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

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

Creep of CMSX-4 superalloy single crystals: effects of rafting at high temperature

Microstructures and mechanical properties of a directionally solidified NiAl–Mo eutectic alloy

The low density and high thermal conductivity of NiAl compared to nickel-base superalloys make NiAl alloys attractive materials for turbine airfoils. The lack of ductility at lower temperatures has been one of the barriers limiting the use of NiAl alloys. The authors identify microalloying additions to NiAl which have demonstrated very significant improvements in the room temperature tensile ductility. The purpose of this paper is to provide details of the experimental data and preliminary information on the potential mechanisms for the ductility improvement.

The effect of iron, gallium and molybdenum on the room temperature tensile ductility of NiAl

Investigation of techniques for measuring lattice mismatch in a rhenium containing nickel base superalloy

An investigation of the change in slip behavior in NiAl with temperature has been conducted, with special emphasis on the 〈001〉 “hard” orientation. Single crystal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations and extensive dislocation analysis performed in the TEM on the 〈001〉 oriented specimens. It was found that, although 〈111〉 slip can occur in RT compression of 〈001〉 oriented specimens, the increased tensile ductility observed at higher temperatures is due to the glide of b = 〈110〉 dislocations. The debris left behind by these dislocations consists of b = 〈100〉 dislocations, making identification of the operative Burgers vector difficult after any appreciable plastic strain. A mechanism for the formation of the b = 〈100〉 debris is presented.

Slip systems in 〈001〉 oriented NiAl single crystals

An investigation of the effect of alloying on slip behavior in NiAl as a function of temperature has been conducted. Single Cyrstal specimens have been deformed in tension and compression in 〈110〉 and 〈001〉 orientations. It was found that the addition of Cr and other alloying additions promote the activation of 〈111〉 slip over deformation by kinking in NiAl based alloys. This is believed to result from differential proportional hardening of the 〈100〉 vs 〈111〉 slip systems. No increase in RT tensile elongation is observed in these alloys. Increased tensile ductility observed at higher temperatures is due to the movement of b = 〈110〉 dislocations.

The effect of alloying on slip systems in 〈001〉 oriented NiAl single crystals

NiAl alloys offer significant payoffs in gas turbine applications. Excellent progress has been made in understanding their mechanical behavior and improving low temperature ductility and high temperature strength. Significant improvements in mechanical properties have been obtained with microalloying. The next challenge is to develop an alloy which has the required balance of ductility, toughness, strength and other properties such as fatigue and impact resistance. Development of design, processing and test methodology for components made out of low ductility and anisotropic materials will also be required. While significant challenges remain, the prognosis for using NiAl alloys as high temperature structural materials is good.

D.F. Lahrman

Papers

The effect of iron, gallium and molybdenum on the room temperature tensile ductility of NiAl

Investigation of techniques for measuring lattice mismatch in a rhenium containing nickel base superalloy

Slip systems in 〈001〉 oriented NiAl single crystals

The effect of alloying on slip systems in 〈001〉 oriented NiAl single crystals

Overview of NiAl Alloys for High Temperature Structural Applications