Internal and effective stresses in high-temperature creep evaluated from transient dip tests and dislocation bowing

Because creep of metals and alloys is modelled on the basis of microstructural observations, it has been shown that there is a difference between the mathematical treatment of high-temperature deformation and the real material behaviour. One idea to consider is to split the applied stress into a part depending on the substructure (the internal stress which has to be reached to start dislocation motion) and a part describing the resistance to the glide motion of dislocations (the effective stress). For ferritic chromium steel these quantities have been measured by means of the stress transient dip test technique. This leads to mean values of internal and effective stresses for the whole specimen. Additionally, local stresses acting on individual dislocations are evaluated from dislocation bowing for a wide range of applied stresses. The results show that the ratio of internal to applied stress decreases with increasing applied stresses, which, on the other hand, causes a large increase of effective stresses. Dislocation bowing stresses show a similar dependence. Compared to the results of dip tests, the determination of local stresses leads to less accurate results and to a large deviation of results within small regions of one specimen. Therefore, it is only valuable for comparison purposes.