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1.) Atomic StructureEach atom consists of a very small nucleus composed of protons and neutrons, which is encircled by

moving electrons. Both electrons and protons are electrically charged, the charge magnitude being 1.60 _ 10_19 C,which is negative in sign for electrons and positive for protons; neutrons are electrically neutral. Masses for thesesubatomic particles are infinitesimally small; protons and neutrons have approximately the same mass, 1.67 _10_27 kg, which is significantly larger than that of an electron, 9.11 _ 10_31 kg.

Each chemical element is characterized by the number of protons in the nucleus,or the atomic number (Z).1For an electrically neutral or complete atom, the atomic number also equals the number of electrons. This atomicnumber ranges in integral units from 1 for hydrogen to 92 for uranium, the highest of the naturally occurringelements.

The atomic mass (A) of a specific atom may be expressed as the sum of themasses of protons andneutrons within the nucleus. Although the number of protons is the same for all atoms of a given element, thenumber of neutrons (N) may be variable. Thus atoms of some elements have two or more different atomic masses,which are called isotopes. The atomic weight of an element corresponds to the weighted average of the atomicmasses of the atoms naturally occurring isotopes.The atomic mass unit (amu) may be used for computations ofatomic weight. A scale has been established whereby 1 amu is defined as ___ of the atomic mass ofthe most

common isotope of carbon, carbon 12 (12C) (A _ 12.00000). Within this scheme, the masses of protons andneutrons are slightly greater than unity, and A _ Z _ N (2.1)

The atomic weight of an element or the molecular weight of a compound may be specified on the basis ofamu per atom (molecule) or mass per mole of material. In one mole of a substance there are 6.023 _ 1023(Avogadros number) atoms or molecules. These two atomic weight schemes are related through the followingequation: 1 amu/atom (or molecule) _ 1 g/mol

For example, the atomic weight of iron is 55.85 amu/atom, or 55.85 g/mol. Sometimes use of amu per atom ormolecule is convenient; on other occasions g (or kg)/molis preferred; the latter is used in this book.

2.) Crystal StructureSolid materials may be classified according to the regularity with which atoms orions are arranged with

respect to one another. A crystalline material is one in whichthe atoms are situated in a repeating or periodic arrayover large atomic distances;that is, long-range order exists, such that upon solidification, the atoms willpositionthemselves in a repetitive three-dimensional pattern, in which each atom is bondedto its nearest-neighboratoms. All metals, many ceramic materials, and certainpolymers form crystalline structures under normalsolidification conditions. Forthose that do not crystallize, this long-range atomic order is absent; thesenoncrystallineor amorphous materials are discussed briefly at the end of this chapter.Some of the properties ofcrystalline solids depend on the crystal structure ofthe material, the manner in which atoms, ions, or molecules arespatially arranged.There is an extremely large number of different crystal structures all having longrangeatomic

order; these vary from relatively simple structures for metals, toexceedingly complex ones, as displayed by someof the ceramic and polymeric

Crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. Thearrangement of atoms, ions, or molecules in a crystal. Crystals are solids having, in all three dimensions of space,a regular repeating internal unit of structure.

Crystals have been studied using x-rays, which excite signals from the atoms. The signals are of differentstrengths and depend on the electron density distribution about atomic cores. Light atoms give weaker signals andhydrogen is invisible to x-rays. However, the mutual atomic arrangements that are called crystal structures can be

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derived once the chemical formulas and physical densities of solids are known, based on the knowledge thatatomic positions are not arbitrary but are dictated by crystal symmetry, and that the diffraction signals received arethe result of systematic constructive interference between the scatterers within the regularly repeating internal unitof pattern.

Simple crystal structures are usually named after the compounds in which they were first discovered(diamond or zinc sulfide, cesium chloride, sodium chloride, and calcium fluoride). Many compounds of the typesA+X and A2+X2 have such structures. They are highly symmetrical, the unit cell is cubic, and the atoms or ions aredisposed at the corners of the unit cell and at points having coordinates that are combinations of 0, 1, , or .

NONCRYSTALLINE SOLIDS

It has been mentioned that noncrystalline solids lack a systematic and regulararrangement of atoms overrelatively large atomic distances. Sometimes such materials are also called amorphous (meaning literally withoutform), or supercooledliquids, inasmuch as their atomic structure resembles that of a liquid. An amorphouscondition may be illustrated by comparison of the crystalline and noncrystalline structures of the ceramiccompound silicon dioxide (SiO2), which may exist in both states. Even though each silicon ion bonds to fouroxygen ions for both states, beyond this, the structure is much more disordered and irregular for thenoncrystalline structure. Whether a crystalline or amorphous solid forms depends on the ease with which arandom atomic structure in the liquid can transform to an ordered state during solidification. Amorphous materials

therefore, are characterized by atomic or molecular structures that are relatively complex and become ordered onlywith some difficulty. Furthermore, rapidly cooling through the freezing temperature favors the formation of anoncrystalline solid, since little time is allowed for the ordering process.Metalsnormally form crystalline solids; butsome ceramic materials are crystalline,whereas others (i.e., the silica glasses) are amorphous. Polymers may becompletely noncrystalline and semicrystalline consisting of varying degrees of crystallinity.

3.) Substructurey point defects, which are places where an atom is missing or irregularly placed in the lattice structure. Pointdefects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atomsy linear defects, which are groups of atoms in irregular positions. Linear defects are commonly calleddislocations.y planar defects, which are interfaces between homogeneous regions of the material. Planar defects includegrain boundaries, stacking faults and external surfaces.

4.) Microstructure

Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by amicroscope above 25 magnification.[1] The microstructure of a material (which can be broadly classifiedinto metallic, polymeric, ceramic and composite) can strongly influence physical properties such as strength,

toughness, ductility, hardness, corrosion resistance, high/low temperature behavior, wear resistance, and so on,which in turn govern the application of these materials in industrial practice.

5.) Atomic Bonding

Atomic bonding is chemical bonding. Chemical bonding is the physical process that is responsible for theinteractions between atoms and molecules. Bonds vary widely. There are covalent, ionic, hydrogen, metallic, aswell as many other types of bonds, and all have a working connection in all living things. There are two differenttypes of atomic bonds: primary and secondary. The primary bonds produce chemical bonds that hold atomstogether.

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All chemical bonds involve electrons. Atoms will stay close together if they have a shared interest in one or

more electrons. Atoms are at their most stable when they have no partially-filled electron shells. If an atom hasonly a few electrons in a shell, it will tend to lose them to empty the shell. These elements are metals. When metalatoms bond, a metallic bond occurs. When an atom has a nearly full electron shell, it will try to find electrons fromanother atom so that it can fill its outer shell. These elements are usually described as nonmetals. The bondbetween two nonmetal atoms is usually a covalent bond. Where metal and nonmetal atom come together an ionicbond occurs. There are also other, less common, types of bond but the details are beyond the scope of this

material. On the next few pages, the Metallic, Covalent and Ionic bonds will be covered in more detail.

Types of Atomic Bonds

o Atomic bonds have two types of bonds: primary and secondary bonds. The primary bonds havethree types of bonds: metallic, covalent and ionic. The secondary bonds also subsections of bonds, and areconsidered the weaker elements.Metallic Bondo Metallic bonds are a metal, and share outer bonds with atoms in a solid. Each atom gives off apositive charge by shedding its outer electrons, and the negatively charges electrons hold the metal atomstogether.

Ionic Bondo Atoms are filled with an outer shell of electrons. Electron shells are filled by transferring electronsfrom one atom to the next. Donor atoms will take on a positive charge, and the acceptors will have a negativecharge. They will attract each other by being positive and negative, and bonding will then occur.

Covalent Bondso Atoms like to share their electrons and this causes their outer shell to be complete.A covalentbond is produced by the sharing of atoms and electrons. This produces a strong covalent bond.Secondary Bondso Secondary bonds are significantly weaker than primary bonds in that they often produce weaklinks, and create deformations in the bond. Secondary bonds include hydrogen and van der waals bonds.Hydrogen Bondso A common bond is a hydrogen bond. They are most common in covalently bonded molecules thatcontain hydrogen. Hydrogen bonds share between covalent and oxygenated atoms. This leads to very smallelectrical charges around the hydrogen bond, and negative charges around the oxygenated bonds.Van der Waals Bondso Van der waals bonds are the weakest bond, but are incredibly important gases, that are cooled atlow temperatures. These bonds are created by small charges of positive and negative electron that produce a weakbond. Van der waals bonds are overwhelmed by thermal energy, causing them to malfunction.

6.) Crystal Defects

A perfect crystal, with every atom of the same type in the correct position, does not exist. All crystals havesome defects. Defects contribute to the mechanical properties of metals. In fact, using the term defect is sort of amisnomer since these features are commonly intentionally used to manipulate the mechanical properties of amaterial.Adding alloying elements to a metal is one way of introducing a crystal defect. Nevertheless, the termdefect will be used, just keep in mind that crystalline defects are not always bad. There are basic classes ofcrystal defects:y point defects, which are places where an atom is missing or irregularly placed in the lattice structure. Pointdefects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atomsy linear defects, which are groups of atoms in irregular positions. Linear defects are commonly calleddislocations.

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y planar defects, which are interfaces between homogeneous regions of the material. Planar defects includegrain boundaries, stacking faults and external surfaces.

It is important to note at this point that plastic deformation in a material occurs due to the movement ofdislocations (linear defects). Millions of dislocations result for plastic forming operations such as rolling andextruding. It is also important to note that any defect in the regular lattice structure disrupts the motion ofdislocation, which makes slip or plastic deformation more difficult. These defects not only include the point andplaner defects mentioned above, and also other dislocations.Dislocation movement produces additionaldislocations, and when dislocations run into each other it often impedes movement of the dislocations. This drives

up the force needed to move the dislocation or, in other words, strengthens the material.

7) Line Defects Dislocations- groups of atoms in irregular positions(e.g. screw and edge dislocations).Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out of

position in the crystal structure. Dislocations are generated and move when a stress is applied. The motion ofdislocations allows slip plastic deformation to occur.

Before the discovery of the dislocation by Taylor, Orowan and Polyani in 1934, no one could figure out howthe plastic deformation properties of a metal could be greatly changed by solely by forming (without changing the

chemical composition). This became even bigger mystery when in the early 1900s scientists estimated that metalsundergo plastic deformation at forces much smaller than the theoretical strength of the forces that are holding themetal atoms together.

Many metallurgists remained skeptical of the dislocation theory until the development of the transmissionelectron microscope in the late 1950s. The TEM allowed experimental evidence to be collected that showed thatthe strength and ductility of metals are controlled by dislocations.

There are two basic types of dislocations, the edge dislocation and the screw dislocation.Actually, edgeand screw dislocations are just extreme forms of the possible dislocation structures that can occur. Mostdislocations are probably a hybrid of the edge and screw forms but this discussion will be limited to these two

types.

Edge DislocationsThe edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a linedefect because the locus of defective points produced in the lattice by the dislocation lie along a line. This lineruns along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediatevicinity of the dislocation line.

Understanding the movement of a dislocation is key to understanding why dislocations allow deformation to occurat much lower stress than in a perfect crystal. Dislocation motion is analogous to movement of a caterpillar. Thecaterpillar would have to exert a large force to move its entire body at once. Instead it moves the rear portion of itsbody forward a small amount and creates a hump. The hump then moves forward and eventual moves all of thebody forward by a small amount.

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The dislocation moves similarly moves a small amount at a time. The dislocation in the top half of the

crystal is slipping one plane at a time as it moves to the right from its position in image (a) to its position in image(b) and finally image (c). In the process of slipping one plane at a time the dislocation propagates across thecrystal. The movement of the dislocation across the plane eventually causes the top half of the crystal to movewith respect to the bottom half. However, only a small fraction of the bonds are broken at any given time.Movement in this manner requires a much smaller force than breaking all the bonds across the middle planesimultaneously.Screw Dislocations

There is a second basic type of dislocation, called screwdislocation. The screw dislocation is slightly more difficult tovisualize. The motion of a screw dislocation is also a result ofshear stress, but the defect line movement is perpendicular todirection of the stress and the atom displacement, rather thanparallel. To visualize a screw dislocation, imagine a block of metalwith a shear stress applied across one end so that the metalbegins to rip. This is shown in the upper right image. The lowerright image shows the plane of atoms just above the rip. Theatoms represented by the blue circles have not yet moved fromtheir original position. The atoms represented by the red circles

have moved to their new position in the lattice and havereestablished metallic bonds. The atoms represented by thegreen circles are in the process of moving. It can be seen thatonly a portion of the bonds are broke at any given time. As wasthe case with the edge dislocation, movement in this mannerrequires a much smaller force than breaking all the bonds acrossthe middle plane simultaneously.

If the shear force is increased, the atoms will continue toslip to the right. A row of the green atoms will find there way backinto a proper spot in the lattice (and become red) and a row of theblue atoms will slip out of position (and become green). In this

way, the screw dislocation will move upward in the image, whichis perpendicular to direction of the stress. Recall that the edgedislocation moves parallel to the direction of stress. As shown inthe image below, the net plastic deformation of both edge andscrew dislocations is the same, however.

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The dislocations move along the densest planes of atoms in a material, because the stress needed to move

the dislocation increases with the spacing between the planes. FCC and BCC metals have many dense planes, sodislocations move relatively easy and these materials have high ductility. Metals are strengthened by making itmore difficult for dislocations to move. This may involve the introduction of obstacles, such as interstitial atoms orgrain boundaries, to pin the dislocations.Also, as a material plastically deforms, more dislocations are producedand they will get into each others way and impede movement. This is why strain or work hardening occurs.

In ionically bonded materials, the ion must move past an area with a repulsive charge in order to get to thenext location of the same charge. Therefore, slip is difficult and the materials are brittle. Likewise, the low densitypacking of covalent materials makes them generally more brittle than metals.

8) Planar Defects- Stacking Faults and Twin Boundaries

A disruption of the long-range stacking sequence can produce two other common types of crystal defects:1) a stacking fault and 2) a twin region. A change in the stacking sequence over a few atomic spacings produces astacking fault whereas a change over many atomic spacings produces a twin region.

A stacking fault is a one or two layer interruption in the stacking sequence of atom planes. Stacking faultsoccur in a number of crystal structures, but it is easiest to see how they occur in close packed structures. Forexample, it is know from a previous discussion that face centered cubic (fcc) structures differ from hexagonalclose packed (hcp) structures only in their stacking order. For hcp and fcc structures, the first two layers arrangethemselves identically, and are said to have an AB arrangement. If the third layer is placed so that its atoms aredirectly above those of the first (A) layer, the stacking will be ABA. This is the hcp structure, and it continuesABABABAB. However it is possible for the third layer atoms to arrange themselves so that they are in line with the

first layer to produce an ABC arrangement which is that of the fcc structure. So, if the hcp structure is going alongas ABABAB and suddenly switches to ABABABCABAB, there is a stacking fault present.Alternately, in the fcc arrangement the pattern is ABCABCABC. A stacking fault in an fcc structure would appear asone of the C planes missing. In other words the pattern would become ABCABCAB_ABCABC.

If a stacking fault does not corrects itself immediately but continues over some number of atomic spacingsit will produce a second stacking fault that is the twin of the first one. For example if the stacking pattern isABABABAB but switches to ABCABCABC for a period of time before switching back to ABABABAB, a pair of twinstacking faults is produced. The red region in the stacking sequence that goes ABCABCACBACBABCABC is thetwin plane and the twin boundaries are the A planes on each end of the highlighted region.

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Grain Boundaries in Polycrystals

Another type of planer defect is the grain boundary. Up to this point, the discussion has focused on defectsof single crystals. However, solids generally consist of a number of crystallites or grains. Grains can range in sizefrom nanometers to millimeters across and their orientations are usually rotated with respect to neighboringgrains. Where one grain stops and another begins is know as a grain boundary. Grain boundaries limit the lengthsand motions of dislocations. Therefore, having smaller grains (more grain boundary surface area) strengthens a

material. The size of the grains can be controlled by the cooling rate when the material cast or heat treated.Generally, rapid cooling produces smaller grains whereas slow cooling result in larger grains. For moreinformation, refer to the discussion on solidification.

9.) Point Defects

Point defects are where an atom is missing or is in an irregularplace in the lattice structure. Point defects include self interstitialatoms, interstitial impurity atoms, substitutional atoms and vacancies.A self interstitial atom is an extra atom that has crowded its way into

an interstitial void in the crystal structure. Self interstitial atoms occuronly in low concentrations in metals because they distort and highlystress the tightly packed lattice structure.

A substitutional impurity atom is an atom of a different typethan the bulk atoms, which has replaced one of the bulk atoms in thelattice. Substitutional impurity atoms are usually close in size (withinapproximately 15%) to the bulk atom. An example of substitutionalimpurity atoms is the zinc atoms in brass. In brass, zinc atoms with aradius of 0.133 nm have replaced some of the copper atoms, whichhave a radius of 0.128 nm.

Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fitinto the open space between the bulk atoms of the lattice structure. An example of interstitial impurity atoms is thecarbon atoms that are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the openspaces between the larger (0.124 nm) iron atoms.

Vacancies are empty spaces where an atom should be, but is missing. They are common, especially at hightemperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. Inmost cases diffusion (mass transport by atomic motion) can only occur because of vacancies.

10.) Bulk Defects

Bulk defects occur on a much bigger scale than the rest ofthe crystal defects discussed in this section. However, for the sakeof completeness and since they do affect the movement ofdislocations, a few of the more common bulk defects will bementioned. Voids are regions where there are a large number ofatoms missing from the lattice. The image to the right is a void in apiece of metal The image was acquired using a Scanning ElectronMicroscope (SEM). Voids can occur for a number of reasons. Whenvoids occur due to air bubbles becoming trapped when a material

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solidifies, it is commonly called porosity. When a void occurs due to the shrinkage of a material as it solidifies, it iscalled cavitation.

Another type of bulk defect occurs when impurity atoms cluster together to form small regions of adifferent phase. The term phase refers to that region of space occupied by a physically homogeneous material.These regions are often called precipitates

Reference(s):

http://www.careercornerstone.org/pdf/matscieng/matscience.pdfhttp://en.wikipedia.org/wiki/Microstructurehttp://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/bonds.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/crystal_defects.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/linear_defects.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/planar_defects.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/point_defects.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/bulk_defects.htm

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TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES

1338 ARLEGUI STREET, QUIAPO, MANILA

FUNDAMENTALS

OFMATERIAL SCIENCE

AND

ENGINEERING(EC353)

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SUBMITTED BY:

VERGARA, RAYMOND JOHN R.BS ECE / 0910750 / ES31FA1

MS. NELOR JANE LAGUNAINSTRUCTOR