Abstract

Eric Stach
Department of Materials Engineering and
Birck Nanotechnology Center
Purdue University


Friday, November 30, 2007
3:00 - 4:00 pm
Wilsdorf Hall, Room 101

"In-situ transmission electron microscopy studies of the deformation response of nanocrystalline metals"

 

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 nanocrystalline 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 nanocrystalline regime. In the first portion of the presentation, we will review our work in the area of in-situ nanoindentation of sub-micron and nanocrystalline 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 nanoindentation into the edge of an electron transparent material. This permits us to dynamically observe the processes by which mechanical deformation proceeds. During the nanoindentation 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 nanograined films suggest that the same mechanisms are operative, though the difficulty of these nanosized grains makes the evidence less clear. 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.

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