MATERIALS SCIENCE AND ENGINEERING I 3_eng.pdf · materials science and engineering i lecture course...
Transcript of MATERIALS SCIENCE AND ENGINEERING I 3_eng.pdf · materials science and engineering i lecture course...
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;