발간논문

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Two types of martensitic alloy, Fe-C based martensitic steel (Fe-1.7%C) and Fe-Ni based martensitic alloy (Fe-27% Ni-0.14%C), were prepared and then the retained austenite decomposition behaviors of these alloys were investigated during tempering. The results obtained in this study are as follows. (1) The retained austenite of Fe-1.7%C martensitic alloy begins to decompose into bainite when tempered at 150℃ for one hour. On tempering at 280℃ for one hour, most amount of retained austenite contained in the alloy is transformed to bainite. (2) The integral breadth of retained austenite diffraction line increases as tempering temperature is increased up to about 280℃. This is because volume expansion occurs when the retained austenite is transformed to bainite. (3) In the case of Fe-27% Ni-0.14%C martensitic alloy, the retained austenite does not decompose although tempering is done up to higher temperature such as 430℃. (4) The line broading of retained austenite in the Fe-27% Ni-0.14%C martensitic alloy maintains a nearly constant value during tempering up to about 360℃, and then shows a decreasing value on further tempering at higher temperature than the point.

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The thermal densification of the sintered UO₂ pellets has been investigated by resintering of them in the terms of variations of pore size and pore size distribution. The pore size and its distribution of presintered pellets were controlled by the addition of 1 weight percentage polyethylene glycol sized in the range of 40-125㎛ as a volatile pore former. The density of the pellets were in the range of 92 to 96% of theoretical density. A review of resintering experiments at 1600℃ for 10 hours in the 98.5 v/o N₂-1.5 v/o H₂ mixed gas indicated that the pellet containing large pores with diameter of 40㎛ had the lowest fractional density change of 6.0×10^(-3), while that of 12.34×10^(-3) had been revealed in the pellet containing finer pores than 2㎛. The observations led to the conclusion that high thermal stability was achieved by forming large pores of around 40㎛ in the pellets and high densification was induced by small pores approximately less than 2㎛.

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The relation between formation and distribution of slip bands in the grains of aluminum polycrystals (purity: 99.9%) and crystal orientation of each grain have been studied after tensile fatigue stressing of the samples. Grains of the samples were grown to the size of 1.5∼3.0㎜ diameter by heat treatment. Crystal orientation and Schmid factor of each grain were determined by means of Laue back reflection. Cyclic tensile stresses were applied to the samples between 189 and 47ｇ/㎟ or 198 and 32ｇ/㎟ at a low frequency of 70 cpm, and the formation and distribution of slip bands were observed by optical microscope after the cyclic stressing of 10², 10³, and 10⁴ cycles. As the value of Schmid factor of a grain was increased, the number of fatigue cycles, in which slip bands are formed, was decreased, Not any slip bands were found in the grains of low Schmid factor values, such as 0.34 and 0.37, even after 10⁴ cycles.

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