A first-principles study of structure, elasticity and thermal decomposition of Ti_1-xTM_xN alloys (TM = Y, Zr, Nb, Hf, and Ta)

A systematic investigation concerning the effects of transition metals (TM=Y, Zr, Nb, Hf, and Ta) on the structure, elasticity and thermal decomposition of the TiN-based nitride coatings with a cubic rock-salt structure has been performed in terms of first-principles calculations. Calculated lattice parameters of Ti_1-xTM_xN as a function of alloying concentration show positive derivations from the linearized Vegard's law, agreeing well with experimental and theoretical results. Positive enthalpies of mixing of Ti_1-xTM_xN (TM=Y, Zr, and Hf) indicate that the formation of these alloys is energetically unfavored with respect to the mixing of the cubic phases. The predicted consolute temperature of Ti_1-xZr_xN agrees reasonably well with previous theoretical findings. The miscibility gaps disappear in the case of alloying TiN with NbN and TaN. Predicted elastic stiffness constants C₁₁, C₁₂, and C₄₄ together with the aggregate polycrystalline properties of Ti_1-xTM_xN are determined by an efficient strain-stress method. The present results indicate that the above nitride alloys are mechanically stable and addition of Nb and Ta increases the ductility, with Ta possessing the largest effect.