The SEM analysis of decply etched specimens of Ni-base superalloys K38 and K19H+Ta in- dicates that both the primary morphologies under normal growth conditions and the final morphologies near equilibrium of MC carbides are octahedral blocks.The classification of MC crystal faces by means of PBC(Periodic Bond Chains)theory confirms that{100}, {110}and{111}are F faces,and that{212}are S faces.The calculations of the attachment energy for each F face show that{111}are most easily appeared.It has also been inferred that one of the MC growth units is a coordination octahedron composed of one metal atom sur- rounded by six carbon atoms.The octahedral growth units can be attached to the growth sur- faces of MC carbides through laver stacking mechanism on the{111}faces.
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