발간논문

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The sintering behavior of Ni-doped tungsten compact as well as its workability were investigated for the application to tungsten products. The relationship between sintering parameters and the structural and mechanical features of the sintered compact was experimentally determined and discussed. By an appropriate control of the sintered structure the activatedly sintered compact could be plastically deformed by hot forging.

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The influences of addition of Fe, Si, Mn and Ti on the age-precipitation in Al-2.6wt%Cu-0.4wt%Mg alloy have been investigated by means of the measurement of electric resistivity, specific heat and transmission electron microscopy. Internal energy changes for the G.P.B. zones, S` and S precipitates occurred during ageing, have been determined from the specific heat versus temperature curves. The results have been analysed in comparision with those from measurement of electrical resistivity and electron microscopy. The results obtained are summarized as follows. 1) The rate of G.P.B. zone formation is accelerated slightly by the addition of Mn and pronouncedly by the addition of Ti. 2) The rate of G.P.B. zone formation is retarded pronouncedly by the addition of Si and Fe. From these results, it can be deduced out that the additions of Si and Fe increase the sinks of quenched-in vacancy such as the grain boundary and dislocation, and decrease the effective vacancy concentration contributing to the formation of G.P.B. zones. 3) The precipitation of metastable S` phase is retarded by the addition of Fe and Si, However, the precipitation of metastable S` phase is more retarded by the addition of Si than Fe, and the precipitation of equilibrium S phase is by the addition of Fe than Si.

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The effect of heat treatment on preferred orientation and mechanical properties has been investigated for sand-cast Mg-9wt%Al-Zn alloys. By solution heat treatment <0001> preferred orientation is increased at the heat flow direction with decreasing <1010> and <1120> orientations. The strength is increased with increasing the Zn content and double aged specimens has higher strength than single aged specimens. The microstructure reveal that mechanical properties depend on continuous precipitates within grains as well as discontinuous precipitates along grain boundaries, and growth kinetics for both of the precipitates is independent to each other.

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Diffusivities of hydrogen in Ti-Al, Ti-Cr and Ti-Ni alloys at room temperature were calculated from published data on hydrogen absorption rate. Apparent diffusivities calculated were much higher than the values which were extrapolated to room temperature from high temperature diffusion data for pure α and α solid solution. Hydrogen diffusivity shows maximum at 67.5% Cr in Ti-Cr system and at 30% Ni in Ti-Ni system. These phenomena may be explained by the presences of γ phase (Ti Cr₂) in Tr-Cr alloy and δ phase (Ti Ni) in Ti-Ni alloy system. These phases have higher hydrogen diffusivity than other phases.

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A study has been made of sintering and mechanical behaviors of heavy alloys whose compositions were (a)94%W-3%Ni-3%Fe, (b)94%W-2.5%Fe-1%Co, (c)94%W-2.5%Ni-2.5%Fe-1%Mo, and (d)94%W-2.5%Ni-2.5%Fe-1%Mn. Liquid phase sintering was performed in hydrogen atmosphere at 1196℃ for 15 to 1200 minutes, Coarsening of tungsten grains during sintering obeyed the diffusion controlled growth mechanism. The growth rate decreased in the order of specimens (b), (c), (a) and (d). The hardness decreased with increasing sintering time due to increase in mean free path of binder phases, following the Hall-Petch type relation. The hardness decreased in the order of specimens (b),(a),(c) and (d) at a given mean free path of binder phases due to difference in hardness of binder phases.

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