Processing, Microstructure, and Thermal Expansion Measurements Of High Temperature Ru-Al-Cr B2 Alloys
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1128-U05-25
Processing, Microstructure, and Thermal Expansion Measurements Of High Temperature Ru-Al-Cr B2 Alloys Y. Hashimoto1, N.L. Okamoto1, M. Acosta2, D. R. Johnson2, and H. Inui1 1
Department of Materials Science and Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-8501, Japan 2
School of Materials Engineering, Purdue University, West Lafayette IN 47907
ABSTRACT The B2 intermetallic compound RuAl has a melting temperature above 2000 ˚C and is a candidate for high temperature structural applications. A large extension of the B2 phase field is found in the Ru-Al-Cr system as was documented by the characterization of arc-melted and heat treated alloys. Two compositions consisting of Ru-35Al-19Cr and Ru-20Al-38Cr (at. %) were directionally solidified in an optical floating zone furnace. Depending upon the processing conditions, single phase, polycrystalline, B2 microstructures could be produced. The coefficient of thermal expansion (CTE) was measured from room temperature to 1250 °C for the Ru-20Al-38Cr alloy, and an average value of 11×10-6 K-1 was found. Additionally, the thermal conductivity was measured as 27 W/mK at room temperature for the Ru-20Al-38Cr B2 alloy and as 89 W/mK for binary RuAl.
INTRODUCTION Ruthenium aluminide with a CsCl (B2) crystal structure has been identified as a candidate material for high temperature structural applications [1-4]. From a recent investigation of Ru-Cr-Al alloys [5, 6], extensive solubility of Cr within the B2 phase was observed. The addition of Cr to RuAl improves the high temperature oxidation resistance, but it has been reported to decrease the toughness [7]. From four-point bend tests of notched samples of twophase (HCP+B2) Ru-Al-Cr alloys, a fracture toughness of less than 13 MPa√m was estimated for the Cr-containing B2 phase [5]. In this investigation, a range of Ru-Al-Cr alloys are investigated to better describe the phase relationships at 1100 and 1500 °C, to investigate if single crystals of RuAl(Cr) could be grown, and to measure selected physical properties of the B2 phase. For example, attempts to grow single crystals of binary RuAl have been hampered by the rapid evaporation of Al from the melt resulting in a loss of stoichiometry and the formation of an intergranular eutectic film [5]. However, decreasing the Al content by the substitution of Cr may improve the solidification processing of these, Ru-Al-Cr, B2 alloys. Although single crystals were not grown in this study, the solidification behavior was detailed, and the coefficient of thermal expansion (CTE) to temperatures of 1250 °C and the room temperature thermal conductivity were measured.
EXPERIMENTAL PROCEDURE Approximately 10 g buttons of Ru-Al-Cr alloys with compositions A to D and H to K (as marked in Figure 1) were produced by arc-melting in flowing Ar-5% H2. Compositions E, F and G were prepared by induction melting and directionally solidified as described previously [5, 6]. The arc-melted and induction melted materials were sectioned and heat treated at 1100 °C for 200 h (composition
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