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 3D microstructures and microstructural evolution


The effect of microstructure on materials properties and the ways in which microstructures can be controlled by appropriate processing lies right at the core of MST. Indeed MST has always prided itself on the quality with which micrographs are reproduced in high definition, originally in the printed form, and more recently, also electronically. Recent research breakthroughs have opened the way for microstructural characterisation in three dimensions; most notably by atom probe and x-ray tomography.

 

Computed micro-tomography opens up high resolution non destructive 3D imaging for the first time. Indeed laboratory X-ray instruments can achieve resolutions as high as 50nm, while synchrotron X-ray facilities open up the possibility of imaging in 3D structural evolutions over timescales as short as 1 second. Tomography is elucidating a wide range of processes, for example the evolution of porosity during homogenisation in Al-6Mg (figure 1). During homogenisation, the regions of high curvature in highly tortuous pore networks tend to become smooth and ultimately more globular. It is possible to map the evolution of microstructures during service life, for example to study stress corrosion cracking (see figure 2), fatigue crack growth (see figure 3) to be studied in 3D in real time.

 

Figure 1: Microtomography observed pore evolution during homogenisation of Al–6Mg a) as cast, after 10 hour and 100 hours of homogenisation (Y.M. Youssef, A. Chaijaruwanich, R.W. Hamilton, H. Nagaumi, R.J. Dashwood and P.D. Lee, X-ray microtomographic characterisation of pore evolution during homogenisation and rolling of Al-6Mg. Materials Science and Technology, 2006. 22(9): p. 1087-1093.).

 

 

Figure 2: Slice from the centre of a stainless steel sample showing the progress of intergranular stress corrosion cracking over time
(L. Babout, T.J. Marrow, D. Engelberg and P.J. Withers, X-ray microtomographic observation of intergranular stress corrosion cracking in sensitised austenitic stainless steel. Materials Science and Technology, 2006. 22: p. 1068-75.)

 

Figure 3: Crack opening constrained by bridging fibres in a Ti/SiC fibre composite (P.J. Withers, J. Bennett, Y.-C. Hung and M. Preuss, Crack opening displacements during fatigue crack growth in Ti-SiC fibre metal matrix composites by X-ray tomography. Materials Science and Technology, 2006. 22: p. 1052-58.)


Examples of the application of these techniques were showcased in a recent special issue of MST on 3D imaging by microtomography. Click here to read the introduction to this issue which discusses the technique in more detail. To reflect this move from 2D to 3D, or even 3D+time, MST now allows the submission of '3D' reconstructions and 'fly-throughs' and animations. For an example see below: 


As a complement to 3D non destructive X-ray imaging, there is also destructive tomography, using the Focused ion beam microscopy to destructively slice the sample and then reconstruct a 3D image. Alternatively, the atom probe allows compositional variations to be mapped atom by atom and reconstructed in 3D, thereby providing new insights into processes such as precipitation, aging and grain boundary segregation (see for example Figure 4).  A recent theme issue of MST on High resolution chemical mapping included work applying the atom probe. Click here to read the introduction to the issue by Peter Hirsch.

 

 

Figure 4: A high strength maraging steel aged for 100 h showing a 3-DAP analysis: nickel (red) atoms, chromium (black) atoms, copper (green) atoms and molybdenum (blue) atoms (dimension of box is 11611664 nm3); b EFTEM jump ratio image of molybdenum  (K. Stiller, H.O. Andren and M. Andersson, Precipitation in maraging and martensitic chromium steels - what can we learn using 3-DAP and EFTEM. Materials Science and Technology, 2008. 24(6): p. 633-640.)

 

In summary, there is little doubt that Materials Science and Technology will increasingly become a showcase for 3D microstructural studies across metals, ceramics, polymers and biomaterials.