발간논문

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The effects of aging treatment on the microstructures and mechanical properties of metastable beta titanium alloy Ti-15-3(Ti-15V-3Al-3Sn-3Cr) has been investigated with hardness measurement, tensile test, and optical and electron microscopy. Precipitate-free beta structure with average grain size about 56 ㎛ was obtained after solution treatment at 800℃ for 15 min followed by air cooling. Solution treated specimens were aged up to 800 hrs in the temperature range between 350 and 600℃. The morphology of preapitates was significantly varied depending on the aging temperature. Fine aggregates of a precipitates were dominant at 350℃, while Widmansta¨tten α precipitates were dominant at above 450℃. Peak hardness values were maintained up to 800 hrs at 500℃, which showed the superior thermal stability of α precipitates. Tensile strength increased up to 1600㎫ along with the decrease of elongation after aging at 350 and 400℃. Fracture surfaces changed from transgranular ductile mode to intergranular brittle mode as the strength increased with aging.

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The deformation characteristics of composite were studied numerically to investigate the dependence of tensile properties on the size, shape, distribution and volume fraction of the second phase. The overall response of the composite with various microstructure were computed using finite element models under plane strain condition. The development of triaxial stress within the matrix due to the strain compatibility provides an important contribution to strengthening and the distribution of second phases has influence on the yield stress, transition strain. The effect of aspect ratio of unit cell and second phase were studied. At spherical second phase, the composite has maximum yield strength and minimum transition strain at aspect ratio of unit cell of 1. At cylindrical second phase, with increasing aspect ration of unit cell, yield strength increases and transition strain decreases.

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The current distribution around rectangular cathode used in permalloy plating for the production of magnetic recording head was predicted by use of boundary element method. Three dimensional grid generator was made for the calculation of complex domain. It can be also used for curved surface. Experimental data were compared with those calculated values and fairy good agreement was observed. The effect of cell geometry and auxiliary electrode on current distribution was simulated. For the plating on the circular wafer, modification of the shape of auxiliary electrode was proposed for maximization of cathode area when active area density of cathode is smaller than that of auxiliary electrode.

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This study was focused to investigate the influence of die casting temperature on microstructure and mechanical properties of SiC_p(10 and 20vol%)/Al alloy composites and to perform statistical evaluation for strength of SiC_p/Al alloy composites in order to predict the die cast Al alloy composites reliability. Die casting was performed using the preheated mold at the pouring temperature range of 620-750℃ under the pressure of 1,039㎏f/㎠. The pre-charging low speed and a following high injection speed were 0.4 and 2.1㎧, respectively. It was found that the SiC particulates were homogeneously distributed in refined Al matrix, resulting from the improvement of segregation due to rapid solidification. The tensile strength of SiC_p(10 and 20vol%)/Al alloy composites was found to be increased with increasing cast temperature. However, the tensile strength was decreased at the cast temperature of 750℃ in 20vol%/Al alloy composites. For the statistical evaluation of strength, the weibull modulus of SiC_p/Al composites was 22.2(10vol.%) and 14.2(20vol%) at cast temperature of 700℃.

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The yttrium (Y) coated Ni₃Al with post heat treatment has shown a good high temperature oxidation resistance. To understand the effect of Y-coating and post heat treatment on the oxidation resistance of Ni₃Al, the specimens were coated with Y by ion-plating method, and heat treatment was performed before or after Y-coating at low oxygen level. Performance of the Y-coated Ni₃Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni₃Al did not change the oxidation kinetics, but the post heat treatment after Y-ion plating decreased significantly the oxidation rate of Ni₃Al. The scale formed on the Y-coated Ni₃Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of the (Y,Al)O-type oxide in the scale. The coated Y layer becomes a Y-Al compound after post heat treatment. The presence of (Y,Al)O-type oxide in grain boundaries of Al₂O₃ modifies the diffusion rates of Al and oxygen, and the oxide microstructure during oxidation. Improvement of cyclic oxidation resistance of Ni₃Al by the Y-coating is attributed to the fact that the finegrained oxides developed by the presence of (Y,Al)O-type oxide can easily relieve the growth stress.

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An investigation was conducted into the effect of microstructural parameters on the cracking behavior which often occurred in the brass layer softfaced on an AISI 4140 steel substrate. Particular emphasis was placed on a role of the microstructural change, which mostly depended on heat input during softfacing, in the cracking phenomenon. Detailed microstructural analyses of the cracked region showed that cracks were observed to be initiated in the heat-affected region of the 4140 steel, and then propagated into softfaced brass layer, resulting in a surface crack. In the case of the lower heat input, the number of cracks was significantly reduced, although many pores were formed in the softfaced layer because of insufficient melting. These findings suggest that the proper heat input condition must be used considering cracking behavior and softfacing defects.

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Electronic states of 3-d transition alloying elements in cobalt was calculated using the DV-Xα cluster method in order to obtain useful information for alloy design. The main feature of calculated electron density of states for pure fcc Co was quite similar to the results of band calculation. Two calculated alloying parameters, the bond order(Bo) which is a measure of the covalent bond strength between atoms and the d-orbital energy level(Md) of alloying transition elements, were obtained for a variety of alloying elements M in cobalt. Md changed with the order of elements in the Periodic Table and correlated with the atomic radius and electronegativity of the metal. The ionicity and the Bo of 3-d transition alloying elements were estimated and used to elucidate the nature of the bonds between atoms in the fcc Co. These results will give some guide to design of Co-based alloys for high temperature applications.

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