High resolution imaging

Fig. 1 Comparison of a 3D image by projection CT (right: displayed as an imaginary section image) and a SEM (scanning electron microscope) image of the same section (left)

 Figure 1 shows an X-ray CT image of a specimen, and a SEM image of the same section after polishing the specimen (Review paper 2). Under optical and scanning electron microscopes (SEM), the actual sections of specimens cannot be observed because sectioning and polishing alter the specimen. 3D imaging does not require special skills to understand tissues correctly.

Fig. 2 Typical setup of (a) projection CT and (b) imaging CT

 The setup of the projection CT used in Fig. 1 is illustrated in Fig. 2(a). The resolution is determined by the Rayleigh diffraction limit of visible light, etc., and is about 1μm at the maximum (Paper 2, Review paper 3,1). On the other hand, Fig. 2(b) illustrates imaging CT that uses Fresnel zone plates, which can produce images of even higher resolutions(Paper 2Review paper 1 ,3).Even γ-Ag2Al precipitates in Al-Ag alloys can be observed. Today, spatial resolutions as high as 100 nm can be achieved, and specimens much larger than those for TEM can be observed. To visualize smaller structures than the resolution permits, we modify the microstructure with elements that differ in X-ray absorption coefficient.

 

 

 

 

 

  Fig. 3 Imaging CT observation of aging precipitation in aluminum alloy

 
 Figure 3 visualizes silicon grains and grain boundaries (decorated with gallium) of AC4CH cast aluminum (Review paper 2, Paper 3).The grain boundaries were not three times wider than the resolution of the system but are clearly visible.

Fig. 4 Visualization of grain boundaries (green) and eutectic silicon grains (pink) in AC4CH cast aluminum alloy

 Figure 4 shows an application of high resolution imaging (Paper 1, Review Paper 2). Even crack propagation of the Paris region (one of the fatigue crack propagation stages), which is generally insensitive to microstructures, was found to undergo tilting of the fracture surface depending on crystal orientation, which had a significant impact both on the formation of minute unevenness on the fracture surface, and on fatigue fracture resistance.

Fig. 5 Interactions between grain boundaries (yellow) and fatigue cracks (green) of 2024 aluminum alloy

There is a tradeoff relationship between resolution and field size in X-ray CT imaging (Paper 4Review paper 2).We are developing a method called ‘reconstruction at the region of interest’ (for observing only the region of interest within a large diameter specimen at a high resolution) to overcome this problem. In the case of Fig. 5, the specimen was four times larger than the field width.

Review paper

  1. Y. Suzuki and H. Toda, Section 7.1, "Fresnel zone-plate microscopy and microtomography” in Chapter 7 "Tomography using magnifying optics", Advanced Tomographic Methods in Materials Research and Engineering, 2008, 181-201, Oxford University Press
  2. H. Toda, M. Kobayashi, Y. Suzuki, A. Takeuchi, K. Uesugi, 3D・4D Materials Science: Its Current State and Prospects, Hihakaikensa, Vol.58,No.10,2009,433-438
  3. H. Toda, M. Kobayashi, Y. Suzuki, A. Takeuchi, K. Uesugi, X-ray Micro- and Nano-tomography Techniques and Their Applications, Kenbikyo, Vol.44,No.3,2009,199-205.
  4. H. Toda, M. Sato, H. Okuda, M. Kobayashi, Observation and analysis of materials with synchrotron radiation, Keikinzoku Vol.61, No.1, 2011

Research paper

  1. K.H. Khor, H. Toda, J.-Y. Buffiere, W. Ludwig, H.S. Ubhi, P.J. Gregson and I. Sinclair, In-situ high resolution synchrotron X-ray tomography of crack closure micromechanisms, Journal of Physics: Condensed Matter, Vol. 16, 2004, S3511-S3515 (Journal of Physics: Condensed Matter TOP PAPERS 2004)
  2. H. Toda, K. Uesugi, A. Takeuchi, K. Minami, M. Kobayashi and T. Kobayashi, Three-dimensional observation of nanoscopic precipitates in an aluminium alloy by microtomography with Fresnel zone plate optics, Applied Physics Letters, Vol.89, Issue 14, 2006, 143112H.
  3. M. Kobayashi, H. Toda, K. Uesugi, T. Ohgaki, T. Kobayashi, Y. Takayama, B.-G. Ahn, Preferential penetration path of gallium into grain boundary in practical aluminium alloy, Philosophical Magazine, Vol. 86, No.28, 2006, 4351-4366
  4. L. Li, H. Toda, T. Ohgaki, M. Kobayashi, T. Kobayashi, K. Uesugi and Y. Suzuki, Wavelet-based local region-of-interest reconstruction for synchrotron radiation X-ray microtomography, Journal of Applied Physics, Vol.102, 2007, 114908-1-9