STRENGTHENING
MECHANISMS
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STRENGTHENING MECHANISMS
● The ability of a metal to plastically deform depends on the ability of dislocations
to move.
● By reducing the mobility of dislocations, the mechanical strength may be
enhanced; that is, greater mechanical forces will be required to initiate plastic
deformation.
● Therefore, restricting dislocation motion makes a material harder and
stronger.
● Strengthening reduces ductility.
STRENGTHENING MECHANISMS
➢ Grain size reduction
➢ Solid solution strengthening
➢ Work hardening (cold working or strain hardening)
➢ Dispersion hardening
➢ Age hardening
For single-phase
metals
For multiphase
alloys
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Strengthening by Grain Size Reduction
● The size of the grains in a polycrystalline metal influences the mechanical
properties.
● The grain boundary act as a barrier to dislocation motion for two reasons:
● Since two grains are of different orientations, dislocation passing into grain B
will have to change its direction of motion (see Fig.).
● The atomic disorder within a grain boundary region will result in a discontinuity
of slip planes from one grain into the other.
!
● Small angle grain boundaries are not very effective in blocking dislocations.
● High-angle grain boundaries block slip and increase of the material.
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Hall-Petch Equation:
σy: yield strength
d: average grain diameter
σ 0 and ky: constant
Affect of Grain Size
● A fine-grained material is harder and stronger than coarse grained material.
●Since, small grained material has a greater total grain boundary area to impede
dislocation motion.
●For many materials, the yield strength varies with grain size according to Hall-
Petch equation.
● Hall-Petch equation is not
valid for both
➢ very large grain, and
➢ extremely fine grain
polycrystalline materials.
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Solid Solution Strengthening: Alloying
●Alloying is a solid-solution strengthening technique.
●Types of solid solution: (1) Interstitial, (2) Substitutional
●Increasing the concentration of the impurity results in an attendant increase in tensile and
yield strengths.
(a) (b) (c)
Variation with nickel content of (a) tensile strength, (b) yield strength, and (c) ductility for Cu-Ni alloys,
showing strengthening
● Alloys are stronger than
pure metals because
impurity atoms that go into
solid solution ordinarily
impose lattice strain on the
surrounding host atoms.
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Solid Solution Strengthening: Alloying
● Lattice strain field interactions
between dislocations and these
impurity atoms result, and,
consequently, dislocation movement
is restricted.
● The resistance to slip is greater
when impurity atoms are present
because the overall lattice strain
must increase if a dislocation is torn
away from them.
●Furthermore, the same lattice
strain interactions will exist between
impurity atoms and dislocations that
are in motion during plastic
deformation.
● Thus, a greater applied stress is necessary to first initiate and then continue
plastic deformation for solid-solution alloys, as opposed to pure metals.
● This is evidenced by the enhancement of strength and hardness.
Smaller substitutional
impurity
Larger substitutional impurity
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WORK (or STRAIN) HARDENING
● Ductile metals become stronger when they are deformed plastically at
temperatures well below the melting points.
● It is also called cold working, because the temperature at which deformation
takes place is cold.
● Most metals strain harden at room temperature.
● The reason for work hardening is the increase of dislocation density with
plastic deformation. The average distance between dislocation decreases and
dislocations start blocking the motion of each other.
● Percent cold work (% CW) is used to express the degree of plastic
deformation.
● Before deformation: dislocation density ~103 mm/mm3
● Heavily deformed sample: dislocation density ~1010 mm/mm3
● Dislocations interact and obstruct each other.
● More dislocations actually increase the strength of a material.
100% x
A
AA
CW
o
do −
=
Ao : original cross-section area
Ad :area after deformation.
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Common forming operations change the cross sectional area
For 1040 steel, brass, and Cu, the increase in yield and tensile
strength, and the decrease in ductility (%EL)
● There is only a certain amount a material can be deformed before it breaks.
● Different materials have varying % allowable cold work.
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Cold Work is Anisotropic
● When a piece of metal is deformed, grains elongate in the deformation direction.
● Slip occurs only in the favored directions.
● Therefore, strength of the metal increases in the direction it is deformed, but
properties in the other directions do not change as much.
Cold rolling: Metal is often rolled into sheets from thicker stock.
● The width of the sheet is usually kept the same, and only the thickness varies.
● During deformation some fraction of the energy used for deformation is stored in
the metal as residual stresses.
● Restoration of the properties and structure to precold-worked state is possible by
heat treatment (Annealing)
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Example:
Solution:
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ANNEALING: Recovery, Recrystallization, Grain Growth
● Annealing is a heat treatment in which a
material is exposed to an elevated temperature
for an extended time period and then slowly
cooled.
● Time is generally 1 hour.
● Annealing is carried out to:
● relieve stresses
● increase softness, ductility and toughness
● produce a specific microstructure
● So, effects of cold work are reversed!
● Three stages in annealing: (1) recovery,
(2) recrystallization, (3) grain growth
RECOVERY
● The low temperature treatment
● Residual stresses are reduced or even
eliminated.
● Dislocations are rearranged, but dislocation
density unchanged.
● Corrosion resistance, electrical and thermal
conductivity are restored
● The mechanical properties (strength and
ductility) are unchanged.
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RECRYSTALLIZATION
● Recrystallization is the nucleation and growth of new, strain free, equiaxed grains with low
dislocation density which is characteristics of precold-worked state.
● Recrystallization process depends on time and temperature.
● Recrystallization temperature is the temperature at which recrystallization completed in one hour
(typically 0.3 – 0.5 Tmelting).
● Mechanical properties are restored to precold-worked state.
● The recrystallized metal has a low strength but a high ductility.
● Dislocation density greatly reduced by recrystallization.
● Recrystallization is used to refine the grain structure of cold worked material.
● Recrystallization temperature and rate decrease when the amount of cold work increases.
● Without a minimum amount of cold work (5-15%), there is no recrystallization.
The variation of recrystallization
temperature with percent cold work
for IRON
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GRAIN GROWTH
● After recrystallization is complete, the
strain-free grains will continue to grow if
the metal specimen is left at elevated
temperature. This phenomenon is called
grain growth.
● Not all grains can enlarge, but big
grains grow at the expense of small ones
● Grain growth during annealing occurs in
all polycrystalline materials (i.e. they do
not have to be deformed first).
● For grain growth, dependence of grain size (d) on time (t):
PROBLEMS (Callister, chapter 7, pp. 202-206)
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Solution:
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