Thermal stability of nanostructured uranium within a surface layer processed using shot peening

A nanostructured α‑uranium surface layer with an average subgrain/grain size of 140 nm was obtained by using shot peening treatment (SPT). While, the nanostructures might be unstable in thermal condition due to dense lattice defects, high density of dislocations and ultrafine grains induced by severe plastic deformation. In this work, the thermal stability of the nanostructured surface layer was investigated using quasi-in situ electron backscatter diffraction (EBSD) after annealing treatments at temperatures ranging from 400 °C to 500 °C. Results show that the nanostructured surface layer has reasonably good thermal stability below 400 °C. Recrystallization and subsequent growth occurred when the temperature increased to 450 °C. The recrystallization nuclei are rooted in the initial nanostructured sites in the top surface layer and from dense dislocation walls in the subsurface consisting of high density of twin boundaries. At 450 °C, the exponent of recrystallization subsequent growth is determined to be 2.86, and the apparent activation energy value is 112.24 kJ/mol. Recrystallization occurs predominantly in a continuous manner. Furthermore, the evolution of dislocation density and grain size accounts for the decaying of surface mechanical properties.