“Characterization of microstructures and deformation mechanisms in additvely manufactured 316L stainless steels ”
Designing next generation materials for evermore demanding applications is of central importance at LLNL. Alloy design via CALPHAD or machine learning techniques is a major avenue for accelerating materials discovery. At the same time, manufacturing process optimization allows to improve existing materials properties. At LLNL, laser powder-bed-fusion (L-PBF) has become an additive manufacturing (AM) technique of choice to manufacture complex components but also obtain materials with enhanced properties. Rapid thermomechanical cycles during L-PBF retain none-equilibrium microstructures in as-fabricated metallic alloys, most often resulting in improved properties. For L-PBF 316L stainless steels (316LSS), this means breaking off the strength/ductility tradeoff. So-called rapid-solidification cellular structures have explained the significant increase in strength, but much is left to understand concerning the high ductility. Cellular structures are complex, with high density dislocation cells containing precipitates and trapped solutes. After giving an overview of several ongoing efforts at LLNL on alloy design and process optimization and present some of our capabilities, the presentation will focus on recent investigations of L-PBF 316LSS plastic behavior. First introducing the multi-scale microstructural features present in the as-fabricated and annealed materials, the discussion will then move on how we test, characterize, and simulate the plastic deformation, with and without post-process heat treatment. We use in-situ electron microscopy (SEM and TEM), in-situ high energy X-ray diffraction (HEXRD), cellular automata, 3D crystal plasticity simulations, and virtual HEXRD.
This work was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344.