Study of Ni/Al Interface Diffusion by Molecular Dynamics Simulation

Molecular dynamics simulation of Ni/Al interface diffusion is carried out by Embedded Atom Method (EAM) potential. The problem how the temperature affects Ni/Al interface diffusion is discussed. The initial dynamic behavior of Ni/Al interface diffusion at high temperature is shown. The study in this letter is helpful to understand the origin of diffusion phenomenon.

• Molecular Dynamics Simulation
• Interface Diffusion
• EAM Potential

1. Introduction

Diffusion is a method of substance transition. Many processes occurring in actual material proceed through it and is under the control of it. The research on the diffusion is significant. Ni based high temperature alloy is widely used in gas engine industry [1-3]. The Al₂O₃ film on the alloy surface is very steady at high temperature. For increasing the using temperature and enlarge its using range, the Alumina coat of Ni based high temperature alloy is developed. This kind of coat can satisfy generic performance requirement with lower cost and good stability. The importance of Ni based high temperature alloy and its alumina coat has made the study on this material become a hotspot in the material area. Previous studies mostly focus on the actual material experiments and thermodynamics characters. The investigation about the dynamics behavior deep to the atom scale is less. Its microcosmic mechanism on atom scale is not very clear. Especially, the study of the initial stages on the diffusion reaction is much less for the limitation of experiment conditions. In this paper, the dynamic behavior of Ni/Al interface diffusion which happens during the initial reaction stage is investigated carefully by molecular dynamics simulation.

2. Molecular Dynamics Simulation

The first process of molecular dynamics simulation is setting the initial condition. As shown in Figure 1(a), the initial model of the simulation is a computational box of 4a₀ × 4a₀ × 11a₀. The below box of 4a₀ × 4a₀ × 9a₀ stands for the base with fcc Ni atoms being placed in it. The fcc Al atoms are placed in the above box of 4a₀ × 4a₀ × 2a₀. The lattice constant is set by that of Al (It is 4.05Å). [001] face is the initial Ni/Al interface. Repeating boundary condition is used in the direction of [100] and [010]. In the direction of [001], free boundary condition is used to avoid appearing two interfaces in the model. Newton moving equation is solved by Verlet method [4]. Time step size is 2.8 × 10⁻¹⁵ s. We select the interact potential of Embedded Atom Method (EAM) [5,6].

To study the behavior of Ni/Al interface diffusion, we have designed the simulation method from two aspects: 1) The function of temperature on Ni/Al interface diffusion is studied. The system is run for 160000 time steps at 300 K, 400 K, 500 K, 600 K, 700 K and 800 K respectively. The coordinates of every atom corresponding to time are remembered in the process of simulation. 2) The Ni/Al interface diffusion at high temperature is studied. First, the system is run at constant 500 K for 20000 steps. For the initial lattice constant is set at 4.05Å. Ni atom lattice constant at the base is smaller than this value actually; the relaxation at lower temperature of 500 K can help Ni lattice adjusting to its actual size. Thus the precision of simulation can be improved. Then, the temperature of the system is changed to the high temperature of 1200 K. At constant 1200 K, the system is run for 480000 steps till the simulation stops. The information such as the atoms’ coordinates is recorded every certain time during the process of simulation.

3. Results and Discussions

3.1. The Function of the Temperature on the Ni/Al Interface Diffusion

The initial model along with the coordinate system is shown in Figure 1(a).

Figure 1(b) to Figure 1(g) are the pictures of atoms position after running 160000 time steps at 300 K, 400 K, 500 K, 600 K, 700 K and 800 K, respectively. From Figure 1(b) to Figure 1(g), we can observe that the vo-

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