Periodic Reporting for period 1 - FracTAlS (High Cycle Fatigue Cracking of Meso- and Micromechanical Testpieces of Aluminide Intermetallics, with in situ Nanoscale Strain Mapping)
Reporting period: 2020-09-01 to 2022-08-31
The aim of FracTAlS is to increase the understanding of the deformation mechanisms leading to and mediating cracking in high cycle fatigue loading, particularly in lightweight, structural aluminide intermetallics, in order to better direct microstructural and alloy development. Such materials are highly desirable for many rotating and airborne engineering applications but often suffer from prohibitive brittleness, particularly in fatigue. The project applies a combination of nanoscale strain mapping techniques on in-situ meso- and micro-mechanical testing, to study deformation behaviour. Currently, the European hub plays a central role in the research and development of advanced gamma titanium aluminide alloys, such as for improved processability, and the large-scale production of γ-TiAl components. However, no significant improvements have been made to the fatigue properties of the lightweight γ-TiAl alloys in the past few decades, effectively limiting their widespread application in higher volume industries where they could result in considerable increases in fuel efficiency. This project is closely aligned with EU policy on climate action and sustainable development: it targets reduced emissions through reduced hydrocarbon fuel consumption; its success will serve to increase the European confidence and knowledge-base in these material systems and, through further interaction with European industry, the extent of their use.
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.
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.
Both the 3D stress mapping with ~100 nm resolution on micro-mechanical testpieces, and the correlative 3D strain mapping of mesoscopic samples are original approaches to evaluating useful plasticity and damaging states of stress. Data analysis to relate, e.g. the measured residual 3D stress states with the corresponding dislocation structures imaged by post-mortem TEM, is ongoing.