyzing the water-quenched micro-structure and then the effects of each annealing section on the micro-structure were studied. Finally, from the analysis of correlation between the volume fraction of retained austenite and the elongation of annealed specimens, it was revealed that the elongation of TRIP steel has a positive correlation with the volume fraction of retained austenite.

keyword : TRIP steel, Mechanical properties, Microstructure, Retained Austenite

Phases of the Fe-Mn alloy vary depending on Mn content of the alloy. Plastic deformation is also known to alter the phase content of the alloy. The ε (HCP) and γ (FCC) phases of the Fe-17Mn alloy are known to transform to the α` phase with deformation, which has a BCC structure. The current research aims to investigate the effect of the strain-induced transformation on the sliding wear of the Fe-17Mn alloy. Wear tests of the alloy were carried out using a pin-on-disk wear tester at various loads (0.5 N-100 N) under a constant sliding speed condition of 0.38 m/s against a glass (83% SiO2) bead. Worn surfaces and wear debris of the alloy were examined using an SEM and XRD. During the wear, at the wearing surface, the ε and γ phases of the alloy transformed into the α` phase by the deformation at applied loads exceeding a critical value. The transformation was found to accelerate the wear of the Fe-17Mn alloy to result in a higher wear rate.

keyword : Fe-17Mn alloy, γ(FCC), ε(HCP), α`(BCC) phases, Strain-Induced Phase transformation, Sliding wear

Cold spray is an emerging technology that addresses many of the shortcomings of conventional thermal spray processes. The coatings in cold spray process are formed only by high-velocity particles impacting on the substrate to be coated. One of the main characteristics of the cold spray is solid state coating process. This article reports on the effects of in-situ powder preheating on the deposition efficiency, tensile bond strength, hardness, and porosity of the coatings deposited by the cold spray using Ni and Cu powders. The test results show that as the powder preheating temperature is increased, the deposition efficiency of the coatings is linearly increased. However, the effect of powder preheating on the hardness and porosity of the coatings seems to be negligible. Failure occurs at the epoxy bond except very weak coating when the tensile bond strength of the coatings is tested. Thus, it can be concluded that tensile bond strength of the coatings by the cold spray is above 1000 kg/㎠.

keyword : Cold spray, Deposition efficiency, Porosity, Bond strength, Hardness, Powder preheating

It is demonstrated that the effect of hydrogen addition on surface structure and residual stress of hexagonal boron nitride(hBN) thin film deposited by UBM(unbalanced magnetron) sputtering method. The incorporated Ar atoms induced compressive residual stress in the hBN film, whose magnitude appeared to be proportional to the Ar concentration. The hydrogen addition during deposition of BN films leads to a partial transformation of Sp² bonds into Sp³ bonds on the surface of hBN and this surface structure interrupts the penetration of Ar atoms. In the result, the residual stress of hBN thin film which transferred from Sp² to Sp³ bonding state due to the hydrogen addition decreases.

keyword : hydrogen, Ar incorporation concentration, residual stress, surface morphology

The thickness dependence of the electrical resistivity of high purity Cu films deposited by applying a negative substrate bias voltage of -50V was evaluated using the surface scattering of Fuch-Sondheimer (F-S) model and the grain boundary scattering of Mayadas-Shatzkes (M-S) model. For fitting the F-S and M-S models to the experimental data, we have used the approximate equations described by F-S and M-S models and have combined two approximate equations to propose a simple form. By fitting the theoretical model with the simple form, the electrical resistivity of Cu films was in a good agreement with the theoretical curve under the following condition; the film thickness is 2.2~2.3 times than the average grain size at the surface scattering coefficient p = 0, and the grain boundary reflection coefficient R = 0.24. Furthermore, it was found that the effect of resistivity increase produced by solute impurities in the Cu films could be ignored to evaluate the resistivity increase.

keyword : Thin film, Copper, Electrical resistivity, Impurity, Surface scattering, Grain boundary scattering, Grain size

Preparation of silver powders by electroreduction of silver compounds was investigated. Three types of silver compounds were prepared by precipitation: silver chloride, silver sulfate, and silver phosphate. Almost complete conversion of silver phosphate to silver was verified by X-ray diffraction analysis and inductively coupled plasma spectrometry. The shape of silver phosphate powder was not changed during the reaction, but the size was reduced. Raising the electrolyte temperature, however, increased the solubility of silver phosphate, leading to an appreciable change in the shape of the reduced silver powder. The proposed electroreduction mechanism of silver phosphate is a solidstate reaction, which is affected by dissolution and crystallization under high solubility conditions.

keyword : Electroreduction, Silver compound, Silver powder, Solid-state reaction, Dissolution and Crystallization