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MATERIALS SCIENCE

AND ENGINEERING I LECTURE COURSE 3

CRYSTALLINE AND AMORPHOUS STRUCTURE

PLASTIC DEFORMATION OF METALS

INTERATOMIC BONDING

Properties of materials –

determined also by the atomic scale, as a resultant of

interatomic bonding

Example of consequences: Metals – conductors

Ceramics – insulators

Interatomic bonding strong bonding

ionic, non-polar covalent, metallic

weak bonding

hydrogen, polar covalent, Van der Waals

1. STRONG BONDING

1.1 IONIC BONDING

Is realised for large differences in electronegativity;

Through electron exchange

→ ionic character ;

→ minimum degree of mobility for electrons:

Example: Na(11): 3s1

Cl(17): 3p5 → 3p6

1. STRONG BONDING

1.2 NON-POLAR COVALENT BONDING

Between atoms of the same family, without electronegativity difference

Through sharing of valency electrons

→ low electrons mobility

Essential for polymers: C-C

1. STRONG BONDING

1.3 METALLIC BONDING

• Between metal atoms (small difference of electronegativity);

• Also through sharing of valency electrons

– between all atoms (over-posed energy levels)

→ positive ions

• Form crystal lattices

• High electrons mobility

• Classic model: ionic lattice, conduction electrons „gas” (Fermi)

1. STRONG BONDING

1.3 METALLIC BONDING

Classic model of

metallic bonding

Consequence – metallic state: metallic glitter

electrical / thermal conductivity

increase of resistivity with temperature

thermoelectronic emission

2. WEAK BONDING

2.1. POLAR COVALENT BONDING

Between atoms with slightly different relative electronegativity

Example: water

2. WEAK BONDING

2.2 HYDROGEN BONDING

Between

strongly electronegative atoms

(O, N, F) in one molecule and a hydrogen atom that is covalently bound to strongly electronegative atoms in another molecule.

Important for polymers –

cross linking, leading to altering of mechanical properties

2. WEAK BONDING

2.3 VAN DER WAALS BONDING (LONDON DISPERSION FORCES)

Caused by short time polarization of atoms due to asymmetrical rotation of

electrons around nucleus;

Examples:

polymers (polyethylene)

metal / ceramic – polymer soldering

CRYSTALLINE STRUCTURE

Order in materials: close-range (around an atom)

long-range

Materials Crystalline close + long-range order

Ex.: metals, some ceramics

Amorphous only close-range order

Ex.: polymers, glasses

CRYSTALLINE STRUCTURE

Crystal unit cell: structural unit that maintains the characteristics of the 3D crystal. By repeating on the 3 axes it generates the lattice.

Crystalline systems : Bravais lattices

7 fundamental

7 primary derived– atoms in the centre of body / faces

+ other derived systems

(atoms in other positions)

Lattice parameters

Sistem cristalin Celule elementare

triclinic

monoclinic

simplu centrat

ortorombic

simplu baze centrate volum centrat fete centrate

Sistem cristalin Celule elementare

hexagonal

romboedric

(trigonal)

tetragonal

simplu volum-centrat

cubic

simple body-centered face-centered

CRYSTALLINE STRUCTURE

Metals: Body-centered cubic (bcc)

Feα, Cr, W, V, Mo, Tiβ, ...

Face-centered cubic (fcc)

Feγ, Al, Cu, Au, Ag, ...

Hexagonal close packed (hcp) – Zn, Mg, Tiα, ...

Allotropy (for metals) = ability to crystallize in different systems;

transition from one allotropic state to another –

allotropic transformation

Example:

)()( 912 cfcFecvcFe

CRYSTALLINE STRUCTURE

bcc fcc hcp

CRYSTALLINE STRUCTURE

Slip plane: plane with a maximum number of atoms inside the unit cell

(= close packed plane)

deformation inside the crystal - mostly along the slip planes

High number of slip planes → good plasticity

fcc (8) – best plasticity, poor strength / hardness

bcc (6) – lower plasticity, higher strength / hardness

hcp (2) (base planes) – poor ductility

STRUCTURE OF REAL CRYSTALS

Lattice defects:

1. Point defects simple vacancy, interstitials

complex

2. LINE DEFECTS DISLOCATIONS

3. Stacking faults

STRUCTURE OF REAL CRYSTALS

Edge dislocation Screw dislocation

Dislocations – determine the plastic behaviour of metals

They move in the slip planes under the shear stresses

Inside crystal – numerous dislocations (from solidification or straining)

Theoretical strength >1000 x Real strength for metals

STRUCTURE OF REAL CRYSTALS

CRYSTALLIZATION OF METALS Melting: Transition from solid to liquid state (by heating, usually)

Partial breaking of atomic bonds

Crystalline materials breaking of long-distance order

Well-defined temperature

(melting temperature)

Amorphous materials passing through a viscous state

Melting latent heat is absorbed

Crystallization: Formation of the crystalline structure.

Solidifying in crystalline materials.

Determined by the diminishing of free energy in the system

Latent heat is released

CRYSTALLIZATION OF METALS

Crystallization Process takes place in 2 stages:

I. Germination (Nucleation) = formation of nuclei of crystallization

II. Growth of nuclei of crystallization

I. II.

Crystallization Process:

I. Germination; II. Growth of nuclei and formation of structure

CRYSTALLIZATION OF METALS

I. Nuclei of crystallization = small solid particles, starting points for crystallization

Nuclei homogeneous

groups of atoms of the same nature as the melt

heterogeneous

solid particles of different nature (ceramic generally)

Heterogeneous nucleation is more probable

II. Growth of viable nuclei >>> Polycrystalline aggregate – microstructure

CRYSTALLIZATION OF METALS

Analysis of cooling transformations – cooling curves:

temperature = f (time)

Cooling curve for a material

(without phase transformation) -

exponential -

Cooling curve for a pure metal

(crystallization at ts)- plateau

CRYSTALLIZATION OF METALS Critical temperatures = temperatures where solid state transformations occur

Example: allotropic transformations

Cooling curve for a metal

with 2 allotropic transformations

Cooling / heating curve for a metal

thermal hysteresis

.)..(.).( 882 cvcTichTi

ALLOYS ELABORATION

Alloys elaboration : obtaining of desired chemical composition

(in molten state usually)

ALLOYS ELABORATION

ALLOYS ELABORATION

After the elaboration stage, alloys are cast in ingot mould

→ INGOT

Cu ingot as an animal skin

(Antique Greece)

Structure of ingot; upper zone = feeder (feeding head)

1 – marginal (chill) grains zone; 2 – columnar grains zone ;

3 –central grains zone ;

DEFECTS OF INGOTS

1. shrink cavity – Cavity resulted through the solidifying shrinkage

upper - in feeder; Principle defect;

central / dispersed; Accidental defects;

2. Segregation – chemical inhomogeneity

macroscopic (ingot’s level)

microscopic (inside grains)

Zonal segregation upper

lower

Feeder: shrink cavity (upper) + upper segregation

DEFECTS OF INGOTS

3. Non-metallic inclusions – exo /endogenous ceramic particles

inclusions macroscopic

microscopic

blow holes = gas inclusions

4. Minimum strength zones – meeting zones for columnar

grains on adjacent sides

PLASTIC DEFORMATION OF METALS

I. Deformation of the single crystal

Single crystal = single grain (continuous crystal lattice)

Anisotropy = property of displaying different properties on different directions;

(opposite = isotropy)

Single crystal – anisotropic;

Polycrystalline aggregate – isotropic (if not textured)

I.1. Slip deformation

Shear stresses over a critical value → dislocations move in slip planes (planes with highest atoms density) → slip deformation

PLASTIC DEFORMATION OF METALS

Slip deformation of the single crystal

AA’ –theoretical plane

BB’ – real plane

Metals: f.c.c. – 8 slip planes

b.c.c. – 6 slip planes

h.c.p. – ~ 2 slip planes

PLASTIC DEFORMATION OF METALS

I.2 Twinning deformation = splitting of the lattice along a

plane, resulting in symmetrical zones → twins

Twinning deformation

Large deformations through twinning

Small deformations through slipping

New orientation of crystal lattice

(favourable for metals with

few slip planes – h.c.p.)

→ slip planes with new orientations

→ deformation can continue

Glossary

• Legături interatomice = atomic bonding (bonds);

• Reţea cristalină = crystal lattice;

• Stare metalică = metallic state (character);

• Lipire = soldering;

• Celulă elementară = crystal unit cell;

• Volum centrat = body-centered;

• Feţe centrate = face-centered;

• Hexagonal compact = hexagonal close packed;

• Alotropie = allotropy;

• Plan de alunecare = slip plane;

• Dislocaţie = dislocation;

• Dislocaţie marginală = edge dislocation;

• Dislocaţie elicoidală = screw dislocation;

• Defecte de împachetare = stacking faults (packing defects);

Glossary • topire = melting;

• căldura latentă = latent heat;

• cristalizare = crystallization;

• germinare = germination;

• germen cristalin = nucleus of crystallization;

• punct critic = critical temperature;

• histerezis (termic) = (thermal) hysteresis;

• elaborare = elaboration;

• lingou = ingot;

• grăunţi marginali / columnari / echiaxiali = chill / columnar / equiaxed grains;

• maselotă = feeder (feeding head);

• retasură = shrink cavity (shrinkage);

• segregaţie (chimică) = segregation;

• sufluri = blow holes;

• monocristal = single crystal;

• (an)izotropie = (an)isotropy;

• maclare = twinning;