Deformation of UO2 at High Temperatures

The effects of temperature, strain rate, and grain size on the mechanical properties of UO₂ were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, T_c , the material fractured in a brittle manner, with no macroscopic plastic deformation. Between T_c and a second transition at a higher temperature, T_t , a small amount of plastic deformation was measured before fracture. Beyond T_t , the strength of UO₂ decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above T_t :
and
where σ_p and σ_88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.