Electron Back Scattered Diffraction (EBSD) in the SEM - A New Tool for IndustryDr. Michael Fahrmann - Haynes Intl.
My talk will be divided into five sections:
1) An introduction to the back scattered technique
2) Some sample preparation issues.
3) Case studies on orientation imaging.
4) Case studies on phase identification.
5) Summary and future work.
1) The SEM has become a workhorse for materials research and production control. Well established is the use of SEM for imaging by direct scanning, backscatter scanning, and transmission. Equally well established is the use of EDS for Microanalysis of specimen material. Fairly new is the field of Electron Back Scattered Diffraction to yield the orientation of crystallographic phases, as well as identify new phases with a specimen. This has been enabled as a near "real time" analytical tool by use of extensive computational analysis of the diffraction image.
The sample is mounted into the SEM at high tilt angle to the Electron source (in the neighborhood of 70 deg). An EBSD camera gathers the backscatter image during the scan, and a FSD detector is also used to track the atomic number of the species excited by the beam.
Electrons form a Kosel cones after interacting with the crystalline structure of the specimen, and are imaged by the camera as Kikuchi Bands. These have various widths and angles on the diffraction image. The centerline of each band is a trace of the reflecting plane of the crystal. Software is used to compare the angles between the band centerlines with a stored library of crystallographic information. Poles (intersections of centerlines on the image) represent intersection of the reflecting planes. The system at use by Haynes is capable of analyzing 100 frames of information / second.
2) Unlike traditional metallography, the samples for EBSD analysis should not be etched. Etching has been found to leave precipitates at boundaries which interfere with the backscatter diffraction information. A very flat and highly polished surface is preferable. The surface should be polished in a way as to minimize strain on the surface (many steps). Strain will distort the diffraction image and diminish results.
3) A case study in grain size determination. Software can be used to translate the diffraction image into grains with some different angles of rotation being assigned to colors. While user selectable, this is often set such that a 5 degree change produces a different color. Other software tools that can be used are a "Clean Data" tool, Minimum Count tool, and a twin recognition and elimination tool. With the use of each tool the EBSD result becomes closer and closer to what a traditional ASTM method of measurement would come up with as the result. Using all four tools the ASTM gain size is nearly matched, in virtually an instant.
A second case study involves the identification of abnormally large "elephant grains" in a ring rolled part. The EBSD tool readily spots the large grain centered in a fine grain matrix. Using an analytical enhancement called an "inverse pole figure map" is can be shown that there is rotation of the crystal within the elephant grain, which is not present in surrounding smaller grains. This led to a conclusion that the elephant grain was the result of insufficient recrystallization in the feed stock for the ring rolling process and distortion occurred during the rolling process itself.
A third case study looked at similar elephant grains in a plate product. In this case the "inverse pole figure map" showed no distortion of the crystal in the elephant grain. From this, the conclusion was reached that the large grain was the result of lack of precipitation in a salute lean area of the alloy.
4) EBSD can also be used for phase identification. Studying nitride dispersion of a nitrated surface, cube on cube orientation was found differentiating matrix and nitride particles.
Studying an FCC matrix alloy with at cubic gamma prime crystal, and a tetragonal gamma double prime strengthening precipitate revealed no phase contrasts in the EBSD phase map. Realizing one can construct the unit cell of the gamma double prime using two unit cells of the matrix, it was determined that the gamma double prime phase merely assumes atomic positions on the original gamma prime matrix and does not build a separate crystal.
In a similar study an A B phase (orthorhombic) was studied in an FCC matrix. Again, no separate crystal were revealed, so is can be stated that the A2 phase is based (or superimposed) on the FCC matrix.
5) Summary:
EBSD is a new technique that adds valuable crystallographic information to SEM studies.
A good minimal strain polish is required to obtain good EBSD image maps.
Phase identification and distortion can give valuable insight into material processing.
In the future Haynes hopes to use this tool for QA and Failure analysis projects.
A German Co. claims to be able to obtain even more information by considering widths (as well as orientation of the Kickuchi Bands. The cost is longer compute times for results (on the order of minutes).
EBSD may be useful for the study of oxide films.
Approximately a 10 nanometer of surface is sufficient to provide the Backscatter information.