OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Martensite in HV-APS coatings is lamellar (Fig. 4), but depending on the size of the grains in which the transformation occurs, it looks different. For example, martensite formed in large NixAl1-x grains consists of plates located in a twinned orientation relative to each other (Fig. 4, a). The distance between microtwins may be from 0.5 nm (Fig. 4, b) to several nanometers (fig. 4, a). The martensite-martensite interface can be located both inside the former NixAl1-x grain and outside it (Fig. 4, c). Unlike large grains (greater than 500 nm), fine grains completely transform into one microtwinning plate (Fig. 4, d). Sometimes there are martensitic grains in which, even with the use of a dark field, it is not possible to detect microtwins in pairs of parallel plates, and these plates appear to be single crystals (Fig. 5). According to data of local chemical analysis, adjacent plates have different chemical compositions. The Ni content in plates with microtwins (type 1) is 77.4 at. %, which corresponds to the NixAl1-x phase, while the Ni content in plates without microtwins (type 2) is 52.5 at. %, which corresponds to the β-phase. Martensite plates can behave differently when collide with each other or with other phases. For example, growth of individual thin plates growing in different directions often does not stop. These plates pass through each other and only the area of its intersection is rearranged (Fig. 4, a). Fig. 6, a, b shows that when a martensitic plate collides with a γ΄-Ni3Al grain, it does not penetrate into it, but continues to transform. On the other hand, Fig. 6, c shows martensitic plates that seem to have grown inside the γ΄-Ni3Al grain. Apparently, in this case, the NixAl1-x plates appeared first, around which the γ΄-Ni3Al phase later formed. Some plates change the direction of its growth, deviating to the side at collision with obstacle (Fig. 6, d). Deformation and elastic distortions occur in areas near curved plates, which contrast is visible near the bend. Fig. 3. Bright field TEM images of HV-APS coatings: a – two-phase area of NiхAl1-х + γ΄-Ni3Al; b – one-phase area of γ΄-Ni3Al; c) twins in γ΄-Ni3Al; d – stacking faults in γ΄-Ni3Al with diffraction pattern а b c d
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