Slip and twinning are two important deformation mechanisms in crystals. Slip involves the sliding of atomic planes over one another along crystallographic planes called slip planes, and occurs when the critical resolved shear stress is exceeded. It is controlled by dislocations. Twinning involves mirror-image reflections on either side of a twinning plane, where successive atomic planes are displaced by increasing amounts. Twinning accommodates deformation by changing the crystal orientation and is important when slip systems are limited. The key differences between slip and twinning are that slip is a line defect controlled by dislocations while twinning is a grain boundary surface defect.

1. DEFORMATION
MECHANISM IN
CRYSTALS
 Mahfooz Alam
 17ETMM10

2. OUTLINE OF PRESENTATION
• What are SLIP & SLIP SYSTEMS?
o SLIP, Line Defect, Edge & screw dislocations, Burger’s circuit, Glide and Climb.
o CRITICALLY RESOLVED SHEAR STRESS
o SCHMID’S LAW
o Slip planes & Slip directions
• What is TWINNING?
• Differences between SLIP & TWINNING

3. SLIPPING
• Slip is a prominent mechanism of plastic deformation in metals.
• It involves sliding of blocks of crystal over one another along definite crystallographic
planes, called slip-planes.
• Generally slip plane is the plane of greatest atomic density, and the slip direction is the
close packed direction within the slip plane.
• Slip occurs when shear stress applied exceeds a critical value.
• During slip each atom usually moves same integral number of atomic distances along the
slip plane producing a step, but the orientation of the crystal remains the same.

5. • Slip can be considered as a result of a distorted boundary, a Line Defect called dislocation, between
two perfect regions of a space lattice.
• The passage of a dislocation through a crystal results in the relative motion of one part of the crystal
past the other part, this process is called a slip.
• Two types of dislocation segments are identified:
edge dislocation
• which causes axial strains in the space lattice,
screw dislocation
• which causes the lattice to experience shear strains and to distort through a helical path.

8. BURGER’S CIRCUIT

9. • The reason that dislocations control the plastic deformation of crystalline solids is that it is relatively
easy to move dislocations to produce shear deformation.
• It would be enormously difficult to shear a crystal by forcing the glide of rigid planes of atoms over
one another; one would have to force the simultaneous reconfiguration of every crystal bond that
crossed the slip plane.
• The same result is more easily achieved by moving dislocations stepwise through the crystal.
Stepwise dislocation motion requires a much smaller force since each elementary step can be
accomplished by reconfiguring only the bonds that neighbor the dislocation line.
• However, an edge dislocation cannot glide in an arbitrary direction. It can only glide in a particular
plane, the slip plane or glide plane, which contains both the Burgers vector and the dislocation line

10. • In order for the dislocation to move one lattice spacing to the right it is only necessary to break the bond
indicated by the long dash in Fig. and establish the bond indicated by the short dash. The new configuration
is shown in Fig. 4.12b.
• Of course one bond must be broken for each plane through which the dislocation threads, so a significant
force is still required.
• But the force is small compared to that required to slip the upper part of the crystal as a rigid body. If the
dislocation moves through the crystal in a sequence of individual steps, it causes a net slip of the material
above its plane of motion by the Burgers vector, b, and hence causes a rigid displacement of the whole
upper part of the crystal
 The type of
motion that is
illustrated is
called
dislocation
glide

11. • When an edge dislocation moves out of its glide plane its motion is called climb.
• The climb of a dislocation is difficult at ordinary temperatures since it requires that atoms be absorbed
on or liberated from the extra half-plane of atoms that defines the dislocation line.
• The mechanism is slightly different depending on whether the dislocation moves up, which contracts
the extra half-plane, or down, which extends it.
• Because of the difficulty of climb at ordinary temperatures the plastic deformation of real crystals
tends to occur through the motion of dislocations on well-defined planes that are the glide planes of
the active dislocations.
• At high temperature climb becomes
possible and the slip planes are less
well-defined.
• When this happens the strength of the
crystal (its resistance to plastic
deformation) decreases dramatically.
• For this reason most solids are
relatively soft at high temperature.

12. • As the screw dislocation is displaced through the width of the body the material above its plane is
slipped in the direction of the Burgers vector, hence along the length of the body.
• It follows that the longitudinal force acts to drive the screw dislocation sideways.
• If a screw dislocation is passed through the full width of the body it causes the shear, which is the
same as that caused by the passage of an equivalent edge dislocation through the length.
• In contrast to an edge dislocation, a screw dislocation can glide in any plane.
• Since the Burgers vector lies parallel to the dislocation line both are in any plane that contains the
dislocation line, and the screw dislocation can move in any direction perpendicular to its line.
• Dislocations in real materials are mixed dislocations.

13. CRITICAL RESOLVED SHEAR STRESS
• Crystalline materials tend to deform or fail by the relative motion of planes of atoms under
the action of stress. This motion is induced by the component of stresses acting across the
slip planes.
• The deformation process is a collective motion of adjacent slip planes. But all the planes do
not start deforming simultaneously. Rather slip starts from a single plane and then other
planes follow. The first slip occurs when the shear stress across the plane exceeds a certain
value.
• This threshold value is called Critical Resolved Shear Stress.
• (CRSS) is the component of shear stress, resolved in the direction of slip, necessary to
initiate slip in a grain.
• The RSS is related to the applied stress by a geometrical factor ‘m’, the Schmid factor.

14. • A: Cross sectional area of the
sample
• φ: Angle between the tensile axis
and the normal to the slip plane
• λ: Angle between the tensile axis
and the slip direction.
• P: Tensile load.
Shear stress on the slip plane along the slip direction is given by:
Shear stress = Load across the plane/Cross sectional area
τ = P · cos λ/(A/cos φ) = (P/A) · cos λ · cos φ
P/A can be substituted for tensile stress (σ), and the equation then
becomes:
τ = σ · cos λ · cos φ
The term cos λ · cos φ is known as Schmid factor (m).

15. SCHMID'S LAW
• It describes the slip plane and the slip direction of a stressed material, which can resolve the most
amount of shear stress.
• Schmid's Law states that the critically resolved shear stress (τ) is equal to the stress applied to the
material (σ) multiplied by the cosine of the angle with the vector normal to the glide plane (Φ) and
the cosine of the angle with the glide direction (λ). 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.

16. SLIP SYSTEMS
• The slip planes and directions, combined are called as the slip systems.
• The slip directions are the crystallographic directions with the shortest distance between like
atoms or ions and the slip planes are usually densely packed planes.

17. For example, in a face-
centered-cubic (fcc)
lattice, slip occurs on the
four planes of ABC type,
(111) planes, and in the
direction of the diagonals
of cube faces or the three
sides of triangle ABC,
directions.

18. TWINNING
• In Twinning each plane of atoms move
through a definite distance and in the
same direction.
• The extent of movement of each plane is
proportional to its distance from the
twining plane, as shown in fig.
• The distance moved by each successive
atomic plane is greater than the previous
plane by a few atomic spacing.

19. • The atomic arrangement on either side of the twinned plane is in such a way they are mirror
reflections of each other. Twins are known as annealing twins when they are produced during
annealing heat treatment and mechanical twins when they are produced by mechanical
deformation of metals.

20. dislocations
Twinning

21. • When a shear stress is applied the crystal will twin about the twinning plane in such a way
that the region to the left of the twinning plane is not deformed where as the region to the
right is deformed
• The important role of twinning in plastic deformation is that it causes changes in plane
orientation so that further slip can occur.
• Twinning is a special type of grain boundary defect, in which a crystal is joined to its
mirror image.
• Twin boundaries are partly responsible for shock hardening and for many of the changes
that occur in cold work of metals with limited slip systems or at very low temperatures.
• their presence is partly responsible for the hardness due to quenching of steel. In certain
types of high strength steels, very fine deformation twins act as primary obstacles against
dislocation motion. These steels are referred to as 'TWIP' steels, where TWIP stands for
Twinning Induced Plasticity.

22. No
Slip Twinning
1 Crystal slip is a line defect. Twinning is a surface defect grain boundary defect.
2 During slip, all atoms in a block move the
same distance.
During twinning, the atoms in each successive plane in a
block move through different distances proportional to their
distance from twinning plane.
3 Slip is commonly observed in Body-
centered Cubic (BCC) and Face Centered
Cubic (FCC) metals.
Twinning is commonly observed in Hexagonal Close Packing
(HCP) metals.
5 The slipped crystal lattice has the same
orientation.
The twinned crystal lattice is the minor image of the original
lattice.
6 The stress required for slip is comparatively
low.
The stress required for twinning is comparatively more.
7 The stress necessary to propagate slip is
usually higher than the stress required to
start slip.
The stress necessary to propagate twinning is lesser than that
required starting it.
Differences between Slip and Twinning

23. Slip Twinning
8. Occurs in metals having more number of slip
systems.
8. Occurs in metals having less number of slip
systems.
9. Slip can be seen as thin lines when viewed
under microscope.
9. Twinning can be seen as broad lines when
viewed under microscope.
10. For slipping to occur, a threshold value of
stress called critical resolved shear stress is
required.
10. For twinning to occur, no such threshold
value of stress is required.
11. Takes place in milli seconds Takes place in less than micro seconds
12. No sound is created 12. A click sound (Tin cry)