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

A re-assessment of the ternary system Al-Cr-Ni following Dupin's thesis work using a single Gibbs energy function for the γ and γ' phases is presented taking into account new experimental liquidus temperatures. The disordered bcc A2 and ordered B2 phases are also modelled with a single equation. The existence of vacancies as defects in this structure is described. The other phases are modelled as substitutional solutions, or as stoichiometric or non-stroichiometric binary compounds. The present assessment is limited to the sub-system AlNi-Cr-Ni where no ternary phase exists. The parameters describing the Gibbs energy of all the assessed phases are given. Extensive comparisons between calculation and experimental data are presented.

Thermodynamic re-assessment of the ternary system Al-Cr-Ni

Thermodynamic properties of the Ni–Al–Cr system

At high temperature, MCrAlX coatings are deposited onto the surface of superalloy to provide oxidation and corrosion protection by forming a thermalgrowing oxide scale. In this project, the oxidati ...

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Modeling of microstructural evolution and lifetime prediction of MCrAlY coatings on nickel based superalloys during high temperature oxidation

Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and 1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence of the Matano planes. The results of the diffusivity measurements showed that
 
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$$\tilde D_{AlAl}^{N1} $$

 is approximately four times greater than
 
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$$\tilde D_{AlCr}^{N1} $$

 , while
 
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$$\tilde D_{CrAl}^{N1} $$

 and
 
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$$\tilde D_{CrCr}^{N1} $$

 are of the same magnitude. For all concentrations,
 
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$$\tilde D_{AlAl}^{N1} $$

 is two to three times greater than
 
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$$\tilde D_{CrCr}^{N1} $$

 . Both
 
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$$\tilde D_{AlAl}^{N1} $$

 and
 
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$$\tilde D_{AlCr}^{N1} $$

 increase with increasing Al concentration, whereas
 
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$$\tilde D_{CrAl}^{N1} $$

 and
 
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$$\tilde D_{CrCr}^{N1} $$

 show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C.

Interdiffusion in Ni-rich, Ni-Cr-Al alloys at 1100 and 1200 °C: Part II. Diffusion coefficients and predicted concentration profiles

Interdiffusion in Ni-rich, Ni-Cr-Al diffusion couples was studied after annealing at 1100 and 1200 °C. Recession of γ′ (Ni3Al structure), β (NiAl structure), or α (bcc) phases was also measured. Aluminum and chromium concentration profiles were measured in the γ (fcc) phase for most of the diffusion couples. The amount and location of Kirkendall porosity suggests that Al diffuses more rapidly than Cr which diffuses more rapidly than Ni in the γ phase of Ni-Cr-Al alloys. The location of maxima and minima in the concentration profiles of several of the diffusion couples indicates that both cross-term diffusion coefficients for Cr and Al are positive and that DCrAl has a greater effect on the diffusion of Cr than does DA1Cr on the diffusion of Al. The γ/γ + β phase boundary has also been determined for 1200 °C through the use of numerous γ/γ+ β diffusion couples.

Interdiffusion in Ni-Rich, Ni-Cr-Al alloys at 1100 and 1200 °C: Part I. Diffusion paths and microstructures

A simplified model has been developed to predict β recession and diffusion paths in ternary γ/γ + β diffusion couples (γ: fcc, β: NiAl structure). The model was tested by predicting β recession and diffusion paths for four γ/γ + β, Ni-Cr-Al couples annealed for 100 hours at 1200 °C. The model predicted β recession within 20 pct of that measured for each of the couples. The model also predicted shifts in the concentration of the y phase at the γ/γ + β interface within 2 at. pct Al and 6 at. pct Cr of that measured in each of the couples. A qualitative explanation based on simple kinetic and mass balance arguments has been given which demonstrates the necessity for diffusion in the two-phaseregion of certain γ/γ + β, Ni-Cr-Al couples.

J. A. Nesbitt

Papers

Interdiffusion in Ni-Rich, Ni-Cr-Al alloys at 1100 and 1200 °C: Part I. Diffusion paths and microstructures

Predicting diffusion paths and interface motion in γ/γ + β, Ni-Cr-Al diffusion couples