Rheology of Earth Materials :

Closing the gap between timescales in the laboratory and in the mantle

30 June 2021 - New publication in Comptes Rendus - Physique

4D electron tomography of dislocations undergoing electron irradiation

 

Dislocation imaging is obtained with diffraction contrast, a very orientation sensitive technique. Keeping a constant contrast of dislocation for several orientations is particularly challenging. It is for this reason that it took until 2006 to perfom the first transmission electron tomography of dislocations (Barnard et al., 2006), whereas the first conventionnal transmission electron tomography dates from the last sixties (De Rosier & Klug, 1968).

We have recently developped transmission electron tomography with few projected images (Mussi et al., 2021). The contrast of dislocation is enhanced prior to reconstruction to improve an acquisition stage with fewer images. This new technique allows to perform fast tomographic acquisitions providing us with a tantalizing sense of the potential possibility to perform electron tomography during in-situ testing.

In this study, we follow the evolutions of the 3D dislocation microstructure of a single crystal of MgO previously deformed at 1500°C under a confining pressure of 10 GPa. Transmission electron tomography characterizations are conducted with a heating double-tilt sample-holder. The shapes of the dislocations become helical with the combination of heating and electron irradiation.

This first experimental study of 4D dislocation allows us to investigate the initial stapes for dislocation to become helical. The two movies below show the continuous evolution of the 3D shape of the dislocations as a function of the irradiation time (the color code refers to the dislocation character: screw dislocation segments are blue and edge dislocation segments are red). In order to obtain continuous evolutions of shapes and colors, interpolations are performed using the DAIN-App software (Bao et al., 2019).

 

 

            Thanks to this 4D study, we can track the variation of the dislocation character with time. The figure below shows a linear evolution with irradiation time.

 

 

            We can also follow the evolution of the habit planes of dislocation segments. For example, we have found a dislocation involved in a cross-slip mechanism between (101) and (1-21). Under irradiation and heating, the red segment glides on the (101) plane, then stabilizes and climbs. The habit plane of the dislocation segment located on the constriction zone (i.e. in a blocked configuration) evolves gradually from (1-71) to (-1-31) after 105 min of irradiation (see figure below).

 

 

            Furthermore, preferential mixed climb planes have been noticed plotting the histogram of the lengths of the dislocation segments as a function of the angles between the normal of habit planes relative to the Burgers vectors, for each irradiation conditions. This suggests the occurrence of favorable planes which is probably due to the core structure and geometry of jogs in mixed climb configuration.

 

 

           

On one hand, the evolution of the helix pitches and the sizes of the cylinders which contain the helical dislocations enables us to verify that the climb mechanism is dominant (Grilhé, 1964). On the other hand, the point defect chemical potential for a supersaturation c > c0 can be deduced from the geometrical parameters of the helix (radius and pitch). During irradiation, we see this quantity decreasing and beginning to reach a saturation value (see figure below).

 

 

            The occurrence of preferential mixed climb planes can also be verified on the figure below (precisely the (12-1), (11-2) and (011) planes).

 

 

            It is worth noticing that the reduction of the acquisition number of images from 10 (in green from the figure below) to 2 (in purple from the figure below) generates minor deterioration of the reconstruction quality.

 

 

References:

- J.S. Barnard, J. Sharp, J.R. Tong, P.A. Midgley, High-Resolution Three-Dimensional Imaging of Dislocations, Science, 313 (5785) (Jul. 2006), p. 319, https://doi.org/10.1126/science.1125783.

- D. J. De Rosier, A. Klug, Reconstruction of three-dimensional structures from electron micrographs, Nature, 217 (1968), p.130-134, https://doi.org/10.1038/217130a0.

- A. Mussi, J. Gallet, O. Castelnau, P. Cordier, Application of electron tomography of dislocations in beam-sensitive quartz to the determination of strain components, Tectonophysics, 803 (2021), 228754, https://doi.org/10.1016/j.tecto.2021.228754.

- W. Bao, W.S. Lai, C. Ma, X. Zhang, Z. Gao, M.H. Yang, Depth-aware video frame interpolation, in Proceedings of the IEEE Computer Society Conference on Computer Vision and Pattern Recognition (2019), p. 3698-3707, https://doi.org/10.1109/CVPR.2019.00382.

- J. Grilhé, Stabilité des dislocations hélicoïdales, Acta Metallurgica. 12 (1964), p. 1081-1088, https://doi.org/10.1016/0001-6160(64)90080-X.

 

To learn more:

A. Mussi, P. Carrez, K. Gouriet, B. Hue, P. Cordier, 4D electron tomography of dislocations undergoing electron irradiation, Comptes Rendus. Physique, Online first (2021), pp. 1-15, https://doi.org/10.5802/crphys.80.

 

This article is published in the special issue "Plasticity and Solid State Physics" of  Comptes Rendus - Physique, handled by guest editors Samuel Forest and David Rodney