What are Crystalline Material
We'll come into the atomic scale of mainly metals because material science a lot of it deals with metals and alloys. We'll see how is the crystal system, how is the crystal structure of a material? Let's go into details and understand what I am talking about. So as I said, the properties of a material are influenced by its structure. How is the structure of a material helps define the end property of the material. Therefore it is quite pivotal to understand and enhance our knowledge about the structure of a material. Now broadly speaking, any material can be classified into two categories on the basis of structure. So the solids exist as either a crystalline material or amorphous material. We'll be focusing on solids, because the abundant amount of elements and all the alloy systems are in the solid state. Most of the material science will deal with that. So we'll focus on that for the time being now.
Characteristics of Crystalline Material
What does each of them mean specifically, solid exist as a crystalline material or amorphous material. Crystalline material means, there is a long range order in the material. Whereas enough amorphous material there is no ordering in the material. Now we'll discuss it further. Let's see crystalline material as I said has long range periodic arrangement of atoms. That is if I give you if there is an atom here and here and the material is crystalline. Then you can predict where will be the next act that is it. It has a specific location where the atom can be present and there are locations where atom cannot be present. That is what is meant by long-range ordering. This long-range ordering is a repetitive 3d pattern. That is the ordering is present in the three dimension. So it will be in x axis y axis and z axis. There will be ordering in this direction. This direction and in this direction and that is typical characteristic a 3d pattern is a typical characteristic of a crystalline material.
Plastic Deformation in Polycrystalline Material
Pattern of Crystalline Material
Now as I said, there is a pattern in a crystalline material. You might ask that there must be a very basic unit in the three dimension, that can be repeated over and over again to get the whole 3d isn't it exactly. That is how it happens, there are what is known as unit cells will come into that. Those unit cell is the smallest block of 3d space, which can be repeated in x axis y axis and z axis to give you the whole 3d pattern of the crystalline material. Therefore it is called the unit cell. It's a very basic building block of the crystalline material. Now to put things into perspective crystalline material composes of. Most of the metals a lot of ceramics and some polymers. When I say metals, I include metal as an elemental metal as well as alloys, which has combination of different metals. Now you can say that it's all fine that we have 3d arrangement. But is it possible to have different kind of 3d arrangement.
What is Crystalline Material and Amorphous Material
Actually as it happens there is a possibility to have different kinds of 3d arrangement. But the different kind of 3d arrangement can be grouped into seven crystal system. Further classified into fourteen prevail so that 14 gravel artists kind of includes all the possible 3d arrangements. That can be present in a crystalline material will not go into the 14 bravais lattice. We'll see there are seven crystal systems. That's what I have written here that there are different crystal structure need not be one type of 3d arrangement. The different crystal structure has a lot of influence in the property. That kind of determines a lot of property the material will have. So it is quite important to understand what category of crystal structure, that's a particular element or a metal come under. Now we have seen what is a crystalline material alternatively, what is a non crystalline or amorphous material. These both are alternative names for the same thing. Those are materials which do not have a long-range ordering. What do I mean long-range ordering. I have written long-range ordering here and long-range order here. So if there can be short-range order ordering like there can be order in a material.
Let's say in ten atom length or hundred atoms length, but it does not extend throughout the material. Right or to a large macroscopic region long range order is absent. In such a scenario and that scenario will result in a non amorphous material. A very simple example of non-emergent irreal which you all have seen is silica or sand.
It is a non amorphous material though the silicon has kind of a SP 3 hybridisation. It should have three bonds attached to it. But it does not have there are many broken bonds. As a result, the 3d repetition does not happen as it should happen thereby it is amorphous. Now that we have got a glimpse into what is a crystalline system and what is an amorphous system.
Types of Crystalline Material
let us dive deep and understand the different kind of crystal structures that I have mentioned here. Before that let's discuss some more thing, the metal system that will be discussing throughout our topic will be based on hard sphere model. Now what is hard sphere model, hard sphere model assumes atoms to be like marbles, hard marbles. If there is one atom here, then another atom is here and this cannot intrude into the space here. Like there is a physical repulsion, these are this is hard sphere model atoms are considered hard spheres unit cell. As I already discussed is the smallest building block, which can be repeated in the three dimension to get the whole crystalline material.
Now a unit cell, what is the characteristic of a unit cell. A characteristic of a unit cell will be that, it will have three dimension. It will have X Y and a Z dimension and it might be something like this. Make a cube which this angle is 90 degree and all the sides equal. It might be cuboid which all with all the angles is 90 degree. But the sides not equal and it can take other configurations too.
We'll see but to give you your idea unit cell composes of three set of parallel faces this face. This face is parallel to each other this face and this face is parallel to each other. The front face and the back face are parallel to each other. So it is a parallelepiped shaved with three set of parallel faces. As I said, there are seven crystal systems and there are 14 types of arrangement of atoms that can take place in the seven crystal systems. A total of 714 kind of arrangement known as 14 bravais lattice exist.
So now let us see what are the seven crystal systems. This is the unit cell which I have shown over here. But there are only difference being this angles are alpha beta gamma and the lengths are in the x-axis. Here they have considered this as the x-axis II. This is the y-axis so the length is B and this is the z axis and the lengths C.
So these are the six variables which define a crystal system. The angles alpha beta gamma and sides a b c. Now the different combination of angles and sides will give rise to seven different kind of crystal system. Let us see, each of them one by one. The cubic system which will be the most popular system in the metal system scenario which day in and day out is the cubic system. That has all the three sides of equal length and all the three angles 90 degree.
Second the hexagonal system this is the second most important crystal system. When we talk about metals in hexagonal system, we have two equal sides and a different third side and two angles as 90 degree and the third angle exactly 120 degree for a hexagonal system which do not has have any stress inside it. C is approximately equal to one point six three three times in.
But you need not remember this the basic idea is two sides are equal. Third is not equal dead dragon L you can think of this as a not exactly a matchbox. But something which has two faces of equal length. This is a square but the third dimension goes to a very large length or what we see here is a is equal to B but C is different and all the three angles are 90 degree.
So that is tetrahedral rhombohedral is three angles are equal three sides are equal. But the three angles though equal are each different from 90-degree. Orthorhombic has no side equal but all the angles equal to 90 degree. Monoclinic has no side equal to angle equal to ninety degree and the third angle is not equal to 90 degree. Finally comes triclinic, this is the most non symmetric system in which none of the angle is equal R. The none of the sides are equal and none of the angles are equal to each other and none of the angle is equal to 90 degree. So we see that these are seven different kind of systems, which can exist and based on which all unit cells can be defined.
Arrangement of Atoms in Crystalline Material
But as I already mentioned, these two the first two will be the most important for our topic and in fact even in advanced topics. When we focus mainly on metals and metal alloy systems cubic and hexagonal are the most important systems. Now there are four different ways in which atoms can be arranged in a system. This basically gives you just the outline of the unit cell, but this does not tell you where the atoms are going to be placed. So what are the four different locations where atom can be placed. Those are one is simple I will take the example of cubic system.
Let's say in the simple cubic system, only atoms are occupied at the eight corners and this is simple. Second system is known as the body centered. In body centered what happens in addition to the eight atoms at the eight corners. We have another atom at the center, this is body centered then we have. What is known as the face Center? Face Center as the name suggests is you have atoms at eight corners and you have atoms at the six faces too.
So this is face centered and there is another which is known as edge Center which does not exist in the cubic system. It does not exist in the cubic system at center, but in the system in which it does exist the earth system. What will happen is something like this, this is not our cubic system.
I'm not going to say what system supports the earth Center its Center system. But just know that there will be systems which will have at center and in those system, we will have eight atoms at the eight corners and two atoms one at the top face and one at the bottom. Bottom face this is edge Center, so these are the four different kind of ways in which atoms can be arranged in a given unit cell. These are the different ways a unit cell itself can exist and this combination will give you 14 brave a lattice.
Now you can ask me that, I said these three systems of atom arrangement exist in the cubic system but the edge Center does not exist why is it so? This is so because, if we try to make a edge Center in a board cubic system, then what will happen is that it will come out to be a replication of. I think the tetragonal system I'm not quite sure about it from the top of the head. But it will be just a repetition of a different kind of crystal bravais system which already exists.
So we do not take it into the body centered system rather into the cubic system. Rather it goes into the other system, that there is no repetition. So this kind of gives you a brief glimpse into the idea about how the atoms are arranged in a crystalline material. What is the difference between crystalline and amorphous material, the different crystal systems in are available, the different web browser lattices that can be available. The idea that the understanding of the arrangement of atoms is crucial is at the heart of the property evaluation or of any given material.