Dependence of dynamic strain ageing on strain amplitudes during the low-cycle fatigue of TP347H austenitic stainless steel at 550 degree C

Low-cycle fatigue (LCF) tests are carried out on TP347H stainless steel at a strain rate of 8x10(-3) s(-1) with total strain amplitudes ( Delta ( epsilon (t))/2) of plus or minus 0.4% and plus or minus 1.0%, at room temperature (RT) and 550 degree C. It is found that the stress responses and dislocation structures under cyclic loading strongly depend on the value of strain amplitude at 550 degree C. Compared with those at the same strain amplitude at RT, the material shows a rapid strain softening, and finally attains a stabilized state at Delta ( epsilon (t))/2= plus or minus 0.4% and 550 degree C, but the one presents an anomalous behavior, i.e., first a rapid hardening to the maximum stress, followed by a reducing softening at Delta ( epsilon (t))/2 = plus or minus 1.0% and 550 degree C. More cells resulting from dislocation cross-slip and planar structures due to dynamic strain ageing (DSA) restricting cross-slip develop at low strain amplitude of plus or minus 0.4% at the first cycle. However, there are more complicated dislocation structures, such as cells, elongated cells, walls/channels and planar structures at Delta ( epsilon (t))/2= plus or minus 1.0%. The observations of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) exclude the effects of martensitic transformation, creep, oxidation, and precipitations on these stress responses and microstructure evolutions, which result from DSA appearing at 550 degree C.