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

The deformation behaviour of intermetallic superlattice compounds

If their properties can be improved, nickel aluminide alloys offer significant payoffs in gas turbine engine applications. For these materials, excellent progress has been made toward understanding their mechanical behavior as well as improving their low-temperature ductility and high-temperature strength. For example, recent work shows that room-temperature ductility can be improved dramatically by microalloying with iron, gallium or molybdenum. The next challenge is to develop an alloy which has the required balance of ductility, toughness and strength. Development of design and test methodologies for components made out of low-ductility, anisotropic materials will also be required. While significant challenges remain, the continuing developments suggest that the prognosis for using NiAl alloys as high-temperature structural materials is good.

NiAl alloys for high-temperature structural applications

Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

D.R. Barker

Papers

A brittle to ductile transition in NiAl of a critical grain size