Microstructures and Mechanical Properties of Co 3 (Al,W) with the L1 2 Structure

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1128-U05-26

Microstructures and Mechanical Properties of Co3(Al,W) with the L12 Structure Takashi Oohashi, Norihiko L. Okamoto, Kyosuke Kishida, Katsushi Tanaka and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT Microstructures and deformation behavior of γ/γ’ Co-Al-W single-crystals and γ’Co3(Al,W) polycrystals have been investigated. Single crystals with the γ/γ’ two-phase microstructure with a large volume fraction of the γ’ phase are obtained by the Bridgman method. These single crystals exhibit anomalies in yield stress in compression at temperatures above 700 °C. Polycrystalline Co3(Al,W) alloys can be rolled in ambient atmosphere such that the nominal thickness was reduced by 40%. The values of tensile elongation obtained for the polycrystalline Co3(Al,W) alloys in air is slightly smaller than that obtained in vacuum, indicating that the Co3(Al,W) alloys are not severely susceptible to the environmental embrittlement in air. INTRODUCTION The most widely used high-temperature structural materials in aircraft engines and power generation systems are Ni-base superalloys consisting of the solid-solution based on Ni with a face-centered cubic structure (γ phase) and the stable L12-ordered intermetallic compound based on Ni3Al (γ’ phase) [1-3]. The excellent high-temperature mechanical properties of these Ni-base superalloys have been believed to be closely related to the γ/γ’ two-phase microstructure in which the cuboidal γ’ precipitates are aligned coherently in the γ matrix [1-3]. Since the ternary intermetallic compound Co3(Al,W) with the L12 structure was discovered [4], two-phase Cobase alloys composed of γ-Co solid-solution and γ’-Co3(Al,W) phases have been investigated as promising high-temperature materials [5,6]. Some Co-base γ/γ’ two-phase alloys have been reported to exhibit high-temperature strength greater than those of conventional Ni-base superalloys [5]. However, almost nothing is known about the pristine physical properties of γ’Co3(Al,W), supposedly due to the difficulties in obtaining single-phase Co3(Al,W). In the present study, we examine microstructures and compression deformation behavior of single-crystals with a large volume fraction of the γ’ phase as well as tensile deformation behavior of polycrystalline Co3(Al,W) alloys in different test environments in order to clarify environmental effects on the tensile ductility. EXPERIMENTAL PROCEDURES Polycrystalline specimens with compositions of Co-9Al-9W (A), Co-10.5Al-11.5W (B), Co-10.5Al-11.75W (C), Co-10.5Al-12W (D), Co-11Al-11W (E), Co-11Al-12W (F) and Co12Al-11W (G) (at%: as marked in figure 1) were prepared by arc-melting elemental Co, Al and W under an Ar gas flow (figure 1). Directional solidification was made for the alloys A-D by the Bridgman method in vacuum at a growth rate of 4.2 mm/h. The alloys D-G were annealed in Ar gas at 900 °C for 168 h. Microstructures of the as-grown and annealed samples were examined by scanning electron microscopy (SEM). Compres

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