Numerical models of the material and its behaviour accelerate the development of functional materials like biodegradable implants. Magnesium based alloys are gaining high interest as implant materials due to their biocompatibility and biodegradability. [1] Understanding the underlying degradation process and their interdependencies during the biodegradation is necessary for the development and validation of numerical models. In situ synchrotron radiation nano computed tomography (SRnanoCT) is a non-destructive 3D imaging method that can be used to investigate dynamic degradation processes with nominal resolutions below 100 nm. The chemical compositions and morphology can be investigated down to the atomic scale with the aid of (scanning) transmission electron microscopy ((S)TEM). The correlation of these techniques enables to achieve a complementary picture of the degradation process, but, at the same time, brings a number of challenges, like identification of region of interest (ROI) (e.g. precipitates), sample transfer and correlative preparation and image registration.

Figure 1 (showing correlative workflow): Mg-2wt.%Ag degraded in simulated body fluid: a) and b) SR nano tomograms with volume renderings at 0 h and 3.8 h degradation time. The red circle marks an insoluble precipitate. c) Bright field image of TEM lamella at the position of the precipitate. Orange box marks area of EDX scan. d) Elemental maps with EDX at the degradation layer – precipitate interface.

Within this study a correlative workflow (see Fig.1) for understanding the biodegradation of Mg-based alloys at different length scales was enabled. A flow-cell setup was used for in situ SRnanoCT, providing information on degradation rates, homogeneity of the degradation layer (DL) formation and formation of insoluble precipitates. Wire samples (80 µm diameter) of Mg-2wt.%Ag and Mg-2wt.%Gd were degraded in two simulated body environments under physiological conditions (37 °C, pH 7.4, flow rate 1ml/min). From the resulting tomographic scans (Fig 1 a, b), which were obtained every 13-30 minutes, degradation-relevant ROIs were selected for further in-depth characterisation. After the transfer into a dual-beam FIB FEI Helios NanoLab 400S, a lamella at the identified location was prepared by a lift-out method. For the correlative STEM imaging (c) and EDX analysis (d) of structural defects and degradation layer a probe-corrected FEI Titan G2 80-200kV was used. [2]

Preliminary results from correlative imaging show inhomogeneous degradation behaviour, the formation of insoluble precipitates and agglomeration of Ag at the surface. In addition, differences in degradation rates have been observed, showing that Mg-based alloys with Gd degrade faster than Ag-containing alloys and that a higher Ag content also increases the degradation rate.