THE NATURE OF MATERIALS Manufacturing Processes, 1311 Dr Simin Nasseri Southern Polytechnic State University

THE NATURE OF MATERIALS Atomic Structure and the Elements Bonding between Atoms and Molecules Crystalline Structures Noncrystalline (Amorphous) Structures

Importance of Materials in Manufacturing Manufacturing 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 Elements

Atomic Structure and the Elements The 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

Metalloids or Semimetals Element Groupings The 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 table Nonmetals are on right Between them is a transition zone containing metalloids or semi‑metals Metals Metalloids or Semimetals NonMetals Beryllium – Be Boron – B Helium – He Lithium – Li Silicon – Si Neon – Ne Magnesium – Mg Arsenic – As Argon – Ar Cadmium – Cd Antimony – Sb Krypton – Kr Copper- Cu Polonium - Po Xenon – Xe Iron – Fe Tellurium - Te Radon – Rn Zinc – Zn Germanium - Ge Fluorine – F Titanium – Ti Chlorine – Cl Gold – Au Oxygen – O

Periodic Table Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.

Question? 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)

Bonding between Atoms and Molecules

Bonding between Atoms and Molecules Atoms are held together in molecules by various types of bonds Primary bonds - generally associated with formation of molecules Secondary bonds - generally associated with attraction between molecules Primary bonds are much stronger than secondary bonds

Bonding between Atoms and Molecules Primary Bonding Secondary Bonding Ionic Covalent Metallic Dipole forces London forces Hydrogen bonding

The ones on the outer shell Primary Bonds Characterized by strong atom‑to‑atom attractions that involve exchange of valence electrons Following forms: Ionic Covalent Metallic The ones on the outer shell

Ionic Bonding 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 Ductility Low Electrical Conductivity Example: Sodium Chloride (NaCl) Figure 2.4 First form of primary bonding: (a) Ionic

Covalent Bonding Outer electrons are shared between two local atoms of different elements. Properties: High Hardness Low Electrical Conductivity Examples: Diamond, Graphite Figure 2.4 Second form of primary bonding: (b) covalent

Metallic Bonding Outer shell electrons are shared by all atoms to form an electron cloud. Properties: - Good Conductor (Heat and Electricity) - Good Ductility Figure 2.4 Third form of primary bonding: (c) metallic

Secondary Bonds Secondary bonds involve attraction forces between molecules (whereas primary bonds involve atom‑to‑atom attractive forces), No transfer or sharing of electrons in secondary bonding Bonds are weaker than primary bonds Three forms: Dipole forces London forces Hydrogen bonding

Macroscopic Structures of Matter Atoms and molecules are the building blocks of more macroscopic structure of matter When materials solidify from the molten state, they tend to close ranks and pack tightly, arranging themselves into one of two structures: Crystalline Noncrystalline

Crystalline Structures

Crystalline Structure Structure 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 polymers

Crystallinity When 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:

What 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!!

Three 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: 17 Figure 2.8 Three types of crystal structure in metals.

Crystal Structures for Common Metals Room temperature crystal structures for some of the common metals: Body‑centered cubic (BCC) Chromium, Iron, Molybdenum, Tungsten Face‑centered cubic (FCC) Aluminum, Copper, Gold, Lead, Silver, Nickel, (Iron at 1670oF) Hexagonal close‑packed (HCP) Magnesium, Titanium, Zinc

Imperfections (Defects) in Crystals Imperfections 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 imperfections Point defects, Line defects, Surface defects.

Point Defects Imperfections in crystal structure involving either a single atom or a few number of atoms Dislocation of an atom Extra atom present Figure 2.9 Point defects: (a) vacancy, (b) ion‑pair vacancy (Schottky), (c) interstitialcy, (d) displaced ion (Frenkel Defect).

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 dislocation

Edge Dislocation Edge of an extra plane of atoms that exists in the lattice Figure 2.10 Line defects: (a) edge dislocation

Screw Dislocation Spiral within the lattice structure wrapped around an imperfection line, like a screw is wrapped around its axis Figure 2.10 Line defects: (b) screw dislocation

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 Strain When a crystal experiences a gradually increasing stress, it first deforms elastically If 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 Strain If 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 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 2.12 Effect of dislocations in the lattice structure under stress.

Slip 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 situation Good news in manufacturing – the metal is easier to form Bad news in design – the metal is not as strong as the designer would like

Twinning A 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 Metals A 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 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 liquid These crystals grow and finally interfere with each other, forming at their interface a surface defect ‑ a grain boundary Grain boundaries are transition zones, perhaps only a few atoms thick Grain Grain boundary Growth of crystals in metals

Noncrystalline (Amorphous) Structures

Noncrystalline (Amorphous) Structures Many materials are noncrystalline Water and air have noncrystalline structures A metal loses its crystalline structure when melted Some important engineering materials have noncrystalline forms in their solid state: Glass Many plastics Rubber

Features of Noncrystalline Structures Two features differentiate noncrystalline (amorphous) from crystalline materials: Absence of long‑range order in molecular structure Differences in melting and thermal expansion characteristics What are the differences between them?

Crystalline versus Noncrystalline The crystal structure is regular, repeating, and denser The noncrystalline structure is random and less tightly packed. Figure 2.14 Difference in structure between: (a) crystalline and (b) noncrystalline materials.

Solidification Alloy Metal Pure Metal

Volumetric Effects Tg=glass temperature Tm=melting temperature Figure 2.15 Characteristic change in volume for a pure metal (a crystalline structure), compared to the same volumetric changes in glass (a noncrystalline structure).

Summary: Characteristics of Metals Crystalline structures in the solid state, almost without exception BCC, FCC, or HCP unit cells Atoms held together by metallic bonding Properties: high strength and hardness, high electrical and thermal conductivity FCC metals are generally ductile

Summary: Characteristics of Ceramics Most 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 inert ? Refractory materials retain their strength at high temperatures. They are used to make crucibles and linings for furnaces, kilns and incinerators.

Summary: Characteristics of Polymers Many repeating mers in molecule held together by covalent bonding Polymers 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