Deformation of UO ₂ at High Temperatures

Abstract

The effects of temperature, strain rate, and grain size on the mechanical properties of UO 2 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 UO 2 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 : image and image where σ p and σ 88f are the proportional limit and steady‐state flow stress, respectively, and temperature T is in °K.