1Tutoring of

METALLURGY

Matter states

gaseous

liquid

solid Metals

CrystallineAtoms are placedaccording to a precise geometrical order.

AmorphousAtoms are placed without a particular order.

2Chemical bonds:

Van der Waals: related to the small variations of atomselectronical charge.

Ionic : electrostatic bond between ions

Covalent: due to interaction of atomic orbitals

Metallic: the atoms valence electrons create an electroniccloud on the whole lattice.

high bond energy + + + + + ++ + + + + +

+ + + + + +

+ + + + + +

Electronic gas

The Body Cubic Centred Lattice (BCC)Iron (untill 912C); Iron (from 1394C to 1538C); Chromium; Molibdenum;

Number of atoms per cell = 1 + 8*1/8 = 2

Volume of a single cell= a3 = 12.32 r3

ra 43 =3

4ra =Diagonal= r = 0.124 nm (Iron)

33 38.8342 rrVatomi =

= pi

a

Volume of atoms inside a single cell

68.0==cell

atoms

VVFC

3The Face Cubic Centred Lattice (FCC)Iron (from 912C to 1394C); Alluminium; Nickel; Copper

Number of atoms per cell = 6*1/2 + 8*1/8 = 4

Volume of a single cell = a3 = 22.63 r3

ra 42 =2

4ra = r = 0.124 nm (Iron)

33om 75.163

44 rrV iat =

= pi

a

Diagonal=

Volume of atoms inside a single cell

74.0==cell

atoms

VVFC

BCC FCC

Cell Volume 12.32 r3 22.63 r3

Numbers of atoms 2 4

Available Volume per atom 6.16 r

3 5.66 r3

FC 68 % 74%

4Hexagonal LatticeMagnesium, Zinc, Titanium (untill about 882C)

Number of atoms per cell = 12*1/6 + 2*1/2+3 = 6

74.0==cell

atoms

VVFC

Crystal

5grans Grain boundary

Defects inside crystal lattice:

point defects

Vacancies: some atoms are lacking inside the orderedlattice

interstitial atoms: atoms, different from the lattice ones, are placed inside void spaces inside the lattice itself

Substitutional atoms: some atoms, different from the lattice ones, replaces one or more atoms inside the lattice.

6 line defects screw dislocation

Edge Dislocation

edge dislocation

Defects inside crystal lattice:

The plastic deformation in metals is related to the dislocationsmovement.

Thanks to the dislocations its possible to deform a material withstresses much lower than a perfect crystal.

7The grab movement

The edge dislocation moves parallel to force direction.

The macroscopic plastic deformation of a metal is relatedto the movement of a very high number of dislocations

8Plastic deformation of polycrystalline metals

The shear bends, along which the dislocations movementoccurs, can be different depending on the kind of lattice.

The deformation of a crystal depends even on the deformability of neighbour crystals.

The dislocation movement is obstructed by grain boundaries.

So, as the number of the grains increases, the dislocationmovement becomes harder and harder

Its more difficult to plastically deform a fine grained material, even if, on the other side, this results in higher mechanical properties

9Strengthening Mechanisms

A metal can deform if the dislocations can move. So, in order to make it stronger, we can do something to change itscondition.

Make the grain finer

Alloying

Strain hardening

Strengthening Mechanisms

If we deform at low temperature (for example roomtemperature) soft materials, their mechanical propertiesimprove.

Strain Hardening

The strain hardening is due to the increased dislocationdensity during plastic deformation

The dislocations can obstruct one each other during their movement

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A metal alloy is a material with metal properties and made by at least two chemical elements; at least one of these two element mustbe a metal.

Alloy

Homogeneous

Solid solution

Substitutional Interstitial

Compounds

Intermetallic Interstitial

Heterogeneous

Combination of differentsolid phases (pure metals, solid solutions, compounds)

Strengthening due to chemical composition (alloying)

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Solid Solution = macroscopically homogeneous mixture created by the addition of a solute inside a pure metal that is the solvent. Thelattice is quite the same of the solvent one.

substitutional: the solute atoms replace solvent atoms inside thesolvent lattice

Ordered Not ordered

interstitial: the solute atoms (usually of small dimensions) goinside the void places inside solvent lattice; this can create a small deformation of the lattice itself

Compound = its a solid solution with a certain chemicalcomposition that can be expressed as AxBy

intermetallics: they are made by different metalslinked by strong chemical bonds (ionic or covalent); their properties are not metallic. Ex.: Mg2Pb, Mg2Sn

interstitial: they are made by metals togheter withsmall dimensions atoms placed in lattice void spacesEx.: TiC, TaC, Fe4N, Fe3C.

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The strengthening by alloying is due to the lattice deformationcaused by solute atoms; this can obstruct dislocation movement