Local and global analysis of plasticity in ferritic-austenitic duplex steels

Abstract
The character of plastic deformation in ferritic-austenitic duplex steels is studied for both mechanical and purely thermal loading. A generation of different deformational regimes is possible even in the absence of the mechanical loading component, linked to the mismatch between the coefficients of thermal expansion of the phases. The specific type of microstructure of duplex steels results in a highly non-uniform spatial strain distribution making impossible an utilisation of standard approaches widely used in analysis of two-phase materials. This necessitates a study of both local and global parameters of the deformational process and a combination of experimental and theoretical (numerical) methods. Electron back-scatter diffraction (EBSD) is used to provide information on grain rotation induced by plastic deformation as an indicator for local plastic flow. A comparison of the misorientation between neighbouring grains (separated by grain or phase boundaries) before and after deformation demonstrates differences in plastic strain evolution in different zones of the polycrystalline aggregate. Purely thermal cyclic loading of ferritic-austenitic duplex steels is studied experimentally by means of dilatometry ; the extension data for specimens of duplex steels are compared with theoretical and numerical results. By means of structural models and finite element analyses, taking into account details of the microstructure at different levels of resolution, the conditions for the local initiation of plastic flow and positions of hot-spots of plastic flow are studied for different phase distributions (matrix-inclusion topology) and various characteristic length scales. These simulations demonstrate the high sensitivity of the effective material/structure response to the arrangement of the phases in the microstructure.