It is difficult to produce the Co-based superalloys consisting of the so-called g/g‘ microstructure because of the absence of a stable strengthening L12 phase in the Col-Al binary system. However, an addition of C to this alloy stabilizes the Perovskite type carbide (E21) with the composition of M3AlC. The carbide has a carbon atom at the body center site in the L12 structure, and its crystallographic similarity makes us expect the formation of a fine microstructure in the Co-based solid solution. Thus in the present study, the Co-Al-C ternary phase diagram has been constructed by combining ab initio energetic calculations with the CALPHAD approach, and the result was compared with the Ni-Al-C ternary phase diagram obtained by the same procedure.
In a thermodynamic analysis, special care was taken to the expression of free energy for the L12 and E21 structures. To treat these two structures as a continuous solution, the free energy was expressed by the (M,Al)3(Al,M)1(C,Va)1 type of sublattice model. For lack of experimental information, the thermodynamic properties concerning the E21 structure were evaluated by the Full Potential Linearized Augmented Plane Wave (FLAPW) method.
The calculated results show that the E21 phase in the Co-Al-C system is in equilibrium with the fcc Co, B2 type of intermetallic compound and the graphite phase. This finding is in good agreement with the previous experimental results [1]. On the other hand, the E21 phase doesn’t exist in the Ni-Al-C ternary system. In this contribution, we will discuss the phase equilibria in the Co-Ni-Al-C quaternary system.
[1] Y. Kimura, M. Takahashi, S. Miura, T. Suzuki and Y. Mishima: Intermetallics, 3(1995) 413-425.