Discontinuous Precipitation Reactions in Co-10Al-4C (At. Pct)
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INTRODUCTION
CO-BASE alloys have higher melting temperatures than Ni-base alloys and offer better weldability, superior thermal fatigue resistance, better wear resistance, and better stress-rupture properties at high temperatures. Further, Co-base alloys often have outstanding hot corrosion resistance in contaminated gas turbine atmospheres.[1] However, they are typically weaker than Ni-base alloys at low and intermediate temperatures due to the lack of stable c¢ precipitates in the microstructure.[1,2] Consequently, while Ni-base alloys have been used more widely for applications at intermediate temperatures due to their higher strength and better strength retention than Co-base alloys, there continues to be interest in developing higher strength Co-base alloys that are precipitation strengthened by L12 (c¢) or related precipitates.[3]
H. KAMALI and S. HOSSEIN NEDJAD are with the Department of Materials Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran. Contact e-mail: [email protected] M.J. KAUFMAN, R.D. FIELD and A.J. CLARKE are with the Department of Metallurgical and Materials Engineering, College of Applied Science and Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted December 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
One of the Co-base alloys which has been studied in this regard is Co-Al-C. A j-Co3AlC phase with the E21 (perovskite) structure was initially reported in the Co-Al-C system by Huetter and Stadelmaier.[4] The E21 structure is similar to the L12 structure of the c¢ phase in Ni-base alloys except for an excess carbon atom located in the octahedral interstices at the body center of the cubic unit cell. For this reason, the E21 structure has also been called L¢12.[5,6] Further, it has also been reported that the carbon concentration of the E21 phase is not stoichiometric, and an alternative j-Co3AlC0.5 phase with E¢21 structure is formed by extra ordering of C atoms taking place in the E21 structure to minimize lattice energy by means of elastic and magnetic contributions.[7–11] Indeed, a super-cell of E¢21 consists of four L12 and four L¢12 unit cells being alternately arranged in three dimensions, and with the Fm3m space group.[5–7] The similarity between the E¢21 and L12 structures suggests the possibility of novel precipitation-strengthened Co-base alloys appropriate for intermediate and high temperatures similar to Ni-base alloys.[7–11] Subsequent to the first paper on the j phase, other authors have reported on the thermal and magnetic stabilities, microstructural evolution, and mechanical properties of Co-Al-C alloys.[5–10] Moreover, some efforts have been devoted to determination of the phase equilibria in the Co-rich corner of the Co-Al-C phase diagram.[11–15] However, the aging behavior and microstructural evolution in Co-Al-C alloys containing
the j phase have not characterized in a systematic manner to date. Therefore, this paper is focused on the microstructural evolution and age hardening behavior of a Co-10Al-4C (at
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