strengthening mechanisms in metals and alloys

1. Strengthening Mechanisms
Hitesh Basitti
2017pmt5094

2.  Strength of Material can be increased by hindering dislocation, which is
responsible for plastic deformation.
 Dislocation: A displacement of part of a crystal lattice structure.
"dislocations are present due to the accidents of imperfect growth"
 Different ways to hinder dislocation motion/Strengthening mechanisms:
In single-phase materials
Grain size reduction
Solid solution strengthening
Strain hardening
In multi-phase materials
Precipitation strengthening
Dispersion strengthening
Martensite strengthening

3.  Ordinarily ductility is sacrificed when an alloy is strengthened.
 The relationship between dislocation motion and mechanical behavior
of metals is significance to the understanding of strengthening
mechanisms.
 The ability of a metal to plastically deform depends on the ability of
dislocations to move.
 Virtually all strengthening techniques rely on this simple principle:
Restricting or Hindering dislocation motion renders a material harder
and stronger.

4. Strengthening by Grain size reduction
Grain boundaries are barriers to slip.
• Barrier "strength" increases with Increasing angle of misorientation.
• Smaller grain size: more barriers to slip
It is based on the fact that
dislocations will experience hindrances
while trying to move from a grain into
the next because of abrupt change in
orientation of planes.
Adapted from Fig. 7.14, Callister 7e.
(Fig. 7.14 is from A Textbook of
Materials Technology, by Van Vlack,
Pearson Education, Inc., Upper Saddle
River, NJ.)
Yield strength is related to grain size
(diameter, d) as Hall-Petch relation:

6. Strengthening by Grain size reduction (Contd…)
Grain Size Reduction Techniques:
•Increase Rate of solidification from the liquid phase.
•Perform Plastic deformation followed by an appropriate heat treatment.
Notes:
 Grain size reduction also improves toughness of many alloys.
Small-angle grain boundaries are not effective in interfering with the slip
process because of the small crystallographic misalignment across the
boundary.
Boundaries between two different phases are also impediments to
movements of dislocations.

7. Solid solution strengthening
Impurity atoms distort the lattice & generate stress.
Stress can produce a barrier to dislocation motion.
Impure foreign atoms in a single phase material produces lattice strains which can
anchor the dislocations.
Effectiveness of this strengthening depends on two factors–size difference and
volume fraction of solute.
Solute atoms interact with dislocations in many ways:
elastic interaction
modulus interaction
stacking-fault interaction
electrical interaction
short-range order interaction
long-range order interaction
Elastic, modulus, and long-range order interactions are of long-range i.e. they are
relatively insensitive to temperature and continue to act about 0.6Tm.

8. Adapted from Fig. 7.4, Callister 7e.
Stress Concentration at Dislocations

9. • Small impurities tend to concentrate at dislocations on the “Compressive stress side”
• Reduce mobility of dislocation increase strength
Strengthening by Alloying
Adapted from Fig. 7.17, Callister 7e.

10. Large impurities concentrate at dislocations on “Tensile Stress” side – pinning dislocation
Adapted from Fig. 7.18, Callister 7e.

11. e.g. : Solid Solution Strengthening in Copper
Tensile strength & yield strength increase with wt% Ni
Empirical relation:
Alloying increases YS and TS.

12. Strain hardening
• Phenomenon where ductile metals become stronger and harder when they are
deformed plastically is called strain hardening or work hardening.
• Increasing temperature lowers the rate of strain hardening. Hence materials are strain
hardened at low temperatures, thus also called cold working.
• During plastic deformation, dislocation density increases. And thus their interaction
with each other resulting in increase in yield stress.
• Dislocation density (ρ) and shear stress (τ) are related as follows:

13.  During strain hardening, in addition to mechanical properties, physical properties
also changes:
A small decrease in density
An appreciable decrease in electrical conductivity
Small increase in thermal coefficient of expansion
Increased chemical reactivity (decrease in corrosion resistance).
 Deleterious effects of cold work can be removed by heating the material to
suitable temperatures–Annealing.
 It restores the original properties into material. It consists of three stages–
recovery, recrystallization and grain growth.
 In industry, alternate cycles of strain hardening and annealing are used to deform
most metals to a very great extent.
Strain hardening (contd…)

14. Impact of Cold Work
As cold work is increased …….
• Yield strength (YS) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
For Low-Carbon Steel, Adapted from Fig.
7.20, Callister 7e.

15. D o =15.2mm
Cold
Work
Dd =12.2mm
Copper
Cold Work Analysis
What is the tensile strength and ductility after cold working?
%6.35100x%
2
22




o
do
r
rr
CW

16. % Cold Work
100
300
500
700
Cu
200 40 60
yield strength (MPa)
% Cold Work
tensile strength (MPa)
200
Cu
0
400
600
800
20 40 60
340MPa
TS = 340MPa
ductility (%EL)
% Cold Work
20
40
60
20 40 6000
Cu
7%
%EL = 7%YS = 300 MPa
Adapted from Fig. 7.19, Callister 7e. (Fig. 7.19 is adapted from Metals Handbook: Properties and Selection: Iron
and Steels, Vol. 1, 9th ed., B. Bardes (Ed.), American Society for Metals, 1978, p. 226; and Metals Handbook:
Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American
Society for Metals, 1979, p. 276 and 327.)
Cold Work Analysis
What is the tensile strength and ductility after cold working?

17. tensilestrength(MPa)
ductility(%EL)
tensile strength
ductility
600
300
400
500
60
50
40
30
20
annealing temperature (ºC)
200100 300 400 500 600 700
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are reversed!
Effect of Heating After %CW
Adapted from Fig. 7.22, Callister 7e. (Fig.
7.22 is adapted from G. Sachs and K.R. van
Horn, Practical Metallurgy, Applied Metallurgy,
and the Industrial Processing of Ferrous and
Nonferrous Metals and Alloys, American Society
for Metals, 1940, p. 139.)
• 3 Annealing
stages to
discuss...

18. Annihilation reduces dislocation density.
Recovery
• Scenario 1
Results from
diffusion
• Scenario 2
4. opposite dislocations
meet and annihilate
Dislocations
annihilate
and form
a perfect
atomic
plane.
extra half-plane
of atoms
extra half-plane
of atoms
atoms
diffuse
to regions
of tension
2. grey atoms leave by
vacancy diffusion
allowing disl. to “climb”
tR
1. dislocation blocked;
can’t move to the right
Obstacle dislocation
3. “Climbed” disl. can now
move on new slip plane

19. • New grains are formed that:
-- have a low dislocation density
-- are small
-- consume cold-worked grains.
Adapted from
Fig. 7.21 (a),(b),
Callister 7e.
(Fig. 7.21 (a),(b)
are courtesy of
J.E. Burke,
General Electric
Company.)
33% cold
worked
brass
New crystals
nucleate after
3 sec. at 580C.
0.6 mm 0.6 mm
Recrystallization

20. • All cold-worked grains are consumed.
Adapted from
Fig. 7.21 (c),(d),
Callister 7e.
(Fig. 7.21 (c),(d)
are courtesy of
J.E. Burke,
General Electric
Company.)
After 4
seconds
After 8
seconds
0.6 mm0.6 mm
Further Recrystallization

21. Recrystallization Temperature, TR
TR = recrystallization temperature = point of highest rate of
property change
1. TR  0.3-0.6 Tm (K)
2. Due to diffusion  annealing time TR = f(t)
shorter annealing time => higher TR
3. Higher %CW => lower TR – strain hardening
4. Pure metals lower TR due to dislocation movements
• Easier to move in pure metals => lower TR

22. • At longer times, larger grains consume smaller ones.
• Why? Grain boundary area (and therefore energy)
is reduced.
After 8 s,
580ºC
After 15 min,
580ºC
0.6 mm 0.6 mm
Adapted from
Fig. 7.21 (d),(e),
Callister 7e.
(Fig. 7.21 (d),(e)
are courtesy of
J.E. Burke,
General Electric
Company.)
Grain Growth
• Empirical Relation:
Ktdd n
o
n

coefficient dependent on
material & Temp.
grain dia. At time t.
elapsed time
exponent typ. ~ 2
This is: Ostwald Ripening

23. TR
Adapted from Fig.
7.22, Callister 7e.
º
º
TR = recrystallization
temperature

24. Precipitation & Dispersion hardening
 Foreign particles can also obstructs movement of dislocations i.e. increases the
strength of the material.
 Foreign particles can be introduced in two ways– precipitation and mixing – and
–consolidation technique.
 Precipitation hardening is also called age hardening because strength increases
with time.
 Requisite for precipitation hardening is that second phase must be soluble at an
elevated temperature but precipitates upon quenching and aging at a lower
temperature.
e.g.: Al-alloys, Cu-Be alloys, Mg-Al alloys, Cu-Sn alloys
 If aging occurs at room temperature – Natural aging
 If material need to be heated during aging – Artificial aging.

25.  In dispersion hardening, fine second particles are mixed with matrix powder,
consolidated, and pressed in powder metallurgy techniques.
 For dispersion hardening, second phase need to have very low solubility at all
temperatures.
e.g.: oxides, carbides, nitrides, borides, etc.
 Dislocation moving through matrix embedded with foreign particles can either
cut through the particles or bend around and by pass them.
 Cutting of particles is easier for small particles which can be considered as
segregated solute atoms.
 Effective strengthening is achieved in the bending process, when the particles
are submicroscopic in size.
Precipitation & Dispersion hardening (contd..)

26. Adapted from Fig. 11.22, Callister .
Schematic temperature-versus-time plot
showing both solution and precipitation
heat treatments for precipitation
hardening.
Adapted from Fig. 11.23, Callister .
Schematic diagram showing strength
and hardness as a function of the logarithm
of aging time at constant temperature
during the precipitation heat treatment.

27. Martensite strengthening
 This strengthening method is based on formation of martensitic phase from the
retained high temperature phase at temperatures lower then the equilibrium
invariant transformation temperature.
 Martensite forms as a result of shearing of lattices.
 Martensite plate lets assumes characteristic lenticular shape that minimizes the
elastic distortion in the matrix.
 These platelets divide and subdivide the grains of the parent phase. Always
touching but never crossing one another.
 Martensite platelets grow at very high speeds (1/3rd of sound speed) i.e.
activation energy for grow this less. Thus volume fraction of martensite exist is
controlled by its nucleation rate.

28. Martensite strengthening (contd…)
 Martensite platelets attain their shape by two successive shear displacements-first
displacement is a homogeneous shear through out the plate which occurs parallel
to a specific plane in the parent phase known as the habit plane, second
displacement, the lesser of the two, can take place by one of two mechanisms: slip
as in Fe-C martensite or twinning as in Fe-Ni Martensite.
 Martensite formation occurs in many systems.
e.g.: Fe-C, Fe-Ni, Fe-Ni-C, Cu-Zn, Au-Cd, and even in pure metals like Li, Zr and
Co. However, only the alloys based on Fe and C show a pronounced
strengthening effect.
 High strength of Martensite is attributed to its characteristic twin structure and to
high dislocation density. In Fe-C system, carbon atoms are also involved in
strengthening.

29. End