Invited Lecture
Molecular dynamics simulation of self-diffusion in BCC and HCP metals

In the first part of the talk, theoretical methods to predict the diffusivity in pure metals will be discussed. Such methods usually involve calculations of point-defect characteristics at T=0 and determination of the diffusivity from the transition state theory. This approach typically relies on the assumption that these point-defect characteristics are temperature-independent. One possible way to introduce temperature corrections is to use the quasi-harmonic (QH) approximation. However, the only way to test the accuracy of the QH approximation is to compare its predictions with more direct calculations. Classical molecular dynamics (MD) simulation offers the most direct approach since MD automatically captures the anharmonicity of atomic vibrations. The presented MD simulation results will show that the QH approximation can considerably underestimate variations of point-defect characteristics with temperature.
In the second part, the vacancy and interstitial mechanisms of self-diffusion in bcc and hcp metals will be compared. MD simulation demonstrates that that self-diffusion in bcc Fe is controlled by the vacancy mechanism at all temperatures because the equilibrium vacancy concentration is always much larger than the equilibrium interstitial concentration. However, the predominance of the vacancy concentration can be explained by the lower vacancy formation energy only at low temperatures. At high temperatures, the interstitial and vacancy formation energies are about the same but the vacancy formation entropy increases with temperature while the interstitial formation entropy decreases. Thus, it is the high vacancy formation entropy that is responsible for the larger vacancy concentration at high temperatures. On contrary, the MD simulation predicts that the self-diffusion proceeds via the interstitial mechanism in hcp Zr and both vacancy and interstitial mechanisms contribute to diffusivity in bcc Zr.