Smaller nanocrystals may not rotate more after all - Dr. Bin Chen

MARCH 2, 2017

For more than 60 years, the conventional belief has been that smaller grains of materials rotate more under stress due to the motion of grain boundary (GB) dislocations. Under the guidance of Dr. Bin Chen of the Center for High Pressure Science & Technology Advanced Research (HPSTAR), Xiaoling Zhou, a Ph.D. student of HPSTAR, and her co-workers observed that nanocrystals of a critical grain size rotate more than any other grain size. The results were published today in the journal Physical Review Letters: “Reversal in the Size Dependence of Grain Rotation”.

Plastic deformation is a common physical process of solid materials that usually determines materials’ strength. General knowledge is well established for the plastic deformation of materials at a micrometer scale. However, the plastic deformation of nanomaterials has long been an outstanding challenge for the research and industrial communities. Grain boundary sliding and grain rotation are thought to be the dominant mechanisms of plastic deformations at the nanoscale. Grain rotation is significantly relevant in nano-mechanics. Classic theories developed in the 1950’s and later (e.g. the Read-Shockley model) stipulate that grain rotation can be accomplished primarily by grain boundary dislocations.  

Although the observation of grain rotation in the deformation of micrometer-sized crystals is feasible [L. Margulies et al. Science 291, 2392 (2001)], in situ probing the grain rotation of ultrafine nanocrystals is difficult, precluding further investigation of nanoscale mechanics. To overcome technical difficulties, Xiaoling Zhou and her co-workers used Laue x-ray micro-diffraction to monitor grain rotation markers in nickel media of various grain sizes. “Under the same stress conditions, the same sized tungsten carbide marker of grain rotation should rotate the same amount on average”, said Dr. Bin Chen, the project team leader, “but surprisingly, we observed that the marker crystals in the 70 nm nickel media rotated more than in any other size of nickel media”. They inferred that the reversal in the size dependence of the grain rotation arose from the crossover between the grain boundary dislocation-mediated and grain interior dislocation-mediated deformation mechanisms. This new finding could reshape our views and knowledge of nanometal plastic deformation, which provides guidance for understanding and designing the structural and physical properties of materials and potentially advances the development of nanotechnology.

Caption: 2D orientation maps of WC markers embedded in the nickel media. The vertical and horizontal axes correspond to the 2D scanning directions of sample stage. Different colors on the maps indicate different crystalline orientations, as referenced in the color triangle on the right. The numbers at the top right corners are pressure in units of GPa. Courtesy of  Xiaoling Zhou.

The work was supported by support NSAF (Grant No. U1530402). X. Z. was partially supported by the ALS Doctoral Fellowship in Residence Program. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231.

Media Report： Berkeley Lab:

                       https://als.lbl.gov/new-insights-nanoscale-deformation/

                          http://today.lbl.gov/2017/05/03/new-insights-into-nanoscale-deformation/

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