1. 3.40 Physical Metallurgy Lecture #5 Review Metallic Bonding 11/29/2015 Presented by Leon Faison Course 2N

2. 11/29/2015 Agenda Metallic Bonding –Indicative Properties –Metastability –Research Work –Embedded Atom Method

3. Metallic Bonding Indicative Properties Shiny Malleable High electrical and thermal conductivity can be contributed to the delocalized, free electrons in the atoms 11/29/2015 No Gap

4. Metallic Bonding Metastability Shiny solids on Earth is a miracle except for gold (most noble) Standard Gibbs free energy of the formation of Fe 2 O 3 reveals that there are -3.8 eV per Fe atom! 11/29/2015

5. 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 11/29/2015 time H (thickness) oxygenH oxidized, “passivation” layer metal

6. 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 11/29/2015 Metallic Bonding

7. 11/29/2015 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).

8. 11/29/2015 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

9. 11/29/2015 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

10. 11/29/2015 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) = glue projection function

11. 11/29/2015 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 ebeb

12. Questions? 11/29/2015