발간논문

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The Ostwald-ripening of tungsten grains in liquid nickel-tungsten matrix at 1540℃ was observed and the results are compared to the LSW (Lifshitz., Slyozov, Wagner) theory and Ardell`s theory. The specimen compositions were 99W-1Ni, 93W-7Ni, and 70W-30Ni by weight percent. The specimens were prepared by usual powder metallurgy techniques using tungsten and nickel powders of 2.6μ and 4.6μ average size, respectively. The sintering was carried out at 1540℃ in hydrogen atmosphere for times ranging from 10 minutes to 15 hours. The measured linear intercept distribution of the tungsten grains was compared to the theoretical values for diffusion contralled and interface reaction controlled mechanisms obtained by Exner and Lukas from the LSW theory. In the specimens of 93W-7Ni composition the linear intercept distribution is wider than those suggested by LSW theory during the initial period of sintering. However, as the sintering time becomes longer it approaches the equilibrium distribution corresponding to the reaction controlled mechanism for the particle growth, and after 15 hours of sintering the observed distribution resembles the reaction controlled case. In the 93W-7Ni specimens the average tungsten grain size γ￣increases with time ｔas γ￣ｔ⅓ predicted by the LSW theory for the diffusion controlled mechanism. However, since the measurements were made up to 15 hours of sintering before reaching the equilibrium distribution it is not possible to conclude definitely that the observed growth rate corresponds to the dissusion controlled mechanism rather than the reaction controlled mechanism. On the other hand previous experimental results on tungsten heavy alloys show tungsten particle growth rates following the diffusion controlled mechanism. Further indication for diffusion controlled growth is provided by the variation of the growth rate with the relative amount of grains and the matrix as shown by previous measurements on heavy alloys and indicated in the present investigation. It thus appears that the observed growth rates correspond to the diffusion controlled mechanism while the grain size distribution at equilibrium resembles the prediction of reaction controlled mechanism. This apparent discrepancy is resolved by Ardell`s recent theoretical result that when the amount of the matrix phase is small and the reaction rate is controlled by diffusion the average particle size follows the ｔ⅓ law as predicted by LSW theory but the distribution closely resembles the reaction controlled case in LSW theory. Thus the results of the present investigation agree in general with the predictions of Ardell`s theory.

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The effect of cold working and subsequent annealing on mechanical properties of W-Ni-Fe heavy alloy has been studied. The specimen composition was 90W-5Ni-5Fe by weight percent. The specimens were prepared by usual powder metallurgy techniques and sintered in liquid phase at 1460℃ for 30 minutes in hydrogen atmosphere. The specimens were then cold worked up to 60% by rolling. The observed UTS increases with cold working from about 94㎏/㎟ to 143㎏/㎟ at 50% deformation. The elongation decreases sharply from 20% to about 2% at 15% deformation. While the microhardness of the tungsten grains increases steadily with degree of cold work, the micro hardness of the matrix appears to increase rapidly with deformation initially. It is therefore suggested that at relatively small deformation the matrix phase work hardens rapidly and affects the deformation behavior of the composite alloy. The alloy specimen cold worked to 50% was annealed at temperatures from 400℃ to 1000℃ for an hour in hydrogen atmosphere and the mechanical properties were measured. Annealing of the cold worked specimen at 400˚increases UTS to 160㎏/㎟ from 143㎏/㎟ for the unannealed specimen, and with increasing annealing temperatures UTS decreases. The elongation begins to increase with annealing temperature at 800℃ and rises sharply to 4% at 1000℃. The microhardness of the tungsten grains remains unchanged with annealing temperature up to 600℃ and from 800℃ begins to decrease slowly. The microhardness of the matrix on the other hand shows maximum after annealing at 400℃ and decreases subsequently with higher annealing temperatures. These observations indicate that the increase of the UTS after annealing at 400℃ is largely due to the increased strength of the matrix. This increase in matrix strength with annealing is most likely caused by some, kind of precipitation hardening which is probably accelerated by cold work. The observed changes of mechanical properties at higher annealing temperatures are attributed mainly to the recovery and recrystallization of the matrix phase.

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Kinetics of the reaction of hydrogen with Zr-Ni alloys have been studied in the temperature range of 900℃-1080℃ at 400 and 600 torr. hydrogen pressure using thermogravimetric method. It is found that the rate controlling step of all reactions is hydrogen diffusion in metals. The diffusion coefficients of hydrogen in Zr-Ni alloys (0.5 a/o 1.09a/o, 1.5 a/o, 2.3 a/o Ni-Zr) are D=0.2902 exp [－(9859±849)/RT] ㎠/sec. :0.5 a/o Ni-Zr D=1.786 exp [－(12010±907)/RT] ㎠/sec. :1.09 a/o Ni-Zr D=6.403 exp [－(13550±587)/RT] ㎠/sec. :1.5 a/o Ni-Zr D=164.0 exp [－(15910±2740)/RT] ㎠/sec. :2.3 a/o Ni-Zr respectively. Nickel increases markedly the activation energy of hydrogen diffusion in Zr-Ni alloys. It can be probably explained by that Ni decreases the interstitial sites of Zr lattice.

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The possibility of producing bronze oilless bearings from prealloyed copper-30% tin powder was investigated. The possibility of studying the dimensional change and homogenization during the sintering of Cu-Sn alloys with this prealloyed powder was also explored. The copper-30% tin prealloyed powder was made by gas atomization. The specimens of bearing shape were prepared by usual powder metallurgy techniques from the mixture of copper powder and the prealloyed powder. The copper powder sizes were similar to the commercial grade powder and the prealloyed powders were -200∼+270 mesh and -400 mesh. The specimens were sintered in hydrogen nitrogen atmosphere at temperatures ranging from 740℃ to 820℃ for times up to 24 minutes. Specimens sintered for 12 minutes showed large dimensional growth between 780℃ and 870℃ and the growth was larger for the specimens made from larger prealloyed powder. During sintering at 820℃ large dimensional growth occurred between 6 and 12 minutes and small growth up to 24 minutes. The changes occurring during sintering could be easily observed from the shape changes in the spherical prealloyed powders showe in the microstructure . The composition homogenization was also revealed in X-ray diffraction measurement. The results crsuggest that the dimensional growth occurs when the tin rich melt flows to copper clusters froming homogeneous & bronze. Measurements of oil impregnation and radial ushing strength indicate that oilless bearings with satisfactory properties can he produced from the copper-30% tin prealloyed powder. The results also show that the Cu-30% Sn prealloyed powder may provide a useful tool for studying the dimensional growth and homogenization during sintering of Cu-Sn mixtures.

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