| Densification of W-Ni-Fe heavy-alloy during sintering at 1300℃ and 1450℃ was studied. The (Ni＋Fe) content of the specimens were 3.5, 10, and 15 weight % while Ni: Fe ratio maintained at 7 : 3. Two grades of tungsten powder with average particle size of 1.23μand 5.4μwere used. In order to observe the effect of compacting pressure on densification rate, the specimens were pressed at 0.5, 2.5, and 5 tons/㎠. During sintering at 1450℃ with some liquid phase present the specimens with small tungsten powder showed faster densification initially. However, after 30 minutes of sintering densities exceeding 99% of the theoretical density were obtained with 1.23μand 5.4μ tungsten powders. The densification rate was little affected by the (Ni＋Fe) content. For the specimen with 3.5% (Ni＋Fe) and 1.23μW powder compacted at different pressures same high density was obtained in short sintering time for all specimens when sintered at 1450℃. The densification rate at 1300℃ is shown to be greatly affected by the tungsten particle size. After sintering for 60 minutes at 1300℃, the specimens with 1.23μW-powder are densified to about 98% of the theoretical density, while the specimens with 5.4μW-powder show densities around 70-80% of the theoretical density. It is therefore concluded that in liquid phase sintering the tungsten powder size is not critical for obtaining densities approaching the theoretical density, but in solid state sintering it is necessary to use small tungsten powder size to obtain high densification. The fractional volume changes during liquid phase sintering plotted against sintering time in log-log scale show slopes ranging between 0 and 15, and varying slightly with composition. This result is inconsistent with any predictions based on Kingery`s model of liquid phase sintering. The observed densification rates during solid state sintering cannot be explained by the activated sintering model of Brophy et al. It is suggested that the densification rate should be dependent on the amount of porosity remaining and therefore the "impingement" effect has to be taken into account in establishing a theoretical model for densification during sintering. Observations of the tungsten grain growth show that with smaller (Ni＋Fe) content, the tungsten grain growth is considerably larger. |
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