Pictures of Grain Boundaries
|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.
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 processjust 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:|
|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.|
|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.|
|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 highresolution 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.|
|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.|
|Things get a bit better with "edge-on" pictures at atomic resolution of some slightly different kind of grain boundary ("small-angle tilt").|
|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.|
|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|
what are you supposed to learn form all this stuff? Not much, really. The messages are simple: |
9.1.1 Things are Complicated
Bravais Lattices and Crystals
6.2.3 Welding with Fire or Hammer
Grain Boundary - Advanced
5.3.1 Grain Boundaries
Transmission Electron Microscopes
Microscopes for Science
5.3.2 Phase Boundaries
© H. Föll (Iron, Steel and Swords script)