Structure of Materials

 Topics: An Introduction to Materials: Nature and Properties

We will start this new course on the nature and properties of materials, and we will start
the first module of this particular course, which is based on a structure of materials.

The recommended reading material for the course have listed three books, first book by Prof. V. Raghavan, which is Material Science Engineering, Second good book is by Callister, which is material science engineering. It is an introductory book, and the third book is by John Wulff, i.e., the structure and properties of materials by Wiley, the first volume is related to the structure of materials. So, it is an excellent book if somebody wants to get into details. So, let us see why all of us know that materials are essential, even our civilizations are named after materials like Bronze age, Stone age, Iron age, and currently we are in silicon age or electronics age.

So, as we know that materials are very important if you go back to our ancestors, they were using stones earlier. Then they started inventing materials, and strangely precious metals came before many of the materials and followed by the development of copper-based alloys such as bronze and brass. The Indus Valley Civilization uses bronzes and brasses. Then the advent of iron gave rise to substantial advantages for human beings because iron is considered a stronger material. It could be used not only in warfare but also in a lot of other practical things. This made hunting easier compared with stones and other objects. For the past 200 years, the advent of silicon-based technological inventions led to the development of electronics or electricity, and because of this, we are utilizing all the technological devices that are silicon-based.

So, materials are very important for us, and that is why it is important to study the science and engineering of materials. So, this course is an introductory course, and we will talk about the basics of materials, which will help you to advance further in this discipline.

 

In 10,000 BC, the man was using things like stone, straw break, wood skins. Strangely, gold came quite early. And then, after 5000 BC or so, a man started developing potteries, which were based on ceramics, glasses, and composites was used in the paper. For example, polymers and elastomers were used. And then Copper, Bronze, and Iron, they came before Indus Valley Civilization in various other civilizations. Then iron age, which expands the use of metals because of the advent of iron and then at the same time and also kept using things like ceramics and glasses in the form of cement factories, because they needed to build houses and mansions and palaces etc.

Furthermore, as we keep moving down the 1900s, iron gave way to cast iron, followed by steel. Steel is a better material as compared to iron alone, and then the development of alloy steels, which in fact, improved the steel. So, this is the age of steel as one can see. Now steel was very good, but the man invented other materials which are even lighter and stronger. So, that is where materials like aluminum alloys, titanium alloys, zirconium alloys, all those came into the picture.

During the 1960s or so, this curve start going upwards, which means metals are slowly shrinking in their domain, and other materials start expanding. Moreover, men also started developing a lot of synthetic polymers, and these synthetic polymers give rise to an array of applications, which are polymer-based because the polymer is a light material. And then composites also by mixing stone and by mixing ceramics and polymers or metals and polymers and metals and ceramics, people made these composites which had different properties, which compromises the properties of both metal and ceramic. So, it is a mixture of both of them. So, you can see that the arena of materials has changed dramatically as a function of time, and today, for example, somewhere around the 1950s, let us say advent in the vacuum technology, processing technologies, gives way to the manufacturing of silicon.

Today we stand in the age of silicon, and perhaps today is the age of molecular engineering as well because we are looking at materials and using materials at the molecular scale, very thin films, 2D structures such as graphene. We can say that we are standing in the age of a completely different era of materials, which is very different from what we had earlier.

     So, if we classify these materials, which are used for most of the engineering applications, there are few categories in which we can classify. So, the first one, of course, that comes to our mind is metals and alloys.

In metals and alloys, for example, copper is metal and brass, which is an alloy of copper and zinc. You can have iron and carbon alloy, which are nothing but steel and cast irons. So, anything copper, nickel, iron, zirconium, titanium, aluminum, etc., all of them are metals, and you can make alloys of them by mixing them with various elements, and they have excellent properties. Metals typically are very ductile, and they are also reasonably strong, and they can be used in applications starting from low temperature to high temperature. Also, metals are electrically and thermally conducting; that is why metals are used extensively in our world. The second category of materials ceramics and glasses, for example, aluminum oxide, silicon oxide, silicon carbide, magnesium oxide, titanium oxide, and all these oxides, nitrides, carbides, basically happen to be ceramics. Ceramics are different from metals because they are much more brittle but very strong; they have high strength or high modulus. So, if you subject them to impact loading, for example, if you are drinking tea in a glass cup, you know that if it breaks, it shatters whereas metal does not do that. So, which means it is brittle. However, there are certain applications in which ceramics are important because ceramics are high-temperature materials, and they also have a low coefficient thermal expansion, So, ceramics are used for refractories, bricks, and kilns. Ceramics are very important for cutting tools, and they also have high hardness so that they could be very important there. Metals, on the other hand, are typically used as structural materials like bridges, houses, rods, automobiles, anything which has to be strong, ductile, and tough.

The third category is polymers, which are light material. They have low elastic modulus. However, they are very flexible, and you can make extremely thin structures, very light structures out of them, they mostly contain light elements like carbon, nitrogen, oxygen, etc. So, examples could be polyethylene, which is used just like a plastic bag that we use daily. PVC is polyvinyl chloride, which is used for ducting, piping, and all that it is a strong material, but it is lightweight, and it does not corrode. So, there is another advantage of polymers that they do not corrode. So, metals, for example, if you make a metal pipe and your savage flush goes through them, they corrode, but polymers do not corrode.

 So, polymers are light, easy to make, do not corrode, and they cost less. Many silicones are used in a variety of applications; these polymers in elastomers are another class of materials that are very different from metals and ceramics, in the sense that they are not so strong. However, they have a light, they have high flexibility, and they are very tough. So, you can make things from them rather easily without going to high-temperature processing of metals and ceramics.

So, polymers have made our life easier; for example, the plastic bag has made our life more comfortable. Then the fourth class of the materials is called composites, hybrid materials, which are a mixture of the above. So, you can mix metal, and ceramic makes a metal matrix composite. So, you utilize the properties of both metals and ceramics. Similarly, you can make a polymer matrix composite when you mix polymer and ceramics. So, you use the advantage of the polymer as well as ceramic. Moreover, you can mix polymer in metal as well.

So, a combination of the two or three classes of these different materials will give composites, and they have their advantages. For example, tennis rackets that we have today are a composite, and many of the parts and the automotive applications or aircraft applications, wherever you require high specific strength or high specific modulus, you predominately tend to use composites. Because composites have high strength per unit weight, similarly, they have high modulus per unit weight, and that is what is useful in certain applications. So, these are some of the applications you can see, The plier for holding things is you can see the head of this is made of metal because it has to be strong, it should not be brittle, but it should provide you a good grip, it should not yield.

 Example of each materials,

So, it is made of steel, but there are a lot of other applications of metals you use them for making bridges, they are used as a construction material, cars are having a lot of parts and cars are made of metals like steel, aluminum, copper. Ceramics you can see that ceramic piece here white piece, which is basically insulator, ceramics are the insulator. So, it insulates the parts of letting us say spark plug, and ceramics are also thermal and electrical insulators. So, they also provide insulation from common electrical. So, for example, on electrical poles, you see white ceramic pieces; they are nothing but ceramic insulators.

Polymers are used to make mugs, plastic bags, pipes, etc. a lot of medical devices are made of polymers. Elastomers, for example, rubber, are put in different clubs because of various reasons, we will explain later. The glasses are typically transparent. And something in between you can see is the hybrid, for example, tennis rackets, airline, aircraft components, automotive components they are all made by mixing these materials to make them light yet strong.

 

These are certain applications, as I explained to you before. To summarize, ceramics have high stiffness, high elastic modulus, hard, high abrasion resistance, good high temperature strength, which means they hold their strength up to higher temperatures, above 1000 0C. They have reasonably good corrosion resistance, but they are brittle; this is a major problem in ceramics because they cannot absorb any shock. So, glasses, on the other hand, are hard, corrosion-resistant, electrically insulating, transparent. So, these are some good attributes of glasses, very similar to what you have in ceramics, but they are also brittle. So, this is again a problem with glasses.

Polymers have low density; they are light because they are made of light elements like carbon, nitrogen, oxygen, and hydrogen. Processes like molding can easily shape them, and they have high strength per unit weight. So, as such, their strength is not very high, but if you look at the perspective of density, they are very strong. They lack stiffness, which means they have low elastic modulus, but they are very flexible, you can make plastic works at large strains. So, they can withstand large strains. However, their properties are highly sensitive to temperature, because they softened with temperature, their melting points are lower. So, plastics are typically not used for applications, wherever you have to subject material to high temperature. So, plastics are typically suitable for temperatures lower than 50 or 100 oC depending upon the type of polymer.

Elastomer is a cousin of polymers. It lacks stiffness, and it has low modulus several times lower than metals, basically rubber, it has this wonderful ability to retain its shape after being stretched, you can provide very large strains to rubber or the elastomer. Furthermore, they are relatively strong and tough as compared to the polymer. So, similar kinds of applications wherever you require a stronger polymer, you use elastomer. However, one difference between polymer and elastomer is, polymers can be melted and reused, but the elastomer cannot be melted. So, typically, an elastomer decomposes whereas, polymer does not decompose.

 

And then we come to metals and hybrids, metals they are very tough, they have high fracture toughness, this is a parameter called KIC, which is a representative of fracture toughness. They have high stiffness, high elastic modulus, very ductile depending upon the composition and processing. What is the metal made or whether it is iron-based, aluminum-based, copper-based, or nickel-based? They can give you strength, which is highly varying from 50 MPa to 1000 MPa even higher depending upon the composition and processing. So, it is very good to have a metal because you can engineer its property depending upon what you want — depending on and which can be varied by changing the composition and processing conditions. They are typically thermally and electrically conductive; that is why they are used in applications wherever you require high electrical conductivity and high thermal conductivity. However, most metals are reactive; they tend to oxidize, or they tend to react with the environment, and that is why most metals have low corrosion resistance.

So, wherever that atmosphere is aggressive, an alkaline environment or you have sludges, you cannot make them out of metal because they will corrode as a function of time. So, this is a drawback of metal. And then we have hybrids, another thing about metals is typically heavy, except for aluminum, titanium most metals tend to be heavy. Iron has a density of about 8, and gold is very heavy, silver is also heavy, nickel is heavy, all of these metals or most of the engineering metals that I am talking about tend to be heavy. Most of the engineering metals, except copper, aluminum, titanium, and magnesium, are tend to be heavier, and metals are typically made by melting route once you made them.

Then we have hybrids, which tend to be expensive because you have to process them in a specific manner by mixing different classes of materials. Since you use different materials, they are not very easy to shape and join because metals have different joining characteristics ceramics, have different joining characteristics, polymers have different joining characteristics, and they all are process the different temperatures as a result it’s very difficult to make a good shape out of polymer out of composite and to join them. So, the processing is a little difficult in the case of hybrids. However, you can achieve very good properties dependent upon the combination of materials. For example, in a tennis racket, what is that you require in a tennis racket? It should be light; it should be strong, and it should not yield. So, when the tennis racket hits, it should be able to flex a little bit without deforming permanently or breaking. So, that is achieved by making a composite, which is let us say polymer carbon composite.

 So, depending upon what you mix and how you mix and what kind of shape and size of materials are, you can tailor their properties extensively. So, they typically give you high specific strength or modulus, which are required basically in automotive and aircraft applications. So, these are certain examples of materials.

Now let us look at what makes materials important, or how can you engineer them? So, this is called materials tetrahedron, which consists of four parts, one is the structure of materials, and what is this? This particular lecture series is all about structure, but a structure is a very wide connotation; there are various meanings of structure. Then second is properties, like mechanical property, thermal property, electronic property, optical property, etc. The third is processing, how do you make a material, how do you process material to bring it to a particular shape and size that you want and then performance. Performance is related to applications like structural applications, electronic applications, etc.

So, they use a variety of materials, which have very different properties and different functionality. So, how do you make them? There are various methods by which you can make materials, like powder processing, you can start from powder and then make a particular component, or you can start by melting route, which has to be cast. After casting, you may provide some more mechanical treatment like rolling. Depending upon the material, there are a variety of processing methods available.

And then we have mechanical properties, electrical properties, magnetic properties, thermal properties. So, the question is how to measure or how to tailor them? And then we have structure at the end. The structure of a material is looked at various scales, and one is the macro-structure. Macro-structure just like see something with the naked eye. If you want to look at it in a little bit more detail to see how different layers are, is there any porosity, is there any crack, which is not visible with the naked eye, you look at the microstructure, then you go to the microscope.

And if you are not happy with looking at the microstructure, if you want to understand the structure even better, then you have to go to atomic structure which means you really have to go to very fine techniques, and then you have to do some modeling as well, and if you want to understand even the atomic structure, the properties, which are emanated in a material then you look at the electronic structure.

The electronic structure is typically a modeling-based exercise. So, the structure of a material depends upon the length scale that you are talking about, and it could be a macro-structure; it could be a microstructure; it could be an atomic structure, electronic structure. So, you can see that length scales decrease as you go from macro to micro to atomic to electronic. And it is the combination of these four attributes of materials, structure, processing, performance, and properties, which determine the potential of a material.

So, for a given application, you need to optimize the properties, and you need to optimize the process, the process has to be simple, cheap, and easy to make. Properties should be according to the application, and properties are affected by structure; the structure is affected by processing.

Today with the understanding of science, physics, and chemistry, we can classify these materials into various classes metals and alloys, ceramics, plastics, polymers, and elastomers in that category and hybrids or composites.

Now, the question that arises is, what is the difference between these four? Why should we categorize these in materials in these four categories? I showed you a fundamental reason is because of properties. However, there is something more fundamental other than the properties, which are bonding characteristics which determine the structure of these materials.

Before we move on to the discussion on bonding, I will show you how the structure of materials is important. So, this is the structure of materials at various length scales. The structure is visible to the naked eye, called a macro-structure, which means the length scales which are beyond the resolution of the human eye. The microscope could be an optical microscope; it could be a scanning electron microscope, which can help you resolve things down to microns and a few hundred nanometers. For example, these are the fibers or pores aligned inside a certain fashion, which is not visible by the naked eye because of a length scale here. So, this length scale here could be a few microns or submicron. This is not resolvable by eye, and then you need to put it in. So, less than let us say a few hundreds of micrometers, you will call it macro. If you want to go to a higher level of detail, and then you can look at the structure for material at more at a  nanoscale or atomic level. So, here this is a transmission electron microscope image of a material, you can see that you can resolve things down to 0.5 nm, that scale bar you can see is about 10 nm. So, you can resolve things down to one half of one nanometer. So, this is called as nano or atomic structure by proper careful imaging. You can also try to look down the atomic arrangement in a material. Moreover, if you want to understand this even better, then you need to do what we call atomic simulations, which tell you about the atomic structure of the materials.

Now, these are atomic structures, which can go to the atomic level, let us say electronic, and these are. So, this was done by TEM. If you want to go below 1nm, you cannot do microscopy, and you need to do simulations. So, this is by simulations or modeling.

So, these are the four levels of structures, which are present in a material, and it is very important to understand these structures because how the structures are made what the distribution of various things is? What is the size of various things? What is their morphology? How they are oriented and various other things? They will determine what the properties of a material are, and those properties will determine the applicability for a particular application, and basically, the processing controls this structure.

 So, that is why I showed you the tetrahedron, which is very important. So, in the next lecture, we will now we will talk about the bonding of materials to have a bit of idea about what the bonding is and how that particular bonding is related to the classification of materials that we did. Then we will move on to the course, in terms of studying the structures, we will start from the smallest scale first and then go to the largest scale later on so.

For Next Lecture Click below

 Structure of Materials : Bonding in Materials

Structure of Materials : Correlation between bond and physical properties 

Thank you.

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