On diffusion in the β-NiAl phase

doi:10.1016/j.jallcom.2005.04.194

Journal of Alloys and Compounds

Volume 403, Issues 1–2, 10 November 2005, Pages 147-153

https://doi.org/10.1016/j.jallcom.2005.04.194 Get rights and content

Abstract

Interdiffusion coefficients in the β-NiAl phase over the homogeneity range are determined by the diffusion couple technique in the temperature range of 1000–1200 °C. Intrinsic diffusivities of the species at 1000 °C at different compositions are measured by Kirkendall marker experiments. The variations of the molar volume with composition and partial molar volumes of the species, required for the determination of the diffusion parameters, are calculated using lattice parameter and vacancy concentration. Tracer diffusion coefficients of the species were calculated from the knowledge on intrinsic diffusivities and compared with the data available in the literature, which were measured directly by tracer methods. The influence of the vacancy wind effect is determined on the calculated results.

Introduction

Materials in the Ni–Al system are attractive for a wide range of applications, because of their high strength to density ratio. Diffusion studies of β-NiAl draw special attention because of the complex defect structure of that phase. The β-NiAl phase with B2 (CsCl-type) structure exists over a wide concentration range extending across the stoichiometric (1:1) composition. The unit cell of this intermetallic consists of two interpenetrating simple cubic sublattices [1] and at the equiatomic composition all Al-atoms occupy the cube corners of one sublattice, and Ni-atoms occupy the corners of the other sublattice. The deviation from stoichiometry is accomplished by essentially two mechanisms. On the Ni-rich side, antisite defects are created, that is Ni-atoms can occupy the Al-sublattice, whereas on the other side of the stoichiometry, structural vacancies are present on the Ni-sublattice. The concentration of these constitutionally generated vacancies can be appreciable.

Such a peculiar defect structure has great influence on the diffusion behaviour of this intermetallic compound. There are several studies on the diffusion phenomena of the phase, however, it was found that sometimes the variation in molar volume with composition was not taken under consideration or otherwise irregularities were observed in the treatment [2], [3], [4], [5].

In this article, we will present the different diffusion parameters determined by the diffusion couple technique. Interdiffusion coefficients over the whole homogeneity range were calculated in the temperature range of 1000–1200 °C. Intrinsic diffusion coefficients of the species Ni and Al at different compositions over the homogeneity range were determined at the Kirkendall marker position in the annealed couple at a single temperature of 1000 °C. In all the calculations proper data on the molar volume of the phase at different compositions and partial molar volumes of the species were used. With the help of the activity data, available in the literature, tracer diffusivities of the species and vacancy wind effect were calculated from the knowledge on the intrinsic diffusivities. These results obtained by the diffusion couple technique were compared with the data available in the literature determined directly by tracer method.

Section snippets

Experimental procedure

Aluminium (99.995%) and Nickel (99.99%) supplied by Goodfellow (UK) were used as starting materials. A number of binary alloys were prepared in an arc melting furnace in argon atmosphere. The ingots prepared in the furnace were remelted three times for better homogeneity. After that, alloys were equilibrated at 1100 °C for 200 h in vacuum and the composition was measured by electron probe microanalysis (EPMA).

Thin slices of 2 mm were cut from the alloys by a slow speed diamond blade. The bonding

Interdiffusion coefficients

Interdiffusion coefficients, $\tilde{D}$ were calculated by the Sauer-Freise treatment [6] adopted by Wagner [7] following: $\tilde{D} (Y^{*}) = \frac{V_{m}}{2 t} {(\frac{d x}{d Y})}^{*} [{(1 - Y)}^{*} \int_{- \infty}^{x^{*}} \frac{Y}{V_{m}} d x + Y^{*} \int_{x^{*}}^{+ \infty} \frac{1 - Y}{V_{m}} d x]$ where the Sauer-Freise variable Y is defined as $(N_{i} - N_{i}^{-}) / (N_{i}^{+} - N_{i}^{-})$ ; N_i is the mole fraction of component i, the superscript “−” and “+” refer to the unreacted left-hand (x = −∞) and right-hand (x = +∞) ends of the diffusion couple, respectively; V_m is the molar volume and (dY/dx)^* is the gradient at the point of interest in the Y versus x

Conclusion remarks

The diffusion couple technique is used to study diffusion properties in the β-NiAl phase. Proper values of the molar volumes at different compositions and partial molar volumes of the species are taken into consideration. This is an important factor to consider in systems where the molar volume changes with composition. In general, very often the variation of molar volume with composition is neglected because of the lack of data available in the literature and one single molar volume calculated

Acknowledgement

The authors are thankful to Mr. H.A.M. van der Palen for his incessant help in the laboratory.

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¹: Present address: University of British Columbia, Vancouver, Canada.

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On diffusion in the β-NiAl phase

Abstract

Introduction

Section snippets

Experimental procedure

Interdiffusion coefficients

Conclusion remarks

Acknowledgement

Prog. Solid-State Chem.

Intermetallics

Acta Met.

Acta Mater.

Acta Mater.

Scripta Met.

Acta Met.

Acta Met.

Acta Met.

Met. Trans. A

Metall. Mater. Trans. A