OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 1 2022 scanning electron microscope, the EVO HD 15 electron microscope (by ZEISS), and the AxioObserver.A1m metallographic microscope (by ZEISS). As for distribution of elements within the samples, it was studied by X-ray microanalysis using the EVO HD 15 electron microscope. Microhardness of the structural constituents was measured by indentation of a tetrahedral diamond pyramid under the load of 10 g (HV0,01) with the DuraScan80 hardness meter (by EmcoTest). Hardness of the materials was measured according to the Rockwell method (scale B) using the TK-2M hardness meter. Results and discussion Effect of mechanical activation on the shape and size of tungsten particles In Figure 1, one can see the changed shape of tungsten powder particles after mechanical activation. Before the activation, the particles of tungsten were equiaxial polyhedra. After being mechanically activated for 60 min at the 800 RPM, most particles have the equiaxial shape and rough surface. A small quantity of the particles has the splintery shape. As the duration of mechanical activation is increased, the quantity of splintery particles goes down. a b Fig. 1. Shape of tungsten particles: a – before mechanical activation; b – after mechanical activation Sizes of the particles were measured using the images obtained with the electron microscope. After the described mechanical activation mode, it ranges within 0.025–12 μm. The particles are distributed according to sizes as follows: d10 = 67 nm; d50 = 220 nm; d90 = 750 nm. Meanwhile, in the mechanically activated powder, the share of nanoparticles sized up to 100 nm exceeds 20% (Figure 2). The minimum size of the particles, which equals to 25 nm, was obtained at the 800 RPM and the mechanical activation duration of 60–120 min (Table 1). After mechanical activation, the signifi cant proportion of the powder sticks together forming loose aggregates of up to 80 μm size. The aggregation of nanoparticles is explained by the presence of numerous uncompensated interatomic bonds on its surface. Combining such particles into aggregates contributes to a decrease in its free energy [13]. The shape and size of the resulting powders indicate the following processes occurring duringmechanical activation: large particles are split up; the fragments are rolled and gain the rounded shape; small particles
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