Creep behaviour of a heat-resistant ferritic chromium steel in terms of stress exponents

Abstract

In many publications the high-temperature deformation behaviour of materials is described by the stress sensitivity of steady-state creep rate, the creep exponent, n. In order to investigate the mechanisms of dislocation motion, it is more promising to evaluate the constant structure creep properties. This leads to the constant structure creep exponent, m′, which is not influenced by the stress dependence of the substructure. Therefore, the investigation of deformation mechanisms is less difficult. Additionally, m′ is the basis for the calculation of the effective stress exponent, m, of dislocation velocity, which permits the investigation of the strength of interactions between alloying atoms and moving dislocations. It is shown that the creep exponent, n, is between 5 and 10 in the power-law creep region (where diffusion-controlled glide processes of dislocations cause deformation). However, it increases to about 50, if exponential creep is working (in this region the glide processes are thermally activated but diffusion is not the rate-controlling mechanism). The constant structure creep exponent, m′, is relatively small and independent of stress in the power-law creep region. It increases almost linearly with the applied stress, if thermally activated glide dominates creep. The evaluation of the stress exponent, m, which can be calculated from m′ and the effective stresses, showed that dislocation motion is influenced by alloying atoms as long as power-law creep works. There is experimental evidence that power-law breakdown is due to a breakdown of the alloying effect, because dislocations can escape from their dragging Cottrell clouds at high applied stresses.