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Materials Science and Testing
Strengthening mechanisms
o Strengthening mechanisms
o Cold deformation – dislocations
o Alloying
o Grain refinement
o Precipitation and dispersion hardening – heat treatment
Today’s topics
Strengthening mechanisms
Strengthening mechanisms
Mechanical properties
High hardness and strength and good toughness
Increase the strength of the materials
Strengthening mechanisms
Methods
Cold deformation
Alloying
Grain refinement
Heat treatment
Strengthening mechanisms
No softening mechanism, the dislocation density is increasing
(dislocation reaction, Cottrel-Lommer junction)
Cold working (deformation):
Strengthening effect of cold working
Strength – dislocation density
Strengthening effect of dislocations
Perfect lattice: only whole lattice planes
can slip (cf.: Frankel model)
Real lattice: deformation through
dislocation’s slip
Strengthening effect of solute elements
Elastic deformation in the lattice in the neighbour region of
the foreign atom.
Two type of solute atoms:
●Interstitial
●Substitutional
Interstitial or substitutional:
Size difference
Strengthening effect of solute elements
AlloyingAlloying
Stress field around the foreign atom
More shear stress is necessary to move the dislocation through this area.
The alloys have higher strength than pure metals.
Strengthening effect of solute elements
Size effect
The stress field and the elastic deformation around the alloying atom
depends on the size difference.
Strengthening effect of solute elements
The higher is the volume change effect of an alloying atom the higher
is the stronger is the interaction between the dislocations and alloying
atoms.
Screw dislocations: only shear deformation
Drag: only interstitial atoms
Edge dislocation: Drag: substitutional and
interstitial atoms
Strengthening effect of solute elements
Strengthening effect of solute elements
Cottrel- atmosphere
Substitutional atoms:
- Compression zone
Smaller atoms
- Tension zone
Larger atoms
Interstitial atoms
Strengthening effect of solute elements
Modulus effect
The solute atom change the elastic modulus of the lattice locally.
Similarly to the size effect it blocks the sliding of the dislocations
(both type, edge and screw).
Other effects:
Change in other physical and chemical properties
Change the crystal structure locally
The foreign atoms by the
stacking faults:
- Lower the energy of the defect
- Block the moving of the dislocations
Effect of stacking faults
Strengthening effect of solute elements
Strengthening effect of the grain size
Important role of grain boundaries
Connection of two crystallite
different orientation – unordered structure of atoms
unordered structure – dislocation structure
different orientation – different elastic moduli:
changes the shear stresses on the slip planes
the slip planes and directions end at the grain boundaries
how can a dislocation move to the other grain?
the grains influence the plastic deformation of the neighbor
grains
Strengthening effect of the grain size
1. Plastic deformation : in high stressed slip planes
2. In less favorably oriented grains the plasticdeformation can begin when the dislocationsin the neighbor grains reached the boundary.
3. A dislocation from the Frank-Read sourcecan’t pass to the other grain because of theorientation difference.
4. Pile-up of the dislocation on the grainboundary
5. The dislocations push the other dislocations before them (the stress increasing)
6. The greater is the number of the dislocation coming from the source, the higher is the stress at the closest dislocation to theboundary.
Strengthening effect of the grain size
Increasing grain
size
Stress field of the piled-up neighbor grain begins
dislocations at the border to deform plastically
number of piled up
dislocations is increasing
plastic deformation
by lower stresses
Strengthening effect of the grain size
2/1
0
dkRRee
Re0 and k are materials constants
Hall-Petch equation
Hall: relation between Yield stress and average grain size for
steels
Petch, later : for wider range of metals
Dispersion and precipitation hardening
Heat treatment
Effect of heat treatment
Alloys without allotropic transformation
presence of a 2nd phase
Cu, Al alloys
- precipitation hardening
- dispersion hardening
Alloys with allotropic transformation
e.g. steels
non-equilibrium transformation
Allotropic transformation:
The material transforms from
one crystal arrangement to a
different one.
e.g. Fe: BCC FCC
704°C
Precipitation Hardening
Precipitation hardening
Conditions (in binary system)
• an appreciable maximum solubility of one component in the other
• the solubility limit rapidly decreases in concentration of the major
component with temperature reduction.
• The solvent metal is soft and tough
• The precipitating phase is hard
• The precipitations are initially coherent
e.g.: Cu-Al, Cu-Be, Cu-Sn, Mg-Al, Al-Ag, Ti-Al
Limited solubility, hard 2nd phase
Al-Cu: Al2Cu Al-Mg-Si: Mg2Si Al-Zn-Mg: Zn2Mg
Supersaturated solid solution,
Metastable state
Precipitation hardening
Technology steps:
Precipitation hardening
3. Quenching
fast cooling to room
temperature
1. Heating to
homogenous
region (1 phase)
2. Homogenization
homogenous distribution of
alloying elements
4. Ageing
produce
precipitations
Structure of the precipitations
Zones - (Guinier Preston zones) atoms cluster together in very small and thin discs
that are only one or two atoms thick and approximately 25 atoms in diameter;
these form at countless positions within the α phase
β’’ - thin disc shaped phases with non-equilibrium composition connected
coherently to the matrix (α phase)
β’ - disc shaped phases with non-equilibrium composition connected
semi-coherently to the matrix (α phase)
β - phases with equilibrium composition adnd different crystal structure
connected incoherently to the matrix (α phase)
incoherent coherent
Microstructure of an aluminum that has been precipitation hardened.
The light matrix phase: aluminum solid solution.
The majority of the small plate-shaped dark precipitate particles are a transition β’’
phase, the remainder being the equilibrium (MgZn2) phase. The grain boundaries
are “decorated” by some of these particles.
Structure of the precipitations
Characteristic curves of metastable
phases
Solution curves Curves of precipitations
concentration Log time
Tem
pera
ture
zones
Thermodynamic process of ageing
α (supersaturated) α + zones α + β’’ α + β’
α (equilibrium) + β (equilibrium)
Fre
e e
nth
alp
y
time
α (supersaturated) α + zones
3 Coherent β’’ precipitations
4 Semi-coherent β’ precipitations
5 Incoherent β precipitations
Process of precipitation
1 Supersaturated
solid solution:
- as quenched
- atoms are dispersed
within the lattice
2 Guinier Preston zones:
- plates (clusters) on the
certain planes
- thickness: 1-10 atom
- diameter: 25-75 atoms
incoherent
semi-coherent
zones
Coherent connection
Log time
Hard
ness,
str
egth
Size-effect: different lattice parameters for the matrix and the precipitation stress
field around the precipitation
Modulus-effect: different elastic modulus parameters for the matrix and the
precipitation
Interface-effect: new surface is created as a dislocation cuts the precipitation
Coherent phase slip planes continues the dislocation cuts the phase
Distorted region obstructs the dislocation movement
Strongest obstacle: θ’ semi-coherent phase
new surface
Surface energy =2πrbγ
Mechanisms of strengthening
Incoherent precipitations
Dislocation – precipitation
reaction:
Orowan-mechanism
Mechanisms of strengthening
dislocation
Precip.
remaining
loop
Dispersion Hardening
The quantity of the second phase is independent from the temperature.
• The increasing of the temperature cause less decrease in the strength)
• These phases are stable compounds like Al2O3 or SiO2
• Fine dispersion
e.g.:
Inner oxidation: dispersion hardenable Al alloys,
Al-Cu and Cu-Si alloys
heating in air
oxygen diffusion inner regions of the material
build stable compounds with Al or Si
Dispersion hardening
Geometric structure of precipitations
good not good
hard
soft
crack
hard
soft
crack
Geometric structure of precipitations
good not good
Thermomechanical Processes
Thermomechanical processes
Plastic deformation and heat treatment in one technological
process
Phase transforamtion
Recrystallisation and recovery
Change of dislocation structure
Change of the grain structure
Strengthening mechanisms
Severe Plastic Deformation
Strengthening mechanisms - techniques
Strengthening mechanism Technique, tool
AlloyingConventional and powder metalurgy
(sintering)
Cold deformationPlastic deformation techniques
Special deformation technic:
severe plastic deformation SPD
Grain refinementSpecial heat treatment techniques
Heat treatment
Precipitation and dispersion
harmening
conventional techniques
…without attempting to be comprehensive…
Dislocation cell structure changes to
grainstructure
High dislocation density at the grain
boundaries
Structure change during SPD
Microstructure of Cu during SPD
Initial grains
Dislocation density reaches a critical value
Annihilation of dislocation with opposite
signs
Principe of the process:
HPT of a bar
Pressure: 1-5 GPa
Sample size: Ø10-20 x 0.2-0.5 mm
Maximal strain: 100-150 (!)
constrained
unconstrained
High Pressure torsion - HPT
Evolution of high angle grain boundaries
Low angle boundaries
High angle boundaries
defo
rmatio
n
θ < 5 °
θ > 15 °
High Pressure torsion - HPT
Deformation process:
Shear deformation
(shear deformation is characteristic for the most of SPD techniques)
Homogeneity of the microstructure
High Pressure Torsion - HPT
High Pressure Torsion - cylindrical sample
toolstools
sample
Equal Channel Angular Pressing - ECAP
The materials is sheared in specified (1,2) plane in
the tool, while the cross section of the sample
remains unchanged.
Principe of the process:
Sample size: Ø5-20 x 100-150 mm
Strain: 8-12
Equal Channel Angular Pressing - ECAP
Idealized case: deformation on the shear plane
Real case: the deformation zone is extended
(circular shape)
Equal Channel Angular Pressing - ECAP
Equal Channel Angular Pressing - ECAP
Multiple pressing:
4 different routesIt allows the altertation of the
microstructure in differet ways.
Effect of the deformation routes on the microstructure:
Acta mater. 46, 9, 3317-3331 (1998)
BC:
equiaxed grains
HAGB
A, C:
elongated grains
lower ratio of HAGB
Equal Channel Angular Pressing - ECAP
Continuous Sheet Shearing - CSS
Regulated texture evolution
Accumulative Roll Bonding - ARB
Repetitive Corrugation and Straigthening -
RCS
High purity Cu
after 12 passes
grain size: