발간논문

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In the our previous studies about the two-phase NiAl/Ni_3Al alloys, the various microstrucutures of polycrystalline NiAl/Ni_3Al alloys could be obtained by the M_s temperature control of NiAl-martensite including titanium, and the mechanical properties were considerably dependent on their microstructures. In this study, the grain boundaries were tried to align by directional solidification to suppress the inter granular fracture at room temperature or grain boundary sliding at high temperature. To effectively maintain the grain boundary alignment in the course of the heat-treatment of a DS ingot, suppressing recrystallization during solutionizing treatment, which is the preceding process of martensitic transformation of a DS alloy, is very important. Based on the orientation relationship between parent β-NiAl(the B2 structure)and product β`-martensite(the L1_0 structure), the optimum heat-treatment conditions for DS ingot were determined through XRD observation. Consequently, the columnar grained NiAl/Ni_3Al alloys could be achieved for the only Ti-added ternary ingot contrary to the recrystallized binary ingot. And, the columnar grained alloys including Ti could show higher yield strength at high temperature than the usual polycrystalline alloys.

keyword : NiAl, Ni3Al, Martensitic transformation, Directional solidification, Microstructure control

The effect of chromium contents on the microstructure, tensile and corrosion behaviors of as-solutionized CD4MCU(Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate test was also conducted at a nominal strain rate of 1×10^-6/sec in air and 3.5% NaCl+5% H_2SO_4 solution for studying the stress corrosion cracking behavior of the present alloy. It was observed that the increasing amount of chromium encouraged the precipitation of fine 2nd austenitic phases in the ferritic matrix. The fine austenitic phases appeared to contribute the homogenization of deformation, and, consequently, the ultimate tensile strength and elongation were largely improved. The resistance to general and pitting corrosion of CD4MCU alloys in 3.5% NaCl+5% H_2SO_4 solution was significantly improved with the addition of chromium. The trend observed in the stress corrosion cracking(SCC) behavior of CD4MCU alloys in 3.5% NaCl+5% H_2SO_4 solution with different chromium contents was two-fold. In the anodic dissolution environment, the higher chromium content was beneficial in improving the SCC resistance. The reverse was true in the hydrogen generating environment.

keyword : CD4MCU cast duplex stainless steel, Tensile behavior, Corrosion behavior, Slow strain rate test

The objective of this study is to investigate the correlation between the microstructure and wear resistance of various molybdenum blend coatings applicable to automotive parts. Five types of molybdenum spray powders, one of which was pure molybdenum powders and the others were blend powders of molybdenum powders mixed with brass, bronze, and aluminum alloy powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that they consisted of curved lamellar structures formed by elongated splats, while hard precipitates formed during spraying were relatively homogeneously distributed in the molybdenum matrix. The wear test results revealed that the blend coatings showed the better wear resistance than the pure molybdenum coating because they contained a number of hard precipitates. In particular, the molybdenum coating blended with bronze and aluminum alloy powders showed the excellent wear resistance, together with the excellent wear resistance of a counterpart material, resulting from the presence of hard precipitates and their homogeneous wear. Therefore, in order to improve the overall wear properties with consideration of the wear resistance of a counterpart material, appropriate spray powders should be blended with molybdenum powders to form hard precipitates in the coatings of this study.

keyword : Atmospheric plasma spraying, Molybdenum blend coating, Pin-on-disc wear test, Wear resistance, Hardness

A series of tensile and load relaxation behaviors in B2 type β-CuZn alloy have been examined to clarify the strain rate & the temperature dependence of yield stress anomaly. It has been found in tensile tests that the region showing anomalous peak of yield stress moves to a higher stress and higher temperature region with increasing strain rate. The anomalous yielding region below peak temperature(T_p) revealed slightly positive strain rate dependence, while the region above T_p showed considerably positive strain rate dependence. The slip lines formed below peak temperatures (T_P) were clearly visible with naked eye(coarse slip lines), and became finer near T_p(fine slip lines), while they disappeared completely above T_p. Based on the different tendency of temperature and strain rate dependence of anomalous yielding as the above-mentioned results, two types of deformation modes were suggested and incorporated into the flow curve analysis, which was in accord well with the experimental results. The slightly positive strain rate dependence of yield stress anomaly indicated that the yield stress anomaly in β-CuZn alloy mainly seemed to be attributed to the vacancy hardening.

keyword : Yield stress anomaly, β-CuZn alloy, Strain rate dependence, Vacancy hardening, Load relaxation test

The electromagnetic wave absorption properties of various amorphous alloy-ferrite-epoxy composites have been investigated in a frequency range from 1 to 5 GH_Z. The fraction of amorphous alloy in the composites was varied from 0 to 50 vol.% at a fixed 50 vol.% fraction of epoxy resin polymer. The complex permeability(μ) and permittivity (ε) of the composites were measured by reflection/transmission technique. Both the matching frequency and thickness for maximizing reflection loss of the composites were theoritically determined by plotting the measured μ and ε on an impedance matching solution map. The increase of amorphous alloy fraction in amorphous alloy-ferrite-epoxy composites improved electromagnetic wave absorption properites, i.e, a larger matching frequency and thinner thickness compared with the ferrite-epoxy composite. On the basis of these results, a new thinner wave absorber is proposed by the optimization of amorphous alloy fraction and thickness in amorphous alloy-ferrite-epoxy composites.

keyword : Electromagnetic wave absorption, Amorphous alloy, Ni-Zn ferrite, Impedance matching frequency and thickness

The degradation characteristics of the base metal and weld of STS316L were investigated by the experimental method of ultrasonic exposure with parameters such as ultrasonic velocity, attenuation factor and frequency. Through this research, degradation characteristics could be evaluated by the combinations of experimental ultrasonic parameters. According to the increase of grain sizes, standard deviation and ferrite contents, the ultrasonic velocities and frequencies were decreased, while the ultrasonic attenuation factors were increased. This results can be to predict the welded structures life time assesmant according to the aged degradation and damage without disjointing or stopping in-service.

keyword : STS316L we1dment, Ultrasonic velocity, Attenuation factor, Short time fourier transformation, Time- frequency analysis, Degradation

In order to characterize the dynamic change of microstructures in commercially-available superalloy 718, a series of compression tests were conducted by varying test temperature and strain rate. Orientation relationship and the misorientations between recrystallized grains as well as unrecrystallized grains were thoroughly investigated with EBSD technique. The dynamically-recrystallized grain size was similar regardless of the compression test condition except for the sample tested at 1066℃-0.005/sec condition, where the dynamically-recrystallized grain size was as large as 20 μm in average diameter. In case of the sample tested at 1066℃, flow stress drops caused by the strain softening were almost same at the strain rates of 0.5/sec and 0.005/sec in spite of the different fraction of dynamic recrystallization. It was concluded from the observation of the misorientation angle distribution that it was due to the active dynamic recovery in the unrecrystallized large grains at high temperature. It was also suggested from the variations of CSL boundary distribution that the characteristic redistribution of the dynamically-recrystallized grain boundary might be taken place during the compression at high temperature and low strain rate.

keyword : Alloy 718, Dynamic recrystallization, Lattice rotation, EBSD, Grain rotation

In this study the effects of some factors on foaming property were investigated in powder compact foaming process. First of all, production of metal foam strongly depends on the green density of precursor and when the density is low, expansion is low or does not occur by leak of hydrogen gas and/or internal oxidation due to inflow of oxygen. The foaming of precursor starts when its temperature arrives at melting point of Al and in viewpoints of expansion ratio, pore size distribution and morphology, the optimum foamed structure is obtained when the foaming process is finished at melting end point. The structure is also affected by heating rate depending on setting temperature of furnace and the most suitable temperature in this work was 800℃ higher about 150℃ than Al melting temperature. It is suggested that foaming agent content is desirable to be below 1 wt.%.

keyword : Aluminum metal foam, Powder process, Precursor density, Heating rate