Dislocations and Strengthening Mechanisms
Plastic
deformation –Dislocations
Permanent
plastic deformation is due to shear process–atoms change their neighbors.
Inter-atomic forces and crystal structure plays an important role during
plastic deformation. Cumulative movement of dislocations leads to gross plastic
deformation. Edge dislocation move by slip and climb, while screw dislocation
move by slip and cross-slip.
During their
movement, dislocations tend to interact. The interaction is very complex
because of number of dislocations moving over many slip systems in different
directions Dislocations
moving on parallel planes may annihilate each other, resulting in either
vacancies or interstitials. Dislocations
moving on non-parallel planes hinder each other’s movement by producing sharp
breaks–jog (break out of slip plane), kink(break in slip plane) . Other hindrances to dislocation motion–interstitiall and
substitutional atoms, foreign particles, grain
boundaries, external grain surface, and change in structure due to phase
change.
Material
strength can be increased by arresting dislocation motion
Plastic
deformation mechanisms -Slip
Mainly two
kinds :slip and twinning.
• Slip is prominent among the two. It involves
sliding of blocks of crystal over other along slip planes.
• Slip occurs when shear stress applied exceeds a
critical value.
• Slip occurs most readily in specific
directions(slip directions)on certain crystallographic planes.Feasible
combination of a slip plane together with a slip direction is considered as a
slip system.
• During slip each atom usually moves same
integral number of atomic distances along the slip plane
Extent of slip depends on many factors-external load and the
corresponding value of shear stress produced by it, crystal structure,
orientation of active slip planes with the direction of shearing stresses generated.
Slip occurs when shear stress applied exceeds a critical value. For single
crystal, Schmid defined critical shear stress,which
can be expressed as:
Both factors τ and σ are measured in stress, which is
calculated the same as pressure by dividing force by area. φ and λ are angles
usually measured in degrees.
In a poly-crystalline material, individual
grains provide a mutual geometrical constraint on one other, and this precludes
plastic deformation at low applied stresses.
Slip in polycrystalline material involves
generation, movement and (re-) arrangement of dislocations. During
deformation, mechanical integrity and coherency are maintained along the grain
boundaries. A minimum of five independent slip systems must be operative for a
polycrystalline solid to exhibit ductility and maintain grain boundary
integrity–von Mises. On the other hand, crystal deform by twinning.
Strengthening
mechanisms of material can be increased by hindering dislocation, which is responsible for plastic
deformation.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
-Fiber
strengthening
-Martnsite
strengthening
Strengthening by Grain size reduction
It is based on the fact that dislocations will
experience hindrances while trying to move from a grain in to the next because
of abrupt change in orientation of planes. Hindrances can be two types:
forcible change of slip direction, and discontinuous slip plane. Smaller the grain size, often a dislocation encounters
a hindrance. Yield strength of material will be increased. Yield strength is related to grain
size(diameter, d)as Hall-Petch
relation:
Grain size can
be tailored by controlled cooling or by plastic deformation followed by
appropriate heat treatment
Solid solution strengthening
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
Yield point phenomenon
Localized,
heterogeneous type of transition from elastic to plastic deformation marked by
abrupt elastic-plastic transition–Yield point phenomenon. It characterizes that
material needs higher stress to initiate plastic flow than to continue it.
The bands are
called Lüders bands/Hartmann lines/stretcher stains, and generally are
approximately 45 to the tensile axis.
Occurrence of yield
point is associated with presence of small amounts of interstitial or
substitutional impurities. It’s been found that either unlocking of
dislocations by a high stress for the case of strong pinning or generation of
new dislocations are the reasons for yield-point phenomenon. Magnitude of
yield-point effect will depend on energy of interaction between solute atoms
and dislocations and on the concentration of solute atoms at the dislocations.
Fracture is the separation of a
single body into pieces by an imposed stress. Information about plastic
deformation and fracture is given in this article. As polycrystalline material
is made up of many grains which may have second phase particles and grain
boundaries. It is therefore easier to study plastic deformation in a single
crystal to eliminate the effects of grain boundaries and second phase particles
Plastic deformation mechanisms –Twinning
It results when
a portion of crystal takes up an orientation that is related to the orientation
of the rest of
The important
role of twinning in plastic deformation is that it causes changes in plane
orientation the untwined lattice in a definite, symmetrical way so that further
slip can occur. Twinning also occurs in a definite direction on a specific
plane for each crystal structure
Deformation by Slip.
If a single crystal of a metal is
stressed in tension beyond its elastic limit, it elongates slightly, a step
appears on the surface indicating relative displacement of one part of the
crystal with respect to the rest, and the elongation stops. Increasing the load
will cause movement on another parallel plane, resulting in another step. It is
as if neighboring thin sections of the crystal had slipped past one another
like sliding cards on a deck. Each successive elongation requires a higher
stress and results in the appearance of another step. With progressive increase
of the load, a stage is reached which causes the material to fracture.
Sliding occurs in certain planes of
atoms in the crystal and along certain directions in these planes. Thus the
mechanism is a new type of flow that depends upon the perfectly repetitive
structure of the crystal . It allows the atoms in one face of a slip plane to
shear away from their original neighbors in the other face, to slide in an
organized way along this face carrying their own half of the crystal with them,
and finally to join up again with a new set of neighbors as nearly perfect as
before. The movement in the crystal takes place along planes having highest
atomic density with greatest distance between similar parallel planes and in
the direction of close-packed atoms. This can be simply stated as: Slip occurs
on planes that have h ighest planer density of atoms and in the direction with
highest linear density of atoms.
Slip occurs in directions in which
the atoms are most closely packed since this requires the least amount of
energy. As shown in the figure, close-packed rows are vertically further apart
from each other (d1) than rows that are not close-packed (d2), therefore they
can slip past each other with less interference.
Black bars between atoms show the
slope of the path of atoms. Since the slope is less steep in case of
closed-packed rows, less force is required for a given horizontal displacement.
In addition, less displacement (D1 < D2) is required by the atoms to move
from one stable position (before slip) to other stable position (after slip).
To get a better grasp of this, think
of gluing ping pong balls to two boards in a widely spaced pattern as shown in
above figure. Put the boards together, ping pong balls to ping pong balls, and
begin to tilt the bottom board. You will have to go to a very steep angle
before the top board will slip from its position of the nestled ping pong
balls. Now do the experiment again, but this time gluing the balls very close
together. The top board will now slip at a much lesser angle.