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lecture #1c3 basic subatomic particles: Proton, electrons and neutronsAtomic structure

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lecture #1cAtomic structureWhat is atom?Basic structural unit of all engineering materials

Atoms = Nucleus (protons+neutrons) + electrons

Electrons:Smallest mass (1/1836 of proton) compare to neutrons (1.675x10-24g) and protons (1.673x10-24g).

Determine most of the electrical, mechanical, chemical and thermal properties of the atoms.

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lecture #1cFoundation of materials science and engineering,William F. Smith,McGrawHill, 2004, pg19-58The Periodic Table

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lecture #1cAtomic Bonding

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lecture #1cValence ElectronsValence electrons are the electrons that occupy the outermost filled shell. These electrons are extremely important as they participate in the bonding between atoms to form atomic and molecular aggregates.Most of the electrical, mechanical, chemical and thermal properties of the atoms of solids are based on these valence electrons.

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lecture #1cPrimary Atomic BondsIonic Bonding Covalent BondingMetallic bondingTypes of BondingSecondary Atomic BondsPermanent dipole bondsFluctuating dipole

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lecture #1cIonic BondingOccurs in compounds that are composed of both metallic and nonmetallic elements which are in the horizontal extremities of the periodic table

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lecture #1cIonic BondingMetallic elements transfer the valence electrons to nonmetallic atoms

To produce ions that are bonded together by coulombic forces(attraction of positively and negatively charged ions).In the process, all the atoms acquire stable or inert gas configurations.Ionic bonding is termed strong and nondirectional, i.e. the magnitude of the bond is equal in all directions around an ion

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lecture #1cFoundation of materials science and engineering,William F. Smith,McGrawHill, 2004, pg19-58Examples:NaCl, MgOThe properties of ionic materials are:i. hard and brittleii. electrically and thermally insulativeIonic Bonding

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lecture #1cIonic Bonding

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lecture #1cIonic materials are poor electrical conductors

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lecture #1c Metallic BondingOccurs in the solid metals and alloys.

In solid state, atoms are packed relatively close together in a systematic pattern or crystal structure.

The valence electrons of metallic materials are not bound to any particular atom in the solid and are more or less free to drift throughout the entire metal.

Valence electrons belong to the metal as a whole, or forming a sea of electrons.

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lecture #1cMetals have low ionization energies, thus they do not have a tight hold on their valence electrons. These outer electrons easily move around, as they do not "belong" to any one atom, but are part of the whole metal crystal Metallic Bonding

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lecture #1cThe remaining non-valence electrons and atomic nuclei form ion cores, which possess a net positive charge equal in magnitude to the total valence electron charge per atom.

The metallic bonding is also nondirectional.Metallic bonding may be strong or weak.The properties of metallic materials are:i. Good ductilityii. Good electrical conductivityiii.Lustrousiv.Malleable

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lecture #1c(a) Atomic arrangement in metallic copper crystal. Each copper atom is coordinated with 12 other copper atoms, producing a crystal structure called face-centered-cubic.(b) Two dimensional schematic diagram of metallically bonded atoms. The circles with the inner positive ion cores, and the charge clouds around the iron cores represent the dispersed valence electrons.

Foundation of materials science and engineering,William F. Smith,McGrawHill, 2004, pg19-58

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lecture #1cMaterials Structure and Bonding Crystal Structure of Metals

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lecture #1cSOLIDSCRYSTALLINEAMORPHOUSShort-range order (SRO) of the positions of the atoms. Examples:ceramic - common window glass, polymers polystyrene & plasticand food - cotton candyThe constituent atoms, molecules, or ions arepacked in a regularlyordered.Examples:Metals, alloys, minerals, sand, clay, limestone, carbon, and salts (NaCl, KCl)Long-range order (LRO) of the positions of the atoms. Random orientation of particles.

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lecture #1cCrystalline SolidsQuartz crystalProperties:Relate to orientation and are dependent on the crystals axes(anisotropic).Crystals become liquids at aspecific temperature (melting point)Single crystalsPoly-crystals

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lecture #1cAmorphous SolidsWax Properties:Identical in all directions along any axis (isotropic).A relatively wide temperature range for the melting point.Soften gradually when they are heated.

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lecture #1cCRYSTALLINESTRUCTUREThe orderly arrangement of atoms in three dimensional space.Space lattice-infinite 3D array of points.-repeating unit cellsEach point in the space lattice has identical surroundings.

Size & shape of unit cell can be described by 3 lattice vectors (a,b,c).

Lattice constants:Axial lengths (a,b,c)Interaxial angles (,,).

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lecture #1cThe arrangement of the atoms is called crystalline structureSpace LatticeUnit cell

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lecture #1cCRYSTAL SYSTEMS14 standard unit cells (Bravais)4 basic types of unit cells:SimpleBody-centeredFace-centeredBase-centered7 different types of unit cells(specify the lattice constants):CubicTetragonalOrthorhombicRhombohedralHexagonalMonoclinicTriclinic

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lecture #1c14 Bravais Conventional Unit Cell

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lecture #1cThe Structure of MetalWhat is metal?What makes metals immediately recognizable?What are the properties of metals that make them such useful materials?What simple experiments can be carried out to distinguish metals from other materials?

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lecture #1cThe Structure of MetalWhy are some metals hard and other soft?Why some metals can withstand high temperatures, while some others not?

Structure - arrangement of atomsInfluence the behavior and structure

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lecture #1cLIQUIDMETALSOLIDIFICATIONUNIT CELL(0.1 nm)BCCFCCHCPCRYSTALSLatticeimperfectionsDislocationsSINGLECRYSTALSPOLY-CRYSTALSGrain boundariesPlastic deformationAnisotropy Products:Solid state devicesTurbine bladesProducts:Paper clips, bolts,springs, aircraft fuselageMetallic Crystal Structures

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lecture #1cMetallic Crystal StructuresMetals crystallize in the dense-packed structures because energy is released. (atoms come close together & bond more tightly with each other)

Body centered cubic (BCC)Face centered cubic (FCC)Hexagonal centered cubic (HCP)

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lecture #1ca=b=c=0.287nmThe cube side of the unit cell of BCC iron at room temperature is 0.287nm. If unit cells of pure iron are lined up side by side, in 1mm there will be:1mm x 1 unit cell = 3.48 x 106 unit cells0.287nm x 10-6mm/nm

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lecture #1cBody centered cubic (BCC)Face centered cubic (FCC)Hexagonal centered cubic (HCP)Metallic Crystal Structure3 densely packed crystal structures:

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lecture #1cBody Centered Cubic (BCC)Atomic-site unit cellHard-sphere unit cellIsolated unit cellHow many atoms ?1(at the center)+ 8 x 1/8 = 2 atoms per unit cell

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lecture #1cBCCProve the relation between the lattice constant a and the atomic radius R..Prove it!

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lecture #1cFace Centered Cubic (FCC)Atomic-site unit cellHard-sphere unit cellIsolated unit cellHow many atoms ?6 x + 8 x 1/8 = 4 atoms per unit cell

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lecture #1cFCCProve it?

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lecture #1cHexagonal Close-Packed (HCP)How many atoms?6 x + 2 x 6 x 1/6 + 2 x 1/2 = 6 atoms per unit cell

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lecture #1cHCPThe ratio of the height c to its basal a called c/a ratioIdeal c/a ratio consisting of uniform spheres: 1.633Atomic-site unit cellBase of Hcp unit cellTriangle ABC removed from base of unit cellProblem: Calculate the volume of the zinc crystal structure unit cell by using the following data; pure zinc has the Hcp crystal structure with lattice constants a = 0.2665 nm and c= 0.4947 nm.

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lecture #1cAnswers..

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lecture #1cAtom Positions in Cubic Unit CellsRectangular x,y,z axes Atom position in BCC unit cell

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lecture #1cComparison of FCC, HCP & BCC Crystal Structures.

Crystal StructureBCCFCC(close packed)HCP(close-packed)Atom positionAtoms are positioned at each corner of the cube and one at the center of the cubeAtoms are positioned at each corner of the cube and one at the center of each cube face ?Atomic Packing Factor=volatoms/volunit cell0.680.740.74Examples:Iron, tungsten, chromium, molybdenum, vanadiumAluminum, copper, lead, nickel, and iron (at 912-1394oC) Cadmium, zinc, magnesium, cobalt, zirconium, titanium, beryllium

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lecture #1cPolymorphism / AllotropyElements and compounds which exist more than one crystalline form under different conditions of temperature and pressureExamples: iron, titanium and cobalt (at elevated temp. and atmospheric pressure).

Temperature Crystal structureAlpha iron-273-912oCBCCGamma iron912-1394oCFCCDelta iron1394-1539oCFCC

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lecture #1cMaterials Structure and BondingDeformation and Strength of Single Crystal

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lecture #1cDeformation and Strength of Single Crystal

Elastic deformation

The lattice structure is shifted, stretched, and distorted but returns to the materials original shape when force removed.

Plastic deformation

Atoms change positions and slip past one another because the atomic bond is broken material does not return to original shape when force removed

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lecture #1cMetallic bonding allows for slip, the basic mechanism by which metals deform plastically when subjected to high stressesThe properties of materials are a function of their:Atomic StructureBonding StructureCrystal StructureImperfectionsIf these various structures are known then the properties of the material can be determined.

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lecture #1cCrystalline ImperfectionsCrystals contain various types of imperfections and defects that affect their physical and mechanical properties.Crystal lattice imperfections are classified according to their geometry and shape. Strength of a material also depends on crystal imperfections, alloying agents, and impurities.

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lecture #1c May be intentional or unintentional. May be present because of an added alloy. Steel is an alloy of Iron (Fe) and Carbon (C). Carbon atoms are much smaller than the iron atoms and appear as interstitial.Crystalline Imperfections

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lecture #1cTypes of Imperfections in the Crystal Structure Zero dimensional / Point defectse.g.: vacancy, interstitialcy (atom or impurity)2.One dimensional / Line defectse.g.: dislocationsTwo dimensional / Planar defectse.g.: grain boundaries, twin boundaries, low angle boundaries, high angle boundaries, twists, and stacking faults.Three dimensional macroscopic / Volume or bulk imperfectione.g.: voids or inclusion (nonmetallic such as oxide,sulfide and silicates)

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lecture #1c1. Point DefectsVacancyMissing atomsProduced during solidification as a result of:local disturbances during crystal growthAtomic rearrangements in an existing crystal due to atomic mobility.Additional vacancies in metals:Plastic deformationRapid cooling from higher temperature to lower temperature to entrap vacancies

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lecture #1cDefects in a Single-Crystal LatticeSchematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional.

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lecture #1cSelf-InterstitialExtra" atoms positioned between atomic sites.

ImpuritiesA foreign atom that has replaced an atom of the pure metal.

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lecture #1c2. Line defectsDislocations Rows of unit cells that become shifted and upset the orderly arrangement. Types of dislocations: EdgeScrewMixedAll crystalline materials contain dislocations introduced during: SolidificationPlastic deformationAs a consequence of thermal stresses resulting from rapid cooling.

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lecture #1cTypes of dislocations in a single crystal: (a) edge dislocation; and (b) screw dislocation.

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lecture #1cMovement of an Edge DislocationMovement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals is much lower than that predicted by theory.

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lecture #1c Dislocations cause slip between crystal plane when they move. They produce permanent (plastic) deformation.(a) Plastic deformation of a single crystal under a tensile load. Note that the slip planes tend to align themselves in the direction of the pulling force. This behavior can be simulated using a deck of cards with a rubber band around them. (b) Twinning in a single crystal in tension.

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lecture #1cSlip and TwinningPlastic deformation of a single crystal subjected to a shear stress: (a) structure before deformation; and (b) permanent deformation by slip. The size of the b/a ratio influences the magnitude of the shear stress required to cause slip.

Elastic deformation

Plastic deformation

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lecture #1cSlip Lines and Slip BandsSchematic illustration of slip lines and slip bands in a single crystal (grain) subjected to a shear stress.

A slip band consists of a number of slip planes. The crystal at the center of the upper illustration is an individual grain surrounded by other grains.

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lecture #1c3. Planar DefectsBoundaries having two dimensions and separate regions of materials having different crystal structure and/or crystallographic orientations.Twin boundaries Occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them.Twin results from atomic displacements produced from mechanical shear force (mechanical twin, found in BCC & HCP) or annealing heat treatment (annealing twin, found in FCC).Responsible for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures.

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lecture #1cTwin plane or boundary and the adjacent atom positions.

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lecture #1cAtoms on one side of a plane (called the twinning plane) are shifted to form a mirror image of the other sideTwinning, involving the formation of an atomic mirror image (i.e., a "twin") on the opposite side of the twinning plane: (a) before, and (b) after twinning

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lecture #1cStacking FaultsFound in FCC when there is an interruption in the ABC ABC stacking sequence of closed-packed planes.

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lecture #1c4. Volume DefectsForm when a cluster of point defects join to form a 3D void or pore. Introduced during processing and fabrication steps.

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lecture #1cSummary Deformation may be elastic or plastic.

Elastic Deformation - lattice structure is distorted but reorients.

Plastic Deformation occurs by slip along lattice where atomic bonds are broken.

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lecture #1cADDITIONAL NOTES

DISLOCATIONA way of explaining two key observations about the plastic deformation of crystalline material: i.The stress required to plastically deform a crystal is much less than the stress one calculates from considering a defect-free crystal structure ii.Materials work-harden: when a material has been plastically deformed it subsequently requires a greater stress to deform further A dislocation in a 2D close-packed plane can be described as an extra 'half-row' of atoms in the structure.

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SLIP & DISLOCATIONExplain how plastic deformation of materials occurs through the mechanism of slip.Slip involves dislocation glide on particular slip planes

Slip occurs when the shear stress acting in the slip direction on the slip plane reaches some critical value. This critical shear stress is related to the stress required to move dislocations across the slip plane.

When a single crystal is deformed under a tensile stress, it is observed that plastic deformation occurs by slip on well-defined parallel crystal planes. Sections of the crystal slide relative to one another, changing the geometry of the sample as shown in the diagram.

The tensile yield stress of a material is the applied stress required to start plastic deformation of the material under a tensile load.

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lecture #1cThank You

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