발간논문

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It has quite often been observed that the interference functions of rare earth metal-transition metal amorphous alloys, which are prepared only by vapor deposition, show considerable deviation from the expected when evaluated directly from their X-ray diffraction intensity patterns. In this study efforts were made to find the causes and solutions thereof and, for that matter, X-ray diffraction experiments were performed both in transmission and reflection geometries on amorphous Co-Gd alloy films sputtered on a Be-substrate. It was found that, for a specimen on a substrate in reflection geometry, an adequate modification on the data reduction procedures or specifically the absorption correction should be made depending on the configuration of the specimen and substrate. Also it was realized that the data reduction procedure is relatively simpler when the substrate faces towards the incoming X-ray beam than the contrary.

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Effects of pearlite banding and sulfide morphology on the anisotropy of tensile properties and impact toughness were studied in hot rolled C-Mn steels. Homogenizing treatment and sulfide shape control by the rare earth metal addition were used to change the pearlite distribution and sulfide morphology. It is believed that the planar anisotropy of tensile ductility is primarily due to the elongated sulfide rather than pearlite banding. However, the anisotropy in impact toughness is associated with both the pearlite banding and sulfide morphology. The pearlite banding is the cause of anisotropy associated only with notch orientation, whilst the elongated sulfide is the cause of anisotropy associated with both notch and specimen orientation. Homogenizing treatment improved the anisotropy in ductility and toughness by eliminating the pearlite banding and by subdividing and rounding the elongated sulfides.

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The grain growth characteristics of dual-phase steels and the effect of alloying elements on the grain growth in these steels have been investigated using 3%Si-0.3%Cr-C system steels which are composed of αand γ-phase at 900-1300℃. The results are as follows: 1) The grain growth rate in dual-phase steels is substantially slower than that of single-phase steels, and it`s rate is the slowest when the volume fractions of α-and γ-grains are almost the same. 2) The major phase in dual-phase steels grows under a pinning action of the minor phase, and the minor phase grows slowly in a mode of Ostwald ripening. Consequently, the major phase grows also slowly in a mode of Ostwald ripening. The grain growth rates of both phases obey the following equation: (r)³- (r_o)³= K₃t 3) The grain growth rates of both the alloyed and unalloyed dual-phase steels obey the Ostwald ripening law. The functions of the alloying elements in dual-phase steels are classified into two types: Partitioning and pinning. The partitioning of alloying elements such as Mn and Mo renders the growth of the minor phase sluggish because these elements make the concentration difference between α-and γ-phase very large. Titanium carbides (TiC) form fine particles and hinder the grain growth rate by the pinning mechanism.

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Cold model experiments of LD convert have been conducted to evaluate the influences of flow rate of bottom blown gas, number and configuration of tuyeres. Mixing rate depends mainly on flow rate of bottom blown gas and is not stronly influenced by other factors. Efficiency of utilization of the energy of top blown gas for the mixing of the liquid bath is considered to be much smaller.

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The steady-state creep mechanism and behaviour of Zircaloy-4 have been investigated in air environment over the temperature and stress ranges from 535℃ to 620℃ (0.38-0.42Tm) and from 6㎏/㎟ to 9㎏/㎟, respectively. Constant stress creep tests were carried out in this experiment. The work lead to the following results; 1) The stress exponents, n were 5.4, 5.2, 5.1, 5.0, and 4.9 at temperatures of 550, 565, 580, 590 and 600℃, respectively. In other words, the stress exponents decreased as the temperature increased. 2) The apparent activation energies, Q were 67.9, 65.7, 63.8, and 62.2 ㎉/mole at stresses of 6, 7, 8, and 9㎏/㎟, respectively. In other words, the apparent activation energy for the creep deformation decreased as the stress increased. 3) The activation energies for the creep deformation were nearly equal to that of the volume self diffusion of Zr in the Zr-Sn-Fe-Cr system. 4) From the above facts, the creep deformation For Zircaloy-4 seemed to be controlled by dislocation climb over the range of experimental conditions.

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The kinetics of stress-corrosion cracking (SCC) of a high purity Al-Zn-Mg alloy was studied as a function of applied potential, temperature and kind of corrosive media. The SCC tests were carried out in the range of applied potential from -550 to -2150 mV_(SHE) and at the temperatures between 288 and 318 K. 3.5 wt.% NaCl solution (PH=1), air and transformer oil were used as corrosive medium. The percentage elongation and the fracture energy due to SCC increased with decreasing applied potential from -550 to -1150 mV_(SHE) and then decreased. The SCC fractography correspondingly revealed a decreased intergranular-brittle fracture fraction with decreasing applied potential up to -1150 mV_(SHE) and then increased one with decreasing applied potential up to -2150 mV_(SHE). Average SC crack propagation rate, γ, was calculated by introducing a new function related to the σ-εcurve in the inert environment. It was also observed to be 100×10^(-6), 39×10^(-6) and 2.1×10^(-4) ㎜ sec^(-1) at the applied potential of -550, -750 and -1150 mV_(SHE), respectively, tested under the strain rate 8.3×10^(-7) sec^(-1) with the underaged specimen. The observed values are in good accord with the calculated values. The intergranular-brittle fracture mode was also observed in oil, probably caused by the water in oil. Stress-corrosion (SC) fracture mode varied from a simply corroded fracture near the surface/subsurface, followed by an intergranular-brittle fracture over the mixed fracture, composed of the intergranular-brittle fracture and a dimpled rupture, to the pure dimpled rupture in the interior of specimen. It is suggested, on the basis of the observation of potential dependence of the fracture energy and SCC fractography, that SCC is caused by hydrogen-assisted embrittlement. Activation energy for hydrogen-assisted embrittlement in the NaCl solution was found to be 18-20 kJ mole^(-1) for the specimen aged at 180℃ for 0.5h (underaged stage).

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The annealing behaviour of type 316 stainless steel prestrained with various degrees 10-30%, after solution treatment was investigated by using quantitative metallographic techniques. With increasing degree of prestraining slip bands are heavily formed and their densities gradually decrease as annealing time elapses. Fine M_(23)C_6 type complex carbides are found to precipitate along grain boundaries, slip bands and twin boundaries and to affect the restraint of recrystallized grain boundary movement. The average values of grain size during annealing are shown to increase as prestraining percentage decreases. In particular, grains tend to be coarsened in the case of prestraining of less than 10%. The effects of prestraining on the increase in hardness appear to be considerable as compared to those of annealing temperature, showing that Vickers hardness values increase with increasing prestraining percentage. For a given prestraining condition changes in hardness with respect to grain size are obtained to be in good agreement with the Hall-Petch type relation irrespective of annealing temperature.

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