OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Fig. 7. Bright field images of coating structure after heating 400 (a, c, d) and 500 °С (b) а b c d is not observed. The structure also contains martensite crystals in which the L10 → B2 transformation has completely occurred. Fig. 8, c, d shows bright-field and dark-field images of former martensite plates. Data of dark-field analysis confirms the presence of the secondary phase in former martensite plates, the chemical composition of which corresponds to the NixAl1-x phase (Fig. 8, d). The completely transformed plates are separated from each other by low-angle boundaries. Conclusions There are two types of particles in HV-APS coatings: with a dendritic and grain structure. The center of particles with a dendritic structure consists of the β-NiAl phase surrounded by a one-phase layer of the NixAl1-x phase and a layer of dendrites (NixAl1-x) with interdendritic space (γ’-Ni3Al). Most of the coatings are particles with a grain structure (NiхAl1-x and γ’-Ni3Al grains). Only NixAl1-x grains experience martensitic transformation when the particles are cooled. In the large grains, larger than 500 nm, martensite consists of plates in a twinned orientation relative to each other, while small grains are completely transformed into one microtwinning plate. In addition, there are grains in which martensite and β-phase plates alternated. The collision behavior of martensitic plates is different. Thin plates at collision pass through each other and only the area of its intersection is rearranged. When a martensitic plate collides with an already formed γ΄-Ni3Al grain, the plate continues to transform without penetration. If martensitic plates were formed first, the γ΄-Ni3Al phase is formed around it. In addition, thin plates at collision with an obstacle can be deflected.
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