발간논문

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Nickel aluminides are a family of materials which offer the potential of improved high temperature performance over conventional nickel based superalloys. The synthesized material has been characterized by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and selected area image analysis. The mechanical properties of the Ni₃Al/NiAl alloy have been determined as a function of temperature through compressive and four point flexural tests. The lamellar aluminide provides synergistic properties of each of the monolithic aluminides resulting in appreciable room temperature tensile ductility, reasonable room and elevated temperature fracture toughness and higher compressive yield strength than those of obtained in monolithic NiAl.

keyword : Nickel aluminides, Ni3Al/NiAl, Compressive test, Four point flexural test

The burst tests of low tin Zircaloy-4 claddings were conducted in order to characterize high temperature deformation behavior and burst strength characteristics in relation to heat treatment condition and testing temperatures, which varies from room temperature to 500℃. While the uniform burst elongation and total circumferential elongation value of stress-relieved tubes are smaller than those of fully-recrystallized tubes, the ultimate hoop stresses are larger than those of fully recrystallized tubes at all test temperatures. The inflection point of elongation was 300℃ for stress relived tubes and the maximum elongation point was 400℃ for fully recrystallized tube. The burst mechanism was believed to be that, after deformation was uniformly exerted on the specimen circumferentially until maximum circumferential stress was reached, pin-hole fracture was induced by the local stress concentration, which results from the decreasing tube thickness.

keyword : Zircaloy-4, Burst test, Burst strength, Uniform burst elongation, Total circumferential elongation, Ultimate hoop stress

The high temperature deformation and thermal shock tests were performed to estimate the integrity of Zircaloy-4 and advanced cladding under LOCA condition. The high temperature deformation test was carried out at the temperature ranges of 650~950℃ and hoop stress ranges of 20~80 MPa. The thermal shock test was carried out at a individual isothermal oxidation temperature for 500~15000s. A maximum of burst strain occurred at around 800℃, and it decreased to 850~950℃ which is known as temperature range of (α+β) phase in Zircaloy-4. The quenched microstructures generally consisted of three distinct layers such as ZrO₂, α-Zr(O) and prior-β layer. The thickness of ZrO₂and α-Zr(O) layer increased while prior-β layer decreased as the oxidation temperature increased. The hardness measurement was conducted to estimate mechanical properties of the quenched claddings. The hardness decreased in a-layers, then the value decreased slowly or were constant in prior-β. The hardness depends on the oxygen content in the layers. The preliminary results on advanced claddings indicated that LOCA resistance of advanced claddings seem to be superior or comparable to Zircaloy-4 cladding.

keyword : Zircaloy-4, Fuel cladding, Loss of coolant accident, Burst, Thermal shock, Isothermal oxidation, Quench, Zirconium oxides

Tensile and low-cycle fatigue (LCF) tests were carried out in a wide temperature range from 20℃ to 750℃ at strain rates of 1×10^(-4)~1×10^(-2)/s for 17% cold worked 316L stainless steel to investigate the conditions for the occurrence of dynamic strain aging (DSA) and its effects on material properties during tensile and LCF deformations. DSA introduced anomalous changes of tensile and LCF properties, and the DSA regimes under tensile and LCF loading conditions coincided with each other. During tensile deformation, DSA can be manifested in the forms of the plateau in the variation of strength with temperature, the minima in the variation of ductility with temperature, serrated yielding in the stress-strain curves, and the negative strain rate sensitivity (SRS). In case of LCF deformation, it can be manifested in the forms of the occurrence of the plateau or the peak in the variation of cyclic peak stress with temperature, the negative temperature dependence of plastic strain amplitude or softening ratio, the negative SRS, and the negative strain rate dependence of plastic strain amplitude or softening ratio.

keyword : Dynamic strain aging, 316L stainless steel, Low-cycle fatigue, Cold work

The effects of microstructural morphology on ultra-high-speed exploding properties of Ti-6Al-4V alloy were investigated in this study. Exploding bridge wire (EBW) test was conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments. From the test, moving velocity of free surface vs time profiles were obtained to measure strength at Hugoniot elastic limit (HEL) and spall strength and to subsequently predict ballistic mass efficiency. The EBW results indicated that impact resistance of the bimodal microstructure was better than that of the equiaxed microstructure when the measured strength at HEL and spall strength were considered. It was also learned from the observation of fractured EBW specimens that fracture occurred by the initiation and propagation of long cracks in the equiaxed microstructure, whereas a number of microcracks were formed and connected each other in a zig-zag pattern to reduce the driving force for crack propagation in the bimodal microstructure. Since the EBW test helped experimentally estimate ultra-high-speed deformation and fracture behavior, it could be effectively used for the prediction of ballistic impact performance.

keyword : Bimodal microstructure, Equiaxed microstructure, Exploding bridge wire test, Spall strength, Hugoniot elastic limit

The effects of microstructural morphology on dynamic deformation and fracture behavior and ballistic performance of a Ti-6Al-4V alloy were investigated in this study. Ballistic impact tests were conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments, and then the test data were analyzed in relation to microstructures, tensile properties, and dynamic torsional test data. In the impacted region of the equiaxed microstructure, a number of adiabatic shear bands and cracks were observed to be formed along plastic flow lines, and delamination occurred because of cracking along the plastic flow lines or shear bands. In the case of the bimodal microstructure, shear bands were found in limited area near the impacted region without occurring delamination, and their number was fewer than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure. The ballistic performance of the two microstructures was also discussed by comparing the possibility of adiabatic shear band formation obtained from dynamic torsional test data.

keyword : Ti-6Al-4V alloy, Ballistic performance, Dynamic torsional test, Equiaxed microstructure, Bimodal microstructure, Adiabatic shear band

Mechanical milling has been performed for Al-14 wt%Ni-14 wt%Mm(Mm=misch metal) alloy powders rapidly solidified by the gas atomization method. The microstructure, hardness, and thermal stability of the powders were investigated as a function of milling time using X-ray diffraction(XRD), transmission electron microscopy(TEM), Vickers micro-hardness tester and differential scanning calorimetry(DSC) methods. In the early stage of milling, a cold-welded layer with a fine microstructure was formed along the edge of the milled powder (Zone A). The interior of the powder remained unworked (Zone B), resulting in a formation of two-zone microstructure. With increasing milling time, the nanocrystallite size decreased gradually reaching a size of about 10-15 nm and the lattice strain increased reaching a maximum value of about 0.7% after milling for 200 hours. The microhardness of the mechanically milled powder was 132 kg/㎟ after milling for 72 hours, and it increased to 290 kg/㎟ after milling for 200 hours because of the presence of fine nano structured phases.

keyword : Nanostructure, Rapid solidification, Mechanical milling, Thermal stability, Powder, Microhardness

Microstructure and texture changes of aluminized coatings on steel sheets having Cr ranging from 0.0% to 1.0% in the coated layer were investigated. The surface morphology of the Al-Cr coated steel is not influenced by the increase of Cr contents in the coated layer. While the coated layer of aluminized steel consisted of two phases of Al and Al_(5)Fe₂, the aluminized coating layer containing Cr was composed of three phases of Al, Al_(13)Cr₂, and Al_(13)Fe₄+Al_(5)Fe₂. The surface layer was Al, the middle layer was Al_(13)Cr₂, and the interfacial layer between the coating and the steel substrate was Al_(13)Fe₄and Al_(5)Fe₂. The texture of the aluminized coating without Cr was characterized as a {001} fiber texture which major orientation was {001}<110>. As the Cr content in the aluminized layer was increased to 1.0%, the texture of the aluminized coating was not changed but the intensity of the {001} fiber texture was increased. The major orientation in the {001} fiber texture was changed from the rotated cube orientation of the aluminized coating with 0.1%Cr to the cube orientation of the aluminized coating with 1.0%Cr.

keyword : Hot-Dip aluminized steel sheet, Hot-Dip Al-Cr steel sheet, Texture

The effect of corrosion products of hot-dip Al-Cr steel sheet on the corrosion resistance were investigated. The results of XRD and XPS measurements showed that there was not found a hydroxide type corrosion product on the surface of aluminized steel after salt spray test(SST) but there was found some cracks in the surface layer which accelerates corrosion during SST. In Al-Cr coated steel case, the main phase of the corrosion products after salt spray test was found to be Al(OH)₃. The polalization curves of aluminized steel after SST shows that there was no passivation layer. For the Al coated layer, a porosity in the Al-oxide film on the surface layer leads to preferential corrosion. After the Al coated layer was completely removed, the Al_(5)Fe₂layer was selectively corroded and then, the steel substrate was exposed to the electrolyte. However, the polalization curves of Al-Cr coated steel after SST shows that the passivation layer was formed, which could increase the corrosion resistance of Al-Cr coated steel. In case of the Al-Cr coated layer, the densely covered hydroxide layer of Al(OH)₃and the intermetallic compound layer of Al_(13)Cr₂became a barrier against further corrosion after the Al layer was removed. In this case, corrosion was not proceeded even through the same current density. The densely covered hydroxide of Al(OH)₃and the intermetallic compound layer of Al_(13)Cr₂were considered to be one of the reasons of high corrosion resistance for Al-Cr coating.

keyword : Hot-Dip Al-Cr Steel Sheet, Corrosion resistance, Corrosion product

Both Al-21Ti-23Cr(L1₂+Cr₂Al) and Al-37Ti-12Cr(γ+TiAlCr) two-phase alloys have been accepted to be the most pertinent coating materials in Al-Ti-Cr alloy system to improve the high-temperature oxidation resistance of TiAl alloy taking into consideration of the oxidation properties, the resistance to thermal stress and the chemical compatibility. The cyclic oxidation tests at 1000℃ and 1200℃ were performed for the specimens coated with both materials of 10 mm thickness, and the breakaway oxidation could be observed although both bulk alloys could form a stable Al₂O₃layer. And it was resulted from depleted Al content in the coating layer due to Al₂O₃oxide growth and interdiffusion with substrate. Considering the decrease of Al content resulting from oxide growth, the Al-21Ti-23Cr coating with higher initial Al content was more effective for oxidation protection of TiAl alloy for long-term service. On the other hand, when the change of Al content due to the interdiffusion with substrate was considered, the Al-37Ti-12Cr coating with smaller compositional gradient with TiAl substrate was more effective than the Al-21Ti-23Cr coating. However, it was confirmed through the cyclic oxidation tests at 1000℃ and 1200℃ that the initial Al content was more important for longer life-time of coating materials than the smaller compositional gradient with substrate. Therefore, the Al-21Ti-23Cr coating would be supposed to be more effective for long-term oxidation resistance of TiAl alloys.

keyword : L12-based Al-21Ti-23Cr, γ-based Al-37Ti-12Cr, Long-term oxidation, Interface reaction, Coating materials