Polyfast

To investigate the microstructural damage, the shafts of the specimens were longitudinally cut in half. These specimens were embedded in Struers Polyfast (Struers Inc., Cleveland, OH, USA) and ground and polished, at first with Silicon carbide paper, then with diamond polishing paste (9 μ, 1 μ) and finally with an OPS solution at a low force for several minutes. The specimens were examined by a Zeiss Ultra 55 (Carl Zeiss AG, Oberkochen, Germany) and a Tescan Mira3 FEG (TESCAN ORSAY HOLDING, Brno, Czech Republic) scanning electron microscope (SEM) with excitation voltages of 3 kV and 5 kV, respectively. The operator identified grain boundaries and captured micrographs when cavities were found along them. Using MATLAB’s image processing toolbox and custom functions, the sizes of these cavities were documented and processed. Cavities in secondary electron imaging appeared as dark circles with white annular highlights, caused by the edge effect at the steep cavity edges. Non-spherical cavities were approximated by the curvature of their edges. The average grain size was also determined from these micrographs.

In order to study the microstructure pack-cementation and conditioning, samples were hot mounted (Struers Poly Fast, Struers GmbH, Willich, Germany) and ground from 180 grit down to 2000 grit, followed by mechanical polishing with a 3 μm and 1 μm diamond suspension and finished using colloidal silica. The microstructures were investigated using a Zeiss Merlin or Zeiss Supra 50 VP (Carl Zeiss Microscopy, Oberkohen, Germany) scanning electron microscope (SEM). SEM images were typically performed in the backscattered electron (BSE) mode. To determining the diffusion path, electron backscatter diffraction (EBSD) measurements (Oxford Instruments, Bognor Rigis, UK) were performed for phase identification after conditioning. For EBSD, electrons were accelerated using 15 kV and a grit of 517 × 376 pixels with as step size of 0.11 µm was used. For phase identification, the following crystallographic parameters and databases has been used, Table 1.