발간논문

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The mechanical properties of ductile east iron can be significantly improved by austempering, isothermal transformation in the temperature range from Ar" to Ar` point flollowing austenitization. Fracture experiments were performed on 0.24 % Mo-0.5 % Cu alloyed ductile cast irons which were austenized for 1.5 hr at 900℃ and austempered for 2 hr in the temperature range from 280℃ to 415℃. Fracture toughness values higher than K_(IC) 80 MPa m＾(½) were obtained with the bainite ductile irons which were austempered in the range of 365 to 385℃, and then values of yield strength, elongation and hardness were in the ranges of 810 to 880 MPa, 8.8 to 10.4 % and 270 to 310 H_B, respectively.

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There has been a great interest recently in the deposition of hard coatings on tools by physical vapor deposition processes. In this study (Ti, Al)N films were deposited onto high speed steels by biased activated reactive evaporation, varying plasma conditions during deposition. Electron density and electron temperature of glow discharge plasma during deposition were measured using a single electric probe technique. Discharge current of auxiliary electrode was a key parameter to control the plasma density. The films were characterized by x-ray diffractometry in order to determine phase, preferred orientation, mean microstrain, lattice parameter, and grain size. The film morphology was observed by scanning electron microscopy. Microhardness and adhesion strength were also measured. Electron density increased with discharge current of auxiliary electrode and average electron energy showed little variation. As the substrate bias voltage increased (200) texture developed and grain size decreased, while mean microstrain decreased and lattice parameter increased, all showing minimum or maximum values at-1000V bias. Microhardness increased with and adhesion was improved by substrate biasing. Also impact indentation modes showed the same tendency. All results would be caused by ion bombardment effects during deposition.

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The microstructural changes of mechanically alloyed (MA) Al-Mg-Al₂O₃composite powders with the processing time were studied by using optical and transmission electron microscope. The mechanical properties and microstruotures of those P/M products con solidated by extrusion were also investigated at room and high temperatures. These composite powders were found to reach a steady state after 240-300 minutes of mechanical alloying process. Up to 120 minutes of processing time, the structural refinement rate of powders seems to follow the logarithmic relation with processing time given by ε= 3.68 ln (1 + 0.043t). After MA powders reach a steady state, the grain size of powders decreased steadily becoming less than 0.1㎛, while dislocations were rearranged to form low angle grain boundaries transforming into a lower state of strain energy. The combined effect of these two phenomena appears to lead into the saturation of hardness. In the case of extruded bar, Al₂O₃and A1₄C₃dispersoid, 10-20㎚ in diameter we e observed to disperse uniformly. The size of recrystallized grain seemed to be directly controlled by the amount of dispersoids. At higher temperature, however, only the Al₂O₃ particles pinned the grain boundaries. Accordingly grain size was smaller and grain boundary sliding was retarded providing a superior high temperature hardness as the amount of A1₂O₃increased.

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A study has made of residual stresses on the surface and characteristics of flash welded chain wish various steels. The measurement of residual stresses applied sectioning technique using electrical resistance strain gages. As a result of experiment, to product high grade chain, the alloying elements in steel must contain Ni, Cr and Mo, the conditions of heat treatment were oil quenching from 870℃ and tempering at 200℃. Distribution of residual stresses of welded chain was very complicate. Tangential residual stresses σ_T, appeared low valve, longitudinal residual stresses, σ_x, the more the distance from the weld line increase, the more tensile residual stresses increased and peak tensile residual stresses appeared about 2-2.5 times distance of heat affected zone from weld line and compressive residual stresses appeared at the bending region. Generally, the behavior of residual stresses of flash welded chain was affected by the deformation, yield stress, hardness and elongation of raw material.

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The effect of low-temperature annealing on magnetic properties has been investigated in order to clarify the structural relaxation of a metal-metalloid type amorphous alloy, For amorphous Fe_(33)Ni_(50)P_(17) alloy, the saturation magnetization at T= OK was estimated to be almost not affected by annealing at 450k for 3hr indicating the little change of the neighbouring metalloid surroundings. The Curie temperature increased with In t_a kinetics for Fe_(33)Ni_(50)P_(17) alloy annealed at 400-450 K. From the composition dependence of the Curie temperature for amorphous (Fe_(1-x)Ni_x)_(83)P_(17) alloys (x=0-0.6), the increase in Curie temperature was interpreted to originate from the coupling between Fe and Ni atoms. The activation energy for the low-temperature relaxation causing the change in Curie temperature was about 1.3eV.

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The solidified and annealed structures of gas atomized Fe-4wt%C-Cr alloy powders were investigated. The solidified structures were mixtures of austenitic dendrite and (Fe, Cr)₃C/austenite eutectic. Metastable ε-phase was observed in small particle, -25㎛, which was massive solidification products. When the alloy powders were annealed at 600-900 C, growth of carbide which, original1y eutectic type, grew by ostwald ripening was observed. And the volume fraction of carbides had a tendency to increase due to precipitation of carbon which was supersaturated in austenite to carbide. The carbide grew rapid1y above 900℃. Before annealing, the micro hardness of powders marked 950-1300 Hv andwas increased with increasing of Cr contents. After annealed at 1000℃ for 1hr., the microhardness was 850-1500Hv. And in alloy of 4% Cr, the micro-hardness of that was above 1500 Hv, so it is expected that Fe-C-Cr alloy powders can be used as wear-resistance material.

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The mechanism of the shear band formation has been studied in the high1y rolled copper. The results obtained from the transmission electron microscopical investigation and the x-ray texture measurement were interpreted by the rotation stability based on the Taylor deformation theory. The formation of the shear hand was found along a crystallographic {111} plane which corresponded to the rotated rolling plane about TD. It was found that shear bands were developed preferential1y in the deformed grains oriented near the {4 4 11}＜11 11 8＞. It was also noted that the orientation inside of a shear band was the same throughout the shear band. Thus the lattice curvature model in a shear band could not be accepted in case of the Cu-type shear band. The shear bands lying almost parallel to the rolling plane at the very high reduction indicated that the strain state was different during and after the formation of the Cu-type shear band.

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This study was carried out to investigate the mechanism of S. R. cracking in HAZ of high strength low alloy steel. Among 3 commercial high strength steels, steel A (HT80), which is equivalent to ASTM A514 Gr.F. was most susceptible to S. R cracking whereas steel C(HT60) showed no crack in oblique Y-groove test even in high restraint. The cause of S. R cracking along the coarsened austenite grain boundary was secondary hardening phenomenon and impurity segregation to the prior austenite grain boundary. The crack susceptibility was reduced as increasing heat input due to the weakening of secondary hardening.

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Rapidly solidified Cu-(0.65, 0.85) wt.% Zr alloy powders have been produced using high pressure gas atomizer and physical properties of the extrusions of these powders have been investigated. A complete solution of 0.65wt.% Zr in Cu was possible in the rapidly solidified powder. This solubility increment is remarkable on considering the maximum Zr solubility, 0.17 wt.% in pure copper at the eutectic temperature, 980˚C. In the case of the alloy powders, sizes of which are less than∼74㎛, a microcellular structure develops with the cell spacing of 2∼3㎛. From this cell spacing, the cooling rate of the rapidly solidified powders was deduced to be ∼3×10＾5K/sec. The extrudates of Cu-0.65wt.% Zr alloy powders have tensile strength of 450 MPa and electrical conductivity of 85 % IACS. These properties are superior to those reported for the conventional Cu-Zr alloys.

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Mechanical alloying was carried out on the mixed Ni-Cr-Y₂O₃powders using high energy ball mill in order to develope a dispersion strengthened Ni base superalloy. The process can be broken down into five stages (1) Initial stage (2) Stage of welding predominace (3) Stage of equiaxed particle formation (4) Start of random welding orientation (5) Steady state processing. The extruded materials of this mechanical1y alloyed powders showed a finer recrystallized structure(about 0.l㎛ grain size). The high temperature strength of dispersion strengthened materials is limited by grain boundary sliding and transverse boundary rupture. These effects are minimized by developing coarse, elongated grain structure. This structure was gained by heat treatment above a critical temperature, or 1300 C for Ni-20Cr and 1340 C for Ni-20Cr-1Y₂O₃. Ultimate tensile strength was extensively increased by adding Y₂O₃ at room temperature and developing a coarse, elongated grain at high temperature.

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