4 - Harper–Dorn Creep

Abstract

A new low-stress mechanism for creep at high temperatures and low stresses in was originally proposed in a 1957 study by Harper and Dorn, perhaps partly based on. This mechanism has since been termed “Harper–Dorn Creep.” Harper–Dorn Creep has been reported to occur in a large number of metals and alloys as well as a variety of ceramics and ice. Interests in ceramics/minerals lies in predicting low-stress creep plasticity in geological systems such as the lower crust, lower mantle and inner core of the Earth. The fact that the activation energy for Harper–Dorn Creep is about equal to that of self diffusion suggests that Harper–Dorn Creep occurs by climb of edge dislocations. Creep is not observed at constant temperatures, but only with low amplitude temperature fluctuations, where the vacancy concentration would not be in thermal equilibrium, thus leading to climb stresses on edge dislocations of the order 3–6 MPa. This explanation does not appear widely accepted, partly due to the observation that H–D creep is consistently observed by a wide assortment of investigators, presumably with different temperature control abilities. The early low-stress experiments, primarily in metals, indicated that Harper–Dorn includes: activation energy about equal to lattice self-diffusion and grain-size independence, with grain-boundary shearing.