Statistical-Thermodynamic Models of the Ni–Al-Based Ordering Phases (L12, L10, B2): Role of Magnetic Ni-Atoms’ Contribution

Abstract

Among known aircraft metal materials, Ni–Al is identified as an ordering intermetallic alloy with several attractive properties including low density (≈ 6 g/cm3), high melting point (≈ 1911 K), excellent oxidation resistance (up to 1573 K), and good thermal conductivity. These and other physical properties are caused by not only the chemical composition, but also the atomic distribution over the crystal-lattice sites. The interactions between atoms of different kinds lead to deviations from their random distribution and the appearance of short-range (correlation) or even long-range (as in the case of Ni–Al) orders. The possible types of ordered phases in Ni–Al alloys are analysed through the obtained expressions for the occupation probability functions of the distribution of Ni (Al) atoms over the sites of the f.c.c. and b.c.c. lattices. The obtained expressions for the configurational free energy of ordering structures of the L12, L10, and B2 take into account both the interaction of substitutional atoms on all (and not only the nearest) co-ordination spheres and the magnetism of Ni atoms. As ascertained after evaluation of the Ni–Ni exchange-interaction energy parameters for the f.c.c.-Ni–Al, the ordering of the subsystem of interacting magnetic moments of Ni atoms somewhat counteracts the long-range order.

Keywords

• ni–al
• statistical thermodynamics
• long-range atomic order
• ‘mixing’ energy
• exchange interaction
• magnetization