OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 equipped with an HV-APS plasma torch. The supersonic spraying mode using air as the working gas ensures the speed of the sprayed particles at a level of 500 m/s and higher. Optimal modes for high-velocity spraying Ni-Al powder are presented in our previous work [25]. To analyze the structural state of the coatings, a scanning electron microscope (Carl Zeiss EVO50 XVP with an EDS X-Act microanalyzer) and a transmission electron microscope (FEI Tecnai G2 20 TWIN) were used. Transverse sections of coatings were the samples for SEM as well as foils, cut from the middle of the coatings, were the samples for TEM. Ni-Al coatings were kept for 1 hour at temperatures of 300, 400, 500 and 600 °C (air cooling) to study the structural transformations that occur upon heating. Results and discussion Previously, we have shown that HV-APS coatings are characterized by the presence of several zones that differ in structure [25]. Fig. 2 shows SEM image and scheme of the microstructure of the HV-APS coating in the initial state. The chemical composition of all areas was determined using micro-X-ray spectral analysis. According to the data obtained, there are particles, the central part of which is the β-NiAl intermetallic compound (section 1 in Fig. 2), surrounded by a single-phase layer of the β-NiAl phase enriched in Ni (referred to as the NixAl1-x phase) (section 2 in Fig. 2). The structure of section 3 in Fig. 2 is dendritic: the chemical composition of the dendrites coincides with the composition of the layer (section 2), and the chemical composition of the interdendritic space corresponds to theγ’-Ni3Al phase. We considered the fine structure of these areas more thoroughly in our work [25]. As a rule, the particles with a similar structure are not common for HV-APS coatings. Predominant particles are particles with two-phase structure consisting of NixAl1-x and γ’-Ni3Al grains (section B in fig. 2). Fig. 3 shows TEM images of section 4. It can be seen that NixAl1-x grains undergo a shear martensitic transformation, during which the high-temperature B2 structure transforms into the low-temperature L10 structure, while the γ΄-Ni3Al grains do not change. There are also one-phase areas consisting only of grains of the γ΄-Ni3Al phase (fig. 3, b). The shape of the grains in section 4 is non-equiaxial, which is typical for material cooled under non-equilibrium conditions. The grain sizes usually do not exceed 500 nm, although sometimes larger γ΄-Ni3Al grains are formed. Such grains often have deformation twins (Fig. 3, c) and stacking faults (Fig. 3, d). a b Fig. 2. SEM image (a) and scheme (b) of HV-APS coatings: 1 – β-NiAl phase; 2 – layer of NiхAl1-х; 3 – area with dendritic structure: NiхAl1-х dendrites, interdendritic region (γ′-Ni3Al phase); 4 – area with grain structure: both NiхAl1-х and Ni3Al grains
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