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Thermal conductivity is one of the important physical properties for high-performance materials at elevated temperatures.[1,2,3] High thermal conductivity is desirable for the materials to avoid degradation by local heating. A material with a low thermal conductivity is incapable of averaging out the heterogeneous temperature distribution often encountered in operation, which causes degradation by oxidation, local melting, etc.
Cobalt-base superalloys are currently used for gas-turbine vanes because of a combination of higher melting temperatures, good weldability, excellent hot-corrosion resistance to contaminated turbine atmospheres, and superior thermalfatigue resistance compared with nickel-base superalloys.[4] They usually consist of a matrix of cobalt solid solutions and a variety of precipitated phases such as carbides or intermetallic compounds.
Contrary to the extensive research work on thermal conductivity in intermetallic compounds,[5,6,7] the amount of available data for cobalt solid solutions is quite limited.[8] Cobalt has an advantage as a solvent material because of its wide solubility for many elements.[9,10] The purpose of the present work is to identify the global feature of the thermal conductivity at higher temperatures in cobalt solid solutions. First, we investigate the composition dependence of thermal conductivities and organize the results according to the periodic table. Second, the temperature dependence of thermal conductivity is surveyed. Pure cobalt is well known to exhibit an allotropic transformation at 695 K between the high-temperature fcc crystal structure (alpha) and the low-temperature hcp structure (epsilon).[11] In the present study, the effect of crystal structure on thermal conductivity is also investigated.
More than 30 types of cobalt alloys were produced by arc melting in an argon atmosphere. The purity of cobalt was 99.9 mass pct, and the highest-purity raw materials were used for solutes. Total weight losses for each casting were less than 0.1 pct, indicating that the actual compositions were very close to the intended compositions. The ingots were encapsulated in an evacuated quartz tube and then solution treated at 1273 K for 24 hours followed by water quenching. Thermal conductivity was determined by a laser flash method[12] on a disc specimen with dimensions of 10 mm (phi) X 2 mm. A detailed description of the laser-flash apparatus and an evaluation scheme for thermal conductivity were presented in our earlier articles. [5,13]
The thermal conductivity at 300 K for cobalt solid solutions...