Evaluation of Solubility Limit of Carbon in Ni 3 AlC 1-x
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Evaluation of Solubility Limit of Carbon in Ni3AlC1-x Hideki Hosoda and Tomonari Inamura Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259-R2-27, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
ABSTRACT In order to clarify the phase stability of E21-type intermetallic carbides, the maximum solid solubility of carbon in Ni3AlC1-x was evaluated by taking into account the strain energy and the chemical energy for the formation of the Ni6C cluster (EM6C). It was found that the maximum carbon content calculated was 0 at.%C at EM6C0, 3.5 at.%C at EM6C = -5 kJ/mol, 6.5 at.%C at EM6C = -10 kJ/mol, 10 at.%C at EM6C = -15 kJ/mol and 13 at.%C at EM6C = -20 kJ/mol, respectively. Experimentally determined maximum carbon contents in Ni3Al in the literature can be explained when EM6C is ranged from -5 to -15 kJ/mol, and the solid solubility is found to be sensitive to EM6C. The attractive interaction between Ni and C seems to be due to covalent bonding. Similar attractive chemical interaction between transition metals and carbon must stabilize E21 phases. INTRODUCTION L12 type intermetallic compounds are promising candidates as high temperature structural materials. This is because L12 compounds may exhibit room temperature ductility due to the high symmetrical L12 structure, low diffusivity resulting in high creep strength due to a closed pack structure [1] and an anomalous temperature dependence of strength based on the KearWilsdorf mechanism [2]. Especially, L12 3d-transition-metal (M) aluminides and silicides are fascinating due to their oxidation resistance and low density. However, only three L12-type compounds exist in the binary systems: Ni3Al, Al3Si and Ni3Si [3]. Several attempts have been made for the quest for new L12 compounds by the addition of substitutional elements. For example, Ishida and co-workers have found L12 Co3(Al,W) for development of new Co based superalloys [4]. On the other hand, we have focused on interstitial additions and studied several E21 type intermetallic carbides [5-9]. E21 M3AlC phases have the Perovskite-type cubic crystal structure in which the M atoms occupy the face-centered sites, Al atoms occupy the corner sites and C atoms occupy the body-centered octahedral interstitial sites surrounded by six M atoms. The E21 structure of M3AlC closely resembles the L12 structure of M3Al, and therefore similar mechanical properties are expected [5]. Reported E21 M3AlC compounds in M-Al-C systems are listed in Table 1 [10] in addition to A3B type compounds in the binary systems [3]. It is clear from Table 1 that E21 compounds appear by carbon addition even if binary L12 M3Al is not stable. Some E21 compounds such as Ti3AlC and Co3AlC exhibit a positive temperature dependence of the strength at intermediate temperature similar to Ni3Al [5, 9]. However, the effect of carbon on phase stability of E21 is not sufficiently revealed yet. Ohtani and coworkers have calculated formation energies and phase diagrams of Ni3AlC and Co3AlC using a first principle calculation combined with
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