발간논문

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The effect of surrounding atmosphere on phase changes during mechanical alloying of Fe-Zr was studied by X-ray diffractometry, transmission electron microscopy and magnetometry. Atmosphere during mechanical alloying was controlled by using either nitrogen or argon gas. Mechanically alloyed Fe-Zr powders showed a mixture of amorphous phase and α-Fe phase. Amorphization during mechanical alloying in nitrogen atmosphere was retarded compared with mechanical alloying in argon atmosphere. Fraction of α-Fe phase was higher in the alloy powders processed in nitrogen atmosphere than in alloy powders in Ar. The retardation of amorphization during mechanical alloying in nitrogen may be attributed to hardening of Fe phase by the increase of N solubility, which results in difficulty of plastic deformation of Fe phase. Variation of magnetization with processing time agrees well with the structural change with processing time. With increasing processing time, magnetization of Fe-Zr alloys processed in argon decreased rapidly due to the formation of amorphous phase, reached a steady values. Variation of magnetization of Fe-Zr powders processed in nitrogen atmosphere was not significant due to the retarded amorphization. Alloy powders processed in nitrogen atmosphere showed higher magnetization than the powders in Ar atmosphere.

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Intermetallic compounds based on Ti_5Si₃ were made by reactive sintering(RS) alone or in combination with pseudo-hot isostatic pressing(PHIP) and their density variation with sintering parameters was studied. Adding Cu, selected as a sintering aid on the ground of the `figure of merit` theory, increased the density significantly during reactive sintering, presumably due to the role of Cu in increasing the inter-solubility of the base metals and catalyzing their diffusivity during sintering. The sintered density could be increased by decreasing the particle size of the elemental powders and increasing the sintering heat treatment time. The maximum relative sintered density, about 93%, was obtained under a condition of 6wt.% Cu, small particle size and 7h of holding time at 1450℃. Also, the sintered density of the Ti_5Si₃ base intermetallic compound could be remarkably improved by using the method of PHIP. A sintered density of approximately 99% was obtained by compacting at 1250℃ for 2h under a pressure of 250MPa, which was explained in terms of adequate particle rearrangement and effective removal of pores aided by the external pressure during the densification process.

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The cyclic reverse double shear creep test was adopted to investigate the grain growth of Zn-22% Al superplastic alloy. The results represent that the shear specimens experience more increment of concurrent grain growth and consequently increase in grain size, as the test cycle increases. The results indicate that stress reversal effect is the major factor influencing grain growth in shear test. The relation between shear strain rate(γ˙) and grain size (d) could be expressed as follow, when the unit of γ˙ and d is 10-⁴/sec and ㎛, respectively : log γ˙=-1.2571og d-3.7

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Dry sliding wear behavior of heat treated Fe-12Cr-Mo-V-1.6C tool steel was investigated. Friction coefficient and wear rate were measured with variations of applied load and sliding speed at ambient temperature. Wear debris and worn surface of counter part were observed using SEM. Friction coefficients according to applied load and sliding speed were 0.27∼0.60. Friction coefficient decreased proportional to the increase of the load and was able to be expressed in the form of power law μ=BN^k at constant sliding speed. As load and sliding velocity increased, attainable flash temperature at asperity rose, which resulted in the increase of the wear rate. The distance-dependent transition from severe wear to mild wear due to surface oxidation at asperity and debris was observed during the running-in process. Increase in the debris size due to increase of load delayed the transition time from severe wear to mild wear.

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Mechanical test data of superalloy A-286 show that there is a significant increase in tensile strength after rapid solidification processing (RSP). The mechanism of mechanical property improvement upon RSP was studied by TEM quantitative defect analysis and the data analysis using strengthening models. The most prominent microstructural alterations by RSP are found in the precipitates and in the grain growth behavior. On average, the precipitates in RSP samples are smaller, have higher volume fractions, and are much more stable than their conventionally processed counterparts. The increased tensile strength of the RSP samples is found to be the result of the increased volume fraction and number density of second phase particles. However, the strengthening came mostly through grain growth inhibition by the second phase particles, which in turn increased the strength via Hall-Petch relationship. This study shows the importance of recrystallization and thus the method of consolidation for effective strengthening.

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A2219 Al alloy was selected in use as the bonding agent for diamond grinding wheel. In order to alleviate the wetting of the Al adhesive with workpiece, Ti was added to the Al alloy for strengthening and minimizing the adhesive trouble, SiC_p for improving wear resistance. 5%Ti, 10% and 15% in weight respectively was added in 2219 Al matrix, and 149 ㎛ diamond was also added in it. Then 5wt%SiC_p was added in the matrix to find the effect of addition. The hot pressing condition was 600℃, 20Mpa and 0.6Ks in the furnace of the electric resistance type. The grinding ratio was increased with the increase of Ti, in case of grinding wheels without SiC_p. The grinding wheel with SiC_p did not show better performance than it without SiC_p. We have found the A2219 alloy bond added 15wt% Ti showed better grinding ratio than resin bond.

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Recently clad(sandwich) type composite sheets are getting attention because they have good mechanical and vibration damping characteristics. The purpose of this work is manufacturing sandwich type composite sheet which has good vibration damping capacity at high temperature. In composite sheet, Zn-22wt%Al alloy (Superplastic Zinc, SPZ) was selected as a damping material because its Specific Damping Capacity(SDC) was good at high temperature. SPZ and steel sheet were rolled to make a sandwich type composite sheet for studing the variation of SDC with the volume fraction of SPZ and the damping test temperature. The SDC of steel-SPZ-steel composite sheet increases with increasing the volume fraction of SPZ. With increasing temperature, SDC also increases. Especially, in the case of the volume fraction of SPZ is 75%, SDC is 5.2% at room temperature but 13.5% at 130℃. And we can predict the SDC from the elastic strain energy of both steel and SPZ.

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Sliding wear of a nitriding steel against heat-resisting alloys, Incoloy 901 and 12%Cr steel, is investigated. Armsler-type wear tests are carried out at 540℃ under dry-air condition. Weight change during wear tests is measured by intermittent balancing method and wear debris is examined by traditional metallographic techniques. Regardless of the composition and surface condition, the heat-resisting alloys adhere to the non-nitrided nitriding steel samples at the early stage of wear, leading to a weight gain of the samples. When the nitriding steel samples are nitrided, they undergo erosion from the early stage of wear tests, leading to a weight loss. In this case, the base materials under nitrided layer become exposed during wear tests to the heat-resisting alloys. Worn surfaces of the non-nitrided samples show a complex morphology following the formation of surface-adhered layer and its successive delamination.

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The effect of Al contents(0∼5wt%) on the mechanical properties such as tensile behaviors and highcycle fatigue properties in a austenitic Fe-25Mn-0.5C alloy was investigated at room temperature. The increase of Al content in this alloy system reduced the austenite grain size and the amount of twin. The phase was a single austenite, which was not affected by the addition of Al. The yield strength was increased according to the increase of Al content. It is believed that this increase of yield strength results from the grain refinement and solid solution hardening. The ultimate tensile strength depends on the quantity of fine mechanical twin and solid solution hardening of Al. The total elongation decreased with increasing the Al content. It is considered that this variation of elongation is related to the quantity of the fine mechanical twin. In the high-cycle fatigue test. The specimen without Al which had an excellent tensile properties showed high fatigue-resistance. This high fatigueresistance was due to the formation of the mechanical twin which decreased crack propagation rate.

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A numerical model has been proposed for the prediction of crystalline microstructure formation in planar flow casting. The present model is based on the coupling of the control volume(CV) method for macroscopic heat flow calculations and a two-dimensional cellular automaton(CA) model for microstructural evolution in solidification. The CA model takes into account nucleation, preferential crystallographic orientation and growth kinetics of a dendrite. At high solidification velocity, the LKT model with growth rate dependent partition coefficient was introduced to describe the growth kinetics of a dendrite which grows from an undercooled alloy melt. At every interval of time, the latent heat released by the solidifying cells after growth stage in the CA model was fed back into the CV model in order to calculate the temperature distribution for the next interval of time. The present model has been applied to predict the resultant microstructures of Al-Cu polycrystalline ribbons spun by planar flow casting. The calculated grain structures were compared with those of experiments.

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