IT 111 MANUFACTURING MATERIALS

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IT 111 MANUFACTURING MATERIALS

• Lecture 2
• Thomas E. Scott

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Structure of Materials

• Bonding Aggregated versus molecules
• Aggregated van der waals forces
• Molecules covalent, ionic, metallic
• Chemical reactivity related to the electron
  deficiency in the outer shell
• If shell is full, or almost not so reactive
• If shell is almost empty pretty reactive

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Structure of Materials

• Metals tends to give up electrons when bonding
• Non-metals gain electrons when bonded
• Metals fewer than 4 electrons
• Non-metals greater than 4 electrons
• Metals alloy with other metals
• Metals react with non-metals

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Structure of Materials

• Physical state is related to bonding
• Gas loosely bonded
• Liquid more tightly bonded
• Solid rigidly held in position
• Adding heat lessons increase the molecular
  oscillatory energy, and overcomes the bonding ties

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Physical State

• Solid versus liquid versus gas
• Energy balance
• Attractive forces (bonding)
• Repulsive forces (electron excitation)
• From heat

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Bonding

• Van der Waals forces gases
• Weak interaction force between molecules
• Covalent H2, O2, N2
• Oxygen needs two electrons
• Shares with another 0- producing 02
• All organic componds are covalent bonded
• Carbon based (CH4, Polymers)

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Bonding (Contd)

• Ionic electron donor/acceptor
• Chlorine one short, Na 1 (NaCl neutral)
• Oxygen two short, Na 1 (Na2O neutral)
• Sand aluminum oxide and sodium oxide
• Ceramics
• Since they are strongly bonded, high melting point

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Bonding (Contd)

• Metallic bond
• Electron cloud
• Helps explain electrical continuity of metals

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Bonding (Contd)

• Bond strength influences material properties
• Bond strength (approximate)
• Van der Waals - weakest
• Metalic
• Covalent
• Ionic - strongest

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Crystalline Structure

• Atoms approach each other until attraction force
  is balanced by the like charges repulsion
  forces
• Greater number of shells
• Larger distance between atoms
• Greater number of valence electrons
• Less distance between atoms
• BALANCE RESULTS IN CRYSTAL LATTICE STRUCTURE

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Crystal Structures
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Crystal Structures
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Crystalline Structure

• Many Types (See Figures 1-5 and 1-6)
• Body Centered Cubic (BCC) - Iron
• Face Centered Cubic (FCC) Aluminum
• Hexagonal Close Pack (HCP) - Magnesium
• Some materials are polymorphs
• For example, iron is BBC to 16700F, then FCC

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Crystalline Structure

• Crystal formation Grain formation
• Growth starts
• If many simultaneously
• Quenching for example
• Then small grains
• If few starts
• Gradual heat treat
• Then large grains
• Grains form in dendrites (tree like structures)
• Grain boundaries form at intersections

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Crystal Growth
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Crystal Growth (Dendrites)
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Crystal Defects

• Single crystal solidification
• Most materials are polycrystalline
• Can grow columnar crystals
• Can grow single crystal

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Metallic Solid Solutions

• Alloys are crystals of two or more metals
• Solid solutions have dispersed particles
• Substitutional solid
• Alloy takes the place of the matrix element
• Interstitial
• Alloy atoms fit between crystalline elements
• Carbon in iron

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Subtitutional and Interstitial
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Crystalline imperfections

• Crystals are never perfect
• Point defects
• Vacancy missing atoms in the structure
• Causes
• Rapid cooling
• Non-equilibrium during solidification (small
  section)

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Defects
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Crystal imperfections

• Line defects - dislocations
• Lattice distortion causes
• Stress during solidification
• Plastic deformation of a solid
• Edge
• V shaped crack at edge
• Screw
• Compression/tension boundary
• Due to bending
• Most are combination edge and screw

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Dislocation motion

• Stress loading causes migration
• Result is accumulation
• Produces visible crack
• Decrease in supporting material results in
  increased stress
• Result is failure at site of accumulated
  dislocations
• Fatique failures and creep are an evidence of
  this time related phenomenon

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Dislocations
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Crystal Imperfections

• Grain boundaries Planer defects
• Polycrystalline structures
• Shape of the grain boundaries can result in
  anisotropy
• Preferential directional strength/weakness
• Grain size
• Smaller grain size at low temperature
• Increased strength
• No clear shear surface load path
• Stronger and tougher