Quantitative in-situ TEM study of stress-assisted grain growth
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esearch Letters
Quantitative in-situ TEM study of stress-assisted grain growth Sandeep Kumar, Department of Mechanical Engineering and Program in Materials Science and Engineering, University of California, Riverside, California 92521 Tarek Alam and Aman Haque, Mechanical and Nuclear Engineering, Penn State University, University Park, Pennsylvania 16802 Address all correspondence to Sandeep Kumar at [email protected] (Received 9 January 2013; accepted 28 March 2013)
Abstract We present a quantitative in-situ transmission electron microscope (TEM) study of stress-assisted grain growth in 75 nm thick platinum thin films. We utilized notch-induced stress concentration to observe the microstructural evolution in real time. From quantitative measurements, we find that rapid grain growth occurred above 290 MPa of far field stress and ~0.14% elongation. This value is found to be higher than that required for stable interface motion but lower than the stress required for unstable grain boundary motion. We attribute such grain growth to geometrical incompatibility arising out of crystallographic misorientation in adjoining grains, or in other words, geometrically necessary grain growth.
Experimental evidence suggests that grain boundary-mediated deformation mechanisms dominate when the grain size is below a certain critical size (40–100 nm, depending upon the material).[1–5] Grain boundary-mediated mechanisms may include grain rotation, grain boundary migration, and grain boundary sliding. The transition from dislocation-based plasticity to grain boundary-mediated processes changes the overall deformation response. Nanoscale thin films showing minimum amount of ductility and failure are found to be due to brittle crack propagation.[6,7] Transition from dislocation plasticity to grain boundary-mediated process may lead to flaw insensitive behavior as reported in recent studies on aluminum thin films for both static and dynamic loading.[8–10] At the grain size range of 50–60 nm, grain rotation is found to be the active deformation mechanism. These studies relate the effect of stress concentration on material deformation. This behavior is very important for the failure characteristics and reliability of nanoscale thin films. However, different materials have different critical grain size[1] for transition and deformation behavior may be determined by the competing mechanisms as mentioned above. Recently, it has been shown that platinum (Pt) thin films exhibit grain growth as a mechanism to relax externally applied mechanical stress.[11–13] In this study, we take an experimental approach to understand the mechanics behind stress-induced grain growth and its effect on resulting deformation behavior. We used a nanofabricated uniaxial tension/fracture setup inside a transmission electron microscope (TEM) to obtain quantitative information on stress and strain, while directly visualizing the qualitative deformation (such as grain growth, rotation, and dislocation motion) characterization. We developed a unique mechanical testing setup
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