THE NATURE OF MATERIALS Manufacturing Processes, MET 1311 Dr Simin Nasseri Southern Polytechnic State University (© Fundamentals of Modern Manufacturing; Materials, Processes and Systems, by M. P. Groover) 1

Manufacturing Processes, Prof Simin Nasseri The nature of Materials 1. Atomic Structure and the Elements 2. Bonding between Atoms and Molecules 3. Crystalline Structures 4. Noncrystalline (Amorphous) Structures 2

Manufacturing Processes, Prof Simin Nasseri 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 3

Atomic Structure and the Elements 4

Manufacturing Processes, Prof Simin Nasseri 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 5

Manufacturing Processes, Prof Simin Nasseri 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 MetalsMetalloids or Semimetals NonMetals Beryllium – BeBoron – BHelium – He Lithium – LiSilicon – SiNeon – Ne Magnesium – MgArsenic – AsArgon – Ar Cadmium – CdAntimony – SbKrypton – Kr Copper- CuPolonium - PoXenon – Xe Iron – FeTellurium - TeRadon – Rn Zinc – ZnGermanium - GeFluorine – F Titanium – TiChlorine – Cl Gold – AuOxygen – O

Manufacturing Processes, Prof Simin Nasseri Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements. Periodic Table 7

Manufacturing Processes, Prof Simin Nasseri 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) 8

Bonding between Atoms and Molecules 9

Manufacturing Processes, Prof Simin Nasseri Bonding between Atoms and Molecules Atoms are held together in molecules by various types of bonds 1. Primary bonds - generally associated with formation of molecules 2. Secondary bonds - generally associated with attraction between molecules Primary bonds are much stronger than secondary bonds 10

Manufacturing Processes, Prof Simin Nasseri 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 11

Manufacturing Processes, Prof Simin Nasseri Ionic Bonding Figure 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. 12 Example: Sodium Chloride (NaCl) Properties: Poor Ductility Low Electrical Conductivity

Manufacturing Processes, Prof Simin Nasseri Covalent Bonding Figure 2.4 Second form of primary bonding: (b) covalent Outer electrons are shared between two local atoms of different elements. 13 Examples: Diamond, Graphite Properties: High Hardness Low Electrical Conductivity

Manufacturing Processes, Prof Simin Nasseri Metallic Bonding Figure 2.4 Third form of primary bonding: (c) metallic Outer shell electrons are shared by all atoms to form an electron cloud. 14 Example: Metals Properties: - Good Conductor (Heat and Electricity) - Good Ductility

Manufacturing Processes, Prof Simin Nasseri 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 15

Crystalline Structures 16

Manufacturing Processes, Prof Simin Nasseri 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 17

Manufacturing Processes, Prof Simin Nasseri 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: 18

Manufacturing Processes, Prof Simin Nasseri 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!! 19

Manufacturing Processes, Prof Simin Nasseri Three Crystal Structures in Metals 1. Body-centered cubic (BCC) 2. Face centered cubic (FCC) 3. 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: 17 (strongest at room temperature) 20

Manufacturing Processes, Prof Simin Nasseri Crystal Structures for Common Metals (FYI) 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 1670 o F) Hexagonal close ‑ packed (HCP) Magnesium, Titanium, Zinc 21

Manufacturing Processes, Prof Simin Nasseri 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 22

Manufacturing Processes, Prof Simin Nasseri Crystalline Structure Growth of crystals in metals Grain or phase 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 boundary 23

Noncrystalline (Amorphous) Structures 24

Manufacturing Processes, Prof Simin Nasseri 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 25

Manufacturing Processes, Prof Simin Nasseri Features of Noncrystalline Structures Two features differentiate noncrystalline (amorphous) from crystalline materials: 1. Absence of long ‑ range order in molecular structure 2. Differences in melting and thermal expansion characteristics What are the differences between them? 26

Manufacturing Processes, Prof Simin Nasseri Crystalline versus Noncrystalline Figure 2.14 Difference in structure between: (a) crystalline and (b) noncrystalline materials. The crystal structure is regular, repeating, and denser The noncrystalline structure is random and less tightly packed. 27

Manufacturing Processes, Prof Simin Nasseri Volumetric Effects 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). T g =glass temperature T m =melting temperature 28 A pure metal melts at a certain temperature. It absorbs the heat of fusion to change it’s state from solid to liquid. Glass starts to get soft at “glass temperature”. Temperature keeps changing as glass’ state changes gradually from solid to liquid.

Manufacturing Processes, Prof Simin Nasseri 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 29

Manufacturing Processes, Prof Simin Nasseri Summary: Characteristics of Ceramics Most ceramics have crystalline structures, while glass (SiO 2 ) 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. ? 30

Manufacturing Processes, Prof Simin Nasseri 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 31