OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Introduction Nowadays, intermetallic materials design for structural purposes is one of the priority areas for the development of modern mechanical engineering. Due to the combination of characteristics such as high heat resistance and thermal conductivity, the ability to maintain strength and rigidity at high temperatures and relatively low density [1–3], nickel aluminides are used as materials for components of aircraft engines, gas turbines and heat exchangers [4–6]. It is vital that Ni-Al system alloys, being high-temperature materials, have low ductility and fracture toughness at room temperature [6] and this limits its use as bulk parts. In turn, one of the solutions to this problem is the use of nickel aluminides as functional coatings. In general, the main methods of applying Ni-Al coatings are high-velocity oxygen-fuel spraying (HVOF) [7–9], highvelocity air-fuel spraying (HVAF) [9, 10], atmospheric plasma spraying (APS) [11–14] and also its modification such as high-velocity atmospheric plasma spraying (HV-APS). There are eight stable and metastable intermetallic compounds in the Ni-Al system [15], the most promising of which are aluminides located in the nickel-rich part of the phase diagram, such as γ΄-Ni3Al and β-NiAl (Fig. 1) [3, 16, 17]. β-NiAl solid solutions have a wide range of homogeneity (43–70 at. % Ni at 1,400 °C), which decreases to 45–60 at. % Ni at a room temperature [3, 16]. Cooling of the β-phase in the range of high Ni concentrations is accompanied by the formation of a mixture of β- and γ΄-phases, while grains of the β-NiAl phase often have different chemical compositions. The martensitic transformation B2 → L10 occurs in β-phase crystals containing more than 62.3 at. % Ni. The onset temperature of this transformation (Ms) varies, according to various sources, from −200 to ~ 650 or 900 °C [17–19] depending on the Ni concentration. Subsequent heating of alloys from 62.5–68.0 at. % Ni promotes the separation of the Ni5Al3 phase or the metastable Ni2Al phase [20–22]. As a rule, coatings with a similar composition are often used as a bonding layer between the base material and a ceramic heat-shielding coating (YSZ) [23]. Chen et al. [24] discovered that the martensitic transformation occurring in the metal sublayer can cause the destruction of the ceramic coating due to changes in volume during the transformation of the β-phase into martensite. Thus, the study of the structural-phase state, as well as the understanding of structural transformations are priority tasks in the Ni-Al coatings design, since both functional and mechanical, as well as technological properties will depend on this. The purpose of this work is to study the features of the martensitic structure of Ni-Al coatings obtained by the HV-APS method. To achieve this purpose, the following tasks were solved: • study of the coatings structure; • study of the features of the martensitic structure depending on the grain size; • study of the behavior of martensitic plates when colliding with other structural components; • study of the influence of heating temperature on the structure of the coatings. Materials and methods Ni-Al coatings 500–600 µm thick were spraying on discs from low carbon steel with a diameter of 20 mm and a thickness of 8 mm. The particle size of powder was 40–100 µm, chemical composition was 75 at. % Ni and 25 at. % Al. To apply the coatings, we used the Termoplasma 50 plasma spraying installation, Fig. 1. Part of Ni-Al phase diagram
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