| | Pictures of Grain Boundaries |
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Old Pictures Taken be
Me |
 | Here are a
few transmission electron microscope
(TEM) pictures of grain boundaries in silicon (Si) that I took around 1980. The HRTEM pictures
were among the very first pictures ever taken at high resolution. Looking at grain boundaries
edge-on you won't
see much, so the first examples are "top-down". |
|  | You are looking right on the grain boundary.
The silicon above and below the boundary is pretty much invisible. It's like looking at an
old-fashioned slide. The glass plates on top and bottom of the film containing the picture
are invisible.
These grain boundaries were artificially made
(by me) by "welding" two single crystals of silicon with a certain misalignment.
This needs high temperatures and pressure, and is not unlike the "hammer welding" of two pieces of steel. In either case you produce a grain boundary
with some inclusions from the "dirt" still present on the surface of the two pieces
to be joined. In the case of silicon, the "dirt" would be silicon dioxide (SiO2),
in the case of iron, we call it iron oxide, scale or slag. |
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| | Grain boundary in Si made by welding. |
| It's low-angle twist boundary on a {111} plane, if you must know. | |
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| | There
is a lot of structure on a rather small scale. The lines forming a kind of six-fold pattern
are "grain boundary dislocations"
with two kinds of embedded stacking faults. You really
don't want to know more about this.
The smooth blobs (one is marked "slag") are
amorphous silicon dioxide (SiO2) particles left over from the welding process—just
like real slag particles are always found in hammer welded blades. Amorphous silicon dioxide,
by the way, is just quartz glass; if it would be crystalline we call it rock crystal. |
| Here is a picture of a very
similar grain boundary: |
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| Grain boundary in Si made by welding | | Low angle twist boundary on {100}, if you must know. | |
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| | Same thing once
more. Grain boundary dislocations forming a very regular pattern; hardly any "slag".
|
| Here are two pictures of naturally occurring
grain boundaries. They are all inclined to the viewing direction. |
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| |
 | | Large-angle
grain boundary in silicon. | |
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| | | The
"chicken wire" structure indicates a network of very special grain boundary dislocations.
The big black line is a "real" dislocation, running through one of the grains and
ending at the grain boundary, interacting with the dislocations there. |
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| Grain boundary junction in Si | |
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A junction of three large-angle grain boundaries. The lower one
has clearly visible grain boundary dislocations With the eye of faith one also sees a fine-mashed
network in the one branching off to the left. The one going up appears to without a structure
(the "zebra" fringes have nothing to do with structures in the boundary) but that
might simply be due to the limitations of the electron microscope. |
| Just for the hell of it, here are high–resolution
transmission electron microscope (HRTEM) pictures at atomic resolution. Those pictures are
among the very first ones taken with atomic resolution around 1979, when electron microscopes
became powerful enough for that. |
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| |  | | HRTEM picture of the grain boundary
above. | | Visible
are 5 screw dislocations, causing the typical shift of the lattice planes. |
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| | What you see are "screw dislocations". Look up the "dislocation science" module if you feel you need
to know what "screw dislocations" are. Otherwise screw them. |
|
Here, just for the hell of it, is an "edge-on"
picture at atomic resolution of the ("small-angle twist") grain boundary in the top most picture. There is really not much to see. |
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| HRTEM picture of the grain boundary above
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| | Things get a bit better with "edge-on" pictures
at atomic resolution of some slightly different kind of grain boundary ("small-angle
tilt"). |
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|  | | HRTEM picture of a small-angle
tilt grain boundary. | |
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| | The boundary runs
from left to right in the middle of the picture. The colored lines are only to guide the eye.
The blue lines indicate that the orientation of the crystal above or below the boundary differs
indeed by a "small angle".
The red lines indicate ending lattice planes, i.e.
edge dislocations.
The picture shows directly (and for the first time) that this kind
of boundary consists indeed of a lot of dislocations in some special array. This was predicted
long before it could be imaged. Now the prediction has been proved. |
| The next picture shows some unusual and unexpected behavior
that could only be found with "edge-on HRTEM". A simple (low-angle tilt) boundary
is actually not so simple but consists of three boundaries close together. |
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| HRTEM picture of complex boundary consisting of three boundaries.
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| | A low-angle tilt boundary composed of individual dislocations
as in the picture above is actually sandwiched between two so-called "twin" boundaries,
the effects of which cancel each other. That is shown by the black lines that bend substantially
at the twin boundaries, but in opposite directions.
The misorientation between the upper
part and the lower part of the crystal is only determined by the low-angle tilt boundary.
It changes position from in-between the twin boundaries to being superimposed on one of the
twins. |
| | This
effect would be easily missed looking at the grain boundary "top-down", so edge-on
views do have some merits |
| So
what are you supposed to learn form all this stuff? Not much, really. The messages are simple:
- The internal structure of grain boundaries is very complex. Trying to understand in detail
what one sees on those pictures will take a lot of time and effort.
- Nevertheless,
it is great fun (not to mention a lot of work) to take pictures like the ones above.
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© H. Föll (Iron, Steel and Swords script)
With frame
9.1.1 Things are Complicated