발간논문

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The influences of heat treating cycle and contents of C, Ti and Mo on tensile properties of Fe-C-1.5 Si steels were investigated. Specimens were intercritically annealed at 750-810℃ for eight minutes, isothermally transformed to bainite at 400-440℃ for five minutes, and then quenched into cold water. Variation of the strength-elongation relation ship according to the contents of C, Ti and Me was interperted by the amount of retained austenite, which was determined by means of X-ray diffraction. Tensile strength and elongation were improved with increase of the amount of retained austenite. The changes of tensile properties and the amount of retained austenite were correlated with the TRIP effect of the Fe-C-1.5 Si steel.

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Magnetostriction of soft magnetic Fe_(80-x)Co_xSi_6B_(14)(10≤X≤78) amorphous alloy was investigated by using a small-angle magnetization rotation apparatus. Rapid quenching technique with using single roller was employed to prepare the amorphous ribbons. Saturation magnetizaion constant, λs, determined for alloys with various compositions. The magnetostriction reaches 22.2 ×10＾(-6) for X= 10 and decreases with increasing Co content. The magnetostriction reaches zero at around X= 74.5 and is -2.60 ×10＾(-6) at X=78. This result was compared with the magnetostriction of Fe-Co alloy. Metalloids do not affect the zero-magnetostriction of the amorphous.

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The abnormal grain growth and magnetic properties of Borax doped Sr-ferrite magnet were investigated. The cause and effect of abnormal grain growth in sintered Sr-ferrite ware examined by magnetic property measurement and microstructure observation. During sintering of Sr-ferrite, the addition of Borax and excess SrO causes the liquid please to form. This liquid phase promoted the abnormal grain growth. When the abnormal grain growth occurs, the remanence and the preferred orientation increase but the coercivity decreases sharply. The sintering must be finished before the abnormal grain growth happens.

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Kinetics and processes of the B2-DO₃ordering in the Fe₃Al intermetallic compound were investigated using transmission electron microscopy. The overall ordering seemed to occur as in the homogeneous nucleation and growth process. However, the initial stage of the ordering also revealed the characteristics of the continuous process. The heterogeneous nucleation of the DO₃ phase was not observed for several aging and quench temperatures. Excess vacancies enhanced the growth rate of the DO₃ antiphase domains.

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Effective permeability and core loss have been investigated for (Co_(0.94-x)Fe_(0.06)M_x)_78 Si₄B_(18)(M = Cr, Ta, Mo) amorphous alloys in order to clarify soft magnetic properties at high frequencies. At the field of 10 mOe, effective permtabilities are large (e.g. 15,000) at 10 Hz, and decrease drastically with increasing frequency. Decrease of initial permeability due to the increase of frequency is minute at frequencies lower than 100 kHz. The initial permeability increases with the increase of M contents. The core losses of the alloys studied are smaller than that of a permalloy, and are the lowest when M=Cr, higher when M=Ta and the highest when M=Mo.

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A method estimating the nucleation ramie acoulately was proposed and the uncleation process during crystallization of Fe_(40)Ni_(40)P_(14)B_6 metallic glass was investigated with transmission electron microscopy. The preferred nucleating phase of the eutectic crystal which is composed of BCT (Fe,Ni)₃(P,B) and γ(Fe,Ni) austenite is BCT (Fe,Ni)₃(P,B). At the temperature range of 621 ∼646 K, the nucleation rate during isothermal crystallization increases monotonically with time. The activation energies of growth rate, steady state nucleation rate and incubation time are 301 KJ/㏖, 802 KJ/㏖ and 390 KJ/㏖, respectively.

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The nucleation rate with temperature and time during crystallization of Fe_(40)Ni_(40) P_(14)B_6 metallic glass investigated in the preceding paper( I ) was discussed quantitatively using the classical homogeneous nucleation theory. The diffusion coefficient, glass-crystal interface energy(σ) and dσ/dT(T : temperature) obtained from the non-steady state homogeneous nucleation theory agree well with the reported values, which means that the nonsteady state homogeneous nucleation occurs during crystallization. We also showed that the glass-crystal interfacial energy in this alloy must decrease with temperature and discussed the exponent n of Johnson-Mehl-Avrami equation.

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The effects of addition of Al and Ce to a Nd-Zr-Fe-Co-B permanent magnet alloy on the magnetic properties was investigated. The variations of Curie temperatures and the temperature coefficients of remanence and coercivity by addition of Al was also examined. The coercivity of Nd-Br-Fe-Co-B alloy increased at a rate of 2kOe/at% Al with increasing Al content. But the Curie temperature and remanence were decreased at a rate of 13℃/at% Al and 0.6 kG/at% Al. The addition of Al did not improve the thermal stability of this, alloy system. The alloy with addition of 4.5 at% Al showed the temperature coefficients of remanence and coercivity of 0.101 and 0.64 respectively. The addition of Ce deteriorated the thermal stability of remanence of the alloy.

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The solidification phenomena during the continuous casting of 150㎜ diameter billet was simulated by heat transfer analysis using the Crank-Nicolson method. The calculated results were compared with experimentally measured data. The mold cooling followed by the more powerful secondary cooling and the air gap formation during the continuous casting of free cutting brass can yield the wider dendrite arm spacing and the coarser lead particle size in the surface layer than in the core region.

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Powders of a bulk ingot of Nd-Fe-B permanent magnet were prepared by a combination of hydrogenation and mechanical milling. In-situ observation revealed that cracks, during hydrogenation, were initiated at the grain bounderies where Nd-rich phases existed and then propagated into the matrix of Nd2Fe14B phase. The powders were aligned and pressed in the hydrided condition and the green compacts were sintered at 1100℃ for 1 hr, followed by heat treatment at 600℃ for 1 hr. The maximum value of the (BH)max, of 37 MGOe was obtained with the sample which was planetary ball milled for 18 min after the hydrogenation at 13 atm. This value of (BH)max is comparable with that of the mag-nets produced by mechanical milling only.

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