B2-和B19'-NiTi表面原子弛豫、表面能、电子结构及性能的理论研究

采用基于密度泛函理论的第一性原理系统研究了B2-和B19'-NiTi合金所有低指数表面的表面能、表面结构稳定性、表面电子结构等性质.计算结果表明两种NiTi合金所有低指数表面的原子弛豫主要集中在表面2-3个原子层,且以Ti原子为终止原子表面构型的原子振荡最为剧烈,Ni和Ti原子共同终止表面构型的原子振荡最小;价电荷密度沿着表面构型向真空层方向快速衰减;表面能计算结果显示其与配位数成反相关.B2-和B19'-NiTi合金的非密排且非化学计量比表面的表面能取决于Ti的化学势,表面能数值较高;而密排面的表面构型符合化学计量比,其表面能较低,表现出卓越的化学稳定性;其中以B2-NiTi(101)密排面的表面稳定性最优.

Theoretical study of atomic relaxation,surface energy,electronic structure and properties of B2- and B19'-NiTi surfaces

NiTi shape memory alloy has been widely used in industrial and biological fields due to its excellent mechanical properties, unique shape memory effect and superelasticity. In this paper, the atomic relaxation, thermodynamic energy, structural stability, electronic structures and other properties of all low-index surfaces of B2- and B19'-NiTi alloys are systematically studied by using the first principles calculations based on density functional theory. The calculated results show that the atomic relaxations on all low-index surfaces of both B2- and B19'-NiTi alloys are mainly concentrated in 2-3 atomic layers on the surface, which means that the surface effect is mainly confined in two or three layers on the surface configuration. In addition, the atomic relaxation of Ti-terminated surface is most remarkable, and followed by Ni-terminated surface, while the atomic relaxation of Ni&Ti-terminated surface is insignificant. Furthermore, the valence charge density decays rapidly from the surface configuration to the vacuum layer. The calculation results of surface energy show that surface energy is inversely related to the coordinate number, and surface stability increases with the coordination number increasing. For B2- and B19'-NiTi, the surface energy of non-dense and non-stoichiometric surface depend on the chemical potential of Ti, and the surface energy is high. Therefore, the stabilities of these surfaces change with the chemical potential of Ti increasing. However, the surface energy values of dense surface configurations with stoichiometric ratio for B2-NiTi (101) and B19'-NiTi (010) are 1.81 J/m² and 1.93 J/m², respectively, which are both lower than those for other non-dense surfaces in the most Ti chemical potentials range, showing excellent structural stability. Moreover, the electron density analysis indicates that the dominant bonding for B2-NiTi (101) surface is the chained Ni-Ti-Ni metallic bond, the distribution of electrons and the distance between Ni and Ti atoms on the B2-NiTi (101) surface are more uniform and smaller, respectively, than those for B19'-NiTi (010) surface. In summary, the B2-NiTi (101) surface shows the high stability.