Tensile and Creep Rupture Properties of Co + Co 3 AlC 0.5 Two-Phase Alloys
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Co 3AIC 0.5 Two-Phase Alloys
R. Ishibashi*, S.Nakamura*, Y.Aono*, S.Miura** and Y.Mishima***
*Hitachi Research Laboratory, Hitachi, Ltd., Hitachi, Japan **Department of Metallurgical Engineering, Hokkaido University, Sapporo, Japan ***Department of Materials Science and Engineering, Tokyo Institute of Technology, Yokohama, Japan ABSTRACT It is expected that the K-phase of the intermetallic compound Co 3A1C 0.5 would strengthen Cobase alloys used at high temperatures like the ' -phase of Ni-base superalloys. Tensile and creep rupture properties of Co+K two-phase alloys with K-phase volume fractions up to 0.75 were investigated. Alloy samples made by directional solidification casting were annealed at 1573 K for 3.6 ks and at 1373 K for 28.8 ks in vacuum, followed by Ar gas cooling. Tensile tests at RT and 1073 K and creep rupture tests at 1089 K under a stress of 172 MPa were conducted with the tensile axis parallel to the solidification direction. In alloys with low K-phase volume fraction, cuboidal K-precipitates with average particle diameters of 0.4 to 1.0 gtm were observed. They were coherent with the Co(fcc) matrix with misfits of about 3%. As the K-phase volume fraction increased, tensile strength also increased. The alloy with K-phase volume fraction of 0.4 had a 0.2% proof stress of 817MPa, tensile strength of 1047 MPa at RT, creep rupture life of 1.43 Ms, and tensile strain higher than 10%. These strengths are better than those of the conventional Co-base alloys. However, ductility of alloys with Kc-phase volume fraction larger than 0.4 decreased due to large eutectic and primary K-phase particles. INTRODUCTION As thermal resistant materials, Co-base alloys are as important as Ni-base alloys. However Co-base alloys are inferior to Ni-base alloys in terms of strength at high temperatures because conventional Co-base alloys don't have a strengthening phase like the L12-type ,'-phase of Ni-base superalloys. Co 3A1C 0.5 , K-phase, has a structure consisting of four L1 2-type sub-unit cells and four E21-type sub-unit cells in which a carbon atom occupies the body center site of the L1 2-type subunit cell. Thus the structure of the K-phase resembles that of the 'y'-phase and K-precipitates are coherent with the Co(fcc) matrix with misfits similar to y-precipitates in Ni matrix[l]. Therefore, K-phase has attracted attention as a possible strengthening phase for advanced Co-base alloys[2]. An experimental investigation of the Co-rich corner of the Co-Al-C ternary phase diagram showed the stability of the K-phase and relations between K-phase and others in detail[3]. Investigations of mechanical properties under compression showed that alloys containing K-phase have an anomalous positive temperature dependence of strength similar to the y'-phase[2,4]. The resolution of { 111 1-type dislocations were observed in deformed K-phase similar to the y'-phase[5]. Thus, K-phase is expected to have a similar strengthening mechanisms as the y'-phase. In the present work, tensile and creep rupture properties of Co+K two-p
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