Primary creep of UO2 and the effect of amorphous grain boundary phases

Creep experiments were conducted on nearly stoichiometric UO₂ helical springs from 1000 to 1600°C and 2.1 to 80 MPa. Entirely transient behaviour was measured in all experiments with the plastic strain,ε = (Aσ/d^1.5) exp(?Q/RT)t^m, where A is a constant that depends on purity, d is the grain size, σ is the applied stress, Q is the apparent activation energy, t is the time, m is a constant, and the other terms have their usual meaning. At T > 1200°C, Q 100 kcal mol^?1, but at T < 1200°C, Q increased dramatically and became strain dependent. The value of m for most experiments was 0.8, but at σ > 48 MPa, m decreased, and for d < 10 μm, it increased. Amorphous or glassy grain boundary phases were observed by transmission electron microscopy in all specimens: specimens containing the largest concentrations of Fe and Si sometimes had anomalously high creep rates. The phases existed as discontinuous, lenticular bodies on grain faces and a continuous network along triple grain junctions. Some instances of precipitation of UO₂ from the phase were observed. At T > 1200°C, glassy phases may accelerate Coble creep by providing short circuit diffusion paths along the grain boundaries or may accelerate superplastic deformation by diffusion along the continuous glassy phase triple line junctions. At low temperatures the glassy phase appears to control grain-boundary sliding.