Ridge direction of thermal conductivity contours in ternary CoAl phase
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Ridge Direction of Thermal Conductivity Contours in Ternary CoAl Phase YOSHIHIRO TERADA, KENJI OHKUBO, TETSUO MOHRI, and TOMOO SUZUKI CoAl has received attention as a high-temperature structural material due to its high melting point, moderate density, and excellent oxidation resistance.[1,2,3] The remarkable advantage of the compound is a high solubility of tertiary additives,[4,5] which can provide the strengthening with greater intensity from solid solution hardening. Thermal conductivity is one of the key parameters required for high-temperature structural applications of metallic material.[6–9] Rapid heat transfer afforded by high thermal conductivity enables efficient cooling, which suppresses the appearance of life limiting heat-attacked spots, resulting in a higher operating temperature. Also, high thermal conductivity assures uniform temperature distribution, which reduces thermally induced stresses and, thereby, improves fatigue properties. The addition of tertiary elements usually decreases the thermal conductivity of the intermetallic compound due to the increase in the scattering site of the conduction electron.[10] It was found in NiAl-X alloys that the contour map of thermal conductivity in ternary  phase characterizes the direction of the contour ridge, and the ridge direction is classified into three categories depending on the tertiary element X.[11] The direction of the ridge in thermal conductivity contours can be an important component in estimating the thermal conductivity for alloys of intermetallic compounds. In this article, we examine the ridge direction of thermal conductivity contours in ternary CoAl phase for nine kinds
YOSHIHIRO TERADA, Research Associate, KENJI OHKUBO, Technician, and TETSUO MOHRI, Professor, are with the Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan. TOMOO SUZUKI, Professor, is with the Department of Environmental Systems Engineering, Kochi University of Technology, Kochi 782-0003, Japan. Manuscript submitted September 27, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Thermal conductivity at 300 K of two types of CoAl-2 at. pct X alloys as a function of the atomic number of the tertiary element X. The data for Co48Al50X2 and Co50Al48X2 are shown by open and solid symbols, respectively. Note that all the alloys have a single phase of B2 crystal structure.
of tertiary elements. The ridge direction is determined by measuring the thermal conductivities of the two types of solid solutions, in which a dilute amount of tertiary element is added for each constituent of intermetallic compound.[11] Two types of CoAl-2 at. pct X, Co48Al50X2 (Co deficient) and Co50Al48X2 (Al deficient), are used to decide the ridge direction in ternary CoAl phase. All alloys were prepared by arc melting high-purity metals in an atmosphere of purified argon. The ingots were subsequently homogenized in a vacuum at 1273 K for 24 hours followed by water quenching. Thermal conductivity was measured at 300 K on
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