Effects of Nb doping and crystal symmetry on the structural stability and mechanical properties at U-Nb twin boundaries

Uranium is commonly alloyed with transition metals such as Nb, Zr, and Mo to improve mechanical properties and corrosion resistance. Among them, U-Nb alloys possess a well-rounded combination of mechanical properties. The unique properties of U-Nb alloys are closely related to their metastable monoclinic phase structure and twin boundary (TB) mediated deformation mechanism. Recent low-temperature U-Nb aging experiments have revealed that significant changes in strength and ductility are induced by the perturbation of solute atoms and TB microstructure at extremely fine scales (<3 nm). These phenomena pose challenges for conventional experimental characterization methods, limiting a fundamental understanding of the correlation between atomic-scale microstructural evolution and the degradation of mechanical behavior in U-Nb alloys. This study employed first-principles density functional theory (DFT) calculations to systematically investigate the effects of Nb doping and crystal symmetry on structural stability and mechanical properties of U-Nb {130} TBs, including surface energy, twin boundary energy, segregation energy, strengthening/embrittling energy, maximum tensile strength, and elongation. The results show that the monoclinic alpha"-U {130} twin structures exhibit enhanced structural stability along with a higher propensity of twin formation compared with the orthorhombic alpha-U {130} twin structures. Furthermore, the alpha"-U {130} twin structures demonstrate a stronger tendency for atomic segregation that promotes TB strengthening, resulting in superior tensile strength and ductility. This study provides atomic-scale theoretical evidence to elucidate the intrinsic mechanism underlying twinning-modulated mechanical behavior in U-Nb alloys.