HIGH TEMPERATURE CREEP OF ALPHA IRON

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, H0 = 105.0 ± 1.9 kcal mol-1, is by a factor of about 1.75 higher than the activation enthalpy of lattice self-diffusion Hsd. In the range of 920°K-TC (TC = 1042 °K, the Curie temperature) the apparent activation energy depends both on stress and temperature, achieving the value of 220 kcal mol-1 at TC. Accepting the model of nonconservative motion of jogs on screw dislocations, the high value of H0 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 H0 = m Hsd. The temperature dependence of H0 in the range of 920 °K-TC is interpreted by the temperature dependence of Hsd in this temperature range. If the power function of σn 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 ρs, to the density of dislocations unbound in subboundaries ρ is independent on stress. © 1969.