Atomistic mechanisms of morphological evolution and segregation in U-Zr alloys

Abstract

In many metallic alloys, complex microstructures form as a consequence of local atomic ordering that depends on the processing path. This research uses an atomistic approach to study microstructural morphology and evolution in order to investigate how temperature and alloy concentration affect ordering. A semi-empirical Modified Embedded Atom Method (MEAM) is used in conjunction with molecular dynamics (MD) and Monte Carlo (MC) simulations to investigate the properties and equilibrium configurations of the high temperature body-centered-cubic (bcc) uranium-zirconium (U-Zr) alloys. Atomic simulations conducted with the MEAM potential show the thermodynamic driving force to the lamellar structure for the melt-casted U-rich alloys and the finely acicular microstructure of the water quenched U-rich alloys. In addition, when the U-rich U-Zr alloy is equilibrated at a lower temperature, the lamellar/acicular microstructures begin to spheroidize, as observed in experiments. In the intermediate Zr concentration region, the ordering seen is able to facilitate the structure to the partially ordered δ-UZr 2 phase seen experimentally. Lastly, the Zr-rich region is able to successfully show the thermodynamic driving force to the acicular, Widmanstätten, and martensitic needles type microstructures observed experimentally. These simulations are able to successfully replicate some of the fundamental thermo-physical and microstructural characteristics following fabrication and irradiation of the U-Zr metallic fuels.

FCRD Advanced Reactor (Transmutation) Fuels Handbook

“…Recent research into phase-transition mechanisms has emphasized the importance of metastable phases, and may explain many of the variations in experimental data. Models of U-Zr phases, phase segregation mechanisms and microstructure development, and the U-Zr phase diagram have been developed using a variety of approaches (e.g., [5,55,[57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75]).…”

Point Defects Stability, Hydrogen Diffusion, Electronic Structure, and Mechanical Properties of Defected Equiatomic γ(U,Zr) from First-Principles

“…The thermal conductivity of γ(U,Zr) and other BCC binary uranium alloys was investigated via a combined ab-initio and empirical model [ 13 ]. A modified embedded-atom method potential was developed to study the structure, thermodynamics as well as the morphological evolution and segregation of U-Zr alloys [ 14 , 15 ]. Recently, the effect of temperature and Zr content on bulk modulus and heat capacity of γ(U,Zr) were calculated by Aly et al [ 16 ] using ab initio molecular dynamics.…”

Molecular dynamics study of the point defects in bcc uranium

“…Body-centered cubic (bcc) uranium (known also as γ uranium) is dynamically stabilized at high temperatures but unstable below 1050 K [21][22][23][24]. Because of this, there are not many DFT studies of properties of bcc uranium with atomic defects.…”