Dislocation microstructure evolution of Dss in biaxial cyclic loadings, experiment and modeling

P. EVRARD,; I. ALVAREZ-ARMAS; S. HEREÑÚ; V. AUBIN; S. DEGALLAIX

Massachussets USA

Conferencia; International Conference on Fatigue Damage of Structural Materials VII; 2008

The mechanical behavior and the dislocation microstructure evolution in low-cyclefatigue at room temperature have been studied in a forged X2 Cr Ni Mo 25-07 duplexstainless. Fatigue tests were performed under equivalent total strain control at the constantequivalent strain rate 6.6 10-4s-1, on the one hand, under fully reversedtension/compression loading, and on the other hand, undertension/compression-torsion loading following a square loading path centered on strain zero at the same equivalent total strain amplitude Dåeq/2 = 0.5 10-2. Thecyclic hardening/softening curves under uniaxial as biaxial loading show anaccommodation phase characterized by a short hardening followed by a softening.Then, a stabilization stage is reached. However, the stress amplitude is notablyhigher under biaxial loading. Thin foils were prepared from slices taken perpendicular to the stress axis in three specimens fatigued under uniaxial cyclic loading up to the hardening, the softening and the stabilization stage. In the case of biaxial fatigue tests, thin foils were taken parallel to the axis in two specimens fatigued up to softening andstabilization stage. TEM observations revealed an evolution of the dislocationstructures with the number of cycles for both types of loading. Under uniaxialloading, the dislocation structure in the austenite is planar, whatever the numberof cycles. Only the dislocation density increases during cycling. On the contrary,in the ferrite, the dislocation structures evolve strongly. During the hardeningstage, few dense homogeneous dislocation arrangements are observed, while during the softening stage, the dislocation density has increased and bundles are observed. At stabilization stage, a structure of walls and channels is observed, with high and low dislocation density respectively. Under biaxial loading, the dislocation structures are similar to previous ones but earlier marked. From these observations, a polycrystalline model taking into account the dislocation microstructure evolution is proposed in order to model hardening/softening curves and stabilized hysteresis loop.