OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Fig. 6. Interaction of martensite with other phases: a, b – collision of martensite plate with grain γ΄-Ni3Al; c – growth of martensite plates into grain; d – martensite plate deformation; a, c, d – bright field; b – dark field а b c d It was shown above that the temperature of martensitic transformation in Ni-Al alloys is determined by the chemical composition of the material. Coating heating at 400–600 °C allows obtaining information about structural changes in the material. We do not observe any noticeable changes at lower heating temperatures. Fig. 7 demonstrates TEM images of the microstructure of HV-APS coatings after heating at 400 and 500 °C. According to structural studies, the beginning of the reverse transition of L10 martensite to the B2 structure is observed at 400 °C. In addition, a secondary phase in the form of elongated disks segregates along the boundaries of microtwins (Fig. 7, a). In some cases, only part of the martensite plate undergoes transformation. This can be explained by differences in the chemical composition within the same crystal. An increase in temperature to 500 °C leads to further decomposition of martensite plates and the growth of an already precipitated secondary phase (Fig. 7, b). The secondary phase in grains, where martensite has completely transformed, is oriented in one direction. The formation of the secondary phase is characteristic only for NixAl1-x grains and is absent in grains of the γ΄-Ni3Al phase (Fig. 7, b) and β-NiAl plates (fig. 7, c). The internal structure of β-NiAl plates is characterized by a relatively uniform distribution of dislocations. Finally, heating to these temperatures does not lead to any noticeable structural changes in small grains of the NixAl1-x phase (fig. 7, d). A significant increase in the width of grain boundaries is observed in two-phase regions after heating at 600 °C (Fig. 8, a, b). The shape of the γ′-Ni3Al and NixAl1-x grains approaches equiaxial, which indicates the occurrence of recrystallization processes. A growth of the secondary phase with increasing temperature
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