Site preference determination in intermetallic compounds by thermal conductivity measurement
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Tomoo Suzuki Kochi University of Technology, Tosayamada, Kochi 782-0003, Japan (Received 1 December 2000; accepted 25 May 2001)
A method for the determination of site preference of substitutional elements in intermetallic compounds is proposed. It is demonstrated in Ni3Al–X alloys that the ridge direction in thermal conductivity contours in the ternary ␥⬘ phase agrees with that of the solubility lobe of the ␥⬘ phase in ternary phase diagrams. The ridge direction is a reliable indication of site preference of substitutional elements in intermetallic compounds. The present method is conveniently applied to a normal polycrystalline specimen with small size, and therefore, a versatile class of brittle compounds can be studied.
I. INTRODUCTION
Intermetallic compounds have been studied extensively for potential use as structural materials under different loading conditions, temperatures, and environments. The mechanical properties of intermetallic compounds can be improved by the addition of ternary elements.1–8 Since intermetallic compounds are usually ordered phases, additions of substitutional elements may exhibit certain site preferences. The nature of the substitutional site is an important component in alloy development of intermetallic compounds. The amount of strengthening produced by a given ternary additive in a binary compound depends on the substitutional nature of the additive.9–13 Thus, an information of the site occupation of substitutional elements in intermetallic compounds is extremely useful in controlling the mechanical properties. The traditional technique for determining the site preference of ternary elements has been based on the direction of the solubility lobe in a corresponding ternary phase diagram.14,15 The technique is favorable to the compounds having a limited single-phase region in the binary systems, such as Ni 3 Al, Ni 3 Ga, Ni 3 Si, Ni3Ge,15 and Ni3Nb.16 However, a practical limitation in implementing the method is that the direction of the solubility lobe is difficult to be determined for the
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Present address: Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 1528552, Japan.
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J. Mater. Res., Vol. 16, No. 8, Aug 2001 Downloaded: 31 Mar 2015
compounds with a wide single-phase region in the binary systems, e.g., NiAl–X, FeAl–X, and TiAl–X.17 In these compounds, the solubility of most additives is too small in the direction of the solubility lobe to be resolved. Several microscopic techniques have been recently attempted to study the site preference of substitutional elements in intermetallic compounds with particular emphasis on Ni3Al–X. The techniques include Mo¨ssbauer spectroscopy,18,19 perturbed angular correlation (PAC) analysis,20 atom probe field-ion microscopy (APFIM), 21–25 channeling-enhanced microanalysis (ALCHEMI),23,26–28 ion channeling,29,30 x-ray diffractometry (XRD),28,31–33 and others.34–36 However, each technique has its own shortcoming. In Mo¨ssbauer spectroscopy and PAC, only a f
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