High-Temperature Creep Mechanisms in α Iron and Other Metals

Abstract

The temperature and the stress dependence of tensile creep has been studied in ferromagnetic α iron (Ferrovac‐E 99.91% pure) in the temperature range 620°–700°C. By using the change‐of‐slope method, activation energies for creep were obtained. The modulus‐compensated activation energy is shown to be essentially independent of the initial stress and of strain up to the necking point. The change‐of‐slope method gave a modulus‐compensated activation energy of 65 kcal/mole while a plot of the logarithm of the modulus‐compensated strain rate versus reciprocal of the absolute temperature gave a straight line yielding a modulus‐compensated activation energy of 62 kcal/mole. These are in essential agreement with the value for self‐diffusion in this temperature range. The logarithm of the temperature‐compensated creep rate versus logarithm of stress gave a straight line with a slope of 6.6, i.e., a power‐law dependence of the strain rate on the stress. The fact that the activation energy for creep is the same as that for self‐diffusion indicates that the creep process is controlled by diffusion and hence by a nonconservative dislocation motion: either the climb of edge dislocations or the dragging of jogs on screw dislocations. It is also found that there is no noticeable drop in the activation energy with increased stress. This means that at the strain rates and temperatures employed here, the average vacancy concentration is but a small perturbation from the equilibrium vacancy concentration in the undeformed specimen. By utilizing the recent data of Ishida et al. which extends to considerably lower temperatures good estimates of vacancy supersaturations can be obtained from 150° to 700°C (0.23T_m–0.54T_m) at different creep rates. From the available creep data on some fcc metals it is shown to be possible to get a good estimate of energy for motion of vacancies. We obtain by this technique E_m^v=15 500 cal/mole for aluminum which is in essential agreement with the value obtained by other techniques and E_m^v=22 800 cal/mole for copper.