IE 21

• Structure of Metals

  1. Crystals: Orderly configuration/arrangement of atoms when metals solidify from a molten state

  2. Crystalline Structure: Arrangement of atoms in a crystal

  3. Unit Cell: Building block of a crystal; smallest group of atoms showing the characteristics of a lattice structure
• 3 Basic Atomic Arrangements of Metals

  • Body-Centered Cubic
  • Face-Centered Cubic
  • Hexagonal Close-Packed Crystal Structure
• Deformation and Strength of Single Crystals

  1. Elastic Deformation: When a crystal is subjected to an external force and it returns to its original shape when the force is removed

  2. 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

  3. Slipping: Slipping of one plane of atoms over an adjacent plane (slip plane) under a shear stress

  4. Twinning: The crystal forms a mirror image across the plane of twinning; usually occurs in HCP metals
• Shear stress required to cause slip

  Directly proportional to the ratio b/a
• 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
• Slip Systems

  • Combination of a slip plane and its direction of slip (slip plane + slip direction)
• Metals with slip systems

  5 or above are ductile, below 5 are not
• Slip Systems for different crystal structures

  • 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: vacancy, interstitial atom, impurity
  • Volume or bulk imperfections: voids, inclusions
  • Planar imperfections: grain boundaries
• Work Hardening

  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
• Commonly used metals are polycrystalline, composed of many crystals or grains in random orientation
• Grains


  When a mass of molten metal solidifies, crystals form independently of each other at various locations within the liquid mass during solidification; number and size of grain developed in a unit volume of metal depend on the rate of progress of nucleation
• Nucleation


  Initial stage of formation of crystals; acts like the "seed" for a grain
• Nucleation


  Initial stage of formation of crystals; acts like the “seed” for a grain
• Rate of progress of nucleation

  Number and size of grain developed in a unit volume of metal depend on it
• Nucleation

  • No. of grains/unit volume
  • Grain size
  • Strength
  • Hardness
  • Ductility
• Rapid cooling


  Results in smaller grains
• Slow cooling

  Results in larger grains
• ASTM Grain Size Number

  • 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
• Grain Boundary Sliding


  At elevated temperatures, plastic deformation takes place through this; 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
• Temper Embrittlement


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


  Heating the piece of metal in a specific temperature range for a period of time to bring metal back to its original state/properties
• Recovery


  Relieves stresses in highly deformed regions at a certain temperature range below the recrystallization temperature of the metal; no appreciable change in mechanical properties
• Recrystallization


  Formation of new equiaxed and strain-free grains at a certain temperature range, replacing the older grains
• Recrystallization Temperature


  The temperature at which complete recrystallization occurs within approximately 1hr
• Grain Growth


  If the temperature of the metal is raised, the grains begin to grow, affecting mechanical properties
• Cold, Warm, and Hot-working

  • Cold-working
  • Warm-working
  • Hot-working
• Pure Metals


  Metals with atoms of all the same type though not 100% pure due to some impurities
• 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
• Structure of Alloys

  Alloying can take two basic forms: Solid Solutions and Intermetallic Compounds
• Alloying


  Done to enhance the properties of pure metals
• Structure of Alloys

  • Solid Solutions: Maintains the crystal structure of the solvent during alloying, two or more elements in solid state (homogenous)
  • Intermetallic Compounds: Complex structures consisting of two metals with solute atoms present among solvent atoms in certain proportions
• Hume-Rothery Rules

  • Two metals must have similar crystal structures
  • The difference in their atomic radii must be less than 15%
• Conditions to Form Complete Interstitial Solid Solutions

  • The solvent atom must have more than one valence
  • The atomic radius of the solute atom must be less than 59% of the atomic radius of the solvent atom
• Phase


  A physically distinct and homogeneous portion in a material with its own characteristics and properties
• Two-Phase System


  A system with two solid phases