OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 1 2022 As it has been demonstrated above, mechanical activation has led to higher fi neness of tungsten powder, with the area of its free surface increasing, respectively. As a result, precipitation of cobalt from the liquid phase against the particles of tungsten has intensifi ed. Thus, mechanical activation of tungsten has reduced the mass transfer from small cobalt particles to larger ones and contributed to the formation of the more dispersed, fi ne-grained structure in the sintered material. The effect of high-melting nanoparticles on dissolution-reprecipitation of another solid phase during liquid phase sintering needs further investigation. This phenomenon opens up new opportunities for acting on structure formation in sintering to obtain materials with the required structure and properties. Effect of mechanical activation of tungsten on the porosity of sintered Sn-Cu-Co-W materials In the sintered Sn-Cu-Co-W materials, there is a minor quantity of isolated closed pores. The material with non-activated tungsten has the density of 8.16 g/cm3 (porosity of 8%). Meanwhile, the material with milled tungsten has the density of 7.72 g/cm3 (porosity of 13 %). With its high chemical activity, fi nely dispersed tungsten tends to adsorb atmospheric gases and get oxidized. The WO2 tungsten oxide is decomposed when heated in vacuum up to the temperature of 800 °C [28]. Apparently, at the temperature of sintering, oxides are decomposed, and gases are extracted in the closed pores subsequently. The pressure of gases in the closed pores prevents it from healing, which results in higher porosity of the sintered material. Effect of mechanical activation of tungsten on the hardness of sintered Sn-Cu-Co-W materials It is clear from Table 2 that the hardest structural constituent of Sn-Cu-Co-W materials is particles of tungsten. Mechanically activated tungsten has a 1.8–2.2 times higher hardness. For technical reasons, hardness of nanoparticles cannot be measured with the 10 g indenter load. As for larger tungsten particles, having the cross dimension of 10–12 μm, its microhardness is 823–1,162 HV0,01. Higher hardness of mechanically activated tungsten is associated with work hardening of its particles. Recrystallization temperature of tungsten is known to be much higher than 820°C, so during sintering of the material, work hardening of tungsten particles was retained. Ta b l e 2 Microhardness HV0,01 of the structural constituents of the sintered Sn-Cu-Co-W material Sintered material Microhardness HV0,01 of the structural constituents (Cu) Cu3Sn Co W without mechanical activation of tungsten 245±12 367±7 137±16 496±29 with mechanical activation of tungsten 259±22 384±14 140±16 992±169 Apart of mechanically activated tungsten occurs within the material in the form of sintered agglomerates; its structure is given in Figure 6 (the light image; the sample was treated with the solution containing 5 g of ferrichloride, FeCl3, 15 ml of hydrochloric acid, HCl, and 100 ml of water). It can be seen that necking was formed between contact tungsten particles during sintering. Microhardness of the agglomerates was measured at the 100–500 g indenter load; meanwhile, hardness impresses have been obtained with the diagonal length exceeding the size of individual tungsten particles (see Figure 6). When the indenter was pressed in, the particles of tungsten did not get disconnected or crumbled away. In spite of its porous structure, the agglomerates feature high microhardness of 582–1,223 HV. In the structure of the materials under study, particles of tungsten occupy a small volume (less than 5 %), so its hardness has little effect on the general hardness of the material. In Figure 5, it can be seen that the largest volume in the structure of the materials belongs to the Cu3Sn intermetallic phase. In the material with mechanically activated tungsten, hardness of the Cu3Sn intermetallic compound is much higher
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