Harper-Dorn creep of zirconium alpha

Harper-Dorn (H-D) creep in zirconium-alpha has been investigated at homologous temperatOres 0.35 to 0.48 (773 to 1023 K) and stresses ranging from 4x10-6 to 9x10-5 G (G is the shear modulus) by the helicoid specimen technique. It has been shown that H-D creep takes place at intercept grain sizes larger than about 125 μm, while at smaller grain sizes Coble creep operates under the same external conditions. The H-D creep is most probably dislocation core diffusion controlled. A threshold stress for steady state creep has been detected increasing with decreasing temperature. The existence of threshold stress has been qualitatively accounted for by high dislocation density (approx. 1012 m-2) in the specimens tested. The nonconservative motion of jogs on screw dislocations dependent on dislocation core diffusion has been suggested as the creep rate controlling mechanism. The energy of jog formation has been estimated to wh ion has been estimated to which a mean distance between jogs approx. 5b corresponds at 1000 K, where b is the Burgers vector. Also the observed transient creep has been briefly discussed. The transient component of the creep strain cannot be accounted for exclusively by inelastic bowing out of links of dislocation network. (author)