| A series of samples from a mixture of 90 w/o W, 5w/o Ni and 5w/o Fe was compacted at 6 tons/㎠ and sintered at 1450℃ in the presence of a liquid phase at sintering times from 10 minutes to 15hours. The densities, tensile strength, elongation and Rockwell C hardness were obtained. The samples were sectioned and polished and the average size and the linear intercept distribution of the spherical tungsten grains in the matrix were determined. The densities of the samples were between 98.6 and 99.8% of theoretical density regardless of sintering time, but there appears to be a slight tendency towards lower densities, i.e. development of porosity, at sintering times longer than 4 hours. The size of the tungsten particles increases with sintering time. The tensile strenght of the samples is between 75 and 95 ㎏/㎟ and the elongation between 7 and 23 percent. It appears probable that the variation in tensile strength and elongation is related to the amount, size and distribution of porosity in the samples. The hardness decreases with increasing sintering time. When hardness is plotted vs average diameter of the tungsten particles it is found that with an average diameter of 14 microns the hardness is R_c27 but, that the hardness gradually decreases until with an average diameter of 30 microns or larger it is about R_c23. An attempt was made to interpret the data on tungsten particle size and particle size distribution as a function of sintering time on the basis of the existing theories of "Ostwald ripening". A plot of average particle size Vs sintering time shows that the relation d³=㏏ is quite well fulfilled. This according to Wagner points toward particle growth by a diffusion controlled mechanism of solution and precipitation. However, in order to make valid conclusions on the mechanism of particle growth, not only the change of average particle size with time, but also the particle size distribution must be taken into account. Therefore the linear interlept distribution was determined on more than 1000 intercepts for the samples sinterd for 1 hour and 15 hours. This intercept distribution was compared with the intercept distributions which Exner and Lakas derived from Wagner`s curves for particle size distribution for the cases of diffusion controlled and of interface controlled Ostwald ripening. It was found that it corresponded more closely to the stationary distribution postulated by Wagner for interface controlled repening than that for diffusion controlled ripening. Wagner`s distribution for diffusion controlled ripening was derived for the case where the mean free path between particles is much larger than the average particle size. Ardell modified the Wagner theory of diffusion controlled ripening for the case where the mean free path between particles is small, which is the case in the alloy of 90 w/o W-5 w/o Fe-5 w/o Ni. He showed that the dependence of average particle size upon sintering time would still follow the d³=㏏ relation, but that the stationary particle size distribution would be different from Wagner`s distribution. The smaller the distance between particles, the more would the particle size distribution approach that postulated by Wagner for interface controlled ripening. When Ardell`s theory is applied to the results for particle size and particle size distribution obtained in this thesis, it would indicate that in the W-Fe-Ni alloy the growth of the particles is due to diffusion controlled Ostwald ripening. |
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