Mechanical and Thermal Properties for Uranium and U–6Nb Alloy from First-Principles Theory

Abstract

Elasticity, lattice dynamics, and thermal expansion for uranium and U–6Nb alloy (elastic moduli) are calculated from density functional theory that is extended to include orbital polarization (DFT+OP). Introducing 12.5 at.% of niobium, substitutionally, in uranium softens all the cii elastic moduli, resulting in a significantly softer shear modulus (G). Combined with a nearly invariant bulk modulus (B), the quotient B/G increases dramatically for U–6Nb, suggesting a more ductile material. Lattice dynamics from a harmonic model coupled with a DFT+OP electronic structure is applied for α uranium, and the obtained phonon density of states compares well with inelastic neutron-scattering measurements. The Debye temperature associated with the lattice dynamics falls within the range of experimentally observed Debye temperatures and it also validates our quasi-harmonic (QH) phonon model. The QH Debye–Grüneisen phonon method is combined with a DFT+OP electronic structure and used to explore the anisotropic thermal expansion in α uranium. The anomalous negative thermal expansion (contraction) of the b lattice parameter of the α-phase orthorhombic cell is relatively well reproduced from a free-energy model consisting of QH-phonon and DFT+OP electronic structure contributions.

High-Temperature Thermodynamics of Uranium from Ab Initio Modeling

“…The stability of uranium and its alloys has been explored using different methods founded on density-functional theory (DFT) . These methods include the all-electron fullpotential linear muffin-tin orbitals (FPLMTO) method [9][10][11][12][13][14][15][16]27,53,60,64,65] and the planewave-based projector augmented wave (PAW) method that utilizes the pseudopotential approximation [17][18][19][20][21][22][23][24][25][26]32,37,39,45,46,52,55,56,58,61,62,66]. The temperature effects on uranium or actinide systems have been studied by means of the self-consistent ab initio lattice dynamics (SCAILD) method [13,53,60,64,65], and classical molecular dynamics with either the embedded atom method (EAM) or with the modified embedded atom method (MEAM) [28][29][30][31][33]…”

“…As was mentioned in Refs. [36,53,60,64,65], anharmonic effects and conduction electrons play significant roles determining the heat capacity of uranium metal and its alloys and compounds. Ren et al [39], using the quasi-harmonic Debye-Grüneisen model, calculated the constant-pressure heat capacity (C p ) of α-U in the temperature range up to 900 K. At low temperatures, up to 300 K, the calculated heat capacity has a reasonable agreement with the experimental data of Flotow and Lohr [67] and Jones et al [68].…”