The analysis of plasticity and stress states under micromechanical fatigue has thus far been surface-limited. Here, a method currently under development at the ID11 beamline of the European Synchrotron Radiation Facility (ESRF) was adapted and exploited to evaluate the internal stress state, voxel-wise with a 150 nm step size (i.e. approximately the spatial resolution), of micromechanical testpieces. This method is named nano-beam three-dimensional X-ray diffraction computed tomography (n3D-XRD-CT). In a first instance, bicrystal cuboidal microcompression testpiece geometries were developed, allowing either site-specific lifting of grain boundaries, focussed ion beam (FIB) milling and subsequent ex-situ deformation before scanning, or free-standing FIBbed pillars located at an electropolished needle-tip of the same material. The latter design was appropriate for in situ microcompression using an Alemnis nanoindenter with a frame adapted to the beamline facility. Materials tested in this microcompression arrangement, as proof of concept, were pure Cu, Mo and 316L stainless steel. Further development with PhD student N. della Ventura achieved micromechanical testing with n3D-XRD-CT in geometries more optimal for fatigue test capabilities, such as micro-tension; micro-cantilever loading remains under development as further modifications to the load frame are necessary in this case.
Following n3D-XRD-CT acquisitions at the ESRF, the microcompression testpieces were analysed using the FIB-3D-HR-EBSD method by S. Kalácska with similar spatial resolution, to compare the lattice rotations (and hence GND arrays) and residual stress states per layer measured by this destructive alternative method. Where this process was not carried out, transmission electron imaging of FIB-thinned lamellae enabled dislocation structure identification for correlation with the residual stress states observed.
In parallel, the capability for in situ meso-scale mechanical testing in an SEM with concurrent measurement of local stress states applied to specific slip systems and the resulting local plastic strain, all at a sample surface (2D approach), was developed. It was applied in a first instance to 50 – 100 µm thick foils of 316L stainless steel as a proof of concept.
Publications on the results from these studies are currently in progress.
