Research Papers

Probing Microstructure Dynamics With X-Ray Diffraction Microscopy

[+] Author and Article Information
R. M. Suter

Department of Physics and Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213suter@andrew.cmu.edu

C. M. Hefferan

Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213cheffera@andrew.cmu.edu

S. F. Li

Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213sfli@andrew.cmu.edu

D. Hennessy

Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213hennessy@anl.gov

C. Xiao

Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213changshi.xiao@gmail.com

U. Lienert

Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439lienert@aps.anl.gov

B. Tieman

Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439tieman@aps.anl.gov

J. Eng. Mater. Technol 130(2), 021007 (Mar 12, 2008) (5 pages) doi:10.1115/1.2840965 History: Received July 31, 2007; Revised December 19, 2007; Published March 12, 2008

We describe our recent work on developing X-ray diffraction microscopy as a tool for studying three dimensional microstructure dynamics. This is a measurement technique that is demanding of experimental hardware and presents a challenging computational problem to reconstruct the sample microstructure. A dedicated apparatus exists at beamline 1-ID of the Advanced Photon Source for performing these measurements. Submicron mechanical precision is combined with focusing optics that yield 2μmhigh×1.3mm wide line focused beam at 50keV. Our forward modeling analysis approach generates diffraction from a simulated two dimensional triangular mesh. Each mesh element is assigned an independent orientation by optimizing the fit to experimental data. The method is computationally demanding but is adaptable to parallel computation. We illustrate the state of development by measuring and reconstructing a planar section of an aluminum polycrystal microstructure. An orientation map of 90 grains is obtained along with a map showing the spatial variation in the quality of the fit to the data. Sensitivity to orientation variations within grains is on the order of 0.1deg. Volumetric studies of the response of microstructures to thermal or mechanical treatment will soon become practical. It should be possible to incorporate explicit treatment of defect distributions and to observe their evolution.

Copyright © 2008 by American Society of Mechanical Engineers
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Figure 1

An example detector image at L=9.11mm. Black pixels are experimental intensity, green pixels are simulated intensity, and red pixels are both, i.e., red pixels are simulated intensity that overlaps experimental intensity. All diffraction emanates from the intersection of the incident beam with the sample; the direct beam projection onto the detector is shown by the horizontal line at the bottom of the image at z=0. This is one of 270 such images comprising the data set for one layer. See the text for discussion.

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Figure 2

Reconstructed layer of microstructure of an aluminum 1050 alloy sample. (a) shows 24,039 triangular area elements color coded according to their orientations. Regions of apparently uniform color correspond to crystalline grains. White regions were not found to satisfy the convergence criteria for any orientation. (b) shows the confidence plot corresponding to the fit in (a). The circles show the 1mm diameter nominal sample size while the hexagons show the simulated sample space. The color orientation scale in (a) is arbitrary since the material is isotropic and crystal orientations are measured relative to the arbitrary sample mounting orientation. See the text for discussion.

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Figure 3

Optimized microstructure corresponding to the fitted structure of Fig. 2

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Figure 4

Five grains shown with expanded color scales so as to allow visualization of intragrain orientation variations. The color scale for each grain is referenced to the average orientation of its elements and shows the misorientation angle and axis of each element from this value. Statistics of these grains are listed in Table 1.




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