발간논문

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Various thermomechanical treatments were performed in order to optimize the strength and ductility of a Ti₃Al-based intermetallic, Super Alpha 2. Mechanical strength of the Super Alpha 2 was gradually improved without loss of ductility as the solution treatment temperatures were approached to the β-transus temperature. However, the elongation of specimens, heat-treated at temperature above β-transus, was reduced to almost zero percent. The improvement of hardness could be achieved depending upon aging time and temperature. Mechanical property improvement was attributed to phase transformation from B2 to α₂ and "O" phases. Not only the phase seperation, but also finely dispersed secondary phase of the lamellar type around the primary α₂ phase contributed to strength and hardness improvement. These phenomena were identified by SEM, TEM, XRD and Young`s modulus variation.

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The effects of martensite morphologies on the microfracture mechanism of a dual phase HSLA steel are investigated in this study. In the intermediate quenching process(IQ : repeating to the two phase range after quenching from austenite), very fine fibrous martensite is well distributed in the ferrite matrix. On the other hand, in the step quenching process(SQ ; two phase annealing before quenching), bulk martensite is mixed with ferrite. After heat treatment, the IQ specimen shows high strength with relatively good ductility, but the SQ results in very poor ductility. In-situ SEM fracture test reveals that microcrack initiation occurs mostly in the ferrite region where strain is localized into shear bands, rather than in the martensite region. In the IQ case, a crack tends to propagate in the uniform manner throughout the whole ferrite/martensite microstructure, yielding good elongation with high strength. However, in the SQ case, deformation is concentrated into the ferrite and a local crack propagates just along the ferrite regions, so that bulk martensite cannot effectively control the crack propagation behavior, resulting in poor ductility.

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A two dimensional finite element model has been developed to calculate the temperature and stress distribution in a continuously cast beam blank. In the stress analysis, a thermo-elasto-plastic model was employed. The effects of a mold and the friction between a mold and beam blank were examined. The beam blank was found to deform asymmetrically during solidification due to its complicated geometry. Thermal stresses were developed at the initial stage, but decreased as solidification proceeded. The maximum tensile stress was found to develop parallel to the surface of a fillet under the constraint of a mold without friction, But the maximum tensile stress shifted toward the web with increasing friction.

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Amorphous Al_(88)Ni_(10-x)Fe_xNd₂(x=0,3,5 at%) alloys with the two-stage crystallization process of amorphous→amorphous plus Al→Al plus compounds were found to exhibit high ultimate tensile strengths(σ_B) above 800MPa in the temperature range of room temperature to 580K and a large fracture elongations(ε_f) above about 20% at the exothermic peak temperature of Al phase(T_(P1)) in the DSC curve. The maximum value of σ_B and ε_f in the temperature range from the precipitation onset temperature of Al phase to 580K shifts toward the higher temperature side with the replacement of Ni by Fe element. This compositional dependence is thought because a number of the defect-free Al particles with a high heat-resistance against mechanical strenghth and growth of the particles at elevated temperatures precipitate at a higher temperature range with increasing Fe content. The appearance of the large elongation at T_(P1) can be explained by a crystallization-induced plasticity phenomenon which occur through the continuous propagation of the deformation site resulting from the hardening effect of the defect-free Al particles which preferentially precipitated on the shear plane by the local adiabatic heating in the stress-concentrated area.

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In starting-up turbines of power plants, the procedures recommended by manufacturer should be carefully observed. however, many utilities often start turbines so fast to cause accelerated crack growth. Hence, to accurately predict crack growth life of turbine rotors, a crack growth prediction model is needed in which crack growth rate change due to variation of load increasing time is considered. In this study, using a typical rotor steel (1Cr-1Mo-0.25V steel), creep-fatigue crack growth tests were performed at 538℃ under triangular and trapezoidal waveshapes with 0.5, 1, 10, 100 second load rise times and 100 second load hold time. The effect of load increasing time on the crack growth rate during the succeeding load hold period was investigated from the test results and a new C_t estimation equation was proposed which considers the observed effects. The effectiveness of the proposed equation was verified by showing that the scatter band of the measured (da/dt)_(avg) data was reduced when the new equation was adopted

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Transformation behaviour of thermo-mechanically treated Ti-Ni-Cu ternary alloys have been investigated by means of electrical resistivity measurements and X-ray diffraction. The obtained results are as follows. (1) The B2→B19´ transformation start temperature in Ti-Ni-Cu ternary alloy does not almost change with increasing Cu-content from 1at% to 5at%. (2) The B2→B19´ transformation start temperature increases by applying thermo-mechanical treatments to the solution treated alloys in Ti-Ni-Cu ternary alloys whose Cu-content is 2at%. (3) Cu addition to the Ti-Ni binary alloy depresses the R phase transformation

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