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Strain field measurements by digital image correlation today offer new possibilities for analysing the mechanical behaviour of materials in situ during mechanical tests. The originality of the present study is to use this technique on the micro-structural scale, in order to understand and to obtain quantitative values of the fatigue surface damage in a two-phased alloy. In this paper, low-cycle fatigue damage micromechanisms in an austenitic-ferritic stainless steel are studied. Surface damage is observed in real time, with an in situ microscopic device, during a low-cycle fatigue test performed at room temperature. Surface displacement and strain fields are calculated using digital image correlation from images taken during cycling. A detailed analysis of optical images and strain fields measured enables us to follow precisely the evolution of surface strain fields and the damage micromechanisms. Firstly, strain heterogeneities are observed in austenitic grains. Initially, the austenitic phase accommodates the cyclic plastic strain and is then followed by the ferritic phase. Microcrack initiation takes place at the ferrite/ferrite grain boundaries. Microcracks propagate to the neighbouring austenitic grains following the slip markings. Displacement and strain gradients indicate probable microcrack initiation sites. [ABSTRACT FROM AUTHOR]

ABSTRACT Cyclic tests performed in the temperature range 300–773 K on duplex stainless steel DIN 1.4460 show that the cyclic stress–strain behaviour of this steel is strongly temperature dependent. At 300 and 473 K an almost constant peak tensile stress stage, is followed by a slight softening that continues up to failure in the case of 300 K, but by a secondary hardening at 473 K. Pronounced initial cyclic hardening followed by secondary hardening was the main feature of the temperature range between 573 and 723 K. At 773 K, after a weak hardening stage, a strong softening continues up to failure. The mechanical behaviour and the evolution of the microstructure were analysed, and the internal and the effective stresses were studied. It was found that the internal stress is responsible for the strong hardening that occurs in the intermediate temperature range and for the softening at 773 K. [ABSTRACT FROM AUTHOR]

Abstract: The correlation between the low-cycle fatigue behavior and microstructure of SAF 2507 duplex stainless steel has been studied. The cyclic behavior has been analyzed through the cyclic flow stress components, i.e. the effective and the internal ones, as well as the activation volume parameter. A change in the deformation mechanism occurs at Δε p =1.5×10−3, where the accommodation of the plastic strain changes from the reversible to the irreversible bow-out of dislocations giving rise to a continuous increase of the internal stress. The analysis of the effective stress has shown that the interaction between nitrogen atoms and dislocations can be explained through a model for weak obstacles. Finally, above Δε p ⩾6.0×10−3, dislocations on two {111} planes in the austenite can interact and form a well-developed cell structure, which increases the hardening of the phase. Consequently, the typical softening process characteristic of duplex steels diminishes with an increase in plastic strain amplitude. [Copyright &y& Elsevier]