발간논문

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To investigate high temperature fracture mechanisms of Al composites, tensile test. were Conducted on a 2124 Al-SiCw composite and a 2124 Al alloy varying the test temperature from 25℃ to 550℃ and the strain rate from 5×10^(-3)s^(-1) to 0.3s^(-1). At test temperatures above 475℃ the tensile strength of the composite became lower than that of a 2124 Al alloy. Also, the strain rate for the maximum elongation to the failure of the Al-SiCw (0.1s^(-1)) is much higher than that of the 2124 Al (5×10^(-3)s^(-1)) at 500℃. These experimental results suggest that the 2124 Al-SiCw can be superplastically formed with a much faster rate at a lower stress level compared with the 2124 Al. Detailed microstructural and fractographic analyses indicate that the major fracture mechanism of the 2124 Al-SiCw shifts from void initiation and coalescence to cavity formation and growth as the test temperature increases. It is of great interest, to find that the cavity initiation sites of the 2124 Al-SiCcw are strongly influenced by the strain race at high temperatures;whisker-sides at high strain rates and whisker-ends at low strain rates. Furthermore, the favorable cavity growth direction is also Found to be a function of the strain rate; at high strain rates perpendicular and at low strain rates 45° to the tensile axis. The change in the favorable cavity initiation sites and cavity growth direction with the strain rate can be explained by the relative superiority between the matrix/whisker interfacial strength and the matrix shear strength.

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Various thermomechanical treatments were performed in order to optimize the ductility and strength of Ti₃Al-Nb alloys, called Alpha 2 and Super Alpha 2. The mechanical properties such as strength, elongation and hardness can be improved by the thermomechanical treatment. Especially Super Alpha 2 which has more β-stabilizing alloy elements than Alpha 2 exhibited gradually improved mechanical strength as the solution heat treatment temperatures are approached to the β-transus temperature. Further improvement of the hardness could be achived depending upon aging time and temperature. This is due to the phase transformation happened during thermomechanical treatment. Formation and quantity of the orthorhombic martensite phase is the most important factor on mechanical properties. The martensite is formed in the matrix after solution heat treatment and quenching. During aging the martensite transforms to stable secondary α₂+$quot;0$quot;+β/B2 phases which are distributed finely as lamellar type. These microstructural refinement. together with proper amount of primary α₂ Phase increases strength and hardness in Super Alpha 2. While in Alpha 2 the martensitic transformation is suppressed and strength improvement can not be achived.

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A study has been made of the superplastic behavior of a rapidly solidified AL-3Li-1Cu-0. 5Mg-0.5Zr(wt.%) alloy. Although rapidly solidified Al-Li alloys have the very fine grain structure desirable for the improved superplasticity, unfavorable oxide morphology often prevents them from being superplastic. The results of superplastic deformation indicate chat the proper thermomechanical treatment (TMT) of the alloy results in a much improved superplastic ductilities, e.g., elongat.ion of approximately 530%, in spite of the unfavorable oxide morphology. In the case of testing at. 520℃, optimum strain rate of forming is 4 × 10^(-2)/s, tvhich is one or two orders of magnitude higher than that of ingot cast Al-Li alloys. Such a high strain rate is thought to be quite advantageous for the practical application of superplastic deformation of the alloy. It can also he seen chat, the microstructure of the deformed specimens is similar to that in the as-received or the TMT conditions since continuous recrystallization is accomplished by subgrain growth and the growth of the primary grains is prevented by the fine β`(Al₃ Zr) particles.

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TiB₂ coatings on graphite substrates were performed by chemical vapor deposition(CVD) process. The CVD process was carried out under low and atmospheric pressure using TiCl_4,-BCl₃-H₂ system. The deposition rate, microstructural morphologies, and microhardness of TiB₂ coated layer were investigated in terms of deposition parameters such as reaction temperature, mole fractions of Ti/(B+Ti) and H/Cl, total pressure, and the distance between induced cube and substrate. The optimum conditions far TiB₂ coatings were achieved with 1㎝ of the distance between tube and substrate, 1/6 of Ti/(B +Ti) mole fraction, 6 of H/Cl, mole fraction and 1100℃ of reaction temperature. The coated layer exhibited preferred orientation, and the preferred orientation in TiB₂ coatings was expanded by increasing H/Cl male fraction, and by decreasing Ti/(B+Ti) mole fraction. The microhardness value of coated layers was 3150 KHN under atmospheric pressure at the reaction temperature of 1200℃.

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TiB₂ coatings on WC substrates were investigated using BCl₃-TiCl₄-H₂ system by CVD process at various pressure conditions. The measured microhardness values of coated layer were 3800 KHN under 40 torr of pressure and 3280 KHN 760 torr of pressure at, the reaction temperature of 1200℃ The coated layer displayed a-axis preferred orientation, and the average lattice parameters were 3.034Å for a-axis and 3.231Å for c-axis. Diffusion controlled process, had a small activation energy of 3㎉/㏖, dominated at higher temperature than 910℃, while chemically controlled process, had a high activation energy of 24㎉/㏖, governed at lower temperature than 910℃.

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