Investigation of the Stability of C15 Laves Phase Structures in BCC Iron by Molecular Dynamics Method

Abstract

A new class of interstitial clusters which present a structure of C15 Laves phase has been theoretically predicted in bcc iron. However, these C15 clusters have never been identified experimentally. In this thesis, molecular dynamic (MD) simulations were explored to investigate the stability of perfect C15 Laves phase structures made of iron atoms and with or without point defects, as well as the stability of nano-sized C15 clusters embedded in bcc iron with point defects nearby. In this work, in first-ever simulations, two empirical Fe-Fe potentials were tested and the Ackland-2004 potential was demonstrated to be suitable. Periodic boundary conditions (PBCs) were used along three axes. For the perfect C15 Laves phase structures, calculations were performed on the total potential energy, temperature dependence of elastic constants and elastic moduli, and process of phase transformation at 300K. For interactions between perfect C15 Laves phase structures and their internal point defects, the formation energies of vacancies and SIAs, phase transformation temperature, and the process of phase transformation at 300K were simulated. Radial distribution function (RDF) calculations were used to check stability and structural changes. For interactions between nano-sized C15 clusters and nearby external point defects, the process of phase transformation at 300K was also simulated. The simulation results suggest that pure C15 Laves phase structure can remain stable up to about 425K. The presence of internal point defects will cause a decrease of phase transformation temperatures to below 260K. With increasing temperatures, perfect C15 Laves phase structures that contain internal point defects will transform into imperfect bcc structures. At 300K, the presence of nearby external vacancies will induce a recombination of the same number of SIAs from nano-sized C15 clusters, therefore possibly eliminating the C15 clusters; nearby external SIAs will induce the nano-sized C15 clusters to rearrange into 1/2<111> loops. As a conclusion, perfect C15 Laves phase structures with internal point defects are likely unstable at the temperatures higher than 260K, thus they may not remain to be observed at room temperature during irradiation.