Mechanical and Structural Response of Radiation-Damaged Pyrochlore to Thermal Annealing

Abstract

Nanoindentation has been employed to probe the mechanical properties [indentation hardness ( H ) and elastic modulus ( E )] of radiation-damaged pyrochlore before and after step-wise thermal annealing up to 900 K. Three natural U and/or Th containing samples with increasing degree of disorder have been investigated (i.e., Panda Hill: 1.8 wt% ThO 2 , maximum life-time alpha-decay event dose ~1.6 × 10 18 -decay g − 1 ; Blue River: 11.9 wt% UO 2 , maximum life-time alpha-decay event dose ~115.4 × 10 18 -decay g − 1 ; and Miass: 7.2 wt% ThO 2 , maximum life-time alpha-decay event dose ~23.1 × 10 18 -decay g − 1 ). Complementary investigations by photoluminescence and Raman spectroscopy and in-situ annealing transmission electron microscopy (TEM) allowed us to follow the structural evolution. Therefore, a comprehensive understanding of the thermally induced structural reorganization process was obtained. Recrystallization has found to start above 500 K in the pyrochlores. Due to the increasing structural order a general hardening of the mechanical properties was observed. Miass pyrochlore (highest degree of structural damage of the investigated samples) reaches a polycrystalline state after annealing. While lesser damaged, but also highly disordered Blue River pyrochlore (containing small preserved crystalline domains) has found to transform into a single crystal. The recrystallization of both pyrochlore samples was followed by in-situ TEM at 800 and 750 K, respectively.