Atomistic Investigation of High Temperature Material Behavior

High temperature mechanical behavior of materials is of critical importance in a variety of contexts: next-generation reentry vehicles, hyper sonic flights, nuclear plants, engines, among many others. Creep is the dominant failure mechanism for materials used in such applications. Atomistic design of next-generation ultra-high-temperature ceramic composites as well as a thorough understanding of the various complex micro-mechanisms of creep damage using state-of-the art atomistic methods is the main goal of this dissertation. There are two challenges that need to be overcome to accomplish this endeavor. The first one is aptly amplified in a quote by Professor Nabarro (2002) “The creep rate in a land-based power station must be less than 10−11s ... The present state of knowledge reveals specific questions that call for experimental investigation. Theory will contribute, but atomic computation, with a time scale of 10−11s, will not handle processes that take 1011s”. The other is that for the materials of interest, the so-called ultrahigh-temperature ceramics (ZrB2 and HfB2), atomistic potentials are not available. No type of atomistic methodology (molecular dynamics, Monte Carlo) can proceed without this. Furthermore, since oxidation and various related chemical reactions play a key role in the damage of such materials at high temperatures, the atomistic potential must be able to account for reactions. First-principle calculations are indeed possible without an empirical force field but such computations present severe limitations of the size scales they can access and of course, the enormous difficulty of modeling finite temperatures. In short, in this dissertation, I will focus on two aspects that can potentially pave the way for modeling high temperature behavior of ceramics: development of ReaxFF potentials for ZrB2 / HfB2 using quantum chemistry tools and implementation of algorithms that allow access to time scales relevant to creep deformation and damage.Mechanical Engineering, Department o