A presentation that gives a breif view about mechanisms of strengthening in materials and various concepts related.

1. STRENTHENING MECHANISMS IN
MATERIALS
R.NIKHITHA
Hindustan Institute of Technology and Science

2. STABILITY OF ATOMS – ARRHENIUS
EQUATION
• In order to gain stability atoms and imperfections
diffuse.
• The rate of movement is given by the Arrhenius
equation.

3. DIFFUSION
• Movement of atoms in a predictable fashion
within the material.
• In order to produce homogeneous uniform
composition.
• Self diffusion, Interchange diffusion, Vacancy
diffusion, Interstitial diffusion are the four
important diffusion mechanisms.

4. DIFFUSION MECHANISMS
1. Self diffusion: atom movement from one lattice
position to another in pure materials. Effect is not so
significant. Detected by radioactive traces.
2. Interchange diffusion: occurs in binary metallic alloys
like Cu-Ni system. Unlike atoms diffusion occurs.
3. Vacancy diffusion: atoms movement from lattice site
to nearby vacany. No. Of vacancies increases with
increase in temperature.
4. Interstitial diffusion: the small atoms present in the
material moves and fits between the voids
(interstices) of large atoms.

5. DISLOCATIONS
• Line imperfections in perfect lattices.
• Formed during solidification or deformation of the
material.
• Affects the mechanical, electronic and phonic
properties of materials.
• EDGE and SCREW dislocations are basic types.

6. EDGE AND SCREW DISLOCATIONS
• Edge dislocation: occurs either due to presence of an extra
plane of atoms or due to loss of half of a plane of atoms in
the middle of the lattice. Glide in any plane. Bends towards
the dislocation. Dislocation area moves parallel to the
stress direction. Defect occurs in the core of dislocation.
• Screw dislocation: the defect occurs when the plane of
atoms in the lattice trace a helical path around the
dislocation line. Dislocation area moves perpendicular to
the direction of stress applied.

7. PLASTIC DEFORMATION –
DISLOCATIONS
• Movement of dislocations lead to Plastic deformation.
• Edge dislocation move by slip and climb while screw
dislocation move by slip and cross slip.
• Interaction is very complex.
• Dislocations moving in parallel planes result in vacancies or
interstices.
• Dislocations moving in non parallel planes are hindered by
sharp break called jog for those out of slip plane and kink
for those in the slip plane.
• Arresting these movements can give materials more
strength.

8. PLASTIC DEFORMATION MECHANISMS
• Slip: occurs when shear stress applied exceeds a critical value.
Involves generation movement and rearrangement of
dislocations. Also involves sliding of blocks of crystal over the
other along the slip plane. Extent of slip depends on external
load, corresponding shear stress value, crystal structure,
orientation of slip planes.
• Twinning: a portion of crsytal takes up an orientation that is
related to the orientation of the rest of the untwined lattice ina
definite, symmetrical forming a mirror image of a portion. The
region where the atoms travel fractions of atomic distance is
called twinning region. This causes changes in the plane
orientation to slip happen later on.

9. SLIP SYSTEMS

10. TWIN SYSTEMS

11. STRENGTHENING MECHANISMS
PRINCIPLE- To hinder dislocations in order to increase the strength of the
material.
Single phase materials:
• Grain reduction
• Solid solution strengthening
• Strain hardening
Multi-phase materials:
• Precipitation strengthening
• Dispersion strengthening
• Fiber strengthening
• Martensite strengthening

12. • Grain Size reduction: Changing the average crystalline
grain size in order to create boundaries between
grains which could hinder activation of dislocations in
neighbouring grains.Hall-Petch relation gives the
general relation between yeild stress and grain size.
• Solid Solution strengthening: Creating solid solution.
Substitutional solid solution- Solute and solvent ats
are similar in size. Solute atoms occupy lattice points
of solvent. Interstitial solid solution- solute atoms are
smaller than solvent atoms. Solute atoms occupy
voids or interstices of the solvent.
• Strain Hardening: Also called work hardening is a
process of strengthening the material by plastic
deformation. In this process the yeild limit is
increased.

13. • Precipitation Hardening: Introduction of extremely small
uniformly dispersed particles called precipitates of a
second phase within the original phase. These precipitates
obstruct the movement of dislocations. Introduction is
done by mixing and consolidation techniques. Also called
age hardening. Room temp- natural aging. Heating-
artificial aging.
• Fiber Strengthening: Introduction of fibers into matrix.
Fiber material has high strength and high modulus. While
matrix material is ductile and non reactive with the fiber.

14. • Martensite Strengthening: by formation of martensitic
phase. Martensites form due to shearing of lattices.
The martensite platelets divide and subdivide the
grains of parent phase obstructing dislocations.
* PERITECTIC: A liquid and a solid phases transform to a
solid phase.
* EUTECTIC: Liquide transfrom to 2 solid phases.
* EUTECTOID: One solid phase to two other solid
phases.

15. IRON CARBON DIAGRAMS
• The graphical representation of different phases with
respect to the temperature.
• Carbon concentration by weight on the X-axis and
temperature on the Y-axis.

16. VARIOUS PHASES:
• AUSTENITE
• FERRITE
• PEARLITE
• CEMENTITE
• MARENSITE
• LEDEBURITE

17. TEMPERING
• Heating hardened steel at a temperature below the
eutectoid temperature and air cooling it.
• To reduce brittleness of hardened steel.
• First stage: 50-200 celsius. Martensite breaks down to low
carbon martensites and transition precipitate.
• Second stage: 205-305 celsius. Decomposition of
AUSTENITE and decrease in hardness.
• Third stage: 250 -500 celsius. Conversion of low carbon
martensites and precipitates to ferrites and cementites.

18. • Secondary hardening: In some steels containing W, Cr,
V etc.., the hardness increases instead of decreasing
during tempering due to precipitation of alloy
carbides. High speed steels are used as tool steels.

19. ANNEALING
• Heat treatment process that alters the microstructure of a
material in order to change its properties.
• In steels, reduces hardness, increases ductility and
elIminates internal stresses
• Subcritical annealing:1000°F- 1200°F. No crystal structure
change.
• Intermediate annealing: 1200°F- 1400°F. Some
transformation to austenite.
• Full annealing: 1500°F- 1700°F. Complete austenizing

20. CARBURIZING
• Widely used case hardening method. To increase hardness
of steel.
• Pack or Solid Carburizing: Components to be packed in
environment with high carbon content like bone char,
charcoal or barium carbonate. Heated at 850°C – 950°C.
Case hardens depth~ 0.1 to 2 mm.
• Gas Carburizing: Gases like carbon monoxide, natural gas,
a mixture of methane ethane and propane. Heated at
900°C to 950°C for a period of 5-10 hrs.

21. LIQUID CARBURIZING
• Components immersed in liquified carbon-rich bath
of molten salts.
• Molten salts contain a mixture of sodium carbonate,
sodium chloride and silicon carbide.
• At temperature870°C -900°C

22. VACUUM CARBURIZING
• Component placed in low pressure vessel and heated
at 900°C-1000°C.
• Gases like acetylene, ethylene and propane are passed
through a vacuum chamber.
• The gas decomposes and free carbon diffuses onto
the surface of the component

23. THANK YOU