Evaluation of Low stress High temperature Creep Insights into the Harper dorn Creep

Abstract

The theory of creep behavior of single crystals and polycrystalline materials having large grains at very low-stresses and at very high temperatures, as originally proposed by Harper and Dorn in 1957, has been under constant debate since its proposition. The major dispute over the initial observations of Harper and Dorn generally concern the value of the reported creep exponent: While Harper and Dorn reported it to be 1, later many observed it to range from 1 to 3. The variation of the dislocation density with the applied creep stress was later reported to be independent of the applied stress by a few investigators, while a few reported an increase in the dislocation density with stress in the so-called Harper-Dorn creep regime. Since its advent, the mechanism of creep in the Harper-Dorn creep regime has been studied using numerous metals and ceramics; however, often inconsistent results have been reported. Hence, a critical examination of the reported creep stress exponents and the dislocation density dependence on the applied stress is imperative to resolve the creep in the Harper-Dorn creep regime, which is perhaps one of the most debated and unresolved issues in the general area of the mechanical behavior of materials. Accordingly, the principal objectives of this work are to study the creep behavior of high purity single crystals in the Harper-Dorn creep regime. Primarily, creep tests are extensively conducted using high purity (100)-oriented LiF single crystals, which has never been examined in the Harper-Dorn creep regime. However, the creep response of LiF in the five -power law regime is comprehensively reported, which proffers an easy comparison of current study with the literature. In addition, iso-thermal annealing of LiF crystals is conducted up to 10,000 h at a homologous temperature of 0.92 (i.e., 0.92 Tm, where Tm is the melting temperature) to observe the evolution of dislocation density. The dislocations were observed using etch-pit method and the evolution of dislocation density was recor...