발간논문

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Declining temperature multistage torsion tests were performed to study the deformational characteristics and static restoration of Invar alloy under 1100∼850℃, 4.0×10^(-2)∼2.6×10^0/sec and ti=2.5∼10sec conditions. From the analysis of multistage curves, the suitable condition for high temperature deformation could be determined. The fractional softening(FS) was obtained with the condition of the declining temperature and accelerating strain rate. The interpass fractional softening was decreased at lower temperature and its range was determined to be 50∼110%. Dynamic precipitation was observed during multipass deformation where the size and distribution of precipitates increased and spreaded as the number of rolling pass increased. The size or distribution of precipitates could be reduced by controlling the deformation condition and the area fraction of precipitates after deformation of Invar alloy was 0.3∼0.9% and its size was 2.5㎛∼3.9㎛.

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As an effort to improve the mechanical properties, the Cr₂O₃ coatings formed on SM45C substrates by the plasma spraying were subjected to annealing or hot press treatment. After each treatment, mechanical properties such as microhardness, abrasion and bond strength of annealed or hot pressed coatings were compared with the as-sprayed coatings. In case of annealing treatment, as the annealing temperature increased, the porosity decreased. Microhardness and abrasion characteristics were also improved upon annealing. In particular, abrasion characteristics of Cr₂O₃ coatings annealed at 1100℃ were improved 5 times as high as the as-sprayed coating. In case of hot pressing treatment, the increase of temperature and pressure leaded to the improvement of porosity, microhardness and abrasion characteristics of the coatings. The best experimental results were obtained at 900℃ and 30MPa. Specifically, at this condition, the porosity was as low as 4.1% and abrasion characteristics were improved 8 times as good as the as-sprayed coating. Bond strength which was not improved by only annealing or hot press treatment was improved by incorporating the Functional Gradient Layer(FGL). Annealing after forming FGL produced about 14MPa higher bond strength than annealing without FGL, and hot press treatment with FGL showed even better results.

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For the production of severely cold-headed ware, such as a phillips screw, the material requires good cold headability, In this paper the optimum austenite grain size range for good cold headability of stable austenitec stainless steel is discussed. After simulating the cold heading process of phillips screw with the aid of ABAQUS 5.2, the effect of austenite grain size on cold headability is investigated by compression test with drop-weight-tear tester. This result has a good agreement with that of tensile test. The optimm austenite grain size for cold heading is considered to be form 45㎛ tp 60㎛ because of the formation of deformation twins and the prevention of orange peel effect at the surface.

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The effects of impurity content and of fabrication processes on the tensile properties and toughness of 7175 Al forgings have been studied using conventionally processed 7075 and 7175 Al forgings and a specially processed 7175 Al forging. A specially processed 7175 Al forging was superior to a conventional 7075 Al forging in both the strength and toughness. These beneficial effects primarily arise from two effects, i.e., the effect of reduction of impurity level and that of application of special fabrication procedures. The reduction of impurity level of Fe and Si has significantly diminished the number of constituent particles containing Cu and Mg. A further reduction of the number of constituent particles has been observed on applying specially designed fabrication process. The reduction of impurity level exerts no significant effect on the dispersion of Cr and Mg containing E phase. However, the application of special fabrication processes results in a coarsening of E phase. This is believed to have an effect to promote a finer grain structure, which may also contribute to the enhancement of strength and touhgness of the alloy.

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The low cycle fatigue behaviors of Ni-base superalloy KM 1557 with two different microstructures were investigated. The fatigue properties can be rationalized in terms of the Coffin-Manson eqaution relating plastic strain with the number of cycles to failure. For a low total strain amplitude(1.0%) the fatigue life of the Ni-base superalloy with α-W and γ′ is substantially less than that of other condition. Microstrucural examination revealed that microcracks initiated at the interface between matrix and α-W. This was attributed to the twin formation around α-W. During low cycle fatigue, the Ni-base superalloy with only α-W precipitates gives rise to a short period cyclic hardening followed by softening. However, the Ni-base superalloy with not only α-W but also γ′ gives rise to cyclic softening. The cyclic softening behavior of the Ni-base superalloy KM 1557 with two different microstructures was related to the strain localization due to γ′ shearing and the interface between α-W and matrix. This study mainly concerned with the role of interface between α-W and matrix, and twin formation.

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Using numberical method, the work condition to make a plate preform in Spray Deposition Method has been optimized in this study. In this calculation we have used the assumptions that spray density is represented by Gaussian funtion and surface grows to normal direction and droplet flys from orifice to calculation points on the preform surface. We proposed two methods to make plate preforms by spray forming process, In each method the nozzle control function is derived as ; Method I ω(θ)=G·v·sinθ_(max)·√b·cos²(θ) Method II ω(θ)=G·v·sinρ_(max)·√b·1/cos(ρ) In these equation v is the substrate velocity, b is spray density constant, θ is the angle between spray axis and z-axis, and ρ is the angle between spray axis and y-axis. The simulation suggests that the constant G must be larger than 3 to make plate without surface modulation.

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Fabrication and tensile properties were studied on SiC whiskers reinforced 2124Al matrix composites. The composites were fabricated by vacuum hot pressing at 520℃ following hot extrusion at an extrusion ratio of 20 : 1 and at 470℃. After solution heat treatment at 495℃ and 4 hours the tensile tests were performed with aging heat treatment at 177±3℃. At peak aged condition(177±3℃, 5 hours), the tensile strength and elastic modulus were 700 MPa and 115 GPa, respectively. The fractures sources were found to be cavities initiated at the ends of SiC whiskers and fractured Mn-containing particles. On solution heat treated conditions the cleavaged fractures were predominant in matrix and cavities initiated at the sides of SiC whiskers, partialy.

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Creep induced instability of hardening precipitates at the grain boundaries occurs at high temperatures. This paper discusses the effect of tensile creep stress on the microstructural instability of a Ni-Cr-W-Al-Ti superalloy(KM 1557). The tensile creep stress results in the formation of the precipitate free zones(PFZs) of α-W and γ′ near grain boundaries, and PFZs are generally found near grain boundaries inclined to the tensile-stress axis or triple points. These PFZs are not formed at secondary stage, but observed at tertiary stage and in the creep-ruptured specimens. In the TEM observations, it is found that the cutting of γ′ particles by dislocations starts at the secondary stage, and slip lines and bands are formed in the tertiary stage. Many PFZs are observed from the region very close to the fracture surface where severe deformation took place. On the other hand, gage section near the grip of creep specimen in which lightly deformed did not contain PFZs. The area fraction of PFZs is directly proportional to the reduction of area in the creep specimens. These experimental results support that the formation of PFZs is dependent on the plastic deformation.

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The high-temperature mechanical behaviour of copper polycrystals was examined using stress relaxation tests in conjunction with microstructural observations on deformed and quenched specimens using transmission electron microscopy. The relationship between the dynamic recrystallization subsructures and the flow behaviour was analysed in detail, and the mechanisms of dynamic recrystallization were also discussed. Nucleation of dynamic recrystallization is attributed to deformation-induced instabilities of the dislocation structure. The formation of dislocation cells and subgrain structures is of great significance to the understanding of high-temperature deformation.

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