발간논문

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Magnetic properties and hardness of sintered cores made of Fe-Si alloy powder containing high silicon were studied and the mechanism bringing about their characteristics is examined by metallographic methods. Fe-6.5%Si powders with different particle sizes were compacted under pressures of 8∼9ton//㎠ and sintered at 1250℃∼1400℃ in hydrogen atmosphere. The magnetic properties of Fe-6.5%Si sintered cores were improved with increasing the green density of core and sintering temperature. Especially, coercive force and hardness of sintered cores compared favorably with compressed iron powder cores and were almost comparable to electrial steel sheets. It may be concluded that the higher the density the better the magnetic properties of compacted core, high temperature sintering provides further improvement to them, and their mechanism is attributed to the elimination of porosity and the coarse-grained microstructures. Therefore, it seems that density (or porosity) is the predominant factor controlling magnetic properties and mechanical properties of compacted core.

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Delayed failure behaviour of the different transformation structures of 0.25C-2.5Ni-2.5Cr-0.5Mo-0.1V low alloy steel has been studied. The studied microstructures are martensite, lower bainite, and mixed structure of 50% martensite and 50% lower bainite. All these structures have been tempered at 450℃ for 40 min to have the same tensile strength level of 143㎏/㎟. Delayed failure testing has been carried out with cantilever bend tester, in distilled water at 25℃. By comparing K_(ISCC) values, lower bainitic structure has shown the highest value, although it is only slightly higher than that of the martensitic structure. Mixed structure has the lowest resistance to delayed failure. The fracture modes of both martensitic and mixed structures have been observed as intergranular. In the martensitic structure, however, it is noticeable that there is a larger amount of ductile tearing between intergranular facets. The fracture mode of lower bainitic structure is the mixed topography of microplastic tearing and microvoid coalescence. The above experimental results are discussed in terms of Oriani`s decohesion theory of hydrogen embrittlement. The lowest resistance of the mixed structure to delayed failure may be due to the enhanced decohesion by hydrogen at the phase boundaries of martensite and lower bainite.

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To investigate the reason which controlls the depth of origin of the hydrogen induced blistering in steel, Fe-1.7% Mn steel is selected. For this study, using Tresca`s yield criterion, a model is proposed and the feasibility of the model is exercised experimentally. It is assumed that the hydrogen induced blistering is taking place due to a modified triaxial stress state around a void in which hydrogen gas generates high hydrostatic pressure. Triaxial stress state around the void can be modified by the external or residual stress to give biaxial stress. This modified biaxial stress induces the shear stress component which is in responsible for the plastic deformation. For the verification of this hypothesis, specimens are cold rolled and pulled in tension to give compressive and tensile residual stress states in the specimens, respectively. It is observed that as the amount of the cold rolling increases the compressive residual stress raises and the depth of hydrogen induced crack is also deepened. However, those specimens which are tensioned is believed to have almost same residual tensile stress regardless the amount of the tensile strain and it is observed that the depth of the crack origin did not change with deformation except for the first small strain. These observed experimental results give a strong confirmation of the proposed hypothesis which is based on the pressure theory of hydrogen embrittlement.

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The influence of solidification condition on solute redistribution (that is, microsegregation) in dendritic growth was investigated with unidirectionally solidified Al-8% Mg alloy. The aim of the present investigation is to interpret the mechanism of the solute redistibution determined as a function of the fraction of solid which is given by two dimensional solidification model, and moreover to provide the quantitative data of the microsegregation and the microstructure in alloys of engineering interest in order to explain the homogenization process. For this purpose XMA analysis for a dendrite element at each distance from the water cooled copper chill face was made. The results obtained are as follows; 1) The segregation index (S) decrease by increasing the solidification rate (R), and the S between primary dendrite arms is more remarkable than that between secondary dendrite arms. 2) Minimum Mg solute concentration is demonstrated at the center of primary dendrite stalk, and increase by increasing solidification rate R(that is, decreasing distance from the chill face) 3) The degree of microsegregation (included β-phase) in Al-8% Mg alloy was determined by the change of the effective partition coefficient Ke. 4) The influence of diffusion layer on the solute movement ahead of the solid-liquid interface near the dendrite tip which was not obtained by means of the prior linear analysis was clearly demonstrated by determining the fraction of solid from two dimensional solidification model, and it was found that the diffusion layer has a remarkable effect upon the solute redistribution. The degree of microsegregation was dependent upon the change of the effective partition coefficient with the change in the rate of solidification, and could be interpreted quantitatively by the partially mixed liquid model.

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Growth of coating layer and wear properties of chemically vapor deposited (CVD) TiC onto WC-Co substrate were investigated. The deposition temperature, time and Co content of the substrate showed pronounced effects on the thickness of coating layer. The texture of TiC layer changed from <100> orientation at low deposition temperature (950℃) to random orientation at high deposition temperature (1050℃). The wear resistance of coated specimen was remarkably increased and enhanced wear resistance was also discussed.

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The steady state creep deformation mechanism of 70Cu-30Ni solid solution alloy was investigated for the temperature range of 0.44∼0.64T_m. For the better understanding of the mechanism the effective stress was measured and the creep rate was expressed with the effective stress as ε˙_s=1.84×10^(-4)(σ^*/σ_(0.05))^(2.6) where σ^* and σ_(0.05) are the effective stress and 0.05 strain offset proof stress, respectively. The observed experimental results indicate that the creep deformation mechanism of the given alloy under the stress range of 15∼336㎫ is controlled by the non-conservative motion of the jogged screw dislocations.

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The solution kinetics of nonequilibrium second phase of a segregated binary alloy in the later stages of solution treatment have not yet been made theoretically clear. This paper attempted mainly to investigate this behaviour by Yoon`s approximate analysis given by a simple model based upon primary arm spacing as an effective diffusion distance of the second phase which has crystallized at the boundary of dendrite element. For this purpose, we used the high magnesium Al-8% Mg alloy which is expected to be used for transport of liguified natural gas. The theoretically calculated results are in the approximate agreement with that of experiments for complete dissolution time and solution kinetics (expecially at the later stages) of β-phase. The Yoon`s approximate analysis was in better agreement with the present experimental results than the Flemings conventional analysis

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Effect of metal mold ratio on the mechanical properties and cast structure of Cu-Al-Si alloy has been studied by using the metal mold which has various thickness. 1. Mechanical properties of specimen show the maximum value at metal mold ratio 1.5 and solidification rate 17℃/sec (1.27㎜/sec) 2. addition of 2.2wt%Si and suitable solidification rate 17℃/sec (1.27㎜/sec) increased equiaxed structure zone and decreased dendrite arm spacing. 3. Mechanical properties of specimen is decreased as metal mold preheating temperature increased. 4. Tensile strength and hardness of specimen which has silicon content of 2.2wt% is increased from 48㎏/㎟ to 53㎏/㎟, from H_B 107 to H_B 143 respectively than that of 1.4wt%Si. 5. β→α+γ₂ eutectoid transformation is suppressed with increasing solidification rate.

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The effect of ultrasonic vibration on mechanical properties and microstructure of 7075 Al alloy by precipitation hardening has been investigated. Ageing treatment with ultrasonic vibration at 120℃ is improving the mechanical properties such as the yield strength and elongation compared with the conventional ageing treatment. It is believed that ultrasonics produces the excess vacancies and these vacancies are accelerating the formation of GP zones. The formation of GP zones has pronounced effect on the refinement of precipitate particle size and inhibition of PFZ(precipitate free gone) and grain boundary precipitate formation. Ageing at 180℃ with ultrasonic vibration produces directly η´ particle without the formation of GP zone. Therefore it is believed that the excess vacancies formed by ultrasonic vibration do not affect on the refinement of matrix precipitate particle size.

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Effects of two intermediate thermomechanical treatments, which are FA-ITMT and ISML-ITMT on mechanical properties and microstructures of 7050 aluminum alloy were studied. The mechanical properties were investigated as function of deformation temperature and degree of deformation during final deformation step of FA-ITMT. The ITMT treated specimens were examined either in the as-recrystallized(AR) condition or in AR plus hot deformed condition (AR+HR). The results showed that fine grained microstructures can be obtained in 7050 aluminum alloy through the FA-ITMT and ISML-ITMT. However, the mechanical properties of ITMT AR treated specimen were inferior to those of conventionally processed specimen. Elongation and fracture toughness of AR+HR treated specimen were superior to those of conventionally processed specimen especially in the T-L direction. Fracture toughness increased as hot deformation temperature increased during FA-ITMT, and it also increased as the degree of hot deformation increased at 390℃. When the degree of hot deformation at 430℃ and 470℃ respectively was 40%, the fracture toughness showed the peak value.

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