Cyclic Hardening/Softening of a TRIP Duplex Stainless Steel

Recently, a series of duplex stainless steels (DSSs) have been developed by replacing Ni–Mo with Mn–N, which makes DSS more economical and produces TRIP effect with higher strength and plasticity. In this work, the mechanical properties of a TRIP-assisted DSS under monotonic and cyclic loading conditions were studied and the micro-structure under cyclic loading was analyzed by transmission electron microscopy (TEM). The test steel exhibits enhanced mechanical properties and a typical “three-stages” hardening characteristic due to TRIP effect under monotonic loading conditions. Cyclic hardening/softening characteristics of the test steel are affected by strain amplitude and the number of cycles. With smaller strain amplitude, cyclic hardening occurs first, and then shifts to cyclic softening and gradually trends to stabilization. With larger strain amplitude, after initial rapidly cyclic hardening, the test steel still changes to cyclic softening, however, no stabilization occurs until failure. The dislocation walls in ferrite during cyclic deformation are responsible for the overall cyclic softening of the test steel; While austenite produces ε martensite transformation at larger strain amplitude, which suppresses the cyclic softening, so that with an increase in strain amplitude the cyclic softening rate increases rapidly, followed by a slow increase and a final decrease.