THE NATURE OF MATERIALS

Industrial Material Applications, IE251Dr M. A. EissaKing Saud UniversityCollege of EngineeringDepartment of Industrial EngineeringLec. 1 THE NATURE OF MATERIALSTHE NATURE OF MATERIALSAtomic Structure and the ElementsBonding between Atoms and MoleculesCrystalline StructuresNoncrystalline (Amorphous) StructuresImportance of Materials in ManufacturingManufacturing is a transformation process

It is the material that is transformed

And it is the behavior of the material when subjected to the forces, temperatures, and other parameters of the process that determines the success of the operation Atomic Structure and the ElementsAtomic Structure and the ElementsThe basic structural unit of matter is the atom Each atom is composed of a positively charged nucleus, surrounded by a sufficient number of negatively charged electrons so the charges are balanced More than 100 elements, and they are the chemical building blocks of all matter

Element GroupingsThe elements can be grouped into families and relationships established between and within the families by means of the Periodic Table

Metals occupy the left and center portions of the tableNonmetals are on rightBetween them is a transition zone containing metalloids or semimetalsMetalsMetalloids or SemimetalsNonMetalsBeryllium BeBoron BHelium HeLithium LiSilicon SiNeon NeMagnesium MgArsenic AsArgon ArCadmium CdAntimony SbKrypton KrCopper- CuPolonium - PoXenon XeIron FeTellurium - TeRadon RnZinc ZnGermanium - GeFluorine FTitanium TiChlorine ClGold AuOxygen OFigure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.

Periodic TableQuestion?What are the noble metals?

CopperSilverGoldNoble metals (precious metals) are metals that are resistant to corrosion or oxidation, unlike most base metals. Platinum (Pt), Palladium (Pd) Bonding between Atoms and MoleculesBonding between Atoms and MoleculesAtoms are held together in molecules by various types of bondsPrimary bonds - generally associated with formation of moleculesSecondary bonds - generally associated with attraction between moleculesPrimary bonds are much stronger than secondary bonds

Bonding between Atoms and MoleculesPrimary BondingSecondaryBondingIonicCovalentMetallicDipole forcesLondon forcesHydrogen bondingPrimary BondsCharacterized by strong atomtoatom attractions that involve exchange of valence electrons Following forms: IonicCovalentMetallic The ones on the outer shell Ionic BondingFigure 2.4 First form of primary bonding: (a) Ionic

Atoms of one element give up their outer electron(s), which are in turn attracted to atoms of some other element to increase electron count in the outermost shell.

Properties:Poor DuctilityLow Electrical ConductivityExample: Sodium Chloride (NaCl)Covalent BondingFigure 2.4 Second form of primary bonding: (b) covalent

Outer electrons are shared between two local atoms of different elements.

Properties:High HardnessLow Electrical ConductivityExamples: Diamond, GraphiteMetallic BondingFigure 2.4 Third form of primary bonding: (c) metallic

Outer shell electrons are shared by all atoms to form an electron cloud.

Properties:- Good Conductor (Heat and Electricity)- Good DuctilitySecondary BondsSecondary bonds involve attraction forces between molecules (whereas primary bonds involve atomtoatom attractive forces), No transfer or sharing of electrons in secondary bondingBonds are weaker than primary bondsThree forms: Dipole forcesLondon forcesHydrogen bonding Macroscopic Structures of MatterAtoms and molecules are the building blocks of more macroscopic structure of matterWhen materials solidify from the molten state, they tend to close ranks and pack tightly, arranging themselves into one of two structures: CrystallineNoncrystallineCrystalline StructuresCrystalline StructureStructure in which atoms are located at regular and recurring positions in three dimensions

Unit cell - basic geometric grouping of atoms that is repeated The pattern may be replicated millions of times within a given crystal Characteristic structure of virtually all metals, as well as many ceramics and some polymersCrystallinityWhen the monomers are arranged in a neat orderly manner, the polymer is crystalline. Polymers are just like socks. Sometimes they are arranged in a neat orderly manner.

An amorphous solid is a solid in which the molecules have no order or arrangement. Some people will just throw their socks in the drawer in one big tangled mess. Their sock drawers look like this:Question?"What is glass... is it a liquid or a solid?" What about glass?! Does glass have a crystalline structure?!

Antique windowpanes are thicker at the bottom, because glass has flowed to the bottom over time! Glass has no crystalline structure, hence it is NOT a solid. Glass is a supercooled liquid. Glass is a liquid that flows very slowly. Glass is a highly viscous liquid!! Three Crystal Structures in MetalsBody-centered cubic (BCC)Face centered cubic (FCC)Hexagonal close-packed (HCP)

Figure 2.8 Three types of crystal structure in metals.# of atoms in unit cell: 9# of atoms: 14# of atoms: 17Crystal Structures for Common Metals Room temperature crystal structures for some of the common metals:Bodycentered cubic (BCC) Chromium, Iron, Molybdenum, TungstenFacecentered cubic (FCC) Aluminum, Copper, Gold, Lead, Silver, Nickel, (Iron at 1670oF)Hexagonal closepacked (HCP)Magnesium, Titanium, ZincBody-Centered Cubic Crystal Structure

Figure 1.2 The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, 1976.Face-Centered Cubic Crystal Structure

Figure 1.3 The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, 1976.Hexagonal Close-Packed Crystal Structure

Figure 1.4 The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, 1976. Imperfections (Defects) in CrystalsImperfections often arise due to inability of solidifying material to continue replication of unit cell, e.g., grain boundaries in metals It is in fact: Deviation in the regular pattern of the crystalline lattice structure.Studying about imperfections is important:Imperfection is bad: a perfect diamond (with no flaws) is more valuable than one containing imperfections.Imperfection is good: the addition of an alloying ingredient in a metal to increase its strength (this is an imperfection which is introduced purposely).

Types of defects or imperfectionsPoint defects,Line defects,Surface defects.

Point DefectsImperfections in crystal structure involving either a single atom or a few number of atoms

Figure 2.9 Point defects: (a) vacancy, (b) ionpair vacancy (Schottky), (c) interstitialcy, (d) displaced ion (Frenkel Defect).Extra atom presentDislocation of an atomDefects in a Single-Crystal Lattice

Figure 1.9 Schematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional.Line Defects Defect happens along a line ( Connected group of point defects that forms a line in the lattice structure)

Most important line defect is a dislocation, which can take two forms: Edge dislocation Screw dislocationEdge DislocationFigure 2.10 Line defects: (a) edge dislocation

Edge of an extra plane of atoms that exists in the lattice

Movement of an Edge Dislocation

Figure 1.10 Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals in much lower than that predicted by theory.Screw DislocationFigure 2.10 Line defects: (b) screw dislocation

Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis

Surface Defects Imperfections that extend in two directions to form a boundary Examples:External: the surface of a crystalline object is an interruption in the lattice structure Internal: grain boundaries are internal surface interruptions Elastic Strain

Elastic StrainWhen a crystal experiences a gradually increasing stress, it first deforms elasticallyIf force is removed lattice structure returns to its original shape

Figure 2.11 Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, with no permanent change in positions of atoms.Plastic StrainIf stress is higher than forces holding atoms in their lattice positions, a permanent shape change occurs

Figure 2.11 Deformation of a crystal structure: (c) plastic deformation (slip), in which atoms in the lattice are forced to move to new "homes.Effect of Dislocations on StrainIn the series of diagrams, the movement of the dislocation allows deformation to occur under a lower stress than in a perfect lattice.Slip involves the relative movement of atoms on the opposite sides of a plane in the lattice, called slip plane.

Figure 2.12 Effect of dislocations in the lattice structure under stress.Slip on a Macroscopic ScaleWhen a lattice structure with an edge dislocation is subjected to a shear stress, the material deforms much more readily than in a perfect structure.

Dislocations are a goodnewsbadnews situation Good news in manufacturing the metal is easier to formBad news in design the metal is not as strong as the designer would likeTwinningA second mechanism of plastic deformation in which atoms on one side of a plane (the twinning plane) are shifted to form a mirror image of the other side

Figure 2.13 Twinning, involving the formation of an atomic mirror image on the opposite side of the twinning plane: (a) before, and (b) after twinning.Polycrystalline Nature of MetalsA block of metal may contain millions of individual crystals, called grains Such a structure is called polycrystalline Each grain has its own unique lattice orientation; but collectively, the grains are randomly oriented in the block

Crystalline StructureHow do polycrystalline structures form? As a block of metal cools from the molten state and begins to solidify, individual crystals nucleate at random positions and orientations throughout the liquid These crystals grow and finally interfere with each other, forming at their interface a surface defect a grain boundaryGrain boundaries are transition zones, perhaps only a few atoms thick

Growth of crystals in metalsGrainGrainboundaryNoncrystalline (Amorphous) StructuresNoncrystalline (Amorphous) StructuresMany materials are noncrystallineWater and air have noncrystalline structuresA metal loses its crystalline structure when melted

Some important engineering materials have noncrystalline forms in their solid state:GlassMany plastics Rubber

Features of Noncrystalline StructuresTwo features differentiate noncrystalline (amorphous) from crystalline materials:

Absence of longrange order in molecular structure Differences in melting and thermal expansion characteristics

What are the differences between them?Crystalline versus NoncrystallineFigure 2.14 Difference in structure between: (a) crystalline and (b) noncrystalline materials.

The crystal structure is regular, repeating, and denserThe noncrystalline structure is random and less tightly packed.Solidification

Alloy MetalPure MetalVolumetric EffectsFigure 2.15 Characteristic change in volume for a pure metal (a crystalline structure), compared to the same volumetric changes in glass (a noncrystalline structure).

Tg=glass temperatureTm=melting temperatureSummary: Characteristics of MetalsCrystalline structures in the solid state, almost without exception BCC, FCC, or HCP unit cellsAtoms held together by metallic bondingProperties: high strength and hardness, high electrical and thermal conductivityFCC metals are generally ductileSummary: Characteristics of CeramicsMost ceramics have crystalline structures, while glass (SiO2) is amorphous Molecules characterized by ionic or covalent bonding, or both Properties: high hardness and stiffness, electrically insulating, refractory, and chemically inertRefractory materials retain their strength at high temperatures. They are used to make crucibles and linings for furnaces, kilns and incinerators.

?Summary: Characteristics of PolymersMany repeating mers in molecule held together by covalent bondingPolymers usually carbon plus one or more other elements: H, N, O, and Cl Amorphous (glassy) structure or mixture of amorphous and crystalline Properties: low density, high electrical resistivity, and low thermal conductivity, strength and stiffness vary widely