The deformation-induced martensite and dynamic strain aging during cyclic deformation in AISI 321

To investigate the deformation-induced martensite and dynamic strain aging (DSA) during cyclic deformation in AISI 321, fatigue tests were carried out at strain amplitude 0.5% and 0.9% at room temperature (RT), 550 degrees C and 650 degrees C. Optical microscope and scanning electron microscope were used to observe the crystal grains and fracture surface microstructure respectively; x-ray diffraction was used to detect the phase transformation. Cyclic stress response curves exhibited different combinations of cyclic hardening, saturation and cyclic softening. Deformation-induced martensite was detected by x-ray diffraction and the amount increased with strain amplitude at RT, which is responsible for the rapid secondary hardening. DSA's manifestation only showed up at 650 degrees C and enhanced at higher strain amplitude, resulting in higher hardening at 650 degrees C than 550 degrees C. Fatigue life reduction at RT is due to rapid crack propagation caused by deformation-induced martensite hardening. The DSA hardening leads to a reduction of fatigue life at elevated temperatures by ways of multiple crack initiation and rapid crack propagation.