발간논문

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Martensitic transformation and effects of thermal fatigue cycles on the transfer mation temperature and shape recovery of Ti-Ni-Cu alloys were investigated by means of electrical resistance measurement, thermal fatigue tests and transmission electron micros copy. Thermo-mechanical treatment at 600℃ for one hour after cold rolling showed the best shape recovery effect irrespective of Cu concentration. Substitution of a small amount of Cu for Ni in the equiatomic composition of Ti-Ni showed folling results ; Ms-temperature of the Ti-Ni-Cu alloy increased slightly with the increase of Cu content, The range of transformation temperature for Ti_(50)Ni_(40)Cu_(10) alloy(i.e., T_(Mf-Af)) was reduced to 7℃. After stress relief treatment at 750 ℃ for one hour, the Ti_(50)Ni_(45)Cu_5 alloy showed good two-way memory effect. Change of the M_s temperature was observed to a certain number of thermal cycles, and it became nearly constant after a critical number of thermal fatigue cycles.

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Dependence of the deformation twinning on the orientation of grains has been studied in Cu and Cu-Bn alloys. The experimental results obtained from the X-ray texture measurement and the electron microscopical investigation were interpreted by the theoretical considerations based on Sachs and Taylor. In most orientations it was showed that the Tailor deformation cannot be achieved only by the deformation twinning. During the rolling deformation the orientations with TD//<110> can be sucessfully deformed by twinning under the Taylor`s strain condition as well as the Sachs`s stress condition. The transition from the pure metal to the alloy type rolling texture was caused by the development of twinning in Cu-position {112}<111> which was the most probable orientation for twinning following the deformation theories.

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The magnetic properties and microstructures of 0.3㎜ thick high Si-Fe alloys sheets made by the twin roll process were studied after annealing the sheets at 1100℃ for 60 minutes in the vacuum atmosphere of 10^(-4) Torr. The microstructure of 6.5 wt%Si-Fe alloy showed the DO₃precipitates in the B₂matrix. When the specimens were heated at a constant rate of 2℃/min, the electrical resistivities of 3, 4, 5 and 6wt%Si alloys increased in the temperature range between 330℃ and 750℃ in comparison to those of the as quenched state. On the contrary, the resistivity of 6.5 wt%Si alloy decreased in the temperature range between 350℃ and 550℃. The coercive force was 0.51 Oe for the 6.5 wt%Si-Fe alloy and the AC core loss W_(15/50) and W_(10/50) were 0.69 w/㎏ and 0.30w/㎏ respectively.

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A 25Cr-20Ni austenitic steel containing 0.59 wt % nitrogen was aged at 973, 1023, 1073 and 1123K to study the growth features of the cellular precipitation of Cr₂N. Experimentally determined were volume fraction of the cell, averge migration rate of cell boundaries, and averge concentration of nitrogen in the untransformed matrix. The partitioning of nitrogen between the cell and the untransformed matrix was computed on the basis of those data. The average concentration of nitrogen within the cell is higher than initial concentration of nitrogen at the onset of the cellular reaction. It decreases linearly with the increasing volume friction of cell and its gradient increases as the aging temperature decreases. At a given aging temperature, it is determined by the average concentration of nitrogen in the untransformed matrix, irrespective of the initial concentration of nitrogen.

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Hg_(0.3)Cd_(0.7)Te epi layers were grown by a slider liquid phase epitaxial growth process with using ramp cooling. A new slider boat was designed to keep the composition of the Te-rech growth solution from changing during growth. It was possible to grow Hg_(0.3)Cd_(0.7)Te epi layers with uniform composition in the thickness direction. The temperature at which CdTe substrate is contacted with the growth solution should be 1∼2℃ higher than the liquidus temperature in order to obtain the surface morphology with uniform terraces.

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The effects of an aluminide coating treatment on creep-rupture properties of a nickel-base superalloy Rene´ 80 were investigated. It was found after creep tests at the ambient atmosphere that the coating treatment reduced rupture lives compared to the uncoated specimens. This is believed to be due to different crack formation mechanism between coated and uncoated specimens. For the uncoated specimens, creep crackings were initiated at the oxidized surfaces, while for the coated specimen, creep fractures were controlled by the crack unclei due to the high hardness of the diffusion zone in the coated layer. Some creep tests were also conducted on the coated and uncoated specimens at the hot corrosion environment of 75wtNa₂SO₄-25wt%NaCl. The improved creep life of the coated specimen especially obtained at 982℃/156.9MPa under conditions of the hot corrosion by a 75wt% Na₂SO₄25wt% NaCl was attributed to the ductile coating layer which flew readily into the widening crack region.

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For the purpose of investigating the effects of the Al content on the elevated temperature mechanical properties of Fe-Al-C alloys, high temperature tensile test and thermal fatigue test were performed using a series of alloys containing various amounts of Al ranging up to ∼4.5 wt%. Thermal conductivity was also measured. At low temperatures, the thermal conductivity of flake graphite cast iron was higher than that of C/V graphite east iron, but both cast irons exhibited a similar value of conductivity at high temperature near 300℃. The thermal conductivity was decreased with the increase in the Al content and the decreasing rate was reduced with the increase in the temperature. The tensile strength at 700℃ was increased with the increase in the Al content in bath cast irons, but the tensile strength was higher in the case of the C/V graphite cast iron as compared to the flake graphite cant iron. The thermal fatigue life of C/V graphite cast iron was, at temperatures from 300℃ to 700℃, superior to t at of flake graphite cast iron, particular1y in the case of low Al content. The thermal fatigue life of the C/V graphite cast iron deteriorated with increasing Al content, but that of the flake graphite cast iron improved with increasing Al content.

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This study investigated effects of Cu and Sm contents and heat treatments on magnetic properties and formation of cell structures of Sm₂(Co, M)_(17) powders. In the Sm (Co_(bal) Fe_(24-x/4)CU_xZr_(028))_(8.35) system, coercive force and residual induction increased as xincreased upto 0.12 and 0.08, respectively. In the Sm(Co_(672)Fe_(220)Cu_(080)Zr_(028))_Z system, coercive force increased as Z increased from 8.05 to 8.50. When Z=8.50, values for i_(Hc) and B, were 17KOe and 9.5KG, respectively, after solution treatment at 1,100℃ followed by isothermal aging at 800℃ for 24 hours.

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The reaction precess and the morphology of vanadium powders obtained from a vapor phase reaction of VCl₄ and NH₃at 200-1400℃, under a pressure of an atmosphere and 4.9 KPa were determined. The vapor phase reaction products and thermally decomposed powders obtained at various temperatures ware investigated by means of X-ray diffraction, TGA, SEM and TEM. Three types of VN were obtained in the vapor phase reaction depending on the temperature amorphous VN at 250℃ 8F-type VN crystals at 500℃ and a mixture of nitrogen-richer VN and 8F-type crystals above 1050℃. The size of the reaction products obtained at 1400℃ was less than 0.04㎛ and about 0.08-0.16㎛ for the vapor phase reaction and the thermal decomposition in NH₃flow system, respectively. The atomic ratio, N/V, decreased with the increase of the reaction temperature and the lattice parameter calculated for various reaction products obtained at different temperatures above 800℃ were almost constant.

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The fabrication and creep chararteristics of composites reinforced with stainless steel and high carbon steel fibers haute been studied. The Ni-coating, Cu-coating and bear fibers have been tested to examine the effect of fiber surface characteristics. In order to make various fiber volume fraction composite specimen, the liquid casting method has been applied. Creep characteristics such as the rupture time and the steady state creep rate have been investigated in the range of stress with 22.47 to 72.95MPa at 537k. The rupture time is increased with V_f. The stress exponent(n) has been found to be 21.2∼114. In case of V_f 1.6, 2.88%, 100hr rupture stress(δ) was less than theoritical value calculated by the rule-of-mixture(R.0.M). On the other hand V_f 3.52%, shows δ value close to R.O.M, and coated composites were even larger than R. O. M. Severa1 intermetallic compound such as A1Ni, Al₂Cu, Al_5Fe₃and Al_(13)Fe₄in the interface were observed by TEM. These improved wettability of fiber on matrix, and creep strength due to obstruction of dislocation moving. Tested specimen shows dislocation cells at the substructure. The dislocation density decreased from interface to matrix.

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