A critical review of the physical and mechanical properties of NiAl is presented. The physical properties examined include electronic structure and bonding, crystal structure and phase stability, thermodynamic properties, elastic properties, and electrical, magnetic, and thermal properties. Discussion of crystal defects in NiAl include both constitutional and thermal point defects, the core structure and energy of line defects, and planar defects (shear faults, grain boundaries, and free surfaces). The mechanical properties, substructure, and mechanisms of ductility of NiAl single crystals and polycrystals are reviewed in detail, while alloying effects and the deformation of NiAl martensite are briefly described. The fracture toughness, modes of fracture, and cyclic properties reported in the literature are assessed. A critical analysis of diffusion data for NiAl is followed by a discussion of the activation energy and mechanisms of diffusion. This information is related to the creep properties of NiAl, and additional critical comments concerning the substructure and creep mechanisms of NiAl are provided. A review of the environmental resistance of NiAl is followed by a brief discussion of several current and potential applications of NiAl. Concluding remarks include suggestions for future research on NiAl.

Overview No. 104 The physical and mechanical properties of NiAl

Considerable work has been performed on NiAl over the last three decades, with an extremely rapid growth in research on this intermetallic occurring in the last few years due to recent interest in this material for electronic and high temperature structural applications. However, many physical properties and the controlling fracture and deformation mechanisms over certain temperature regimes are still in question. Part of this problem lies in the incomplete characterization of many of the alloys previously investigated. Fragmentary data on processing conditions, chemistry, microstructure and the apparent difficulty in accurately measuring composition has made direct comparison between individual studies sometimes tenuous. Therefore, the purpose of this review is to summarize all available mechanical and pertinent physical properties on NiAl, stressing the most recent investigations, in an attempt to understand the behavior of NiAl and its alloys over a broad temperature range.

Physical and mechanical properties of the B2 compound NiAl

It is demonstrated that a cellular model gives an excellent account of the heat of formation of binary alloys of 27 transition metals. The energy effects are described by two terms, the first representing the difference in electronegativity between the two types of atoms in an alloy and the second term reflecting the discontinuity in the density of electrons at the boundary between dissimilar Wigner-Seitz atomic cells. The electronegativity is expressed in a scale which closely resembles that of the experimental work functions of pure metallic elements.

The use of this scale also gives a clear insight into the heat effects which accompany the alloying of transition metals with non-transition metals. If the latter have p-electrons a negative term adds up to the heat of formation which apparently does not depend on the particular metals considered. As a result, simple rules for the alloying behaviour of transition metals are formulated which have an accuracy between 96 and 100%; the electronegativity concept is shown to apply also for hydrogen in metals.

The relation between the charge transferred per atom, ΔZa, and the difference in electronegativity, Δφ∗, is discussed quantitatively. As an example, the result for solid solutions of two transition metals is ΔZa = 1.2 (1 − ca)Δφ∗, where ca is the concentration of metal A. Our analysis supports the idea that the work function of a metal is, in principle, proportional to the chemical potential for electrons in Wigner-Seitz atomic cells; the proportionality factor is found to be about 1·4.

The electronegativity parameter for transition metals: Heat of formation and charge transfer in alloys

The theoretical proposal of chiral fermions in topological semimetals has led to a significant effort towards their experimental realization. In particular, the Fermi surfaces of chiral semimetals carry quantized Chern numbers, making them an attractive platform for the observation of exotic transport and optical phenomena. While the simplest example of a chiral fermion in condensed matter is a conventional $|C|=1$ Weyl fermion, recent theoretical works have proposed a number of unconventional chiral fermions beyond the standard model which are protected by unique combinations of topology and crystalline symmetries. However, materials candidates for experimentally probing the transport and response signatures of these unconventional fermions have thus far remained elusive. In this Letter, we propose the RhSi family in space group No. 198 as the ideal platform for the experimental examination of unconventional chiral fermions. We find that RhSi is a filling-enforced semimetal that features near its Fermi surface a chiral double sixfold-degenerate spin-1 Weyl node at $R$ and a previously uncharacterized fourfold-degenerate chiral fermion at $\mathrm{\ensuremath{\Gamma}}$. Each unconventional fermion displays Chern number $\ifmmode\pm\else\textpm\fi{}4$ at the Fermi level. We also show that RhSi displays the largest possible momentum separation of compensative chiral fermions, the largest proposed topologically nontrivial energy window, and the longest possible Fermi arcs on its surface. We conclude by proposing signatures of an exotic bulk photogalvanic response in RhSi.

https://link.aps.org/accepted/10.1103/PhysRevLett.119.206401

Unconventional Chiral Fermions and Large Topological Fermi Arcs in RhSi

Thermodynamics and defect structure of intermetallic phases with the B2 (CsCl) structure

The activity of aluminum in the Pd-Al system was determined by an isopiestic method from 30 to 80 at pct Al between 1090° and 1490°K It shows a strong negative deviation from ideality, with a decrease in activity of aluminum of four orders of magnitude around the stoichiometric composition of PdAl The defect structure in the CsCl-structure PdAl compound has been determined by lattice parameter measurements Equations were derived relating the degree of intrinsic disorder (α) to the shape of the activity curve Excellent agreement between the calculated curve and the activity data was obtained for anα = 25 × 10−4 The results from this investigation and from previous studies indicate that the degree of intrinsic disorder in the equiatomic transition metal-aluminum compounds is controlled by electronic effects

E. Miller

Papers

Thermodynamic properties and ordering in PdAl

Thermodynamic properties of nickel-tellurium alloys

Thermodynamic properties of cobalt-tellurium alloys