The Science and Engineering of Materials [Revised]

Chapter 2 Questions

*CHAPTER 2 QUESTIONS*

3-5 What is a liquid crystal material?

A state of matter which has properties between those of conventional liquids and those of solid crystals. For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way.

2.9 Define electronegativity.

An atom which has a type of behavior to gain an electron is called electron negativity. When the electrons in the outer most shell of an atom is incompletely filled, the atom can gain or lose the electrons to become stable. If the atom can gain electrons, it has the property of electron negativity.

4-72 What is meant by the term strain hardening?

As we know, that the dislocation disrupts the perfection of the crystal structures, the dislocation density can be shown to increase as we strain or deform a material. The strengthening of the material is done by the increasing the number of dislocations by deformation or by cold working.

5-8 A certain mechanical component is heat treated using carburization. A commonly encountered engineering problem is that we need to machine a certain part of the component and this part of the surface should not be hardened. Explain how we can achieve this objective.

Case hardening is the best method when we want to harden a certain part of the material and the rest of the parts should not be hardened because it is easy to control the depth, and it is best for complicated parts of the structure.

Chapter 3 Questions

Chapter 3 Questions

Chapter 4 Questions

Chapter 4 Questions

2.1 What is meant by the term COMPOSTION of a material?

Composition can be defined as the combination of one or more materials, which comprise a material's physical, chemical, mechanical, or engineering properties, which can remain distinct and discrete within the completed structure. The properties depend upon all the materials included in the structure. For example, the composition of cement is lime, silica, alumina, iron oxide, magnesium oxide, Sulphur dioxide and alkalis.

2.48 Name at least four allotropes of carbon. Why is graphite electrically conductive while diamond is not if both are pure forms of carbon?

Depending upon temperature and pressure, pure carbon exists as several allotropes. The allotropes of carbon are diamond, graphite, grapheme, Bucky balls and carbon Nano tubes. In graphite each carbon atom is linked to three other carbon atoms forming hexagonal rings. Thus, graphite has layered like structure. These layers are held together by van der Waal's forces which makes each layer to slide over another Due to the electronic delocalization the electrical conductivity is more in each layer. In diamond, each carbon atom is linked to other carbon atoms by forming tetrahedrons with localized covalent bonds which results in relatively smaller electrical conductivity.

5-1 What is the driving force for diffusion?

Diffusion is the spontaneous process in which flux moves from high concentration to lower concentration. Therefore, the driving force for the diffusion is the concentration gradient. It is noticed that temperature can also contribute to the diffusion.

4-48 Arrange the following metals in the expected order of increasing ductility: Cu, Ti, and Fe. Explain

Ductility is the ability of the material to deform under tensile stress. It is often characterized as the material's ability to be stretched into sire. The material which have young's modulus has better ductility. Cu > Ti > Fe

4-49 What are the imperfections in the atomic arrangements that have a significant effect on the material behavior? Give an example of each.

Imperfections that have significant effect on metal behavior are: Stacking Faults: It is an error occurred due to change in the sequence of layers (planes). Example: Suppose the correct sequence of layers in an arrangement is, XYZ XYZ XYZ and suppose the sequence is altered like, XYZ XZY XYZ these types of faults represent the stacking faults. These faults interfere with the slip process of the materials. They mostly occur in metals having FCC structure.

2.29 Materials such as silicon carbide (SiC) and silicon nitride (Si3N4) are used for grinding and polishing applications. Rationalize the choice of these materials for this use.

In grinding and polishing process lots of plastic deformation takes place, hence the grinding material is to be harder and stronger than the material which to be grind or polish. The material, such as silicon carbide and silicon nitride are very strong, very hard, very brittle materials, and have high melting temperatures due to the covalent bonding between their molecules. Due to the brittleness of silicon carbide and silicon nitride, they form sharp and angular grinding particles on the grinding wheel, which gives effective grinding and polishing of the material during the process and due to high melting temperature of silicon carbide and silicon nitride they can withstand under the significant heating at the ground surface.

5-13 What are the different mechanisms for diffusion?

Interdiffusion Vacancy Diffusion Interstitial diffusion

2.38 Would you expect iron or silicon nitride (SiN) to have a higher modulus of elasticity? Why?

Iron has metallic bond mechanism and silicon has a covalent bond mechanism. Compared to the metallic bond, covalent bond is stronger. As the bond is stronger, higher the energy is required to break the bond also the melting point is high. Thus, more force is required to deform the material which results in higher modulus of elasticity. Hence, due to the strong bond mechanism, SiN has comparatively higher modulus of elasticity than iron.

5-7 What is a nitriding heat treatment?

It is the diffusion process in which a heated surface of the material coming in contact with a nitrogen rich compound.

5-47 What factors, other than permeability, are important in selecting a polymer for making plastic bottles?

It should have good malleability, so that it can be used to make any shape of bottle. It should be made from the high density polyethylene because these bottles do not have carcinogens hence good for recycling. Polymer should have good chemical resistance. Poly have have high thermal capacity.

3-11 Define the terms lattice, unit cell, basis, and crystal structure.

LATTICE: It can be defined as a collection of points known as lattice points that are arranged in a periodic pattern so that the surroundings of each point in the lattice are identical. It is purely a mathematical construction and is infinite. It can be one dimensional, two dimensional, and three dimensional. UNIT CELL: It can be defined as the smallest building block by which a crystal is constructed; it consists of molecules, ions or atoms, whose repeated arrangement makes a crystal lattice. BASIS: It can be defined as a group of one or more atoms located in a particular way with respect to each other and it is also associated with each lattice point. It should contain at least one atom; sometimes it contains many atoms of one or more types. CRYSTAL STRUCTURE: When we place the atoms of the basis on every lattice point, then we will obtain a crystal structure. It is an addition of lattice and basis.

5-42 Why is it that inorganic glasses form upon relatively slow cooling of melts, while rapid solidification is necessary to form metallic glasses?

Manufacturing process of metallic glass is different from inorganic glass, because diffusion of atoms in metals increases with the increase in temperature, and when the temperature is increased, the time for diffusion is decreased. At a high temperature, atoms start diffusing to form a uniform structure. Metals heated to a high temperature are cooled at a high rate to decrease the temperature and lower the activation energy. As a result of the sudden cooling process, atoms get encouraged to arrange themselves in non-equilibrium amorphous arrangement and form metallic glasses. However, molten inorganic glass is viscous, and due to the diffusion is slow in the melt silica. Therefore, there is no need to quench inorganic glass.

2.27 What are the bonding mechanisms in thermoplastics?

Materials which become soft when they were given some heat energy and become hard when cooled are called thermoplastics. Thermoplastics can be melted and cooled several times. The bonding mechanism is different for different molecules that form the thermoplastics. The bonding mechanisms in thermoplastics is COVALENT BONDING and VAN DER WAAL'S BONDING.

5-73 Most metals and alloys can be processed using the melting and casting route, but we typically do not choose to apply this method for the processing of specific metals (e.g., W) and most ceramics. Explain.

Metals like tungsten and ceramics have very high melting temperature; therefore they cannot be melted and casted economically, hence for casting such material specific process called sintering is used. In this process powdered metal is pressed and sintered in to the monolithic component reducing the volume of the pores. Many ceramics and metals are manufactured using this process.

2.41 Would you expect MgO or magnesium to have the higher modulus of elasticity? Explain.

Mg has metallic bonds while MgO has ionic bonds. But the metallic bonds in Mg are weaker than the MgO ionic bonds. Also, we know that the separation of bonds needs a higher force which exists in MgO but not in Mg. Hence, the MgO will have a higher modulus of elasticity.

2.5 What is the difference between the microstructure and the macrostructure of a material?

Microstructure is expressed as microns while macrostructure is expressed as macros. Microstructure length scale is 100-100,000 [nm], while macrostructure length scale is greater than microstructure. Microstructure has features such as grain size and defects in materials. Macrostructure has features such as porosity, surface coatings, internal and external micro cracks. Both are written in [um].

2.46 Why is the modulus of elasticity considered a relatively structure-insensitive property?

Modulus of elasticity depends on the binding energy of the atoms in the material that is related to the strength of the bonds rather than the microstructure of the material. It does not change in grain size and shapes. Hence, it is a microstructure insensitive property.

2.19 Compare and contrast metallic and covalent primary bonds in terms of (a) the nature of the bond, (b) the valence of the atoms involved (c) the ductility of the materials bonded in these ways.

NATURE OF BOND: Metallic Bonds = They are strong bonds as they donate their valence electrons, which are positively charged Covalent Bonds = They are also strong as they are formed by the sharing of valence electrons. VALENCE OF THE ELECTRON: Metallic Bonds = Not fixed in one position, and they are good electrical conductors. Covalent Bonds = They are locked in bonds between the atoms and are not available for conduction. They have very high melting point. DUCTILITY: Metallic Bonds = They have good ductility as they are non-directional. Covalent Bonds = They have limited ductility the bonds tend to be directional.

4-20 Do amorphous and crystalline materials plastically deform by the same mechanisms? Explain.

No, amorphous and crystalline materials plastically deform due to different mechanisms. Plastic deformation in amorphous materials is due to sliding of atoms over one another due to the applied stress. This mechanism is called as viscous flow mechanism. In crystalline materials plastic deformation is due to the process of slip which results from the motion of dislocations.

5-4 Why do we use PET plastic to make carbonated beverage bottles?

PET stands for polyethylene terephthalate. PET plastic bottles are used for storing the carbonated, beverages, because PET plastic is excellent moisture barrier material, it does not react with carbon containing drinks or any other beverages. Container made from PET is also used for packaging mouthwash, salad dressing, water, juices, and tennis balls.

4-66 What makes plain carbon steel harder than pure iron?

Plain carbon steel includes the carbon which mainly inhabits interstitial sites and depending on the carbon content in the iron, there is also some Fe3C, due to which eutectic lamellar microstructure is present. All these combines to form very high barriers for the dislocation motion, this makes the steel harder than the pure iron.

5-75 Why does grain growth occur? What is meant by the terms normal and abnormal grain growth?

Polycrystalline material have high energy because of the large number of grain boundary present in it and the atoms in it are not efficiently packed, so this system of grain boundary reduces the energy by grain growth and achieves the stable state. This process produces larger grains from the smaller grains. NORMAL GRAIN GROWTH: This is type of grain growth process in which grains joins in such a way that grain size increases at constant rate but width of grain size distribution does not increases significantly. ABNORMAL GRAIN GROWTH: This is type of process in which grain growth is not equal at every place, at some points grain growth is very large and whereas other site it is small.

5-79 What are the advantages of using hot pressing and hot isostatic pressing compared to using normal sintering?

Produced materials have high density. Materials have isotropic properties. Any type of geometry can be produced which is not possible with normal sintering. High rate of production. Can be used for metal as well as ceramic.

4-69 In structural applications (e.g., steel for bridges and buildings or aluminum alloys for aircraft), why do we use alloys rather than pure metals?

Pure metals are naturally very soft. The alloys are mainly tough and stronger due to the alloying element which act as a barrier to the dislocation motion because they will either occupy the interstitial sites or substitution sites. Therefore we use alloys more than the pure steel in structural applications.

5-15 How is self-diffusion of atoms in metals verified experimentally?

Self-diffusion generally occurs in pure metals, in which the atoms of the same type will migrate in random manner throughout the crystal. Due to no change in the composition, it is difficult to observe self-diffusion. Special techniques like radioactive tracers or negative-ion secondary ion mass spectroscopy are used to verify the self-diffusion process experimentally. In Radioactive Tracers, one radioactive isotope of a metal is placed on the surface of normal metal. The radioactive atoms will move into the normal metal with respect to time. Eventually, all the radio active atoms are distributed throughout the normal metal. This shows that self-diffusion has occurred. Thus, the self-diffusion is verified experimentally.

2.21 What type of bonding does KCl have? Fully explain your reasoning by referring to the electronic structure and electronic properties of each element.

Since there is a transfer of an electron from K to Cl, KCL has ionic bonding.

4-45 Why is it that single crystal and polycrystalline copper are both ductile; however, only single crystal, but not polycrystalline, zinc can exhibit considerable ductility?

Single crystalline materials are those which have only one grain or crystal without grain boundaries whereas polycrystalline materials are those which a number of crystals which are separated by visible boundaries across which there is a possibility for the orientation of crystal plane to change. Copper has FCC structure. This FCC structure supports cross slip mechanism in both single crystalline and poly crystalline copper material to occur. Cross slip occurs in copper because of the presence of several intersecting slip systems. Due to such cross-slip mechanism, copper is said to exhibit ductility. HCP crystal structures like zinc do not intersect the slip system; thus, the dislocation movement does not happen from one crystal of one slip plane to another slip plane of the neighboring crystal. Hence, zinc can exhibit limited ductility.

5-74 What is sintering? What is the driving force for sintering?

Sintering is the process which is used for the metals like tungsten and ceramics which have very high melting temperature; therefore they cannot be melted and casted economically, hence for casting such material sintering is used. In this process powered metal is pressed and sintered in to the monolithic component reducing the volume of the pores. When pressure is applied to the powdered material it gets compacted and forms a shape and particles inside the shape are in contact with each other at many sites with pores between them. Diffusion of atom takes place at a point of contact to reduce the total energy hence as a result of that, pores get shrunk. Many ceramics and metals are manufactured using this process. The driving force for sintering is "loss of total energy by reduction of total surface of the powder particles."

2.45 Steel is coated with a thin layer of ceramic to help protect against corrosion. What do you expect to happen to the coating when the temperature of the steel is increased significantly? Explain.

Steel has a high thermal expansion coefficient and ceramics have low coefficients of thermal expansion. Now, when the steel member is heated, the steel began to expand, but the coating on the other part of the steel expands less as compared to the inner steel. As a result of which the member began to crack in uncertain direction due to which the inner surface of the steel is exposed and unreasonable corrosion occurs to that parts.

4-47 Why is it that cross slip in BCC and FCC metals is easier than in HCP metals? How does this influence the ductility of BCC, FCC, and HCP metals?

The cross slips in hexagonal close-packed (HCP) metals are not easy because the slip planes do not intersect. They appear to be parallel. The cross slips in case of face-centered cubic (FCC) and body-centered cubic (BCC) metals are comparatively possible because they intersect. They create sections of different number of intersections. The HCP being parallel in the slip planes makes polycrystalline brittle. In case of FCC and BCC, the presence of slips creates intersection. They are not found to be brittle at any stage.

2.26 What is the type of bonding in diamond? Are the properties of diamond commensurate with the nature of the bonding?

The type of bonding that exists in diamond is COVALENT BONDING, the bonding that is formed by the sharing of electrons among two or more atoms. YES, the properties of diamond commensurate with the nature of bonding. Diamond is very strong as the covalent bonds are very strong. It has very high melting point and can be used as a semiconductor as covalent bonds have high melting points. They have very high electrical conductivity, and they are also very light weight.

5-27 What is the difference between diffusivity and the diffusion coefficient?

There is no difference.

4-73 To which mechanism of strengthening is the Hall-Petch equation related?

This equation is related to the grain-size strengthening mechanism.

Chapter 5 Questions

Chapter 5 Questions

2.3 What are the different levels of structure of a material?

1. Atomic Structure The structure of an atom that contains negatively charged electrons and positively charged nucleus that is enclosed in the revolving orbits at changing distances from the nucleus, the composition of the nucleus and the arrangement of the electrons differ with several chemical elements. 2. Short and Long Range In short range atomic arrangement, the ions or the atoms have only short range distances in the range of 1A to 10A. In the long range atomic arrangement, the ions or the atoms have a three dimension pattern. They are in the range of 10 [nm] to 10 [cm]. 3. Nanostructure It can be defined as a structure of intermediary dimension between the microscopic and molecular structures. It has a structure of length scale 1 [nm] to 100 [nm] 4. Microstructure They are pronounced as microns. Its length scale is 100 to 100,000 [nm]. 5. Macrostructure They sometimes pronounced as macros.

3-37 What are the different polymorphs of zirconia?

1. MONOCLINIC It is stable up to room temperature at 25 degrees Celsius. 2. TETRAGONAL At temperature 1170 degrees Celsius, the monoclinic zirconia transforms to tetragonal. It is stable up to 2370 degrees Celsius. 3. CUBIC At a temperature of 2370 degrees Celsius, zirconia transforms to cubic. It remains stable from 1170 degrees Celsius to a melting temperature of 2680 degrees Celsius. 4. Orthorhombic This form occurs when high pressure is applied.

3-3 State any two applications in which single crystals are used.

1. Optoelectronic devices which are manufactured or prepared from the crystals of lithium niobate. It can also be made as thin films and can be used for many other applications. 2. Many types of turbine blades can be made from single crystals of nickel-based super alloys. A compute chip is made up of large single crystals.

5-2 Give three examples of materials processes that rely on diffusion in solids and explain how diffusion plays a critical role for one of those processes.

1. Surface Hardening of Steels by the process of Carburization 2. Dopant Diffusion for Semiconductor Devices 3. Oxidation of Aluminum SURFACE HARDENING OF STEELS BY THE PROCESS OF CARBURIZATION: The process of hardening the surface of steel by exposing the outer surface to impurities like carbon or nitrogen is called hardening. If the impurity used consists of carbon particles such as graphite powder or any other gaseous phases containing carbon, the process is called carburization. Generally, the outer surfaces of gears are carburized by the process of diffusion to withstand the wear and tear conditions. Diffusion also plays a key role in the processing of ceramics, joining of materials, and in the control of phase transformations during the heat treatment of various metals and alloys.

3-2 What is a single crystal?

A single grain in a solid material is know as a crystal or a single crystal. Therefore, in a single crystal there are no grain boundaries. The single crystal is 3 dimensional and has a definite shape and pattern and it is the same throughout the volume. Solids are polycrystalline materials. A polycrystalline solid is made of many single crystals. Based on the shape of a single crystal there are seven crystal systems.

3-36 What is the difference between an allotrope and a polymorph?

ALLOTROPE: It is the presence of an element in more than one physical form. It is used for pure metals. There exists multiple bonding in the same phase of matter. They have similar chemical properties. Examples... mercuric iodide, calcium, carbonate POLYMORPH: It is the presence of a substance in one or more crystalline form. It is used for compounds. These are the molecules which have the same chemical formula, but at the same time there will be different arrangements of atoms. They have similar physical properties. Examples... coke, diamond

2.42 Would you expect Al2O3 or aluminum to have the higher coefficient of thermal expansion? Explain.

Al2O3 is a ceramic with strong ionic bonds and a lower thermal expansion coefficient. Aluminum is a metal with weaker metallic bonds and higher thermal expansion coefficient.

3-38 A number of metals undergo an allotropic transformation from one crystal structure to another at a specific temperature. There is generally a volume change that accompanies the transformation. What is the practical significance of such data?

Allotropy is a property of material which has more than one crystal structure. Metals have more than one crystal structure. At higher temperatures, metals can change their crystal structure from one type into another type. The change in volume of material due to temperature results in the change in properties of materials and form the basis for the heat treatment of steels and alloys. The brittleness of the material can be determined using the change in volume.

3-9 Can an alloy exist in both crystalline and amorphous forms?

Alloy is a substance made by the mixture of metals. Alloy has a property tend to form crystalline materials. Amorphous substance exhibits only a short-range order of atoms. Extremely rapid can produce an amorphous alloy. Many engineering materials labeled as amorphous may contain a fraction that is crystalline. Therefore, an alloy exist in both crystalline and amorphous forms.

5-5 Why is it that aluminum metal oxidizes more readily than iron but aluminum is considered to be a metal that usually does not "rust"?

Aluminum has higher oxidation potential than that of iron, so aluminum can lose electrons easily in comparison of iron when both metals come in contact with oxygen, present in the environment. Hence, the oxidation of aluminum to aluminum oxide takes place quicker than iron to iron oxide. As a result of this reaction formed product aluminum oxide forms a protective layer on the surface of aluminum which is invisible and prevents the further corrosion of aluminum, on the other hand, iron oxide exaggerate further rusting.

3-7 Why do some materials assume an amorphous structure?

Amorphous materials tend to form when, for one reason or other, the kinetics of the process by which the material was made did not allow for the formation of periodic arrangements.

2.39 Beryllium and magnesium, both in the 2A column of the periodic table, are lightweight metals. Which would you expect to have the higher modulus of elasticity?

Berylliums' electrons are smaller than the Mg electrons, because of their smaller size they are attached nearer to the core. Each electron being smaller and closer, they are also attached more tightly; therefore, they give a higher binding energy. So Beryllium has a higher a modulus of elasticity than Magnesium.

4-64 Every time we alloy a metal, it gets stronger. Is this true or false? Explain your answer fully.

An alloy is a homogenous mixture of two or more elements with at least one of the elements being a metal. An alloy usually has different properties than its parent metal. Most alloys are formed by heating a metal till they melt and mix them with other elements also in liquid state. It is then solidified. When an alloy is formed, the atoms of one metal/element will cause point defect (or interstitial defects) to another metal and this will form an imperfection. These imperfections will prevent further dislocations in the alloy; they act as 'stop signs' for further dislocations. Any mechanism that starts a dislocation will make a metal stronger. Therefore, every time we alloy a metal, it gets stronger. Therefore, the statement is true.

4-67 Why is jewelry made from gold or silver alloyed with copper?

As we know, that the pure gold is highly malleable and highly dense metal. Malleable means hammer able, as gold can be beaten without difficulty. Dense means more gold for the equal volume of other metal. If jeweler is made from pure gold, it can be crushed with no trouble and be very weighty to wear. Copper has about half of the density of that of gold and it is also ductile in nature so they can drawing into wires or sheets. It is also far low-priced than gold. That's why in jewelry copper is added. Copper is liquefied with gold to make it an alloy, which can then be twisted, drawn or beaten into countless shapes and designs.

4-38 Why is it that dislocations play an important role in controlling the mechanical properties of metallic materials, however, they do not play a role in determining the mechanical properties of glasses?

As, we know that the slip in the materials plays an important role in the mechanical behavior of metals. In metals, slip is present and also dislocations are present which control the mechanical behavior, but in case of amorphous materials where there is no periodic arrangement of ions and also no dislocation, hence it does not play a role in determining the mechanical properties of glasses.

5-46 What are barrier polymers?

Barrier polymers are the macromolecules which have the ability to control the passage of vapor, liquid, and gas. Definition of barrier polymer depends upon its application. Generally, these polymers are used in the food industry to prevent the moisture from outside to enter in the food. This polymer is generally used because of its light weight, usability, and printability.

3-13 Explain why there is no face-centered tetragonal Bravais lattice.

Bravais lattice is an arrangement of lattice points in 14 unique ways. In this lattice, tetragonal crystal system, simple tetragonal, and body centered tetragonal Bravais lattice arrangements. In tetragonal crystal system, the addition of lattice points to get face-centered tetragonal Bravais lattice would result in body-centered tetragonal Bravais lattice. Hence, there is no face-centered tetragonal Bravais Lattice arrangement in a tetragonal crystal system.

4-37 Why would metals behave as brittle materials without dislocations?

Brittleness is defined as the property of the material that breaks without significant deformation when the material is subjected to stress. Even brittle materials have high strength; the material absorbs relatively little energy prior to fracture. As we know that the slip in the materials plays an important role in the mechanical behavior of metals. Dislocation slip is defined as the plastic deformation that occurs in the material. It absorbs energy before the material fracture. Without dislocation, only slight plastic deformation occurs in the material. As the crystals are aligned in a row, the metal behaves as more brittle due to the crystals which are not aligned. Hence, they absorb only small amount of energy before fracture. Therefore, the metal behaves as brittle materials without dislocation.

2.28 Why are covalently bonded materials generally less dense than those that are bonded ionically or metallically?

COVALENT BOND MECHANISM: In the covalent bond mechanism, the atom shares the valence electrons between two or more atoms which have a definite direction and hence covalent bond is directional. Thus, there is a limit for the number of atoms that they can bond. IONIC BOND MECHANISM and METALLIC BOND MECHANISM: The ionic bond mechanism takes place when the material contains more than one atom. One atom donates valence electrons to the other atoms and both atoms acquire electrical charge and behave as ions, whereas, in the metallic bonding mechanism, the elements have a tendency to donate their valence electrons to the surrounding atoms and forms positively charged ions. Thus, metallic and ionic bonds are non-directional and try to increase the possible number of neighbors. As there are more neighbors in the metals and ionic solids, they tend to be closely packed and hence higher density than covalent solids.

5-3 In the carburization treatment of steels, what are the diffusing species?

Carburization treatment of steels is the process in which steel is hardened by the use of diffusion phenomenon. In this process, steel that is having low concentration of carbon is diffused by graphite powder, or any other gaseous like carbon monoxide phase containing high concentration of carbon followed by the heat treatment. As a result of this, carbon from high concentration (Graphite powder or Carbon monoxide) is absorbed at the steel surface, which is having lower concentration of carbon. The diffusing species are charcoal, carbon monoxide gas (CO), sodium cyanide, and barium chloride.

4-34 Why is the theoretical strength of metals much higher than that observed experimentally?

DUE TO THE SLIPPING PROCESS: If the slipping process exists, only a small segment of all the metallic bonds through the interface wants to be fragmented at any one time and the force that is required to deform the metal is small. In practicality, the slipping process occurs in large segment of metal. It has been found that due to the slipping process the actual strength of the metal is 10,000 to 100,000 times lower than that of the expected strength of the metallic bonds. DUE TO VOIDS AND DEFECTS

5-41 In solids, the process of diffusion of atoms and ions takes time. Explain how this is used to our advantage while forming metallic glasses.

Diffusion of atoms in metals will change if temperature changes. For example, when the temperature is increased, the time for the diffusion is decreased. At a high temperature, atoms start diffusing to form a uniform structure. Metals heated to a high temperature are cooled at a high rate to decrease the temperature and lower the activation energy. As a result of the sudden cooling process, atoms get encouraged to arrange themselves in non-equilibrium amorphous arrangement and form metallic glasses.

4-31 Can ceramic and polymeric materials contain dislocations?

Dislocations can be said as the line imperfections in a perfect crystal, which are introduced into the crystals during the process of solidification of the material or at the time of deformation, they are generally present in all materials. There are mainly three types of dislocations: edge, screw, and mix dislocations. Yes, ceramic and polymeric materials can contain dislocations.

5-43 In ionic materials, is diffusion expected to occur at a faster rate for cations or anions? Explain.

Due to the size of cations being small, it is easier for them to squeeze past other ions, move past anions, and finally reach the positive sites. Therefore, in ionic materials, diffusion is expected to occur at a faster rate for cations.

3-45 State any two applications of stabilized zirconia ceramics.

EXAMPLES: 1. They are used as thermal coating blades in the manufacturing of turbine blades. 2. They are also used as electrodes for oxygen sensors and solid oxide fuel cells due to their high oxygen ion conductivity.

4-21 What is the Burger's vector orientation relationship with the dislocation axis for both edge and screw dislocations?

Edge dislocation can be described as an edge of an extra plane of atoms within a crystal structure. And it is characterized by a Burger's vector that is perpendicular to the dislocation axis. Screw dislocations result when planes are displaced relative to each other due to shear. The burgers vector is parallel to the dislocation axis.

5-16 Why is it that the activation energy for diffusion via the interstitial mechanism is less than those for other mechanisms?

For diffusion process, the amount of energy required to break the energy barrier to allow atoms to move from initial position to its new position is known as activation energy for diffusion. In the interstitial mechanism, small interstitial ions or atoms are present in the structure, these interstitial atoms can move from one interstitial site to another without requirement of any vacancies the reason for this is that there are more interstitial sites than number of vacancies. Hence, less amount of activation energy is required as compared to vacancy diffusion.

3-6 What is an amorphous material?

It belongs to the Greek word which means without form. These are the materials that have no shape, but rather to materials with no particular structure. The molecules or atoms of amorphous materials are organized in the same way as they are in a liquid. In this, only short range order is present, no long range order is present.

3-1 What is a crystalline material?

It can be said as the material whose basic atoms, ions, or molecules are organized in an organized configuration that spreads in all 3 spatial dimensions. They help in determining the properties and shape of the material. The large crystals are generally recognizable by macroscopic geometrical shape, which consists of plane faces with detailed and exact representative orientations. For example, a snow flake and a diamond

3-4 What is a polycrystalline material?

It is a kind of material that is composed of many small crystals with varying orientation in space and these minor crystals are identified as grains. The margins among the crystals, wherever the crystals stay in misalignment are recognized as grain borders.

5-6 What is a thermal barrier coating? Where are such coatings used?

It is the process in which coating of a insulator is applied to metal that operates at a high temperature, this coating serves as a protective cover for the structural component against the high temperature and effect of the environment. Thermal barrier coating extends the life of structural parts of machine. They are using in air craft engines, and turbine blades.

4-70 Assuming that we could obtain a 1 wt% solid solution of each of the following elements in aluminum, which of the elements would be expected to produce the greatest strengthening effect: Mg, Mn, Cu, and Zn? Why?

Magnesium will cause the most strengthening effect due to its larger atomic size/radius.

4-33 What is meant by the terms plastic and elastic deformation?

PLASTIC DEFORMATION: It is a process in which sufficient stress is retained on plastic or metal to affect the material or object to alter its shape or size in a way which is not alterable. In other words, we can say that the variations are sometime permanent, also, if the even stress is removed, the material will not retain back to its unique size or shape. It is also referred as plasticity. ELASTIC DEFORMATION: It is a process in which sufficient stress is applied on plastic, metal, or other material to affect the material or object to alter its shape or size in a way which is alterable or reversible. In other words, we can say that the variations are not permanent, also, if the even stress is removed, the material will retain back to its unique size or shape. It is also referred as elasticity.

3-12 List the seven different crystal systems and the types of Bravais lattices that are associated with their groups

Placing atoms on the basis on every lattice point will produce a crystal structure. There are seven crystal structures, which are 1. Cubic Crystal 2. Tetragonal 3. Hexagonal 4. Orthorhombic 5. Rhombohedral 6. Monoclinic 7. Triclinic The three dimensional arrangements of lattice points are call Bravais Lattice. There are fourteen types of Bravais lattices grouped into seven crystal systems. Those are, 1. Simple Cubic 2. Face-Centered Cubic 3. Body-Centered Cubic 4. Simple-Tetragonal 5. Body-Centered Tetragonal 6. Hexagonal 7. Simple Orthorhombic 8. Body-Centered Orthorhombic 9. Base-Centered Orthorhombic 10. Face-Centered Orthorhombic 11. Rhombohedral 12. Simple Monoclinic 13. Base-Centered Monoclinic 14. Triclinic

2.44 Explain why the modulus of elasticity of simple thermoplastic polymers, such as polyethylene and polystyrene, is expected to be very low compared to that of metals and ceramics.

Polymers like polyethylene and polystyrene are bonded with Van der Waals Bonds, which are very weak bonds compared to metallic, covalent, and ionic bonds. Because the bonding is weak, very less force is required to break it.

4-46 Explain why hexagonal close-packed metals tend to have a limited ability to be strain hardened

Strain hardening is increasing the strength of a material by increasing the number of dislocations by plastic deformation. Metals with HCP structures have limited number of slip systems and less number of dislocations. Hence, metals with HCP structure will have relatively lower strain-hardening exponent and they have a limited ability to be strain hardened.

4-65 How do the strengthening mechanisms strain hardening, alloying and grain size strengthening increase materials strength? What do they all have in common? Can more than one mechanism be used in a given alloy? Explain.

Strain hardening, alloying, and grainsize strengthening are the mechanisms for making a material stronger. After a material undergoes said mechanisms, a higher stress is required to force further dislocations past already existing dislocations. The defects that are caused due to strain hardening, alloying, and grain size strengthening serve as stop signs for further dislocations in materials because those materials are now resistant to dislocation motion, and any mechanism delays dislocation motion only makes that material stronger. They all have one thing in common - they are used to increase the strength of materials by imperfections, defects and dislocations. More than one mechanism cannot be used in a given alloy. There is a possibility that subsequent mechanisms can weaken the alloy when they facilitate further dislocations. It is therefore, not possible to control the strength of a alloy by using more than one mechanism on it.

2.2 What is meant by the term STRUCTURE of a material?

Structure can be defined as the fundamental property associated with the identification observation, nature, or permanency of arrangement and relationships of an entity. It can also be the organization of atoms in a molecule of a chemical compound.

2.4 Why is it important to consider the structure of a material when designing and fabricating engineering components?

The atomic structure affects the properties of the materials, their behavior, and their applications. It also affects the type of bonds which holds the materials together. It also helps us to differentiate between the type and class of material such as amorphous or crystalline, metals, semiconductors, ceramics, and polymers. With the help of atomic structure, conclusions can be drawn regarding the physical and mechanical properties of different classes of materials. Hence, it is important to consider the structure of the material in designing and fabrication.

2.20 Differentiate the three principle bonding mechanisms in solids. What is van der Waal's bonding? What are the relative binding energies of the different mechanisms?

The bonding mechanisms in the solids are divided in to four types. Out of these four, three mechanisms are relatively strong are said to be primary bonds. They are, metallic bonds, covalent bonds and ionic bonds. METALLIC BONDING MECHANISM: The elements have a tendency to donate their valence electrons to the surrounding atoms and form a positive charged ion, because these elements are electro positive atoms. This type of bonding is called metallic bonding. COVALENT BONDING MECHANISM: In the covalent bond mechanism, the atom shares the valence electrons among two or more atoms. IONIC BONDING MECHANISM: Ionic bond mechanism takes place when the material contains more than one atom. One atom donates valence electrons to the other atoms and the both atoms acquired electrical charge and behave as ions (cation and anion). VAN DER WAAL'S FORCES MECHANISM: If an atom or molecule possesses either permanent dipole moment or induced dipole moment due to electric field, then there will be attraction force between the atoms. These attraction forces are called van der Waal's forces and the interaction is know as van der Waal's bonding. BINDING ENERGIES OF DIFFERENT MECHANISMS: Ionic Bonding Binding Energy = 150-370 Covalent Bonding Binding Energy = 125-300 Metallic Bonding Binding Energy = 25-200 Van Der Waals < 10

4-32 Why is it that ceramic materials are brittle?

The ceramics materials are normally brittle as the type of bonding that exists in them holds the atoms together. The bonding in them can be covalent bonding, ionic bonding or both kinds. The covalent bonds in ceramics material are directional, as they can make bonds only in certain directions. Whenever a force or load is applied, the bond that exists in them tries to resists the deformation due to which the material is brittle. Also, the potential slip planes may contain like changes which move each other that cause the separation and brittleness to occur.

3-46 Explain the significance of crystallographic directions using an example of an application.

The crystallographic directions are helpful in specifying the particular orientation of a single crystal. As we know how to describe using these crystallographic directions, the orientation becomes helpful in many applications. Let us take an example in which the metal deforms easily in the direction in which their atom is in closest contact. It is easier to magnetize iron in the [100] direction than in the [111] and [110] directions.

2.34 Is there a trend in the number of electrons in the outermost energy shell of atoms or ions that have formed bonds?

The electrons in the outermost shells are loosely bound to the nucleus. The electrons in the inner most shells are tightly bound to the nucleus and needs more energy to remove or add the electron into the inner most shells. The atom gets stability to lose or gain the electrons. So the atom will try to lose or gain the electrons.

3-48 How is the influence of crystallographic direction on magnetic properties used in magnetic materials for recording media applications?

The influence of crystallographic direction on magnetic properties used in magnetic materials for recording media application is valid. It is mostly found in case of polycrystalline materials as they depict a particular orientation.

3-8 Explain how it is possible for a substance to exhibit short-range, but not long-range order.

The material displays short range order when the atoms in the substance are arranged in an irregular manner. Substance like gases has short range order. The substance which contains regular and grid like pattern arrangement atoms, displays long range order. Over large distances, the order is blurred and gives way to disorder. Thus, short range order is observed in most substances. Therefore, a substance exhibits short range order but not long range order.

2.36 Aluminum and silicon are side by side on the periodic table. Compare the melting temperatures of the two elements and explain the difference in terms of atomic bonding.

The melting point of Aluminum is 660 degrees Celsius. Aluminum has metallic bonding mechanism, and it is electropositive element. It will lose its valence electrons at low temperatures and acts as a good conductor of heat. The valence electrons in the outermost shell of aluminum is 3, it will donate three electrons and gets the positive charge of 3. The melting point of silicon is 1410 degrees Celsius. Silicon has a covalent bond mechanism, the valence electrons in the outermost shell of Silicon is four. It will achieve complete outer energy shell by sharing it valence electrons with four surrounding atoms. Since the covalent bond is a strong bond, it is tightly bonded with each neighboring atom. Hence, the melting temperature of silicon is more compared to that of Aluminum as each silicon atom is bonded with four neighboring atoms.

4-68 Why do we prefer to use semiconductor crystals that contain as small a number of dislocations as possible?

The presence of dislocations in the semiconductor has a deleterious effect on the performance of photo detectors, lasers, light emitting diodes and solar cells. The dislocation in these materials originate from the concentration inequalities in the melted form which the crystals are grown and stressed induced because of the thermal gradients that the crystals are exposed during the cooling process from the temperature growth, that is why we prefer the use of semiconductor semi crystals.

5-76 Why is the strength of many metallic materials expected to decrease with increasing grain size?

The reduction in the grain boundary area is the driving force for the grain growth. Free energy of material increases due to presence of the grain boundaries hence grain boundaries are treated as the type of defects. Grain boundaries' have a tendency to transform in the larger grain size and this transformation is thermodynamically favorable. Due to low activation energy, the size of the grain size increases which result in decreases in the grain boundary area hence due to this resistance to the motion of dislocation decreases and also the strength of material decreases.

5-72 Arrange the following materials in increasing order of self-diffusion coefficient: Ar gas, water, single crystal aluminum, and liquid aluminum at 700°C.

The self-diffusion depends upon some factors such as diffusion constant and the concentration gradient. Ar gas < Liquid Aluminum < Single Crystal Aluminum < Water

4-71 Do dislocations control the strength of a silicate glass? Explain.

The silicate glasses are amorphous materials in which elastic deformation exists, means that the change in shape is the result of stretching of interatomic bonds. They also do not have a periodic arrangement of ions and do not contain dislocations, there dislocations cannot control the strength of the silicate glass.

4-74 Pure copper is strengthened by the addition of a small concentration of Be. To which mechanism of strengthening is this related?

When small amount of BE added to the pure copper to strengthened it, then the BE which is present in this copper is mainly in the solution form, hence, we can say that this mechanism of strengthening is related to the solid solution strengthening.

2.35 In order to increase the operating temperature of an engine, it is suggested that some of the aluminum components be coated with a ceramic. What kinds of problems could this pose? How could you overcome these problems?

To increase the operational temperature of an engine, aluminum components are coated with ceramic material. But this will impose some problems because ceramic materials cannot be adhered directly on the engine parts and also the coefficient of thermal expansion is different for ceramic and the aluminum which leads to more expansion of aluminum than coating. Thus, coating may crack. These problems can be avoided by coating the ceramics either by soldering or brazing rather than using adhesives and this process should be done at temperature below 400 degrees Celsius.

5-14 (a) Compare interstitial and vacancy atomic mechanisms for diffusion and (b) cite two reasons why interstitial diffusion is normally more rapid than vacancy diffusion.

VACANCY DIFFUSION: In vacancy diffusion, an atom leaves its lattice site and fills the nearby vacancy, thereby creating a new vacancy at the original lattice site. The atoms and vacancies move in counterclockwise direction as diffusion progresses. Temperature plays an important role for the rate of self-diffusion and the diffusion of substitutional atoms in a crystal. INTERSTITIAL DIFFUSION: In interstitial diffusion, a small interstitial atom or ion moves from one interstitial site to another interstitial site. No need of vacancies for this diffusion to occur. The rate of diffusion increases as the temperature increases.

2.30 Explain the role of van der Waals forces in determining the properties of PVC plastic.

Van der Waals bond is known as secondary bond that consists of different mechanism than the primary bonds and week in nature. In PVC, van der Waals forces are relatively stronger, which leads to stiffness of PVC and provides comparatively higher glass transition temperature. Since the glass temperatures are the temperatures at which PVC tends to act as brittle. So the PVC is comparatively brittle at atmospheric temperature. Hence the role of van der Waal forces is to find the ductility and brittleness of the PVCs.

2.43 Aluminum and silicon are side-by-side in the periodic table. Which would you expect to have the higher modulus of elasticity? Explain.

We know that, in aluminum metallic bonding exists while in silicon covalent bonding exists. Again, the covalent bonds are stronger than the metallic bonds. Also, a higher force is required to cause the separation between the ions. Hence, the modulus of elasticity for silicon is higher than the aluminum.

3-43 Monoclinic zirconia cannot be used effectively for manufacturing oxygen sensors or other devices. Explain

When ceramics components made from pure zirconia lead to fracture at lowered temperatures and also the zirconia transforms from the tetragonal to monoclinic form as there exists a volume expansion. It means the cubic to the tetragonal phase does not cause much change in the volume, so pure zirconia cannot be used directly, hence stabilization is needed by doping it with yttria at room temperature. Hence, the monoclinic zirconia cannot be used effectively for the formation of the oxygen sensors and other devices.

4-22 What is a slip system and what role does it play in plastic deformation?

When there is a dislocation motion in certain crystal structures, dislocation occurs in a preferred plane and in a specific direction. This plane and the direction of the dislocation motion together constituent a slip system. Slip plane is generally taken as the closest packed plane in the system. Slip direction is taken as the direction on the slip plane with the highest linear density. When a sufficiently large shear stress is applied on a material, dislocation in crystal structures takes place in a crystallographic plane. Due to this dislocation, slip occurs between the planes of the crystal structure. Due to the applied stress the bond across the slip plane between atoms break which results in slip of numerous dislocations. The effect of this slip of numerous dislocations is plastic deformation.

3-47 Why are Fe-Si alloys used in magnetic applications "grain oriented"?

With the help of crystallographic directions to indicate the direction, the metal deforms easily in the direction in which their atom is in closest contact. It is easier to magnetize iron in the [100] direction in the [111] and [110] directions. This is why the grains in the Fe-Si alloys used in the magnetic applications are "grain oriented".