The low-stress and high-temperature creep in LiF single crystals: An explanation for the So-called Harper-Dorn creep

Abstract

The theory of creep behavior in single and poly-crystalline materials at very low-stresses and at very high temperatures as originally proposed by Harper and Dorn has been under constant debate since its proposition. Accordingly, the principal objectives of this work are to study the creep behavior of high purity single crystals of LiF in the Harper-Dorn regime, which has never been studied. It is observed that stress exponent for LiF at 0.95 Tm changes from ~3.5 (“five”-power law) to ~1.5 at stresses below 10−5 G, which is typical transition stress for the Harper-Dorn creep in other materials. The evolution of dislocation density at very high temperatures (~0.92 Tm) under static annealing conditions reveals a saturation of the dislocation density after ~150 h up to 10,000 h. Furthermore, the dislocation density in the Harper-Dorn regime remains independent of the applied stress, with a value only slightly higher than the saturation dislocation density measured under static annealing conditions. Thus, the typical constant dislocation density observed in the Harper-Dorn regime may be a consequence of “frustration” of the Frank network in LiF, just as concluded in our Al work at very low stresses. If the initial dislocation density is lower than the frustration value, then the “five”-power law creep is expected. A unified dislocation-network based model is developed to interpret the creep response of LiF in the Harper-Dorn and “five”-power law regimes.