Identification of phases in titanium alloy and correlation between EBSD, High Voltage, Medium Voltage and Low Voltage BSE imaging

abstract

English

The imaging of composite materials is steadily growing in importance and since the contribution of the multi-phase composites combining metallic and non-metallic is rising therefore imaging of these materials becoming more significant. When ceramic (or generally non-conductive) matrix contains metallic phase components problems with phase analysis due to charging of the sample is evident. Coating the sample with conductive layer is the standard solution which may obscure fine surface details. This problem can be reduced using electron microscopy at low landing energy while the information quality of signals usually connected with high landing energies is preserved. The aim of this investigation is to evaluate the possibility to distinguish different phases in intermetallic alloy Ti - 46Al - 7Nb - 0.7Cr - 0.2Ni - 0.1Si using energy filtered Low Voltage Backscattered electron images (impact energy less than 5 kV). Microstructure of the specimens was examined using an Electron Backscatter Diffraction (EBSD) and results were correlated with High Voltage BSE (HVBSE – impact energy 20 kV), Medium Voltage BSE (MVBSE –impact energy 12 kV) and Low Voltage BSE (LVBSE – impact energy 2 kV) images. The intermetallic alloy samples were analysed by electron diffraction technique and four main phases were identified: Alpha – Phase (hexagonal; SG 194), Beta – Phase (BCC; SG 229), Gamma – Phase (tetragonal; SG 123) and Ti3Al – Phase (hexagonal; SG 194). These phases exhibit different chemical composition, therefore at 20 keV in HVBSE image an obvious difference in contrast between phases is generated. A decrease in impact energy to 12 keV of the primary electrons causes an increase in image contrast. For this case, the increase in contrast is caused by combination of composition contrast and channelling contrast. When the impact energy is lowered to 2 keV, detector positioned in the electron optics (in-lens) for BSE has to be used. For low impact energies the contrast corresponding to phase differences is still evident although it is accompanied by surface features (scratches, surface deformation). The contrast mechanism at low impact energies does not depend on atomic number or material density, but only on the bonding structure of the outer shell electrons. There we get ionization losses, resonances (phonons, plasmons) or band gap losses. Therefore contrast in LVBSE images is still visible and can even bring additional information in some cases. Low Voltage BSE imaging allows phase contrast imaging while decreasing charging of the non-conductive components in composite materials.

EBSD, Low Voltage BSE, Medium Voltage BSE, High Voltage BSE, band gap losses, resonances, ionisation losses

ČUPERA, J.; JAN, V.

8. 9. 2014

2940

2940

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