On the origin of crack initiation in duplex stainless steel during cyclic loading in the VHCF regime

Formation of micro-cracks of a duplex stainless steel (DSS) in the very high cycle fatigue (VHCF) regime was studied and discussed. Several high-resolution techniques were applied to fatigued specimens, identifying the mechanical properties of the DSS and relating them to the mechanisms of the micro-crack initiation. Results acquired from optical microscopy, nanoindentation tests and synchrotron radiation investigations revealed that the austenite phase of the DSS in VHCF exhibits cyclic softening-hardening-softening behaviour. The first cyclic softening takes place during the initial millions of load cycles, where a continuous growth of slip markings and a decrease in micro-hardness were observed. Afterwards, austenite hardens in the tested austenite grains which show a stop in the growth of slip markings and an increase in the micro-hardness. Synchrotron radiation experiments performed at this stage revealed that the internal residual stresses between the two phases increase compared to the initial state. After this period, slip markings in austenite continue to grow and the micro-hardness decreases, indicating the reoccurrence of cyclic softening. The synchrotron radiation experiments executed at this stage showed a decrease in internal residual stresses. The cyclic hardening and softening behaviour of the austenite can be attributed to dislocation motion and rearrangement, and was supported by transmission electron microscopy (TEM) micrographs. Phenomenological observations applied to the specimens showed two types of micro-cracks: intergranular micro-cracks form continuously from an early to a relatively late stage, and transgranular micro-cracks that initiate suddenly. The formation of these cracks is interpreted by the resultant shear stress τ at the phase boundary, which consists of 1) shear stress τ 1 due to the accumulation of intrusions and extrusions (slip of dislocations), 2) shear stress τ 2 due to dislocation pile-ups and 3) shear stress τ 3 due to external load stress. The three components evolve during cyclic hardening and cyclic softening processes, yielding different resultant shear stresses and resulting in the formation of dis-tinct plastic deformation in the ferrite grains. The shear stress τ 1 increases dramatically within the initial 10 5 cycles, which can be expected from the observation of extrusion-intrusion growth. The shear stress τ 3 in ferrite decreases when austenite cyclically hardens, and increases when austenite cyclically softens. These were observed in the results obtained for stress and strain partitioning experiments applying in-situ X-ray diffraction (XRD) and in-situ digital image correlation (DIC) techniques. Importantly, the shear stress τ 2 which is largely dependent on phase boundary orientations determines the morphology of the micro-cracks. On the occasion of an austenitic grain forming slip markings, the formation of micro-cracks in the adjacent ferritic grain will depend on the following conditions. 1) If the angle between the phase boundary and the ferritic surface is less than 90°, plastic deformation easily occurs and intergranular micro-cracks gradually form at the phase boundary. 2) If the angle between the phase boundary and the ferritic surface is greater than or equal to 90°, slip markings are frequently arrested by the phase boundary and no plastic deformation takes place. When the resultant shear stress is large enough to overcome the barrier effect, transgranular micro-cracks are mostly present. These hypotheses were confirmed by scanning electron microscopy (SEM) images acquired with focused ion beam (FIB)-cutting.