Activated slip systems and microcrack path in LCF of a duplex stainless steel

A low-cycle fatigue test was carried out at room temperature on a forged duplex stainless steel type 25-07 alloyed with 0.17% nitrogen. From EBSD measurements performed before the test and in-situ microscopic observations performed during the test, activated slip systems were identified in austenite and ferrite, respectively. Crystallographic Kurjumov–Sachs (K–S) relationships were rarely observed between the austenitic and ferritic phases and thus the flow of plasticity was nearly inhibited between them. Although the cyclic plastic activity begins firstly in the austenite and then continues to the ferrite, a strong relief was developed rapidly in the ferrite. As a consequence, microcracks initiate preferentially at α/α grain boundaries and then propagate along slip markings formed successively in the austenitic and ferritic grains. Microcracks seldom nucleate at phase boundaries because they lay mainly perpendicular to loading axis.