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Shedding Light on Nanocrystal Defects Print

Nanocrystals have been the focus of much scientific interest lately, given their various advantageous mechanical properties. Their resistance to stress has had researchers proposing nanocrystals as a promising new protective coating for advanced gas turbine and jet engines. But recent studies conducted at the ALS show that the tiny size of nanocrystals does not safeguard them from defects.

Engineering Nanocrystal Materials

Most nanocrystal materials are made up of small crystals, called “grains,” and what happens at the boundaries between these grains is critical to material properties. Based on computer simulations and electron microscopy analysis, the belief has been that dislocation-mediated plastic deformation becomes inactive below a grain size of at least 10 nanometers, and possibly as large as 30 nanometers.

Nickel particles of three different sizes were studied up to 38.6 gigapascals. Samples suffered permanent change in shape or size. This change is a function of both pressure and particle size, and particle size can be smaller than computer modeling had anticipated.

How these changes in shape affect nanocrystal performance is the next step in the research process. Although the pressures on the materials in real applications are usually lower than the pressures in this study, the shear stress (mainly responsible for the generation of defects) in materials such as protective coatings for turbines or engines can be comparable to the shear stress scale in this study. Enhanced dislocation activities can make materials stronger, but the resultant plastic anisotropy due to texturing may not be favorable in some applications. So engineering the mechanical properties of nanomaterials with particle sizes and external stress load is very significant.  The findings from this research could aid in the creation of stronger, more durable materials.

A group of researchers, led by materials scientist Bin Chen, studied nanocrystals of nickel subjected to high pressure and found that dislocations—defects or irregularities—can form in the finest of nanocrystals when stress is applied. The nickel nanocrystals suffered dislocation-mediated plastic deformation even when the crystals were only three nanometers in size. The likelihood of plastic deformation, a permanent change in the shape or size of a material as the result of an applied stress, increases with the presence of dislocations within the material’s structure. A dislocation strongly influences many of the properties of materials, but previous research based on computer simulations has led researchers to believe that dislocations were not an issue for nanocrystals below a certain size. The results from these ALS experiments demonstrate that dislocation-mediated deformation persists to smaller crystal sizes than anticipated and that computer models used to predict nanocrystal behavior thus far have not given enough consideration to the effects of external stress and grain boundaries.

Stress-induced deformation of nanocrystalline nickel reflects the dislocation activity
observed by researchers using a radial diamond-anvil-cell x-ray diffraction
experimental station. (Image courtesy of NDT Education Resource Center)

The plastic behavior of coarse-grained metals is mainly controlled by the nucleation and motion of lattice dislocations. Plastic deformation by dislocation glide, in which dislocations move through the crystal lattice of the material, results in crystallite rotations, generating a lattice-preferred orientation. The anisotropic physical properties of a polycrystalline material are strongly related to the preferred alignment of its crystallites. Although it is commonly believed that the intrinsic deformation behaviors of nanomaterials arise from the interplay between defects and grain-boundary processes, the precise trade-offs between these deformation mechanisms are still unclear, as is the effect of pressure on these different mechanisms.

To investigate grain-size and pressure effects on the plastic deformation of nanometals, Chen and his team used ALS Beamline 12.2.2, a superconducting bend-magnet beamline that supports radial diamond-anvil-cell x-ray diffraction experiments. This research method represents a huge technical hurdle, moving nanocrystal research beyond computer modeling. The researchers recorded in situ observations of texturing (when the crystalline grains have preferred orientations) under a range of high pressures in stressed polycrystalline nickel samples featuring grain sizes of 500, 20, and 3 nanometers.

A radial diamond-anvil cell allows for in situ x-ray diffraction experiments at Beamline 12.2.2.

When stress was applied to the nanocrystals, individual crystals deformed preferentially on slip planes. Dislocations have a preferred direction of travel within a grain of the material, which results in slip that occurs along parallel planes within the grain. The resulting crystal rotations lead to texture development. Radial diffraction images from Beamline 12.2.2 show variations in diffraction peak position with respect to the compression direction, indicating differential stresses in the material. They also display systematic intensity variations that can be used to deduce texture.

Substantial texturing was observed at pressures above 3.0 gigapascals for nickel with 500-nanometer grain size and at greater than 11.0 gigapascals for nickel with 20-nanometer grain size. When compressed above 18.5 gigapascals, even nickel with 3-nanometer grain size showed texturing. Under high external pressures, dislocation activity can be extended down to a length scale of a few nanometers.

Chen and his colleagues started with nanocrystalline nickel because its face-centered cubic structure remains stable under a wide pressure range. They are now applying their techniques to the study of other nanocrystalline materials, both metals and nonmetals. The in situ, high-pressure textural studies they’re able to perform at Beamline 12.2.2 provide the means to investigate deformation mechanisms and help constrain the fundamental physics of deformation at the nanoscale.

Researchers Bin Chen and Katie Lutker at Beamline 12.2.2.

 


 

Research conducted by: B. Chen and J. Yan (ALS and Univ. of California, Santa Cruz); K. Lutker, H. Wenk, and W. Kanitpanyacharoen (Univ. of California, Berkeley); S. Vennila Raju (ALS, Univ. of California, Santa Cruz, and Univ. of Nevada, Las Vegas); J. Lei and S. Yang (LONI Institute, Southern Univ., Baton Rouge); H. Mao (Carnegie Institution of Washington, Washington, DC and Center for High Pressure Science and Technology Advanced Research, Shanghai, China); and Q. Williams (Univ. of California, Santa Cruz).

Research funding: National Science Foundation, NASA, and the DOE Office of Science. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: B. Chen, K. Lutker, S. Vennila Raju, J. Yan, W. Kanitpanyacharoen, J. Lei, S. Yang, H. Wenk, H. Mao, and Q. Williams, “Texture of Nanocrystalline Nickel: Probing the Lower Size Limit of Dislocation Activity,” Science. 338, 1448 (2012).

ALS Science Highlight #273

 

ALSNews Vol. 343