발간논문

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Multi-layer coating of silicon carbide and graphite on UO₂ nuclear pellet was carried out with a fluidized bed type chemical vapor deposition unit to enhance the inherent safety of the nuclear pellet and to develop a renovative coating process. Silane, propane and argon gases were used as the source and carrier gases to deposit silicon and pyrolytic carbon. Combustion reaction was performed in an inert argon atmosphere, which conditions were pre-determined by numerical evaluation of the combustion reaction between silicon and carbon to synthesize silicon carbide. Microstructural observation of the combustion products by scanning electron microscopy showed that the inner and outer pyrolytic carbon layers were equiaxed structure. Chemical analysis by Auger electron spectroscopy and X-ray diffractometry revealed that the middle layer formed by a combustion reaction was fine crystalline beta-silicon carbide.

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A phase mixture model was considered in which nanocrystalline materials are treated as a mixture of crystalline phase, intercrystalline phases (grain boundary, triple line junction and quadruple node) and pores. In order to investigate the effects of grain size and porosity on the elastic modulus, Budiansky`s self-consistent method in conjunction with the phase mixture model was employed. The calculated results are compared with the experimental measurements in the literature. The elastic modulus of nanocrystalline materials decreases with a decrease of the grain size and the decrement is relatively large at grain sizes below about 10 ㎚. The effect of porosity, however, is substantially greater than the grain size effect.

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The cyclic creep behaviour of Zircaloy-4 was investigated over the temperature range of 390∼430℃ (0.32 Tm-0.34 Tm) and the stress range of 15-18 ㎏/㎟ (147.1∼176.5 ㎫). Constant stress creep tests were carried out in the experiment. Square wave cyclic stress was chosen with the minimum stress equals to 10% of the maximum stress and frequency was 3 cpm. The stress exponents for the cyclic creep, n_c, were 7.59, 7.14, 6.79 and 6.48 at the temperature of 663, 678, 693 and 703K, respectively, The apparent activation energies for the cyclic Creep, Qc, were 68.59, 65.82, 63.01 and 59.74 ㎉/㏖ at the stresses of 15. 16. 17 and 18 ㎏/㎟, respectively. Cyclic creep rate was higher than static creep rate because of a cyclic creep acceleration. Larson-Miller parameter of zircaloy-4 under the cyclic creep conditions was obtained as P=(T+460) (log t_r+21). Transgranular fracture was dominated over the experimental range.

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The present study is concerned with the effect of Al composition on the deformation behavior of ultra high pure (UHP) Ti-(48,50,52) at.%Al alloys using ultra high pure Ti with 30 wt. ppm oxygen. It is suggested that the deformation mechanism of UHP binary TiAl is strongly dependent on Al compositions. The deformation substructure of UHP Ti-48 at.%Al is shown to be dominated by ordinary dislocation with b = 1/2$lt;11￣0] as well as deformation twinning and small portion of 1/2$lt;112￣] superdislocation. Deformation mechanism in UHP Ti-50 at.%Al alloy consists of 1/2$lt;11￣0] type ordinary dislocation and 1/2$lt;112￣] superdislocation and a small portion of $lt;01￣1] superdislocation, but deformation twinning is not observed. Most of dislocation structure in UHP Ti-52 at.%Al alloy consisted of the faulted dipoles. Major deformation mode of UHP Ti-48 at.%Al and UHP Ti-50 at.%Al alloys was ordinary dislocation in deformation orientation, which take an advantage of ordinary dislocation slip. However, the major deformation of UHP Ti-52 at.%Al alloys was superdislocation slip in this orientation. It seems that there is little difference between major deformation mode of nominal purity TiAl and that of TiAl contained ultra low oxygen.

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A study was made of the effects of carbon and chromium addition on microstructural factors and wear properties of five high speed steel(HSS) rolls manufactured by centrifugal casting method. Based on the basic chemical composition of Fe-2.0C-5.0V-3.0Mo-1.5W-6.2Cr-0.75Si-0.35Mn(wt.%), the rolls were made with varying additions of carbon and chromium in order to examine their respective effects. They were then austenitized at 1050℃, air cooled, and double-tempered at the temperature range from 470℃ to 600℃. A simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the HSS roll containing a smaller amount of chromium showed the best wear resistance because it contained a number of hard MC carbides. However, it showed a very rough roll surface because of the preferential removal of the matrix and the sticking of the rolled material to the roll surface during the wear process, thereby leading to increase in friction coefficient and rolling force. In order to improve wear resistance with consideration of the roll surface roughness, the increase in the chromium content, i.e., the increase the amount of M_7C₃ carbides which can give good surface roughness, was suggested.

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The effect of orientation relationship between two adjacent grains on the brittle crack propagation was systematically investigated in the viewpoint of misorientation at the boundaries using Electron Back-Scatter Diffraction(EBSD) technique. Crystallographic orientation around the path of secondary crack under the fracture surface has been observed with fractured 3 point band specimen at -100℃. Use of EBSD technique allows the crystallographic cracking plane of the material to be inferred. For every crack observed, the trace of the plane was perpendicular to a $lt;001$gt; direction of the cracked packet. The concept of the geometric compatibility factor has been introduced in order to explain and predict the crack propagation between two adjacent grains. This factor can be calculated from the measuring the two angles from {001} pole figure. A possible cracking plane predicted from calculation of a geometric compatibility factor was coincident with those found in observation.

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The Al-Mg-Si alloys for autobody sheet application are capable of providing a good paint bake hardenability. However, in practical usages, it is difficult to perform paint bake cycle immediately after solution heat treatment and a delay at room temperature before the paint bake cycle is usually unavoidable. Also, the condition of paint bake cycle is insufficient to exert completely their age hardenability, although the alloys have a hardening potential. Therefore, in order to improve the paint bake hardenability, it is necessary to prevent the detrimental effect of natural aging before paint bake cycle. In this study, effects of Cu and Sn alloying elements and pre-aging treatment on paint bake hardening in Al-Mg-Si alloys have been studied by means of micro-hardness measurement and transmission electron microscopy. The results showed that the paint bake hardenability of the Al-0.5Mg-1.3Si-0.1Zr alloy is enhanced by the addition of Cu and Sn alloying elements and the adoption of the preaging treatments. It suggested that the clustering process occurred during natural aging is effectively inhibited by Cu and Sn alloying elements and the preaging treatments.

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Effects of niobium addition and reverted austenite on the tempering characteristics of martensitic Fe-16Cr-5Ni-0.06C(in weight percent) stainless steels were investigated over the temperature range from 400 to 680℃. Tempering stages of these steels can be divided into three distinct regions, namely 1) region I (up to 440℃) : mechanical poperties remained constant, 2) region II (500-620℃) : strength abruptly dropped and ductility increased with increasing temperature resulting from the formation of reverted austenite and 3) region III (above 620℃) : strength increased and ductility decreased again with increasing temperature due to the martensitic transformation of reverted austenite during cooling. In Nb-free alloy (M1), hardness and strength were dropped rapidly after the reversion of austenite started. On the contrary, the strength drop in Nb-containing alloy (M2) was rather gradual, meaning that the softening effect due to the formation of reverted austenite was compensated by the solid-solution hardening and the precipitation hardening of NbC. Moreover, the Nb addition not only improved mechanical properties but affected the formation of reverted austenite.

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Microstructural evolution of martensitic Fe-16Cr-5Ni-0.06C-(0.1Nb) stainless steels during tempering were investigated utilizing scanning electron microscopy and transmission electron microscopy. The microstructure consists of tempered martensite laths together with reverted austenite, delta ferrite and some carbides. On tempering above 500℃, the lath-like reverted austenite formed preferentially at martensite lath boundaries and grew along lath boundaries, showing mainly K-S orientation relationship with matrix. Another type of reverted austenite, namely granular austenite, containing very low density of defects and twin was also found in higher tempering temperatures and the orientation relationship with matrix was found to be N-W relation. In both specimens wish and without Nb, M_(23)C_6 carbide precipitates along the martensite / reverted austenite boundaries or in the lath interior. On the other hand, in Nb containing specimen tempered above 550℃, very fine, spherical NbC carbides are also observed in the martensite lath interior, and this type of carbide presumably contribute to the small decrease of strength in tempering temperature between 500 and 620℃, as shown in previous paper.

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In the ductile-brittle transition temperature region of SA508 Cl.3 Mn-Mo-Ni low alloy steels, the relationship of the local fracture stress and carbides influencing the cleavage fracture behavior was investigated. Based on the ASTM E1921-97 standard method, the reference transition temperatures were determined by three point bending fracture toughness tests. A local fracture stress, σ^*_f, was determined from a theoretical stress distribution in front of crack tip using the cleavage initiation distance measured in each fractured specimen surface. The local fracture stress values showed a strong relationship with toughness characteristics of the materials and those were larger in the materials of smaller carbide size. Quantitative analysis of carbides showed that carbides larger than a certain size are mainly responsible for the cleavage fracture in the ductile-brittle transition temperature region.

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