Point-defect properties and diffusion mechanism in intermetallic compounds of L1_2 type

本文研究了L1_2有序结构的A_3B金属间化合物、Ni_3Al、Ni_3Ga和Ni_3Ge中点缺陷的性质和扩散行为。1.用计算机模拟计算了Ni_3Al中空位和替位缺陷的形成能、迁移能以及表征缺陷性质的其它参量。用改进的Ni_3Al原子间势组，对Ni_3Ga进行了正电子湮没实验，测定了空位的形成能。3.在较宽的温度范围内测量了Ni_3Ga和Ni_3Ge的组元示踪扩散系数。Ni的扩散系数接近于纯Ni中的自扩散系数。Ga在Ni_3Ga中的扩散系数约为Ni的1/2，而Ge在Ni_3Ge中的扩散系数比Ni小两个数量级以上。化学扩散系数随微量元素（A_3 B中的B）浓度的增加而增加，这种依赖性在Ni_3Ge中最强。5.在有序合金热力学的成对相互作用模型的框架下，给出了化学扩散的热力学因子。通过对热力学活度实验数据的分析，求得了Ni_3Al的热力学因子，并由此估算出Al在Ni_3Al中的扩散系数约为Ni扩散系数的1/3。两种A原子A和B的扩散通过普通空位机制发生在原子的亚晶格中。该模型可以解释观测到的扩散系数的大小及其随组成的变化。

We have studied the properties of point defects and diffusion behavior in A_3B itermetallic compounds of L1_2 ordered structure, Ni_3Al, Ni_3Ga and Ni_3Ge.1.The formation energies and migration energies of vacncies and substitutional defects in Ni_3Al, as well as other quantities characterzing the defect properties, have been calculated by computer simulation, using an improved set of inter-atomic potntials for Ni_3Al.2.Positron annihilation experiments have been carried out to determine the formation energy of vacancies in Ni_3Ga. It is in the range between 1.7 and 1.8eV.3.The tracer-diffusion coefficients of the constituent elements have been measured for Ni_3Ga and Ni_3Ge over a wide temperature range. The diffusivity of Ni is close to the self-diffusivity in pure Ni. The diffusion coefficient of Ga in Ni_3Ga is about 1/2 times that of Ni, whereas the diffusion coefficient of Ge in Ni_3Ge is less more than two orders of magnitude smaller than that of Ni.4.Chemical diffusion coefficient has been measured for the three compounds by single-phase interdiffusion experiments. The chemical diffusion coefficient increases with increasing the concentration of the minor element (B in A_3B), the dependency being the strongest in Ni_3Ge.5.The thermodynamic factor in chemical diffusion has been formulated in the framework of the pairwise interaction model for the thermodynamics of ordered alloys. By analysing experimental data of thermodynamic activity, the thermodynamic factor for Ni_3Al has been evaluated, and using this value the diffusivity of Al in Ni_3Al has been estimated to be about 1/3 times the diffusivity of Ni.6.A model for the atomic diffusion processes in these materials has been proposed : the diffusion of both species of A atoms, A and B,occur in the sublattice of atoms via ordinary vacancy mechanism. The model can account for the oberved magnitudes of the diffusion coefficients and their variations with composition.