Module 3

• Crystals


  Orderly configuration/arrangement of atoms when metals solidify from a molten state
• Crystalline structure


  Arrangement of atoms in a crystal
• Unit cell


  Building block of a crystal; smallest group of atoms showing the characteristics of a lattice structure
• Metals form different crystal structures to minimize the energy required to fit together in a regular pattern and can take different crystal structures at different temperature levels
• 3 Basic atomic arrangements of metals

  • FCC (Face-Centered Cubic)
  • HCP (Hexagonal Close-Packed)
• FCC and HCP crystals

  • Most densely packed configurations
• 3 Basic Atomic Arrangements of Metals

  • Body-Centered Cubic
  • Face-Centered Cubic
  • Hexagonal Close-Packed Crystal Structure
• Elastic Deformation


  • When a crystal is subjected to an external force and it returns to its original shape when the force is removed
• Plastic/Permanent Deformation


  • Experienced when the force on the crystal structure is increased sufficiently and the crystal structure does not return to its original shape even if the force is removed
• Slipping


  Slipping of one plane of atoms over an adjacent plane (slip plane) under a shear stress
• Twinning


  The crystal forms a mirror image across the plane of twinning; usually occurs in HCP metals
• Shear stress required to cause slip is directly proportional to the ratio b/a. Thus, slip in crystal takes place along planes of maximum atomic density or in closely packed planes and directions
• Anisotropic


  • A single crystal has different properties when tested in different directions since the b/a ratio is different for different directions within the crystal (e.g. plywood, woven cloth)
• Slip Systems


  • Combination of a slip plane and its direction of slip (slip plane + slip direction)
  • Metals with slip systems of 5 or above are ductile, below 5 are not
  • BCC: 48 slip systems
  • FCC: 12 slip systems
  • HCP: 3 slip systems
• Imperfections in the Crystal Structure of Metals

  • Line defects called dislocations
  • Point defects, such as vacancy (missing atom), an interstitial atom (extra atom in the lattice), or an impurity (foreign atom) that has replaced the atom of a pure metal
  • Volume or bulk imperfections, such as voids or inclusions (nonmetallic elements as oxides, sulfides, silicates)
  • Planar imperfections such as in grain boundaries
• Work Hardening


  Also known as Strain Hardening, the effect of an increase in shear stress that causes an increase in the overall strength of the metal
• Entanglements and impediments caused by dislocations increase the shear stress required for slip
• Grains


  • Commonly used metals are polycrystalline— that is, they are composed of many crystals or grains in random orientation
• Nucleation


  Initial stage of formation of crystals; acts like the "seed" for a grain
• High Nucleation rate = More No. of grains/unit volume = Bigger Grain size = Low Strength = Low Hardness = Low Ductility = Rough surface appearance
• Rapid cooling = smaller grains; slow cooling = larger grains
• ASTM Grain Size Number

  1. 5-8: fine grains; 7: acceptable for sheet metals for making car bodies, appliances, and kitchen utensils
• Grain Boundaries


  • More reactive than the grains themselves; atoms along the grain are packed-less efficiently and are more disordered; High Energy = Rougher Surface
• Grain Boundary Sliding


  At elevated temperatures, and in materials whose properties depend on the deformation rate, plastic deformation takes place through this. It is also the reason for creep
• Grain Boundary Embrittlement


  • A normally ductile and strong metal can crack under very low stresses when brought into close atomic contact with certain low-melting point metals
• Hot shortness


  • Crumbling/disintegration of a metal caused by local melting of a constituent or an impurity in the grain boundary at a temperature below the melting point of the metal itself (e.g. antimony in copper)
• Temper Embrittlement


  • A form of embrittlement in alloy steels caused by the segregation (movement) of impurities to the grain boundaries
• Annealing


  Heating the piece of metal in a specific temperature range for a period of time; Properties of metals which were cold-worked (permanently deformed at room temp) can be brought back to their original state; means to bring metal back to its original state/properties
• Recovery


  The stresses in the highly deformed regions are relieved; occurs at a certain temperature range below the recrystallization temperature of the metal; no appreciable change in mechanical properties (hardness and strength)
• Recrystallization


  Process in which, at a certain temperature range, new equiaxed and strain-free grains are formed, replacing the older grains
• Recrystallization Temperature


  Generally defined as the temperature at which complete recrystallization occurs within approximately 1hr (ranges between approx. 0.3 Tm and 0.5 Tm)
• Grain Growth


  If we continue to raise the temperature of the metal, the grains begin to grow, and their size may eventually exceed their original size which affects mechanical properties
• Orange Peel


  • Large grains which produce a rough surface appearance on sheet metals when stretched or compressed
• Types of Metal Working

  • Cold-working
  • Warm-working
  • Hot-working
• Homologous Temperature


  Defines whether a material is being cold, warm, or hot-worked (Homologous Temperature=T/Tm)
• Pure Metals


  Metals with atoms of all the same type though not 100% pure due to some impurities (e.g. aluminum foil, Ni or Cr for plating, Cu for electrical conductors)
• Alloy


  Composed of two or more chemical elements, at least one of which is a metal; alloying is done to enhance the properties of pure metals
• Forms of Alloying

  • Solid Solutions
  • Intermetallic Compounds
• Hume-Rothery Rules

  Two conditions to form complete substitutional solid solutions: 1) Two metals must have similar crystal structures, 2) The difference in their atomic radii must be less than 15%
• Conditions to Form Complete Interstitial Solid Solutions

  1. The solvent atom must have more than one valence, 2) The atomic radius of the solute atom must be less than 59% of the atomic radius of the solvent atom