3.19 Polycrystalline Materials and Liquid Crystals

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Del curso dictado por Georgia Institute of Technology

Material Behavior

360 calificaciones

Georgia Institute of Technology

Material Behavior

360 calificaciones

Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the first of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.

De la lección

Crystalline Structure [Level of Difficulty: Medium || Student Effort: 2hrs 30mins]

This module covers how atoms are arranged in crystalline materials. Many of the materials that we deal with on a daily basis are crystalline, meaning that they are made up of a regularly repeating array of atoms. The "building block" of a crystal, which is called the Bravais lattice, dtermines some of the physical properties of a material. An understanding of these crystallographic principles will be vital to discussions of defects and diffusion, which are covered in the next module.

3.16 Crystals with 2 Atoms per Lattice Point6:24

3.17 Crystals with 2 Ions or 2 Different Atoms per Lattice Point7:02

3.18 Crystals with Several Atoms per Lattice Point9:42

3.19 Polycrystalline Materials and Liquid Crystals10:20

3.20 X-Ray Diffraction and Crystal Structure14:26

Conoce a los instructores

Thomas H. Sanders, Jr.
Regents' Professor
School of Materials Science and Engineering

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.

And what we have is in the first case, we have a unit cell.

In which all the grains are oriented as given in the diagram.

And when we look at Grain B, that's what the orientation is,

associated with Grain B.

Where the two of them come together then,

results in something that we refer to as a grain boundary.

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.

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.

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.

The last concept I want to introduce in this particular lesson

is the idea of a liquid crystal.

And this has to do with polymer material.

So we have a solution in which we have polymer chains that have been aligned.

But associated with this, they're all organized parallel to the z direction,

but there is nothing crystallographic if we

look in a direction perpendicular to these axes.

So, one carbon chain, though it is parallel and macroscopically aligned,

the carbon atoms in that particular chain are not in equivalent

translational positions as the carbon in a parallel polymer chain.

Now, what we can also do, is to take the polymers and

they can be oriented in a slightly different way.

In which we align the chains, and

that chain alignment results in the structure that you have on the screen.

But once again, we have periodicity along the length of the chain

associated with the but perpendicular to that direction,

we have no long range crystallographic translations.

We have seen a number of important ideas that have been illustrated in this

particular lesson.

And we'll see and

we'll have references to these structures as we go through the course.

Thank you.

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