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.
Manufacturing Processes, 1311
Dr Simin Nasseri
Southern Polytechnic State University
The elements can be grouped into families and relationships established between and within the families by means of the Periodic Table
Figure 2.1 Periodic Table of Elements. Atomic number and symbol are listed for the 103 elements.
Atoms are held together in molecules by various types of bonds
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.Ionic Bonding
Outer electrons are shared between two local atoms of different elements.Covalent Bonding
Outer shell electrons are shared by all atoms to form an electron cloud.Metallic Bonding
Secondary bonds involve attraction forces between molecules(whereas primary bonds involve atom‑to‑atom attractive forces),
Structure in which atoms are located at regular and recurring positions in three dimensions
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?!
"What is glass... is it a liquid or a solid?"
Room temperature crystal structures for some of the common metals:
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).
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).
Defect happens along a line ( Connected group of point defects that forms a line in the lattice structure)
Imperfections that extend in two directions to form a boundary
When a crystal experiences a gradually increasing stress, it first deforms elastically
Figure 2.11 Deformation of a crystal structure: (a) original lattice: (b) elastic deformation, with no permanent change in positions of atoms.
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“.
Figure 2.12 Effect of dislocations in the lattice structure under stress.
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.
Growth of crystals in metals
What are the differences between them?
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.
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).
Refractory materials retain their strength at high temperatures. They are used to make crucibles and linings for furnaces, kilns and incinerators.