Microcosmic mechanism of carbon influencing on NiTiNb9 alloy

Highlights: ► An obvious NbTiC carbonization "cluster" is detected in NiTi matrix phase. ► The Nb element can enhances the influence of the C element on NiTiNb shape memory alloy. ► The interaction between Nb and C can decrease mechanics performance of NiTiNb alloy. ► This method is helpful for alloying shape memory alloy. - Abstract: The microcosmic mechanism, by which the C impurity decreases the Ms and fracture toughness, remains mysterious at present. Using first-principles pseudo-potential plane wave method, the formation enthalpy ΔH, binding energy ΔE, electronic structure and diffusivity of the C element in NiTiNb9 shape memory alloy have been systematically calculated and analyzed in the thermodynamic and kinetic processes. The results show that the addition of C trends to compose carbonization "cluster" in NiTi matrix phase, which not only can decrease the Ms of alloy by enhancing the ratio Ni/Ti, but also it can slack down the ductility by its special "jujube nut " system. Otherwise, because of the unfilled s orbits, the Nb element can enhance the formation ability and diffusivity of carbonization "cluster" structure, and promote the impact of the C element to the shape memory effect and mechanics performance in NiTiNb alloy. Thus, our findings open an avenue for detailed and comprehensive studies of alloying shape memory alloy.