3.40 Physical Metallurgy

Lecture #5 Review

Metallic Bonding

04/22/23

Presented by

Leon FaisonCourse 2N

04/22/23

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!

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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

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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

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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

Questions?

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