materials science and testingstrengthening mechanisms mechanical properties high hardness and...

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


Mechanical properties

High hardness and strength and good toughness

Increase the strength of the materials


Methods

Cold deformation

Alloying

Grain refinement

Heat treatment


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


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.


Size effect

The stress field and the elastic deformation around the alloying atom

depends on the size difference.


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


Cottrel- atmosphere

Substitutional atoms:

- Compression zone

Smaller atoms

- Tension zone

Larger atoms

Interstitial atoms


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 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


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.


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


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


Technology steps:


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


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


Idealized case: deformation on the shear plane

Real case: the deformation zone is extended

(circular shape)



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


Continuous Sheet Shearing - CSS

Regulated texture evolution

Accumulative Roll Bonding - ARB

Repetitive Corrugation and Straigthening -

RCS

High purity Cu

after 12 passes

grain size:

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