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1. MECHANISMS OF
STRENGTHENING IN
METALS
Presented by-
Muveen

2. Abstract-
The understanding of the strengthening mechanisms is
crucial both in the development of new materials with
improved mechanical properties and in the
development of better material models in the simulation
of industrial processes. Three different mechanisms
namely, solid solution strengthening, grain size
strengthening and strain hardening have been
examined in detail.

3. The size of the grains, or average grain diameter, in a
polycrystalline metal influences the mechanical properties.
Adjacent grains normally have different crystallographic
orientations and, of course, a common grain boundary.
The grain boundary acts as a barrier to dislocation motion
for two reasons:
1. Because the two grains are of different orientations, a
dislocation passing into grain B will have to change its
direction of motion; this becomes more difficult as the
crystallographic disorientation increases.
2. The atomic disorder within a grain boundary region will
result in a discontinuity of slip planes from one grain into
the other.
A fine-grained material (one that has small grains) is
harder and stronger than one that is coarse
grained, because the former has a greater total grain
boundary area to impede dislocation motion.

4. For many materials, the yield strength σy varies with grain size
according to
σy = σ0 + kyd-1/2
In this expression, termed the Hall – Petch equation, d is the average
grain diameter, and σ0 and ky are constants for a particular material.
The above equation is not valid for both very large (i.e., coarse) grain
and extremely fine grain polycrystalline materials.
Grain size reduction improves not only strength, but also the
toughness of many alloys. Small-angle grain boundaries are not
effective in interfering with the slip process because of the slight
crystallographic misalignment across the boundary. On the other
hand, twin boundaries will effectively block slip and increase the
strength of the material.

6. SOLID-SOLUTION STRENGTHENING:
Another technique to strengthen and harden metals is
alloying with impurity atoms that go into either substitutional
or interstitial solid solution. Accordingly, this is called solid-
solution strengthening. Increasing the concentration of the
impurity results in an attendant increase in tensile and yield
strengths, as indicated in figures, for nickel in copper

7. Alloys are stronger than pure metals because impurity atoms that go
into solid solution ordinarily impose lattice strains on the surrounding
host atoms. Lattice strain field interactions between dislocations and
these impurity atoms result, and, consequently, dislocation movement is
restricted. An impurity atom that is smaller than a host atom for which
it substitutes exerts tensile strains on the surrounding crystal lattice.
Conversely, a larger substitutional atom imposes compressive strains in
its vicinity. These solute atoms tend to diffuse to and segregate around
dislocations in a way so as to reduce the overall strain energy—that
is, to cancel some of the strain in the lattice surrounding a dislocation.
To accomplish this, a smaller impurity atom is located where its tensile
strain will partially nullify some of the dislocation’s compressive strain.

9. Strain hardening is the phenomenon whereby a ductile
metal becomes harder and stronger as it is plastically
deformed. Most metals strain harden at room temperature.
It is sometimes convenient to express the degree of plastic
deformation as percent cold work rather than as strain.
Percent cold work (%CW) is defined as
%CW= ((A0 – Ad) / A0) * 100
where A0 is the original area of the cross section that
experiences deformation and Ad is the area after
deformation. The price for this enhancement of hardness
and strength is in the ductility of the metal. The strain-
hardening phenomenon is explained on the basis of
dislocation– dislocation strain field interactions.

10. The dislocation density in a metal increases with deformation or
cold work, because of dislocation multiplication or the formation of
new dislocations. Consequently, the average distance of separation
between dislocations decreases— the dislocations are positioned
closer together. On the average, dislocation–dislocation strain
interactions are repulsive. The net result is that the motion of a
dislocation is hindered by the presence of other dislocations. As the
dislocation density increases, this resistance to dislocation motion
by other dislocations becomes more pronounced. Thus, the imposed
stress necessary to deform a metal increases with increasing cold
work. Strain hardening is often utilized commercially to enhance the
mechanical properties of metals during fabrication procedures.

12. CONCLUSION
Understanding the mechanisms behind the strengthening
in metals is crucial in the development of new materials
with better mechanical properties. A systematic analysis of
different mechanisms and how they depend on external
variables like temperature and strain rate combined with
experimental work on the evolution of plastic deformation
at small strains has been the focus of this work. Three
strengthening mechanisms namely, solid solution
strengthening, grain size strengthening & strain hardening
have been dealt with in detail. In both these cases existing
models have been reviewed with aspect to their predicting
capability and physical meaning.

13. 1. William D Callister, Jr. , Callister’s Material Science and
Engineering, John Wiley & Sons Inc.
2. Dilip Chandrasekaran, Grain Size and Solid Solution
Strengthening in Metals, Division of Mechanical
Metallurgy, Department of Materials Science and
Engineering, Royal Institute of Technology
BIBLIOGRAPHY