Stress dependence of steady-state creep rate in zirconium alloys at low stress

In zirconium alloys for applications in nuclear reactor technology the applied stress sensitivity of steady-state creep rate characterized by the parameter m'=[delta ln(d epsilonsub(s)/dt)/delta ln sigma]sub(T) (d epsilonsub(s)/dt is steady-state creep rate, sigma applied stress and T absolute temperature) decreases with decreasing applied stress. The value of m' approaches 1 at steady-state creep rates 10-11 to 10-12 s-1. This strongly suggests that Nabarro-Herring or Coble creep mechanisms operate at the above creep rates. Some alloying elements as niobium and molybdenum improve creep strength of zirconium alloys at stresses at m' approximately equal to 1, and m'>>1. This cannot be explained by simply applying the Nabarro-Herring or the Coble creep models in their original form. The said models must be modified with regard to the fact that emission and absorption of vacancies at grain boundaries are a rate controlling process, due to the restriction of grain boundary dislocation motion caused by secondary phase particles situated in grain boundaries. According to models modified along this line by Ashby, creep rate linearly depends on stress and decreases with decreasing mean interparticle spacing. This is in qualitative agreement with the experiment. Another factor that may influence the creep strength at m' approximately equal to 1 stresses is the mean effective grain diameter. The possible presence of secondary phase particles affecting the effective grain diameter is also considered. (author)