Atomistic simulations of structural rearrangements at solid-solid interfaces
A common problem in high-performance materials such as Ni-base superalloys is the formation of topologically close-packed (TCP) phases, which deprive the bulk matrix of the alloying refractory metals and cause the material to become brittle. It is therefore of utmost importance to understand the atomistic processes at work in solid-solid phase transitions leading to the formation of TCP phases. To this end, we are currently investigating interfaces between the TCP A15 and the cubic BCC phases in the refractory metal molybdenum.
We use molecular dynamics (MD) and adaptive kinetic Monte Carlo (aKMC) simulations to discover atomistic processes at the interfaces and to model the time evolution of the systems in question. MD simulations are simpler and can relatively quickly produce results at high temperatures (600 K and above) where the system evolves at a nanosecond time scale or faster; however, to see changes at room temperature, the time scale must be extended beyond the reach of MD. Here, aKMC shows its usefulness as a method with a speed unaffected by the choice of simulation temperature.
aKMC is still a fairly novel method with few results to validate it. We are working in close cooperation with the group of Graeme Henkelman at the University of Texas (Austin) to test and improve the software, in an effort to streamline it for the purpose of more general applicability. As the first fruits of this labor, a comparison with MD at a high temperatu1re shows that the two methods produce the same result. This paves the way for an array of reliable simulations that is not limited by time scale constraints caused by temperature.