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 kevinhcunningham@gmail.com. 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)

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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.