Micromechanics Based Modeling of Effect of Sigma Phase on Mechanical and Failure Behavior of Duplex Stainless Steel

Duplex stainless steels (DSSs) have been widely used in many industries, since their combining austenitic and ferritic microstructure provided superior mechanical properties and corrosion resistance. However, after welding or heat-treating DSSs undesirable secondary phases such as chi phase, CrN and sigma-phase easily occurred which most likely led to deteriorated toughness. In this work, effects of sigma phase on mechanical characteristics of DSS grade SAF 2507 were described by a micromechanics based FE approach. Tensile, Charpy impact and micro-hardness tests of DSS samples with varying percentages of sigma phase were initially performed. Representative volume element (RVE) models on the microstructure level of investigated DSS specimens were generated. Then, RVE simulations coupled with the Gurson-Tvergaard-Needleman ductile damage model were conducted under uniaxial deformation. Flow stress curves of the single phases were defined with regard to the results of nano-indentation tests. In addition, damage parameters were determined for each individual phase by means of hybrid method between experiment and simulation. The predicted overall stress-strain responses and observed damage occurrences were fairly validated with those from the tensile test results. Finally, the proposed RVE modeling was applied to a DSS welded specimen. It was found that local failure behaviors of different zones on deformed sample containing varying amounts of sigma phase could be well described.