Simulating di usion at low temperature in metallic glasses : the activation-relaxation technique

Understanding the nature of di usion in complex and compact systems, such as alloyed metals, remains a serious challenge in contemporary materials science. Since these processes take place in the bulk, direct observation, using any of the scanning probes, is ruled out and most experimental probes have to be indirect. On the simulation side, progress has been hampered by the long time scale typical of di usion, a scale often beyond the reach of standard methods such as molecular dynamics. In this talk, I will present the activation relaxation technique (ART), a method speci cally designed for simulating activation mechanisms , and a recent application of it to binary Lennard-Jones glasses. Since ART is not sensitive to the height of the activation barriers, it is possible to simulate di usion over similar distance above and below the glass transition temperature. Based on these simulations, a clear distinction emerges in the nature of di usion in these two regimes. In agreement with recent experiments, the simulation show that events become much less collected below the glass transition, involving typically one or two atoms, compared with 10 or more above melting. This and other recent results will be discussed.