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In this lesson we'e going to be talking about polycrystalline materials and
liquid crystals.
So we're moving away from the idea of simple single crystals.
Before we can begin to understand what we mean by a polycrystalline material,
let's take a look at a schematic.
And this would be a view as you would see, for example, under the microscope.
What I've illustrated here I'm going to call Grain A and Grain B.
Grain A has a specific orientation associated with it, and
Grain B has another orientation.
And so, because those two orientations are different, in this particular
region when they come together, they form a boundary that separates the two.
And as a result, what we find is we have the development of the grain boundary.
So a polycrystalline material then is composed of many grains,
which are really made up of individual single crystals.
But they're separated by a region, and
that region is called or referred to as a grain boundary.
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Now, for example, I've illustrated in this particular slide
a series of aluminum alloys that have each been processed differently.
But what you see is, that when these materials are photographed,
all at the same magnification, what you see is there is
a clear distinction in these size of the features that you see.
Or alternatively, what we're looking at,
is the fact that we have an increase in the grain size.
Along with that increase in grain size going from left to right,
what we have is a decrease in the grain boundary surface area per unit volume.
So as the grains are getting smaller, we're seeing a decrease
in the total energy that is in that polycrystalline material.
And if we relate that then, we can relate the size of the grain with
the amount of energy that we have inside of the boundary.
In these particular four figures, all of them are polycrystalline and
they are all equiaxed grain structures.
That is, they are all about the same size in the two
directions that we are examining them.
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Now it turns out,
that we may have something that we refer to as a duplex grain structure.
And duplex grain structure is really nothing more than two
different grain sizes.
So we have some very course grains and
we have some very fine grains that separate them.
Generally speaking, we would not like to have a structure like this.
We would like to have structure illustrated by the examples on the left,
where all the grains are approximately the same size.
So consequently, we're looking for
polycrystalline materials with equiaxed grains.
And in addition to that, we're looking at very small grain sizes in our
structures to get the best performance out of the material in many instances.
Because grain size becomes so important,
we often find a parameter of the grain size number.
The grain size number, which is capital n,
is determined by looking at a field of view and
counting the number of grains that you see in that field of view at 100X.
So what you find is, as n becomes smaller and smaller,
what we'll find is that N also becomes smaller and smaller.
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And so here's our examples.
We have a coarse grain material on the left, and
it has an associated grain size number of N = 1.
When we go all the way over to a grain size of N = 8,
now we have a very fine-grain material.
So we can characterize, then, our materials based upon a grain size number.
Now because of the importance of examining these grain structures,
one of the things that we'll be seeing from time to time are microstructures.
And some of these microstructures illustrate a number of very important
points.
What we have here is, again, a polycrystalline material, but
in order to enhance the particular structure that we're interested in,
namely the grain size, what we do is take the particular material of interest.
In this case, it was a sectioned through an aluminum alloy extrusion.
And then what we would do is to mechanically polish them so
that when we looked at the surface it appeared to be like a mirror.
And then we apply an electrolytic etch.
And as a result of applying that electrolytic etch,
what we find is we put an oxide film on the surface, and
that oxide film winds up being a polarized oxide film.
And the orientation of each one of those grains being different,
the region is going to have a slightly different orientation.
And so consequently, we'll be able to see differences
when we observe these structures using a polarizer.
So that we can bring out the differences between the different grains, and hence,
we can characterize it pretty simply.
So, sometimes we're also interested in, what do these grain structures look like?
Not just their size, but what is their spatial orientation?
And we have here an illustration, it's an aluminum alloy.
It's extruded,
and we're looking at a section perpendicular to the extrusion direction.
Again, what we see is a structure which is
made up of course grains and they're elongated in a particular direction.
And the grains are actually duplex in the sense that we have
a collection of fine and coarse grains.
Again, what we can do, because of this importance, we can use a very simple
system where we can put a little grid, and those are hexagonal grids.
Each grid is associated with a particular grain size number.
And as you're examining your microstructure, you can then begin to look
at these grids to describe what you are calling a particular grain size.
I would like to introduce another term and
that is the term that refers to the type of structure that an element can have.
So, I've illustrated a material, graphite.
Graphite is one of the forms of carbon.
And when we compare that structure to another allotropic form of carbon, namely
when we have the diamond structure, we see two different structural characteristics.
In the case of graphite, we have the hexagonal planes that are associated with
very strong bonds in those planes.
Perpendicular to those planes we have very weak bonds.
Whereas in the case of the diamond material,
we have the face center cubic with two carbon atoms.
And we've discussed this previously.
Now what we know about these two different materials, is that the structure on
the left, graphite, is a material that we often use to lubricate.
Because the graphite winds up breaking between those hexagonal networks and
the material then can protect the surface.
On the other hand, when we look at diamond, diamond is a very hard material.
And so if we were to put diamond powder between two moving structures,
what we would find is, the material would begin to scratch the surfaces.
So the same element, just two different crystallographic forms.
When we have an element or a compound like silica,
SiO2, it exists in several different crystal forms.
And in that case, we refer to that material behavior as polymorphism.
Thomas H. Sanders, Jr.Regents' Professor
