발간논문

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Formation and disorder-order transformation of nanocrystalline FeAl have been investigated by a combination of scanning/transmission electron microscopy, X-ray diffraction analysis, differential scanning calorimetry and saturated magnetization measurement. Fe-50at% Al powders mechanically alloyed for 90 hours consist of 5-10㎚ size grains with a disordered b.c.c. structure. Lattice parameter of FeAl increases with mechanical alloying time not only due to solutionizing of Al into Fe, but also due to increasing of internal defects. X-ray diffraction, TEM analysis and magnetization values of nanocrystalline FeAl show that disorder-order transformation occurs at the temperature range of 300℃-320℃. Such a low-temperature ordering behavior is attributable to the ㎚-scale structure with a large amount of defects accumulated during mechanical alloying process. Present study shows that saturated magenization value is a good indicator of the ordering process.

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As a basic research to develop the material for heavy water reactor pressure tube application the effect of Mo addition to Zr-Nb-Sn alloy was studied for the purpose of minimizing the amount of cold working while maintaining a high strength. To select the target alloy system we first designed various alloy compositions and chose Zr-Nb-Sn and Zr-Nb-Sn-Mo through multi-regression analysis of the relationship between the basic properties and the compositions. Plasma arc melting was used to produce the alloys and the microstructure change introduced by the processing steps including hot forging, beta-heat treatment, hot rolling, cold rolling and recrystallization heat treatment was investigated. Recrystallization of Zr-Nb-Sn was retarded by adding Mo and this resulted in a fine grain structure in Zr-Nb-Sn-Mo alloy. Beside the retarding effect of recrystallization. Mo increased the amount of residual beta phase and showed an indication of precipitation hardening, which added up to the possibility of applying the alloy for the desired usage.

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This article aims at investigating the effects of martensite morphology on the dynamic deformation behavior of dual phase HSLA steels. In the intermediate quenched (IQ) steel sample, very fine fibrous martensites were well distributed in the ferrite matrix, whereas bulk martensites were mixed with ferrite grains in the step quenched (SQ) sample. Dynamic torsional tests were conducted using torsional Kolsky bar for two steel specimens, and then the test data were compared via microstructures, tensile properties, and fracture mode. After the heat-treatment, IQ-sample showed higher tensile strengths with relatively good ductility than those of the as-received sample, but SQ-sample had very poor ductility. The dynamic torsional test results indicated that in IQ-sample, fracture occurred in a ductile mode, irrespective of the loading type and rate. On the other hand, ductile fracture was predominated in SQ-sample under dynamic torsional loading, although cleavage fracture occurred with little plastic deformation under quasi-static tensile loading. This was because the constraint of the ferrite deformation was released by the deformation of martensites themselves, resulting in considerably high fracture shear strain in the dynamic torsional test. The deformation and fracture behavior of SQ-sample also correlated well with the dynamic shear stress-strain curve and observation of the central area of the gage section, i.e., void initiation at martensite ends and propagation through ferrite grains.

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The possibility of formation of the a phase in the Rene´ 77 superalloy, when exposed to high temperature: and under the applied stress, has been evaluated. The Phacomp calculations on the Rene´ 77 used in the present study indicates that an electron vacancy number of this alloy is 2.45, which is not immune to formation of the a phase. Among the four stress-rupture test conditions, the σ phase was observed when the sample was exposed at 871℃ under the applied stress of 276 ㎫. The shape of the σ phase observed in the present study was more-or-less close to blocky shape, and the detrimental effect on stress-rupture property of the Rene´ 77 was minimal. It was interesting to find that the fracture modes were different with stress conditions. While fracture initiated near the carbide in High stress condition, it appeared to grow near the grain boundary or triple point in low stress condition.

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An Investigation was made to determine the effects of annealing parameters on the mechanical properties of dual-phase steels with two different compositions: Fe-0.13% C-1.31% Si-1.91% Mn steel and Fe-0.13% C-1.11% Si-1.91% Mn-0.05% Nb steel. It was found that Nb retarded recrystallization of ferrite and refined the duplex microstructure of ferrite and martensite. Dual-phase properties were obtained in the former steel cooled at a rate of about 40℃/sec after intercritical annealing in the temperature range of 750℃ to 850℃ for 30 seconds. Nb-added steel showed higher strength and lower elongation than the steel without Nb. Good combinations of strength and elongation were obtained in specimens cooled at rates ranging from 10 to 20℃/sec in both steels. Overaging treatment at 400℃ resulted in the decrease in strength and increase in total elongation.

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Fatigue crack growth behavior of various Al-Li alloys was examined with different environments, including air, vacuum and 3.5% NaCl solution, and load ratios. Al 2090 and RSP 644-B showed the better resistance to fatigue crack growth in air than conventional high strength Al alloys such as Al 7075, but MA 905-XL with the fine grain size showed the higher fatigue crack growth rate because of low crack closure level coming from smooth crack path and crack plane. These Al-Li alloys showed the lower fatigue crack growth rate in vacuum and the higher fatigue crack growth rate in 3.5% NaCl solution than in air. But MA 905-XL showed the lower fatigue crack growth rate in 3.5% NaCl solution than in air, which is thought to come from :he corrosion product induced crack closure in 3.5% NaCl solution. In Al 2090 alloys, △K_(th) value decreased linearly in air with increasing the load ratio, but △K_(eff,th) after compensating for the crack closure showed an almost constant value independent of the load ratio. MD345 showed the lowest fatigue crack growth rate in air among the three Al-Cu-Li-Mg-Ag alloys, which is thought to be clue to the highest crack closure level. Weldalite 049 with the highest strength showed the highest △K_(th) among the three alloys, an d also showed the highest fatigue crack growth rate in 3.5% NaCl solution because of the highest corrosion rate.

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Effects of different heat treatment conditions on the microstructure and mechanical properties of IN792+Hf alloy were examined. IN792+Hf alloy with standard heat treatment showed a considerable amount of platelet η phases in the interdendritic regions, which acts as sources of secondary cracks. In order to eliminate the detrimental η phases, solution heat treatment was performed at higher temperature and for longer time. IN792+Hf with solution treatment at 1200℃ for 5 hours did not exhibit any η phases, and showed improved yield strength at elevated temperatures. However, the tensile ductility, as represented by %elongation, was reduced, which is believed to be due to the grain growth and coarse MC carbide formation during solution heat treatment.

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Changes in microstructures and mechanical properties of HY180 steels with tempering condition have been investigated by means of scanning electron microscopy, transmission electron microscopy, vickers hardness test, tensile test and Charpy impact test. Rehardening phenomena distingushed from the secondary hardening occurred in HY180 steels on tempering below 813˚K The maximum hardness(Hv 540) and tensile strength(1370㎫) were obtained in a 0.05wt%Ti-HY180 steel tempered at 753 K for 20 hours. This is attributed to the very fine(50㎚ M₂C(M=Mo,Cr)) carbides which appeared to be peculiar tc the rehardening phenomena. The maximum impact absorption energy(208J) was obtained in a 0.05wt%Ti-HY180 steel tempered at 753 K for 10 hours. This was due to complete dissolution of brittle cementite.

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A modified analytical model for the bulge forming process was made by reflecting the microstructural evolution and applied to the superplastic bulge forming of Ti-6Al-4V alloy. Bulge forming of Ti-6Al-4V alloy sheets with a fine grain size(2.5㎛) was conducted at 900℃ with three different strain rates, e.g., 2.5×10^(-4)s^(-1), 5×10^(-4)s^(-1) and 1×10^(-3)s^(-1), respectively. It was found that the grain growth rate of uniaxially tested specimens was quite different from that of biaxially bulge formed specimens. After incorporating the biaxial grain growth rate into the modified analytical model, bulge farming process was successfully controlled at desired strain rate. Also, the prediction by the modified model for thickness distribution agreed well with the experimental results.

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The present study was performed on B390(Al-17wt%Si) pretreated with P to establish the condition needed to obtain the refinement of primary and the modification of the eutectic Si simulataneously. Tensile and machinability test were carried out to examine the interrelationship between the microstructures and the mechanical properties. A refiner of primary Si and a modifier of eutectic Si were added in the form of AlCuP and Al-10wt%Sr master alloy respectively. The size of primary Si increased, as the addition temperature of Sr lowered and the amount of Sr increased. The optimum Sr addition content was 100pprr.. At this condition, the size of primary Si was lower than 30㎛, and the eutectic Si was modified. The result of tensile test revealed that tensile strength was the highest at this condition. And machinability was enhanced by the simultaneous refinement of primary and eutectic Si.

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