Atomistic Computer Simulations of Zr-Nb-Fe Intermetallic Compounds

Abstract

The behavior and properties of zirconium-niobium based alloys are influenced by the presence of iron and chromium. The chemical elements lead to the formation of intermetallic Zr-Nb-Fe(-Cr) precipitates. This thesis undertakes a comprehensive analysis of the precipitates through theoretical modeling supported by empirical imaging experiments. Using atomistic computer simulation tools such as density functional theory (DFT), Metropolis Monte Carlo (MC) and simulated diffraction patterns, we show that this intermetallic compound should be referred to as Zr(Nb, Fe)2 if of equiatomic composition and referred to as Zr(Zr, Nb, Fe, Cr)2 if it has the composition Zr9Nb9Fe5Cr1, which closely approximates the composition of a subset of secondary phase particles found in Zr-2.5Nb. Further, we studied the Zr(Zr, Nb, Fe)2 secondary phase particles in greater detail. To do so, we used computational tools to calculate the electron and phonon contributions to their free energy. According to our calculations, the secondary phase particles are C14 Laves phase at temperatures greater than 600K and undergo a thermally activated displacive phase transition to C15 at lower temperatures. These calculations are consistent with the experimentally observed mixed crystallographic character of the precipitates. We used finite element analysis (FEA) coupled with high-resolution transmission electron microscopy to demonstrate that the secondary phase particles form an incoherent interface with their surrounding Zr matrix. Finally, we developed moment tensor potentials (MTPs) to study anharmonic effects on the free energy of the aforementioned secondary phase particles. We perform thermodynamic integration and upsampled its accuracy to the DFT level. Thermodynamic integration points to a 900K C14-C15 phase transition temperature, as compared to 600K in the harmonic approximation, as mentioned above.