First-principles study of NiAl alloyed with Co
Graphical abstract
Influence of Co in NiAl on structural, elastic, electronic and thermal properties.
Introduction
The intermetallic compound B2-NiAl is one of the most promising engineering materials with attractive chemical and mechanical properties, and it is a contender of next generation of high temperature structure material [1]. The properties include high melting temperature (1638 °C), low density (5.86 g/cm3), high modulus, high thermal conductivity and excellent environmental resistance. However, room temperature brittleness and high temperature strength shortage seriously limit its practical application, but it can be improved by alloying additions [2], [3], [4]. It is known that solid solubility varies with different dopant atoms, and Co has high solubility in NiAl.
The understanding of the alloying effects requires the knowledge of site distributions of ternary elements. The site preference of Co has been investigated extensively by various experimental techniques, such as X-ray diffraction (XRD) [5], Extended X-ray absorption edge fine structure (EXAFS) [6]. Thermal conductivity measurement [7]. The results showed that Co substitute for a Ni site. Theoretical evaluations of the site preference of Co have been conducted by many researchers [8], [9], [10], [11], [12], [13]. They get the same conclusion that Co has a consistent preference for the Ni sublattice. There are some investigations about the effect of alloying on properties of NiAl. Kovalev et al. [14] pointed out that alloying with Co has a beneficial effect on the micro-mechanism of fracture and ductile–brittle transition temperature, and this improvement is tied with the change of density of electron states N(EF). The microhardness of NiAl alloys was increased due to solid solution hardening of Co [15].
Through the theoretical and experimental research above, it is worth noting that the theoretical calculations only gave material properties at 0 K and zero pressure, without any thermal effects included. As a potential high-temperature material, the thermal properties cannot be over-emphasized. Practical use of NiAl alloy often occurs at high temperature. Therefore, it is necessary to investigate the influence of Co on the thermal properties. To our knowledge, there is no correlative report about this aspect. To address this interest, we investigate the structural and thermodynamic properties at high pressures and temperatures, by using first-principle calculations combined with the quasi-harmonic Debye model.
In this paper, the site preference of Co in NiAl and its effects on structural, elastic, electronic and thermal properties are investigated by performing first principles calculations using density functional theory (DFT), which would be useful for the understanding and design of the relevant alloys. The influence of Co on the thermal expansion coefficient, bulk modulus, Debye temperature and heat capacity at different pressure and temperature were studied for the first time.
Section snippets
Calculation parameters
NiAl has an ordered B2 structure that consists of two interpenetrating simple cubic sublattices, with Ni and Al atoms occupying the corners and the center of the body, respectively. The lattice constants are a0 = 0.2887 nm, α = β = γ = 90. A supercell consisting of 16 atoms is devised for the present study, a 2 × 2 × 2 cubic representation of the ordered structure, with one single point defect such as an antisite (NiAl or AlNi) or the ternary element Co on the sublattice. One alloying atom in each supercell
Site preferences of ternary elements in NiAl at ground state
Formation enthalpy is the entropy of the compound or solution in relation to the composition-weighted pure elements. The bulk stability of the compound is determined by formation enthalpy. The negative formation enthalpy value implies the structure is thermodynamically stable and the positive value means the structure is instable. In this work, the formation enthalpy of (Ni7Co)Al8 was calculated by using the following expression:
The formation
Conclusions
In this paper, the site preference of Co in NiAl and its effects on electronic and elastic properties were investigated by first principles. The formation enthalpy is negative, implying that the compounds can exist from the energetic point of view. The formation enthalpy of doping system is bigger than NiAl means that stability is reduced after Co doping. The site preference were investigated through calculating the transfer energy of NiAl alloyed by Co element. The results show that Co shows
Acknowledgment
This work is supported by program of excellent team at Kunming University of Science and Technology.
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