Comparison of the thermodunamic assesstments of the NiAl system at the initial stage of nucleation

О. М. Римар

Abstract


The paper presents the comparative results of the phenomenological modeling of the presence or absence of contact melting in the Ni-Al system using the Calphad method. Сurves of the Gibbs thermodynamic potential versus nickel concentration were constructed.
Today the nickel-aluminum system has found a remarkably wide application in the study of SHS phenomena. It was found that the method of preparation of thin films (flow velocity, its density, temperature of substrate) thickness of the multilayers and their period affects on velocity of the front combustion, temperature and phase composition. At isothermal reactions in this system the order of formation equilibrium phases with increase in the share of nickel are often observed ( Al3Ni … Ni3 Al ). Knowing the order of phase formation we can predict the physical properties of thin films. Investigating of the Ni-Al system by Calphad method it was observed that the use of different articles from thermodynamic evaluation of this system gives quite different results: according to sources [1], [2], [3], we can observe the phenomenon of contact melting even at room temperature, and on the results of the source [4] it is absent. It is considered that the oxide film on the surface of nickel and aluminum are absent. The difference in results is not in the mathematical model that describes the thermodynamics of the process (they are the same), but in adjustable the data that contribute excess to the Gibbs energy (which is associated with account imperfection of lattice).
By the molecular-dynamic research of the F. Baras and O. Politano it was found that the addition of nickel to pure aluminum reduces the melting point of the latter. According to the thermodynamic data [1] the melting temperature of the solution is approximately equal to 700 K, while pure aluminum melts at 933 K. According to this fact, we can assume that even at the stage of preparation of thin films by different methods liquid layer can exists from the beginning on the contact of nickel-aluminum pair.
Using data from each source a metastable phase diagrams were built for the comparison with each other. Based on the parallel tangents rules, the driving forces of intermetallic compounds nucleation from liquid nickel aluminum solution were found. It was assumed that the intermetallic compounds appear by the mechanism of precipitation from the liquid solution.


Keywords


Calphad; metastable phase diagram; contact melting; driving force of the nucleation; parallel tangents rule

References


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