OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 1 2022 is the dissolution-reprecipitation process. This process consists in small particles of the solid phase getting dissolved in the liquid phase and its substance reprecipitating against the surface of larger particles [24, 25]. In the material with tungsten not exposed to mechanical activation, the size of cobalt particles has increased owing to dissolution-reprecipitation from 1.6 μm up to 9–15 μm (see Figure 3 a). In the sintered material containing mechanically activated tungsten, cobalt particles are smaller, about 3–10 μm (Figure 3 b). The effect of mechanical activation of tungsten on the dissolution-reprecipitation of cobalt is explained by the following. In systems containing two solid metals and a liquid phase, the mass transfer is directed toward the metal having the largest free energy [26]. In the ascending order of the surface energy, the components of the system in question are located as follows: Sn, Cu, Co, W [27]. Under such conditions, it is transfer of cobalt via the liquid phase to tungsten particles that is the most favorable in terms of energy. The mass transfer of cobalt to tungsten through the liquid phase is confi rmed by component distribution maps given in Figure 5. In the maps, one can see sintered agglomerates of tungsten particles; notably, spaces between the particles are mainly fi lled up with cobalt. Obviously, cobalt penetrated deep into the tungsten agglomerates together with the liquid phase. Precipitation of cobalt led to blocking of pores of the agglomerates. After that, cobalt could fi nd its way deep into the agglomerates owing to its diffusion over the surface of tungsten particles. The mass transfer of cobalt to tungsten through the liquid phase occurs without mechanical activation of tungsten, too. In Figure 4 a, it can be seen that particles of tungsten not exposed to mechanical activation are surrounded by “enclosures” formed as a result of precipitation of cobalt from the liquid phase. Fig. 5. Element distribution maps for the material with mechanically activated tungsten: 1 – intermetallic compound Cu3Sn; 2 – cobalt particles; 3 – tungsten particles
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