발간논문

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Kinetic energy analysis of mass filtered ions emitting from ion plating plasmas was carried out in search of optimum discharge condition that gives maximal reactivity of TiN formation and enhanced adhesion due to the fast depositing particles. The ion energy distribution was analyzed by the cylindrical energy analyzer. In a gas discharge of Ar at a pressure of 50mTorr and high anode voltage, Ar^+ and Ar^(++) show two main peaks at the energy equivalent to 90∼120% of anode voltage, which seem to have different ionization region. In a mixed gas discharge of Ar and N₂, the addition of Ar into N₂discharge enhances N₂^+`s mean kinetic energy, but the addition of N₂into Ar discharge certainly decreases the main peak energy of Ar^+ and broadens the energy distribution. This phenomenon was interpreted using charge exchange collision in a cathode sheath and local potential distribution in glow region.

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In the hot W-filament CVD(HFCVD) diamond thin film growing, the filament becomes carburized and changes its resistance when the vol. % of CH₄gas is higher than the critical value. The effects of tmeperature and the negative bias of W-filament on the critical CH₄concentration were studied. The critical CHa concentration increased as temperature dropped and was almost proportional to the negative bias applied to W-filament. The diamond thin films grown with the assistance of electron bombardment (EACVD) were compared with those grown by usual HFCVD. The EACVD method gave higher film growth rate and smaller grain size than HFCVD method, and as the voltage difference between the filament and the Si substrate increased the higher growth rate and the smaller grain size were resulted. In the diamond films grwon by HFCVD the triangular microstructures were frequently seen on {111} plane, while none was found on the films grown by EACVD, suggesting that EACVD produces better quality diamond thin film than HFCVD. The XRD and RHEED diffraction patterns of all these films grown by EACVD showed the same patterns as those of natural diamond, confirming that the diamond films grown in this study were of good quality.

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The combined use of high resolution transmission electron microscopy and the image processing are applied to determine the atomic-scale structure of the short-range order diffuse scattering, Microdomain was not detected in quenched Ni₄Mo alloy which showed the diffuse scattering at {11/20} positions. The lattice modulation observed in this alloy seems to be attributed to the concentration waves appeared parallel to the {210} planes. In quenched Cu₃Pt alloy which showed the diffuse scatterings with twofold and fourfold splittings of intensity maxima, the L1₂- type microdomains were recognized. This microdomains were believed to have out-of-step relation between neighbors.

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Purpose of this study was to develop non-destructive material evaluation method of aluminum alloy base metal matrix composite(MMC) by ultrasonics. Five aluminum base MMC specimen were fubricated in which the fractional ratios of fiber were changed from 0% to 31%. Relation between acoustic properties, microstructural features, and elastic constant were compared. Ultrasonic velocity method was, then, useful for nondestructive elastic constant measurement of composite materials, since the method had as same measurement accuracy as conventional strain measurement method. Furthermore, velocity and attenuation parameter also related to the fractional ratio of fiber and these relation could utilize ultrasonic non-destructive evaluation of fiber structure in MMC.

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Superplastic properties of 7475 Al alloy were evaluated at 516℃ over the strain-rate range of 6.3×10^(-5) to 2.1×10^(-2)s^(-1). The strain-rate sensitivity(m) was determined either from the true stresses and the true strain rates at the points of maximum load, or from change in strain rates. The latter method shows clear distinction between Region I(low stress region) and Region II (superplastic region). Observation of 7475 Al alloy after superplastic deformation was done by optical, scanning electron and transmission electron microscopy. Optical micrographs revealed that at low stress($lt;2 MPa) dispersoidfree zones(DFZ) formed at grain boundaries, primarily normal to the tensile direction. This result demonstrates the importance of diffusional flow at low stress. Also, scanning electron micrographs revealed that cavitation was reduced in the DFZ. Finally, transmission electron micrographs showed that dispersoid particles were accumulated at grain boundaries during the superplastic deformation. This result suggested that the deformation behavior at low stress was most likely a consequence of the existence of a thershold stress.

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The change of thermal expansion behavior by addition of Mn, Cr and Co within 5 wt% has been investigated to Fe-36% Ni Invar alloy. The amount of thermal expansion for the addition of Mn is larger than that far the addition of Cr. In the case of Co addition, thermal expansion decreases up to 4% Co but increases at 5% Co. The increase in the thermal expansion coefficient(α) per 1 wt% is 1.1×10^(-6) and 0.55×10^(-6) for the addition of Mn and Cr, respectively, in the temperature range of 30 to 100˚C. On the other hand the decrease in α for the addition of Co per 1 wt% up to 4% Co is 0.12×10^(-6) and then increases remarkably thereafter. The change of α with the kind and the amount of additives is discussed using some magnetic parameters. α decreases with increasing the ratio of specific magnetization to Curie temperature. In the range of the electron concentration(e/a) below 8.7, α for Mn or Cr added alloys increases with decreasing e/a but for Co added alloys the inverse tendency is shown.

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Transport phenomena in the UCC process are important in that they effect the quality of the crystal. Hence the heat, fluid flow and mass transfer must be considered in order to understand crystal growth. A numerical model has been developed for the prediction of temperature, velocity, and concentration fields. The governing equations have been discretized using the control-volume based on the finite difference method. The effects of convection on the solute profiles both in the melt and in the crystal have been studied. Attention is also focused both on the initial transient stage of solte redistribtuion and on the radial inhomogeneity of the solute concentration in the grown crystal. The effects of process variables, such as mold length, crucible wall temperature, and crystal pulling speed upon transport phenomena have been investigated.

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Phase transformation in Fe-40wt.%Cr binary alloy thermally aged at 475℃ was investigated by Mo¨ssbauer spectroscopy, superconductivity quantum interference device(SQUID) and vibrating sample magnetometer(VSM). Microhardness change accompanying its thermal aging was also studied. Microhardness of the binary alloy increased immediately and without saturation with aging up to 2000 hours. Specific saturation magnetization value(σ_s) measured at room temperature by VSM increased during the initial aging time of 2 hours to 1000 hours whereas it increased slowly during aging of 1000 to 2000 hours. Broadening of the six-line Mo¨ssbauer spectrum obtained at room temperature and additional appearance of paramagnetic peaks were observed for binary alloy specimens aged for a time range of 400 to 2000 hours. These observations indicate occurrence of phase separation into Fe-rich and Cr-rich region. Results of Curie temperature measurements for the Cr-rich region by SQUID indicate that the volume fraction and Cr concentration of the Cr-rich region increased with increasing aging time. These results lead to a conclusion that spinodal decomposition progresses during aging at 475℃ and the Fe-40wt.%Cr alloy and that gives rise to hardening of the alloy.

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In order to get basic data for application of a CuZnAl shape memory alloy colied helical spring, the relationship between weight and displacement, recovery energy and thermal cycle properties are investigated. The results obtained in this study are as follows; The relation between weight and displacement is nun-linear and weight with change of temperature is showed hysteresis curve. Slope of weight displacement curve tends to increase with increasing spring diameter. Small spring diameter is showed maximum recovery energy and large displacement at low weight compares to large spring diameter. But with increasing weight the maximum recovery energy and large displacement tend to move large spring diameter. Shape memory ability by thermal cycle radically decreases with increasing thermal cycle and has 30∼50% after 100th thermal cycle.

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The mechanical properties of Ni_(76)Al_(24)-0.5B intermetallic compound by elemental powder process were studied. Benefited with the fine grain structure the hot extruded alloy showed a combination of high tensile yield strength, about 850㎫, and tensile ductility, more than 40% elongation. To improve the strength further, a dispersion hardening was also studied. The strengthening species was selected on the basis of thermal expansion compatibility with the matrix. Based on the thermal stress and strain due to mismatch of the thermal expansion coefficient between the matrix and the dispersoids, we determined ZrO₂as the most suitable species. More than 140㎫increase in the yield strength was obtained with a tensile elongation of 30% by adding 3.2vol% of the reinforcing phase to the Ni_(76)Al_(24)-o.5B matrix.

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