Speaker
Description
The new generation of synchrotron sources offers the ultimate state-of-the-art instrumentation in terms of resolution, brilliance and coherence. Coupled with a nano-probe, it allows new mechanisms and processes to be studied in materials sciences and engineering. Beamline ID16B at the ESRF - The European Synchrotron - proposes a multi-modal approach al-lowing the combination of several nano-scale techniques such as X-ray fluorescence (XRF), X-ray absorption spectroscopy (XANES), X-ray diffraction (XRD), X-ray Excited Optical Luminescence (XEOL), X-ray Beam Induced Current (XBIC) and 3D phase contrast nano-imaging. The presentation will detail the use of synchrotron X-ray nano-tomography to understand the mechanical response of an additively manufactured Al alloy with heterogenous microstructures.
Indeed, the microstructures of aluminum alloys designed for laser beam powder bed fusion (LB-PBF) can be heterogeneous across all scales. A bimodal grain structure is often observed with fine-grained regions (FG) consisting of submicron grains near the melt pool boundaries, and coarser columnar grains (CG) in the melt pool interiors. In the FG and CG regions, the morphology and size of intermetallic particles differ as does the composition of the solid solution due to changes in local solidification conditions at the melt pool scale. This specific grain arrangement leads to a 3D architecture at the mesoscale. The mechanical response of such microstructures was found to be influenced by the microstructure heterogeneity using digital image correlation based on SEM images collected during in situ tensile tests. The response of such heterogeneous microstructures is affected by the relative mechanical behaviour of the FG and CG regions.
However, the complexity of the microstructure topology requires a 3D approach to further improve our understanding of the mechanical response of such Al alloys. Thanks to 3D images collected using synchrotron X-ray nano-tomography (voxel size of 25nm3) at the beamline ID16B after different strain increments, we determined the 3D strain maps, at the scale of few melt pools, in a high strength aluminum alloy designed for LB-PBF. The high resolution of the 3D images allowed capturing the fine intermetallic network decorating the microstructure. This enabled to perform high resolution digital volume correlation (DVC) since these intermetallics act as a natural speckle and allow subsets as small as 2 µm3 to be used. The evolution of the 3D strain fields as a function of the macroscopic stress is considered and the heterogenous distribution of strain is explored.
