High temperature creep of alpha iron

Abstract

The creep of decarburized ARMCO iron has been, studied in the temperature range of 820–1170 °K by the isothermal tests technique. In the range of 820–920°K the apparent activation energy of creep depends linearly on stress; the value corresponding to the stress σ= 0, H 0 = 105.0 ± 1.9 kcal mol −1, is by a factor of about 1.75 higher than the activation enthalpy of lattice self-diffusion H sd . In the range of 920° K- T C ( T C = 1042 ° K, the Curie temperature) the apparent activation energy depends both on stress and temperature, achieving the value of 220 kcal mol −1 at T C . Accepting the model of nonconservative motion of jogs on screw dislocations, the high value of H 0 is interpreted on the basis of a suggestion that the nonconservative jump of a jog on a screw dislocation by one interatomic distance is connected with co-operative absorption or emission of an effective number of m vacancies, so that H 0 = m H sd . The temperature dependence of H 0 in the range of 920 ° K- T C is interpreted by the temperature dependence of H sd in this temperature range. If the power function of σ n type is chosen to describe the experimentally determined stress dependence of the minimum creep rate, n depends both on stress and temperature; at a given temperature it decreases initially with the increasing stress, achieves a minimum and then increases again. The experimentally determined dependence n = n( σ, T) is correlated with the dependence following from the accepted model of nonconservative motion of jogs on screw dislocations under the assumption that the ratio of the density of moving screw dislocations ρ s , to the density of dislocations unbound in subboundaries ρ is independent on stress.