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Characterizing the Microstructure of a Corrosion-Resistant Alloy
Kevin Cunningham, Michael Short, Ph.D., and Professor Ronald Ballinger
Nuclear Science and Engineering
Massachusetts Institute of Technology, Cambridge, MA 02139
This work was supported by the CMSE Research Experience for Undergraduates
Program, as part of the MRSEC Program of the National Science Foundation under grant
number DMR-08-19762, and by the MIT Materials Processing Center.
This work was performed in part at the Center for Nanoscale Systems (CNS), a member
of the National Nanotechnology Infrastructure Network (NNIN), which is supported by
the National Science Foundation under NSF award no. ECS-0335765. CNS is part of
Harvard University.
Material preparation Samples of the forged Fe-12Cr-2Si alloy were cut with
a low-speed diamond saw and mounted in conductive
thermosetting plastic. Next, mechanical grinding and
polishing were done with increasingly fine SiC
sandpaper down to 1200 grit, or 6.5 micron particle
size. Further polishing with diamond paste down to
0.25 microns in size gave samples a mirror finish. To
prepare for EBSD, even further polishing is required.
Vibratory polishing
Electrolytic polishing
Preliminary results
Shown to the right are two images generated using EBSD
data collection software. These data were collected from
a sample that had been polished using both vibratory and
electrolytic methods.
In color is a grain map, showing at each point the
orientation of the crystal lattice. White spaces show
points where the diffraction pattern could not be indexed.
The map in grayscale shows the image quality of the
diffraction pattern for each point, where lighter areas are
of higher quality.
The quality of the diffraction patterns can suffer from
several factors, the most significant of which is the
smoothness of the sample. Any minor variation in the
surface can make the diffraction pattern too diffuse for the
software to interpret.
EBSD analysis
Future work In order to generate the most precise images using
EBSD, custom sample preparation techniques must be
perfected for the Fe-12Cr-2Si alloy. Vibratory and
electrolytic polishing are only useful if the proper
settings are used. For electrolytic polishing in
particular, the electrolyte choice and polishing voltage
must be optimized for the material. Care must be
taken to maintain a very clean environment during
preparation and storage, so that the surfaces do not
become fouled before EBSD analysis.
A greater facility with the technique of EBSD is also
required to optimize the collection and indexing of
diffraction patterns. The poor image quality of most of
the diffraction patterns is due to inefficient image
processing as well as imperfect samples. Once setup
times decrease, scans may be performed for longer
durations, allowing for larger scan areas at higher
resolutions. Considering the large grains of this alloy,
increasing scan area is especially important.
References M. P. Short. The Design of a Functionally Graded
Composite for Service in High Temperature Lead and
Lead-Bismuth Cooled Nuclear Reactors. Ph.D.
Dissertation, Massachusetts Institute of Technology,
Cambridge, MA, 2011.
F. J. Humphreys. Grain and Subgrain Characterisation
by Electron Backscatter Diffraction. Journal of
Materials Science, 36:3833-3854, 2001.
United States Steel Corporation. Method of Improving
Magnetic Permeability of Cube-on-Edge Oriented,
Silicon-Iron Sheet Stock. U.S. Patent 3,644,185,
February 22, 1972
Acknowledgements Hannu Hanninen, for his guidance with EBSD theory
Tapio Saukkonen, for insight on vibratory polishing
David Lange, for training and access to the Harvard
Center for Nanoscale Systems imaging facilities
Jonathan Gibbs, Peter Stahle, and Brandon
Sorbom, for teaching me far beyond my own project
For further information Please contact [email protected]. More
information on this and related projects in the H. H.
Uhlig Corrosion Laboratory can be found at
uhliglab.scripts.mit.edu.
Misorientation Angle
Misorientation Angle (degrees)
Nu
mb
er
Fra
ctio
n
The EBSD technique
EBSD allows software to calculate the crystal
orientation at thousands of points on a surface
within minutes. The spatial resolution for a field
emission SEM operating at 20 kV is under 10 nm.
The direction, width, and intersections of diffraction
bands correspond respectively to the crystal
direction, plane spacing, and zone axis directions
at the scan point on the surface.
Left: current vs. voltage curve used to determine
polishing voltage, with schematic of surface
removal. (Image credit: Oxford Instruments)
Above: current vs. time curve used to determine
duration of polishing. Surface removal occurs
from A to B. Current near 0 from B to C
indicates smooth surface.
A
B C
Above, from left to right: mechanical grinding, diamond polishing, and
vibratory polishing. Grinding makes surfaces smooth enough to polish,
but induces significant surface damage. Diamond polishing removes the
damage left by grinding but still can damage the surface itself. Vibratory
polishing uses chemomechanical action to remove all residual surface
imperfections without leaving any damage. (Image credit: Struers)
Background The Ballinger lab has developed an alloy that resists
corrosion in lead and lead-bismuth cooled nuclear
reactors. While this material has demonstrated
remarkable corrosion resistance during exposure times
of over 500 hours, not much is known about how the
microstructure of the alloy contributes to this chemical
durability. The purpose of this project is to examine
the crystal grain structure of the alloy using electron
backscatter diffraction (EBSD).
Left: body-centered cubic (BCC) unit
cell schematic. The alloy studied in this
project is single-phase ferrite, which is a
crystal structure of iron that has a BCC
lattice. (Image credit: Lawrence
Livermore National Laboratory)
Minimizing the grain boundary density at the surface of a material is vital to the process of
designing for corrosion resistance. Grain boundaries are the most vulnerable sites for
corrosive attack. Grain size and misorientation angle between grains indicate the density
and vulnerability of grain boundaries, respectively. In this alloy, the grains of several
hundred microns in size and the low number of boundaries with misorientation between 10
and 25 degrees may be the primary reasons for its strong corrosion resistance.
Above: Inside the SEM chamber,
the sample is tilted 70 degrees
from horizontal. (Image credit:
Oxford Instruments)
Above: A high-quality
diffraction pattern for a
sample of iron. (Image
credit: Oxford Instruments)
Above: EBSD grain map (left) and image quality
map (right) for Fe-12Cr-2Si sample. The legend
of the grain map is an inverse pole figure that
describes the crystal direction at each point.