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26COMBUSTION SYNTHESIS AND STRUCTURE FORMATION IN Ni-Al-C SYSTEM
A. E. Sytschev*", N. A. Kochetov", A. S. Shchukin", M. L. Busurina", and A. V. AborkinA
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia
2Vladimir State University named after Alexander and Nikolay Stoletovs, Vladimir, Russia *e-mail: sytschev@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10171
Ni-Al intermetallics have a significant drawback due to insufficient ductility especially at the room temperature. Mechanical properties, such as strength, modulus of elasticity, viscosity can vary significantly even when very small amounts of carbon are added. The small presence of carbon in the Ni-Al alloy leads to an increase in strength by 30%. Carbon is a promising solid lubricant due to its excellent tribological properties. It is proved that graphene as a solid lubricant not only causes low wear but provides easy shearing and thus results in reduced friction. The formation of a chemical bond between carbon components and matrix, which strongly affects the mechanical properties of the material, can vary depending on the conditions for obtaining the material. The presence of the melt greatly accelerates the process of mutual diffusion of the reaction components and increases the contact area due to the capillary spreading of the melt over the carbon. The formation of a melt in the Ni-Al system is easily realized during SHS process. The most important stage in the preparation of the reactive blends can also be mechanical activation (MA), the parameters of which ultimately affect the kinetics of solid-phase combustion (sintering). The peculiarities of the formation of the structure of intermetallic Ni-Al alloys modified by carbon components in the mode of self-propagating high-temperature synthesis (SHS) and spark plasma sintering (SPS), and the influence on the structure of preliminary MA are investigated in this study. SHS experiments in the Ni-Al-C system showed the possibility of obtaining a material using carbon components in the combustion regime. The reaction mixture was prepared from ASD-1 aluminum powders of about 20 ^m in size, containing about 99.7% Al, and a metallic carbonyl nickel grade PNA (not less than 99.9 wt %) of about 10 p,m in size for NiAl production. In a mixture of Ni and Al powders, carbon (0^6 wt %) was added in the form of carbon black. The reaction mixture was exposed to MA for 1 min on the ATO-2 attritor. Cylindrical samples with a diameter of 1 cm were pressed from activated mixtures by cold pressing. The combustion velocity is noticeably decreased with increasing of the soot content. XRD analysis of the synthesized material showed only the presence of the NiAl phase. NiAl diffraction lines are shifted to smaller angles with an increase in the carbon content in the initial charge, which indicates an increase in the cell parameters. Due to the low amount of soot in the starting mixture (2-6 wt %), carbon was not detected in the synthesized product. The microstructural studies showed that as a result of the synthesis, a close-packed intermetallic matrix based on NiAl with a grain size of about 5-10 p,m was formed. Carbon is predominantly located along the boundaries of intermetallic NiAl grains and forms a continuous multilayer graphene-like film on the surface with a
thickness of about 66 nm (Fig. 1). Fig.L Cross section of NiAl/C comp°site-
-SUS 2019_Moscow, Russia
It can be assumed that the formation of multilayer carbon nanofilms is occurred as a result of allotropic conversion of carbon black to graphite only at the point of direct contact of the NiAl melt with soot. Figure 2 shows the graphite film on the surface of NiAl grain after complete solidification of the melt. The film has a specific morphology of smooth surface areas separated from each other by out of plane faceted wrinkles. The structure at the joint between the wrinkles is more complicated, but the faceted structure is still evident. The graphitic nanofilm is wrinkled due to its thermal expansion coefficient mismatch with NiAl intermetallic.
Fig. 2. The view of graphite film on the surface of NiAl grains.
It is known that if the carbon concentration in the solid exceeds the limit of solubility at a given temperature, it is possible to segregate excess of carbon to the surface to form graphene-like structure. A well-known example of this approach is segregation on the Ni surface. The solubility of carbon in nickel increases with increasing temperature. Therefore, it is possible to create an excess concentration of carbon in Ni as a result of cooling the metal saturated with carbon at high temperature. The mechanism of carbon film formation during SHS can be associated with dissolution of carbon (soot, graphite, carbon nanotubes) in the Ni-Al melt at the combustion temperature that is followed by the growth as graphite films on the melt surface during cooling down.
Spark plasma sintering (SPS) experiments in the Ni-Al-C (2 and 4 wt % C) system are also showed the possibility of synthesizing dense material with 2D carbon film. SHS product shows similar microstructures of NiAl intermetallic with different carbon content. NiAl grains have regular shape with the size about 20 |im.
Ni and Al particles are melted in contact with a carbon during SHS to saturate with carbon at a concentration correspond to the binary phase diagram of metal-carbon. Upon cooling, as the solubility of carbon in the Ni-Al melt decreased, the excess carbon formed on top of the Ni-Al melt as well as other sites within the melt. The solubility of carbon in Ni-Al melts increases with increasing combustion temperature. It is possible to create an excess concentration of carbon in Ni-Al melt as a result of cooling the melt saturated with carbon at high temperature.
According to the tribological tests (Fig. 3), the friction coefficient of NiAl-C composite is about 0.5. The friction coefficient of the sample with 2 wt % C increases sharply on the sliding distance of 0^25 m to 0.75. Microstructural analysis showed (Figs. 4, 5) that tribological test resulted to formation of noticeable friction grooves. EDA analysis of the friction surface shows the presence of elements contained in the composition of the counterbody - iron Fe and oxygen O, which indicates their transfer. It is observed that metal oxides and carbon components are present in different regions in the wear track. Thus, visual analysis of friction surfaces, peculiarities of changes in the chemical composition of the friction surface, and the layer on the counterbody indicate the adhesion-abrasive wear mechanism of the contact pair.
Sliding distance, m Sliding distance, m
Fig. 3. Tribological test of Ni-Al-C (2 and 4 wt % C samples).
Fig. 4. Microstructure and concentration profiles of elements in the friction area of Ni-Al-C (2 wt % C).
Fig. 5. Microstructure and concentration profiles of elements in the friction area of Ni-Al-C (4 wt % C).
The work was supported by the Russian Foundation for Basic Research (project
no. 18-08-00181\18).
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