B. Winiarski

Large volume serial section tomography by Xe Plasma FIB dual beam microscopy

Ga(+) Focused Ion Beam-Scanning Electron Microscopes (FIB-SEM) have revolutionised the level of microstructural information that can be recovered in 3D by block face serial section tomography (SST), as well as enabling the site-specific removal of smaller regions for subsequent transmission electron microscope (TEM) examination. However, Ga(+) FIB material removal rates limit the volumes and depths that can be probed to dimensions in the tens of microns range. Emerging Xe(+) Plasma Focused Ion Beam-Scanning Electron Microscope (PFIB-SEM) systems promise faster removal rates. Here we examine the potential of the method for large volume serial section tomography as applied to bainitic steel and WC-Co hard metals. Our studies demonstrate that with careful control of milling parameters precise automated serial sectioning can be achieved with low levels of milling artefacts at removal rates some 60× faster. Volumes that are hundreds of microns in dimension have been collected using fully automated SST routines in feasible timescales (<24h) showing good grain orientation contrast and capturing microstructural features at the tens of nanometres to the tens of microns scale. Accompanying electron back scattered diffraction (EBSD) maps show high indexing rates suggesting low levels of surface damage. Further, under high current Ga(+) FIB milling WC-Co is prone to amorphisation of WC surface layers and phase transformation of the Co phase, neither of which have been observed at PFIB currents as high as 60nA at 30kV. Xe(+) PFIB dual beam microscopes promise to radically extend our capability for 3D tomography, 3D EDX, 3D EBSD as well as correlative tomography.

Mapping Residual Stress Profiles at the Micron Scale Using FIB Micro-Hole Drilling

Measuring residual stress at the sub-micron scale imposes experimental challenges. We propose a new technique, namely the incremental micro-hole-drilling method (IµHM), for measurement of residual stress profiles as a function of depth with high spatial definition. Like its macroscale counterpart, it is applicable to either crystalline or amorphous materials, but at the sub-micron scale. Our method involves micro-hole milling using the focused ion beam of a dual beam FEGSEM/FIB microscope. The surface displacements are tracked by digital image correlation of SEM images recorded during milling. The displacement fields mapped around the whole are used to reconstruct the variation of the in-plane stress tensor as a function of depth. In this way the multi-axial state of residual stress has been characterised around drilled holes of 2 microns or so, enabling the profiling of the stress variation at the sub-micron scale to a depth of 2 microns. Here we demonstrate the efficacy of this method by measuring the stresses in a surface-severe-plastically-deformed (S2PD) Zr50Cu40Al10 bulk metallic glass (in atomic percent, at.%) sample after failure under four-point-bending-fatigue.

Broad ion beam serial section tomography.

Here we examine the potential of serial Broad Ion Beam (BIB) Ar+ ion polishing as an advanced serial section tomography (SST) technique for destructive 3D material characterisation for collecting data from volumes with lateral dimensions significantly greater than 100µm and potentially over millimetre sized areas. Further, the associated low level of damage introduced makes BIB milling very well suited to 3D EBSD acquisition with very high indexing rates. Block face serial sectioning data registration schemes usually assume that the data comprises a series of parallel, planar slices. We quantify the variations in slice thickness and parallelity which can arise when using BIB systems comparing Gatan PECS and Ilion BIB systems for large volume serial sectioning and 3D-EBSD data acquisition. As a test case we obtain 3D morphologies and grain orientations for both phases of a WC-11%wt. Co hardmetal. In our case we have carried out the data acquisition through the manual transfer of the sample between SEM and BIB which is a very slow process (1-2 slice per day), however forthcoming automated procedures will markedly speed up the process. We show that irrespective of the sectioning method raw large area 2D-EBSD maps are affected by distortions and artefacts which affect 3D-EBSD such that quantitative analyses and visualisation can give misleading and erroneous results. Addressing and correcting these issues will offer real benefits when large area (millimetre sized) automated serial section BIBS is developed.

Comparative Analysis of Shot-Peened Residual Stresses Using Micro-Hole Drilling, Micro-Slot Cutting, X-ray Diffraction Methods and Finite-Element Modelling

Micro-Hole Drilling (μHD) and Micro-Slot Cutting (μSC) methods for the measurement of residual stress at the micron scale have recently been proposed but have yet to be evaluated and validated against other methods such as X-ray Diffraction (XRD). In this paper near surface and sub-surface residual stresses were measured in ceramic-shot peened and fatigued Al-7075-T651 double notched samples using μHD and μSC methods, and compared to XRD micro-diffractometer results and Finite-Element (FE) predictions. The micron-seized sampling volumes enabled the stress to be evaluated in individual impact craters (dimples) showing significant point-to-point variation (~±150 MPa) (with certain dimples even recording tensile stresses). At a depth of around 30 μm the heavily deformed region had largely been removed and the stress became much more homogeneous. At this depth the μHD and μSC results were in good accord with those from XRD and FE modelling showing a stress of around 150 MPa far from the notch with stress increasing at the notch being to about 200 MPa for the blunt (2 mm) notch and 500 MPa for the sharp (0.15 mm).

Modelling the Residual Stress Field Ahead of the Notch Root in Shot Peened V-Notched Samples

The knowledge of the residual stress field in the vicinity of the notch of shot peened fatigue specimens is of paramount importance to understand the fatigue resistance of components carrying stress raisers and subjected to surface treatments aimed at introducing compressive residual stresses. For this purpose, residual stresses were measured along the notch bisector using two experimental techniques, namely micro-XRD and FIB-DIC micro-slot cutting and micro-hole drilling. The measurements were used to reconstruct the residual stress field through FE analyses. The obtained results accord with both experimental techniques and indicate an increasing concentration of the longitudinal residual stress component with increasing sharpness of the notch. The proposed approach can be very useful to estimate the notch fatigue resistance of shot peened component on the basis of local stress and fracture mechanics approaches.

Metrology of Three-Dimensional Techniques in Focused Ion Beam Microscopy

The use of focused ion beam (FIB) microscopes to characterise the microstructure of materials in three dimensions, by reconstruction of serial sections, has rapidly grown during the last decade. This is due to improved capabilities in material characterisation and more effective control of the ion beam to cut cross sections in a wide range of materials. It is easy to assume that a visual reconstruction of a stack of images produced by FIB is a fairly accurate representation of the true 3D structure and subsequently carry out measurements based on these data. However, it will be shown that this is not straightforward and in practice, errors or uncertainties in the sectioning, imaging or mapping, and reconstruction can combine to produce misleading results. This paper discusses the metrological challenges faced, but often disregarded, in measurement of the errors and uncertainties that occur throughout FIB 3D characterisation of materials. This was done by studying image stacks and 3D reconstructions from purpose-made structures of known geometries and composition.