Post on 13-Dec-2015
3.40 Physical Metallurgy
Lecture #5 Review
Metallic Bonding
04/18/23
Presented by
Leon FaisonCourse 2N
04/18/23
Agenda
• Metallic Bonding
– Indicative Properties
– Metastability
– Research Work
– Embedded Atom Method
Metallic Bonding
Indicative Properties
• Shiny
• Malleable
• High electrical and thermal conductivity can be contributed to the delocalized, free electrons in the atoms
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No Gap
Metallic Bonding
Metastability
• Shiny solids on Earth is a miracle except for gold (most noble)
• Standard Gibbs free energy of the formation of Fe2O3 reveals that there are -3.8 eV per Fe atom!
04/18/23
Metallic Bonding
Metastability
• Parabolic Kinetics acts as the defense mechanism for metals from further oxidation
• It does not always work, for example, localized pitting corrosion
• Combination of chemical and mechanical attack can manifest in the metal’s passivation layer
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time
H(thickness)
oxygenH
oxidized, “passivation” layer
metal
• U.S. Navy warships are vulnerable to this “joint attack” of chemical and mechanical manifestations
• Superstructures are made from aluminum (5456-H116) and exposed to the marine environment
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Metallic Bonding
04/18/23
Metallic Bonding
• Alloying elements of the 5456-H116 such as Magnesium precipitate toward the grain boundaries
• High elevated temperatures accelerate the onset of “sensitization”
• Due to the potential difference between Aluminum and Magnesium, a galvanic cell is created by the presence of salt water in the marine environment
• Cracks begin to form on the deck plating and others areas of the superstructure (e.g. high stress areas).
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Possible Solutions
• Weld repairs worsen already affected aluminum
• GMAW/P - Pulsed Gas Metal Arc Welding– Rapidly switches the welding
output from high peak current to low background current
Metallic Bonding
04/18/23
Metallic Bonding
Embedded Atom Model
• The free electrons are shared between all atoms and do not “belong” to a particular ion
(Communism!)
• Like glue, this sea of electrons provides bonding energy which is less than individual isolated atomic states
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Metallic Bonding
Embedded Atom Model (Mathmatically)
e.g.
Note Must also consider bond angle dependence when dealing with electron gas/electron liquid in a 3-atom quantity where it is shown
that metallic bonding tends to be more isotropic than covalent.Xi
j
j
j ρ(rij)
ρ(rij)ρ(rij)
ρ(rij) = glue projection function
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Metallic Bonding
Embedded Atom Model Applications
1. Energy potential over lattices sites to
compare the stability of different crystal
structures at zero pressure
2. Molecular dynamics for non-perfectly arranged atoms
(ma = F)
Ω
eb