A New Approach to Low Cycle Fatigue Behavior of a Duplex Stainless Steel Based on the Deformation Mechanisms of the Individual Phases

Abstract

This paper concerns the low-cycle fatigue (LCF) behavior of a duplex (austenitic-ferritic) stainless steel with a 50% ferrite content. The study focuses on the damage mechanisms leading to crack initiation in the duplex alloy cycled at imposed plastic strain amplitude. Furthermore, the behavior of two other alloys of compositions close to those of the α and γ phases is simultaneously examined and correlated to the duplex alloy cyclic properties. The LCF properties of the duplex alloy approach those of the ferrite phase at high strain amplitudes and those of the austenite phase at low strain amplitudes. Many observations confirm this result: the Coffin-Manson curves, the hardening/softening curves, the cyclic stress-strain curves, and the crack initiation sites indicate a change in the duplex behavior near a plastic strain amplitude of 10−3. The proposed analysis of the LCF behavior of the duplex alloy is based on the cyclic plastic deformation mechanisms of the α phase (which exhibits twinning and pencil glide) and of the γ phase (with planar slip). Finally, the influence of a 3.5% NaCl solution on the LCF properties of the duplex alloy clearly underlines the occurrence in this alloy of two different types of behavior according to the applied plastic strain amplitude.