Structure of Metals

Atom

Types of atomic bonds

• Ionic bonds: Example : NaClpoor ductility and low thermal and

electrical conductivity.• Covalent bonds Example: H2O, N2

Low electrical conductivity, high hardness• Metallic bonds:,

High thermal and electrical conductivity. Example: metals

The crystal structure of metals

• Body-centered Cubic (BCC): alpha iron, chromium, etc

• Face-centered cubic (FCC): gamma iron, aluminium, copper, etc

• Hexagonal close-packed (HCP): Cobalt, Zinc, etc

Body-centered cubic

(a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.

How many atoms in one unit cell?

http://www.youtube.com/watch?v=Co550Yn7QVc&feature=related

Face-centered Cubic

(a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells.

• http://www.youtube.com/watch?v=RoyzZUJfo-Y

Hexagonal close-packed crystal

(a) unit cell; and (b) single crystal with many unit cells

http://www.youtube.com/watch?v=rIeRqoWaGg4&feature=related

Atomic Packing Fraction for FCC

Face-Centered-CubicArrangement

APF = vol. of atomic spheres in unit cell total unit cell vol.

Depends on: • Crystal structure.• How “close” packed the atoms are.• In simple close-packed structures with hard

sphere atoms, independent of atomic radius

Unit cell contains: 6 x 1/2 + 8 x 1/8 = 4 atoms/unit cell

11

a

a

2R

R22

R22

3

3

44 RVatoms 333

_ 216)22( RRaV cellunit

Basic Geometry for FCC

2a 4R

Geometry:

4 atoms/unit cell

Ra 22

Geometry along close-packed direction give relation between a and R.

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Atomic Packing Fraction for FCC

Face-Centered-CubicArrangementAPF = vol. of atomic spheres in unit cell

total unit cell vol.

How many spheres (i.e. atoms)? What is volume/atom? What is cube volume/cell? How is “R” related to “a”?

4/cell4R3/3

a3

= 0.74APF =

a3

4

3( 2a/4)34

atoms

unit cell atomvolume

unit cell

volumeIndependent of R!

2a 4RUnit cell contains: 6 x 1/2 + 8 x 1/8 = 4 atoms/unit cell

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Deformation and strength of single crystal

(a) Deformation by slip. The b/a ratio influences the magnitude of the shear stress required to cause slip. (b) Deformation by twinning, involving the generation of a “twin” around a line of symmetry subjected to shear.

Types of defects

• Point defects or zero dimensional defects: vacancy, interstitial atom or impurity

• Linear or one dimensional defects: dislocations.• Planar or two dimensional defects: such as

grain boundaries and phase boundaries.• Volume, bulk imperfection or three

dimensional defects, such as voids, inclusions, other phases, or cracks.

Dislocation

(a) edge dislocation; and (b) screw dislocation

Movement of an edge dislocation across the crystal lattice under a shear stress

Schematic illustration of the stages during the solidification of molten metal

(a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries

Grain SizeGrain size significantly influences the mechanical properties of metals. In general, a large grain size is associated with low strength, low hardness, and low ductility.The ASTM grain size number: N=2n-1

n: grain size number.N: number of grain, per square inch at a magnification of 100X (equal to 0.0645mm2 of actual area)

Question: Number of Grains in the ball of a ballpoint pen? Assume that the ball of a ballpoint pen is 1mm in diameter and has an ASTM grain size of 10. Calculate the number of grains in the ball?

Plastic Deformation of Polycrystalline Metals

(a) before deformation; and (b) after deformation

Recovery, Recrystallization and Grain growth

Cold, Warm and Hot Working

Cold working refers to plastic deformation that is usually, but not necessarily, carried out at room temperature.

Hot working refers to plastic deformation that carried out above the recrystallization temperature.

Warm working is carried out at intermediate temperatures.

Summary• Bond: ionic bond, covalent bond, metallic bond• Crystal structure: BCC, FCC, HCP• No metal is perfect (including ourselves)• Plastic deformation in single crystal: slip and

twinning.• Grain size has a significant effect on the strength

of metals.• Grain boundaries have a major influence on the

behavior of metals.

Reading assignment

1. Review Slides2. Chapter 1 of your book