Presentation on theme: "THE NATURE OF MATERIALS"— Presentation transcript:
1THE NATURE OF MATERIALS Manufacturing Processes, 1311Dr Simin NasseriSouthern Polytechnic State University
2THE NATURE OF MATERIALS Atomic Structure and the ElementsBonding between Atoms and MoleculesCrystalline StructuresNoncrystalline (Amorphous) Structures
3Importance of Materials in Manufacturing Manufacturing is a transformation processIt is the material that is transformedAnd 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
5Atomic Structure and the Elements The basic structural unit of matter is the atomEach atom is composed of a positively charged nucleus, surrounded by a sufficient number of negatively charged electrons so the charges are balancedMore than 100 elements, and they are the chemical building blocks of all matter
6Metalloids or Semimetals Element GroupingsThe elements can be grouped into families and relationships established between and within the families by means of the Periodic TableMetals occupy the left and center portions of the tableNonmetals are on rightBetween them is a transition zone containing metalloids or semi‑metalsMetalsMetalloids 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 – O
7Periodic TableFigure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.
8Question? What are the noble metals? Copper Silver Gold Noble metals (precious metals) are metals that are resistant to corrosion or oxidation, unlike most base metals.Platinum (Pt), Palladium (Pd)
10Bonding between Atoms and Molecules Atoms 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
11Bonding between Atoms and Molecules PrimaryBondingSecondaryBondingIonicCovalentMetallicDipole forcesLondon forcesHydrogen bonding
12The ones on the outer shell Primary BondsCharacterized by strong atom‑to‑atom attractions that involve exchange of valence electronsFollowing forms:IonicCovalentMetallicThe ones on the outer shell
13Ionic BondingAtoms 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)Figure 2.4 First form of primary bonding: (a) Ionic
14Covalent BondingOuter electrons are shared between two local atoms of different elements.Properties:High HardnessLow Electrical ConductivityExamples: Diamond, GraphiteFigure 2.4 Second form of primary bonding: (b) covalent
15Metallic BondingOuter shell electrons are shared by all atoms to form an electron cloud.Properties:- Good Conductor (Heat and Electricity)- Good DuctilityFigure 2.4 Third form of primary bonding: (c) metallic
16Secondary BondsSecondary bonds involve attraction forces between molecules (whereas primary bonds involve atom‑to‑atom attractive forces),No transfer or sharing of electrons in secondary bondingBonds are weaker than primary bondsThree forms:Dipole forcesLondon forcesHydrogen bonding
17Macroscopic Structures of Matter Atoms 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:CrystallineNoncrystalline
19Crystalline Structure Structure in which atoms are located at regular and recurring positions in three dimensionsUnit cell - basic geometric grouping of atoms that is repeatedThe pattern may be replicated millions of times within a given crystalCharacteristic structure of virtually all metals, as well as many ceramics and some polymers
20CrystallinityWhen 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:
21What about glass?! Does glass have a crystalline structure?! Question?What about glass?! Does glass have a crystalline structure?!"What is glass... is it a liquid or a solid?"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!!
22Three Crystal Structures in Metals Body-centered cubic (BCC)Face centered cubic (FCC)Hexagonal close-packed (HCP)# of atoms in unit cell: 9# of atoms: 14# of atoms: 17Figure 2.8 Three types of crystal structure in metals.
23Crystal Structures for Common Metals Room temperature crystal structures for some of the common metals:Body‑centered cubic (BCC)Chromium, Iron, Molybdenum, TungstenFace‑centered cubic (FCC)Aluminum, Copper, Gold, Lead, Silver, Nickel, (Iron at 1670oF)Hexagonal close‑packed (HCP)Magnesium, Titanium, Zinc
24Imperfections (Defects) in Crystals Imperfections often arise due to inability of solidifying material to continue replication of unit cell, e.g., grain boundaries in metalsIt 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).
25Types of defects or imperfections Point defects,Line defects,Surface defects.
26Point DefectsImperfections in crystal structure involving either a single atom or a few number of atomsDislocation of an atomExtra atom presentFigure 2.9 Point defects: (a) vacancy, (b) ion‑pair vacancy (Schottky), (c) interstitialcy, (d) displaced ion (Frenkel Defect).
27Line DefectsDefect 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 dislocationScrew dislocation
28Edge DislocationEdge of an extra plane of atoms that exists in the latticeFigure Line defects: (a) edge dislocation
29Screw DislocationSpiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axisFigure Line defects: (b) screw dislocation
30Surface DefectsImperfections that extend in two directions to form a boundaryExamples:External: the surface of a crystalline object is an interruption in the lattice structureInternal: grain boundaries are internal surface interruptions
32Elastic StrainWhen a crystal experiences a gradually increasing stress, it first deforms elasticallyIf force is removed lattice structure returns to its original shapeFigure 2.11 Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, with no permanent change in positions of atoms.
33Plastic StrainIf stress is higher than forces holding atoms in their lattice positions, a permanent shape change occursFigure 2.11 Deformation of a crystal structure: (c) plastic deformation (slip), in which atoms in the lattice are forced to move to new "homes“.
34Effect of Dislocations on Strain In 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 Effect of dislocations in the lattice structure under stress.
35Slip on a Macroscopic Scale When 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 good‑news‑bad‑news situationGood news in manufacturing – the metal is easier to formBad news in design – the metal is not as strong as the designer would like
36TwinningA 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 sideFigure Twinning, involving the formation of an atomic mirror image on the opposite side of the twinning plane: (a) before, and (b) after twinning.
37Polycrystalline Nature of Metals A block of metal may contain millions of individual crystals, called grainsSuch a structure is called polycrystallineEach grain has its own unique lattice orientation; but collectively, the grains are randomly oriented in the block
38Crystalline Structure How 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 liquidThese 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 thickGrainGrainboundaryGrowth of crystals in metals
40Noncrystalline (Amorphous) Structures Many materials are noncrystallineWater and air have noncrystalline structuresA metal loses its crystalline structure when meltedSome important engineering materials have noncrystalline forms in their solid state:GlassMany plasticsRubber
41Features of Noncrystalline Structures Two features differentiate noncrystalline (amorphous) from crystalline materials:Absence of long‑range order in molecular structureDifferences in melting and thermal expansion characteristicsWhat are the differences between them?
42Crystalline versus Noncrystalline The crystal structure is regular, repeating, and denserThe noncrystalline structure is random and less tightly packed.Figure 2.14 Difference in structure between: (a) crystalline and (b) noncrystalline materials.
44Volumetric EffectsTg=glass temperatureTm=melting temperatureFigure Characteristic change in volume for a pure metal (a crystalline structure), compared to the same volumetric changes in glass (a noncrystalline structure).
45Summary: Characteristics of Metals Crystalline structures in the solid state, almost without exceptionBCC, FCC, or HCP unit cellsAtoms held together by metallic bondingProperties: high strength and hardness, high electrical and thermal conductivityFCC metals are generally ductile
46Summary: Characteristics of Ceramics Most ceramics have crystalline structures, while glass (SiO2) is amorphousMolecules characterized by ionic or covalent bonding, or bothProperties: high hardness and stiffness, electrically insulating, refractory, and chemically inert?Refractory materials retain their strength at high temperatures. They are used to make crucibles and linings for furnaces, kilns and incinerators.
47Summary: Characteristics of Polymers Many repeating mers in molecule held together by covalent bondingPolymers usually carbon plus one or more other elements: H, N, O, and ClAmorphous (glassy) structure or mixture of amorphous and crystallineProperties: low density, high electrical resistivity, and low thermal conductivity, strength and stiffness vary widely