Understanding Deformation Processes in Nanocrystalline Metals Through the Use of Real-time Electron Microscopy Techniques

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It is has long been known that the grain size of a material has a substantial effect on its mechanical strength, through the well-established Hall-Petch relationship. In the past decade or so, there has been a resurgence of interest in this topic resulting from the ability to create metals with grain sizes in the deep sub-micron to nano-crystalline scale via a variety of processing techniques. In these materials, it has been conjectured that it may no longer be possible to deform individual grains via simple unit dislocation processes, and other mechanisms may be required to achieve plastic flow.

Here we utilize the technique of in-situ transmission electron microscopy to directly image how deformation proceeds in materials with grains sizes in the sub-micron and deep nano-crystalline regime. In the first portion of the presentation, we will review our work in the area of in-situ nano-indentation of sub-micron and nano-crystalline evaporated aluminum, while in the second portion, we will compare these results with in-situ uniaxial straining of pulsed-laser deposited (PLD) nickel.

We have constructed a unique sample holder for transmission electron microscopy that allows us to perform localized nano-indentation into the edge of an electron transparent material. This permits us to dynamically observe the processes by which mechanical deformation proceeds (Minor, et al., Nat. Mat, 2006). During the nano-indentation of sub-micron grains, we find that deformation induces grain growth, resulting from grain boundary migration, grain rotation and grain coalescence. In-situ studies of nano-grained films suggest that the same mechanisms are operative, though the difficulty of these nano-sized grains makes the evidence less clear (Jin, et al. Acat Mat, 2004).

Uniaxial straining experiments of PLD nickel provide additional strong evidence of grain rotation and grain agglomeration. Through the use of dark-field imaging, we have conclusively demonstrated that that when the grain size is on the order of 10 nm grain rotation can become a prominent deformation response. However, even at these small grain sizes, we find that dislocations are trapped within the grains, indicating that dislocation processes are still active. (Shan, et al. Science, 2004)


Dr. Eric Stach is an Associate Professor of Materials Engineering at Purdue University and directs the electron microscopy center at Purdue's Birck Nanotechnology Center. He received his B. S. M. E. from Duke University in 1992, his M.S.M.S.E from the University of Washington in 1994, and his Ph.D. from the University of Virginia in 1998. Prior to his recent appointment to Purdue, he was Principal Investigator and Program Leader within the Materials Science Division and the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory. Professor Stach's research focuses on the development and application of advanced electron microscopy techniques for the real time observation of nano-structured crystal growth and nano-materials reliability.

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Researchers should cite this work as follows:

  • Eric Stach (2007), "Understanding Deformation Processes in Nanocrystalline Metals Through the Use of Real-time Electron Microscopy Techniques," http://nanohub.org/resources/2689.

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