발간논문

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Unidirectional solidification experiments were conducted on Al-Fe-Mn ternary eutectic alloys at the temperature gradient 80℃/cm in the range of solidification rates of 1-38 mm/min in order to investigate the solidification and mechanical behaviors. It has been obtained in the present work that the morphology of the composite structure changed from the plate type into the square piller or rod type as the solidification rate increased. The chemical composition of fibers at low solidification rates was Fe(Mn)Al₃, and that at high solidifcation rates was (FeMn)Al_6. The faster solidification rate produced the higher ultimate tensile strength. But the ultimate tensile strength decreased by misalignment of fibers at very high solidification rates. The ultimate tensile strength of unidirectionally solidified and aged specimens have the same value as that of only unidirectionally solidifed specimens.

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This is the first article of a series dealing with studies on the quantitative representation of fiber-and sheet-type textures in metals of cubic crystal system. Texture measurements by neutron diffraction method are analyzed using Bunge`s series expansion method and the effect of series truncation is studied for samples of various texture sharpness. The present article describes two computer programs, TXFIB and TXSHT, developed for the analysis of the respective fiber-and sheet-type texture. Using these computer programs, the orientation distrbution function can be expanded in the series of generalized spherical harmonics up to 58th term from 6 experimental pole figures as input. Estimations of various errors involved in the texture analysis and texture sharpness index are also included in the programs.

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Experiments were carried out to investigate the effect of various casting processes such as sand casting, chill casting and ESR, and also of nickel addition on the structure and sliding wear properties of Al-Sn alloy containing up to 20% tin. The results obtained are as follows; 1) Addition of nickel to the Al-Sn alloy produced by sand casting and ESR was effective to refine tin particles, while there was no structural change in chill cast alloy. 2) The wear resistance of the alloy produced by ESR was superior to that by sand casting and chill casting. The wear resistance varied with sliding conditions, but better wear resistance was achieved with continuously networked tin structure. 3) In the sliding wear of Al-Sn alloy, abrasive and adhesive wear were occured, while oxidizing wear was not recognized, resulting an increased sliding speed raised the wear amount. 4) In the wear test of Al-Sn alloy, the average size of wear debris and the wear amount increased with increasing sliding speed, and both of them were also found to be in proportional.

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Tenslle test and microstructural analysis were performed to investigate the effects of dual phase heat treatment and cold drawing so as to promote the mechanical properties of 0.07%C steel wire. The strength of the dual phase steel wires varied with volume fraction, shape, and size of martensite and grain size of ferrite. The strength of the dual phase steel wire was appromiately proportional to a reciprocal of square root of mean free path. The elongation of dual phase steel wires varied with volume fraction, distribution, and size of martensite. The carbon content in martensite changed the strength of martensite, which affected the elongation of the specimens. Low carbon steel wire of 1257 MPa in tensile strength and 4 percent in elongation was obtained, when low carbon steel rod with 475 MPa and 38% in tensile strength and elongation, respectively, was heat treated and cold drawn.

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Morphological changes of intermediate tungstic oxide particles, prepared by reducing WO₃with hydrogen and a mixture of hydrogen and nitrogen over the temperature range from 800℃ to 1000℃, were studied using scanning electron microscope. Two different morphologies of W_(18)O_(49), bundles of needle crystals and short pole-type particles, were obtained at 800℃, whereas W_(18)O_(49), obtained at 900℃ and above, consisted of pale-type particles and fine whiskers. The growth of W_(18)O_(49) particles was faster in the length than the width with increasing reduction temperature. The size and morphologies of W_(18)O_(49) particles were greatly influenced by those of WO₃particles. Transformation of W_(18)O_(49) to WO₂and WO₂to α-W was accompanied by the rounding of W_(18)O_(49) particles and the recrystallization of α-W in spherical shape respectively. Extremely fine α-W powder was obtained by reduction of WO₃powder at 500℃ for 4 hrs in hydrogen followed by immediate reduction at 650℃ for 1 hr.

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Al-Si alloys have been modified by Na. However, because of fading tendency of Na, modification of Al-Si alloys by Sr is recently being researched. Besides Na addition, high colling rate also results in modification. Accordingly, this experiment was performed to study the modification of Al-Si alloys by Sr in permanent mold casting and was focused to Sr content for complete modification, improvement of mechanical properties, thermal analysis and shrinkage. At 0.3% Sr, when the mold is preheated to 200℃, complete modification was accomplished and mechanical properties were considerably improved. Thermal analysis curve and shrinkage tendency may be used for determining the degree of modification in the permanent mold casting.

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