Deformation Behaviors Of Duplex Stainless Steel Studied By In-situ Experiments And Self-Consistent Modeling

With modern analytical tools and computational methods it is possible to estimate the stresses to which a component is subjected in service. However, this is not sufficient for the reliable prediction of component performance, due to the fact that in many cases an unexpected failure would occur because of the presence of internal stresses. These internal stresses are in microscopic scales, and combined with the service stresses of the material, thus the component life may be shortened seriously. Furthermore, as for multiphase materials, microstresses can arise from differences in thermal expansion abilities, yield stress, and stiffness among the different phase constituents. Therefore, considerable effort is now being made to explore a framework within which stresses in microscopic scales can be incorporated into design in aerospace, marine, nuclear, and other critical engineering industries.The microstresses can be determined non-destructively utilizing diffraction techniques. In these techniques, the Bragg diffraction of X-rays and neutrons in the crystal structure of materials is mostly applied for measuring the lattice spacings in grains within the crystal material. The penetration depth of X-rays in normal structure materials is in the order of micron limiting the measurements to the surface of the components, whereas the penetration depth of neutrons in the same materials is usually in the order of centimeter, making it possible to measure a bulk average of the elastic strains within sub-sets of grains for the component. In a diffraction measurement, the grains, those participate in the measurement are the ones that have a specific lattice plane normal in a given direction. This selective nature of diffraction techniques therefore permits the elastic tensor in macroscopic directions of the material to be determined for a series of grain sub-sets along different crystallographic orientations.Based on the diffraction data, the simplest way for estimating the stress state is to multiply the measured elastic lattice strain component with the Young's modulus for the used reflection (for the grain sub-set). The moduli for the specific reflections are known as the diffraction elastic constants. If elastic strain components are determined in multiple directions, it is possible to use the generalized Hooke's law in the calculations of the stress state, and thereby to obtain the three-dimensional stress state of the engineering component. However, this assumes isotropic materials, which means that the intergranular strains existing in the real polycrystalline materials with crystallographic texture or crystalline anisotropy are neglected. Another way to determine the stress state is to perform numerical modeling of a polycrystal deformation. Using micromechanical models, which are based on the deformation of the constituent grains of the polycrystalline material, it is possible to predict the elastic and plastic deformation of the polycrystal and, thereby, of specific grain sub-sets within the material. For modeling the micromechanical behaviors of multiphase materials now in most extensive engineering applications, the microstresses characterizing the elastoplastic properties of materials on different microscopic scales, i.e., considering the interactions of phase to phase and grain to grain, have to be described.In the current work, the evolution of lattice strains in a superduplex stainless steel of austenite and ferrite, SAF 2507, during uniaxial compressive loading were measured by using the in-situ neutron diffraction technique. The results provide unambiguous evidence for the existence of large intergranular stresses and valuable experimental inputs for the numerical modeling aiming at accurate evaluations of both the grain-orientation-dependent stress and the phase stress existing in two-phase materials. Based on the experimental lattice strain distributions as well as the laboratorial mechanical tests, the thesis details the implementation of a newly developed two-phase Visco-Plastic Self-Consistent (VPSC) model for simulating the heterogeneous stress within the duplex material. In the presented simulations, thermal residual stresses, initial microstructure and grain orientation, together with texture evolution of the material are involved in the mechanical process; The elastic and plastic interactions among grains with their specific crystal orientations and mechanical performances are considered within the mixed phases, according to which the stress partition between phases and among orientated grains are characterized by the phase stress and the grain-orientation-dependent stress. Therefore, a clear and quantitative exploration into the micromechanical behavior of two-phase materials is achieved.In-situ tensile tests were also carried out with the electron back scattering diffraction (EBSD) technique to characterize the evolution of microstructures and local elastic and plastic strain during deformation of the duplex steel. It was observed that as deformation proceeded low angle boundary continuously increased at some characteristic regions in both phases. Still using the developed VPSC model, the evolution of heterogeneous stresses within the material during tensile was simulated. Based on the calculated distributions of grain-orientation-dependent stress in respective phases and stress partition between the phases, the experimental results are explained by the accommodation of micromechanical properties of grains of different orientations and phases. Good agreement of the measured and the simulated average strains for specifically orientated grains is achieved for both phases.The model was also applied for simulating rolling textures of the duplex steel. It is confirmed that the model with featured slip systems and stress/strain states could characterize the texture development of the ferritic phase at moderate and large reductions. However, for the austenitic phase a reliable prediction could only be achieved at low strain levels when shear banding is not the dominant mechanism. For modeling deformation textures of the austenite, a simplified approach which incorporated the micro-scale shear banding mechanism was applied by performing the two-phase VPSC model on the single-phase material. The prediction of a transition from the Copper-type texture to the Brass-type texture was achieved at large deformation.