발간논문

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In these sequential papers, the crystallographic features of Cr₂N precipitates in high-nitrogen austenitic stainless steel (HNS) were investigated with particular emphasis on the crystal structure of Cr₂N. In the first part of this study, the precipitation behavior of the second phases were examined utilizing electron microscopy and thermodynamic calculation. The precipitation sequence of the Cr₂N was found to be intergranular, cellular, and intragranular precipitation with aging time. Based on the analyses of SAD patterns, the orientation relationship between y and Cr₂N was found to be [011], [1100]_(Cr₂N) and (111)γ (0001)_(Cr₂N). On prolonged aging, the coarse a-phase with bet structure precipitated along grain and cell boundaries. The precipitation of a-phase could be explained by the mechanism that formation of nitrogen-depleted zone near Cr₂N induced nucleation of the a-phase, which was supported by thermodynamic calculation and electron microscopy. (Received August 23, 2004)

keyword : High nitrogen steel, HNS, Precipitation, Cr2N, σ-phase and Thermodynamic calculation

In the second part of this study, the crystal structure of Cr₂N precipitates was investigated utilizing transmission electron microscopy (TEM). Based on the analyses of SAD patterns with various zone axes, the crystal structure of Cr₂N was confirmed to be trigonal belonging to the space group P31m, and could be explained in terms of E-type occupational-ordering model. Three sets of superlattice reflections (001), (⅓⅓0) and (⅓⅓1) distinguishing trigonal from hcp were identified in Cr₂N superstructure, and the important characteristics of these reflections were discussed in comparison with the previous studies. (Received August 23, 2004)

keyword : Crystal structure, Superlattice reflection, trigonal, ε-type ordering and TEM

In the third part of this study, a theoretical model for the crystal structure of Cr₂N was derived based on the static concentration waves (SCWs) method, and discussed in comparison with other E-type ordering models suggested in Fe-N system. The modified occupation probability function (OPF) for describing the distribution of nitrogen atoms in Cr₂N superstructure was calculated using the superlattice reflections found in the previous study, and could be expressed as: n(r)=c－1/6η₁cos2πz＋4/3η₃cos2/3π(x＋y＋3z). And, this OPF function has four components depending on the distribution of nitrogen atoms within Cr2N superstructure: n₁=c－1/6η₁＋4/3η₃,n₂=c＋1/6η₁－4/3η₃,n₃=c－1/6η₁－2/3η₃and n₄=c＋1/6η₁＋2/3η₃.

keyword : Cr2N superstructure, Static concentration waves (SCWs) method, and Occupation probability function, OPF

In this study, the effect of microstructure on the high strain-rate impact resistance of Ti-6A1-4V-lFe alloy were investigated. For this purpose, a series of dynamic torsion test and exploding bridge wire (EBW) test was conducted on the three typical microstructures, i.e., equiaxed, Widmanstda¨tten and bimodal microstructures. After the dynamic torsion test, thick adiabatic shear bands were well developed along and near the fracture surfaces in the equiaxed and Widmansta¨tten microstructures, while bimodal microstructure revealed many micro-voids near the fracture surface. The EBW test, a method to estimate ballistic impact resistance, also showed higher impact resistance of bimodal microstructure, confirming the result of dynamic torsion test. The different impact resistance of the three microstructures was discussed in detail in terms of microstructural development during the tests. (Received August 10, 2004)

keyword : Ti-6Al-4V-1Fe alloy, Equiaxed, Widmanstatten, Bimodal, Dynamic Deformation Behavior, EBW, Explosive Bridge Wire, test, Adiabatic Shear Band

Wear resistance and fracture toughness of heat-treated duo-cast materials composed of a high-chromium white cast iron and a low-chromium steel were investigated in this study. Different size, volume fraction, and distribution of M7C3 carbides were employed to the wear resistant part by changing the amount of chromium and molybdenum, and the volume fraction of martensite in the austenitic matrix was varied by the heat treatment. In the alloys containing a small amount of chromium, a network structure of eutectic M_(7)C₃carbides was formed along solidification cell boundaries, and led to the improvement of wear resistance and fracture toughness. After the heat-treatment, the selective wear of the matrix and the cracking or fallen-offs of carbides were considerably reduced since the hardness difference between carbides and matrix decreased by the increase in the matrix hardness, thereby leading to the improvement of the wear resistance. However, the fracture toughness of the heat-treated alloys was lower than that of the as-cast alloys because the matrix containing a considerable amount of martensite did not prevent effectively the crack propagation. (Received July 20, 2004)

keyword : Duo-cast alloys, Wear resistance, Fracture toughness, High-chromium white cast iron

A mechno-luminescent SrAl₂O₄:Eu,Dy thin film phosphor on the surface of mechanical test specimen was developed for stress or crack indicating applications. As a preliminary step, SrAl₂O₄:Eu,Dy thin film phosphors were deposited on several conventional substrates such as quartz glass, Si(111), Si(001), and corrugated quartz glass using a pulsed laser deposition (PLD) technique. Processing parameters were optimized, in an attempt to enhance the efficiency of luminescent of the SrAl₂O₄:Eu,Dy thin film phosphor. The SrAl₂O₄:Eu,Dy thin film phosphor was also deposited on the surface of a compact tension specimen composed of polycrystalline Al₂O₃ and the applicability of the SrAl₂O4:Eu,Dy thin film phosphor as a stress or crack indicator was verified in an actual mechanical test. (Received July 14, 2004)

keyword : Mechano-luminescence (ML) material, Pulsed Laser Deposition, PLD, SrAl2O4:Eu,Dy thin film phosphor

The effect of strain rate on the room temperature deformation behavior of a Fe-27.5 at.% Al has been investigated in this study. Phase composition and degree of order in Fe-27.5 at.% Al alloy were varied by aging at various temperatures corresponding to phase fields of D03 and B2 to elucidate the effects of microstructure and degree of order on the room temperature deformation behavior. A series of load relaxation tests was conducted and the results were analyzed based on the internal variable theory of inelastic deformation. Room temperature strength of the Fe-27.5 at.% Al alloy was found to increase with decrease in degree of order. All the flow curves could be successfully described by the constitutive equations of the internal variable theory of inelastic deformation and were found to be governed by the frictional stress element. (Received June 20, 2004)

keyword : Fe-27.5 at.% Al alloy, Degree of order, Load relaxation test, Internal variable theory, Frictional stress

An internal variable theory has been used in this study to investigate quantitatively the major deformation mechanism of a duplex stainless steel. The flow curves obtained from load relaxation tests were found separable into two parts, viz. grain matrix deformation (GMD) curve and grain/phase boundary sliding (G/PBS) curve as was predicted by the internal variable theory. The major deformation mechanism of duplex stainless steel at high temperature is found to be a dynamic recrystallization (DRX) at an early stage of deformation, but grain/phase boundary sliding becomes the major deformation mechanism at the late stage of deformation. Additionally, x phase precipitated first by replacing Mo with W in duplex stainless steels appears to improve the superplastic deformation characteristics when it exists below a critical level. (Received May 19, 2004)

keyword : Duplex stainless steel, Superplasticity, χ phase, Internal variable theory, Dynamic recrystallization Grain/phase boundary sliding

A new model for delayed hydride cracking (DHC) of zirconium alloys is suggested where a driving force for the DHC is not the stress gradient but a supersaturated hydrogen concentration AC over the terminal solid solubility (TSSD) arising from a hysteresis of the terminal solid solubility (TSS) on a heating and a cooling. To demonstrate the feasibility of the newly proposed DHC model, DHC velocity (DHCV) and the threshold stress intensity factor or KIH were determined at different temperatures ranging from 100 to 300℃ on compact tension specimens of Zr-2.5Nb tubes with different yield strengths and hydrogen concentrations of 6 to 100 ppm. As expected, the DHCV and KIH increased and decreased, respectively, with the AC and then leveled off to a constant with the AC in excess of the TSS for precipitation (TSSP) subtracted by the TSS for dissolution (TSSD). Through a supplementary experiment investigating the effect of the degree of supercooling on the DHC velocity which is intentionally introduced on reaching the test temperature by cooling from the peak temperature, the new DHC model is confirmed to be valid. The striation spacing and DHCV increased with the square of the yield strength, and with temperature. By correlating the striation spacings and a plastic zone ahead of the crack tip in the Zr-2.5Nb tubes with the stress gradient we demonstrate that the DHCV is governed by the hydrogen concentration gradient determined by the AC and the plastic zone size which is governed by the square of the yield strength and the Km. Therefore, the activation energy of the DHCV turns out to be a sum of the activation energy of the hydrogen diffusion and that of the striation spacing. (Received January 19, 2004)

keyword : Delayed hydride cracking, Zirconium, Yield strength, Striation spacing, Hydrogen concentration

Nanocrystalline Cu-doped TiO₂ was synthesized by mechanical alloying (MA) to shift the absorption threshold into the visible light region. The synthesized powders were characterized by XRD, TEM and BET for structural analysis, UV-DRS and PL for the optical study and TOC for photocatalytic abilities. The Cu-doped powder had a rutile phase in which the added Cu atoms were uniformly distributed and had a large surface area. Optical studies showed that the absorption wavelength of Cu-doped powders moved to visible light range (red shift), which was believed to be induced by the Cu impurity energy level. The degradation rate of carbon concentration in Cu-doped powders was faster than pure that of TiO₂. (Received July 26, 2004)

keyword : Titanium dioxide, Mechanical allying, Copper doped, Nano-size