Lec3-Dislocation, Slip Systems and Twining

7/30/2019 Lec3-Dislocation, Slip Systems and Twining

1/32

L-03

ENGINEERINGMATERIALS

DISLOCATION, SLIP SYSTEMS ANDTWINING


2/32

2

4.3 IMPURITIES IN SOLIDS-L-03 A pure metal consisting of only one type of atom just

isnt possible; impurity or foreign atoms will always bepresent, and some will exist as crystalline point defects.

In fact, even with relatively sophisticated techniques, it is

difficult to refine metals to a purity in excess of

99.9999%. At this level, on the order of 1022 to 1023 impurity atoms

will be present in one cubic meter of material.

Most familiar metals are not highly pure; rather, they

are alloys, in which impurity atoms have been addedintentionally to impart specific characteristics to the

material.


3/32

Crystal Defects A perfect crystal, with every atom of the same

type in the correct position, does not exist.

All crystals have some defects. Defects

contribute to the mechanical properties of

metals. these defects are commonly intentionally

used to manipulate the mechanical properties

of a material. Adding alloying elements to a metal is one

way of introducing a crystal defect.3


4/32

Crystal Defects just keep in mind that crystalline defects

are not always bad.

There are basic classes of crystal defects:

point defects, which are places where anatom is missing or irregularly placed in

the lattice structure. Point defects

include lattice vacancies, self-interstitialatoms, substitution impurity atoms, and

interstitial impurity atoms

4


5/32

5

Figure 4.1 Two-dimensional

representations of a

vacancy and a

self-interstitial.


6/32

6

Figure 4.2 Two-dimensional schematic representations of

substitutional and interstitial impurity atoms.


7/32

Crystal Defects linear defects, which are groups of atoms in

irregular positions. Linear defects are

commonly called dislocations.

planar defects, which are interfaces between

homogeneous regions of the material. Planar

defects include grain boundaries, stacking

faults and external surfaces.

It is important to note at this point that plastic

deformation in a material occurs due to the

movement of dislocations (linear defects).7


8/32

Crystal Defects Millions of dislocations result for plastic

forming operations such as rolling andextruding.

It is also important to note that any defect in

the regular lattice structure disrupts themotion of dislocation, which makes slip or

plastic deformation more difficult.

These defects not only include the point andplaner defects mentioned above, and also

other dislocations.8


9/32

Crystal Defects Dislocation movement produces

additional dislocations, and when

dislocations run into each other it

often impedes movement of thedislocations.

This drives up the force needed to

move the dislocation or, in other

words, strengthens the material.9


10/32


11/32

Point Defects A substitutional impurity atom is an atom of a

different type than the bulk atoms, which has

replaced one of the bulk atoms in the lattice.

Substitutional impurity atoms are usually close in

size (within approximately 15%) to the bulk atom.

An example of substitutional impurity atoms is

the zinc atoms in brass. In brass, zinc atoms with

a radius of 0.133 nm have replaced some of thecopper atoms, which have a radius of 0.128 nm.

11


12/32

Point Defects Interstitial impurity atoms are much smaller than the

atoms in the bulk matrix. Interstitial impurity atoms

fit into the open space between the bulk atoms ofthe lattice structure. An example of interstitialimpurity atoms is the carbon atoms that are addedto iron to make steel. Carbon atoms, with a radius of0.071 nm, fit nicely in the open spaces between thelarger (0.124 nm) iron atoms.

Vacancies are empty spaces where an atom shouldbe, but is missing. They are common, especially athigh temperatures when atoms are frequently and

randomly change their positions leaving behindempty lattice sites. In most cases diffusion (masstransport by atomic motion) can only occur becauseof vacancies.

12


13/32

Linear Defects - Dislocations Dislocations are another type of defect in

crystals. Dislocations are areas were the atomsare out of position in the crystal structure.Dislocations are generated and move when astress is applied. The motion of dislocations

allows slip plastic deformation to occur. The TEM (Transmission Electron Microscope-

image resolutions of 1 - 2 Angstroms) allowedexperimental evidence to be collected that

showed that the strength and ductility of metalsare controlled by dislocations.

13


14/32

Dislocations

why study Dislocations andStrengthening Mechanisms?With a knowledge of the nature of dislocations and

the role they play in the plastic deformation process,

we are able to understand the underlying

mechanisms of the techniques that are used to

strengthen and harden metals and their alloys.

Thus, it becomes possible to design and tailor the

mechanical properties of materialsfor example, the

strength or toughness of a metalmatrix composite.

14Underlying-


15/32

materials may experience two kinds ofdeformation: elastic and plastic.

Plastic deformation is permanent, and strengthand hardness are measures of a materialsresistance to this deformation.

On a microscopic scale, plastic deformationcorresponds to the net movement of largenumbers of atoms in response to an applied

stress. During this process, inter atomic bonds must be

ruptured and then reformed.

15


16/32

DISLOCATION

In crystalline solids, plastic deformation

most often involves the motion of

dislocations, linear crystalline defects.

Dislocations and Plastic Deformation.

a type of linear crystalline defect is

known as dislocation.

Edge and screw are the two

fundamental dislocation types.

16


17/32

17

1. the nature of a dislocation (i.e., edge, screw, or

mixed) is defined by the relative orientations of

dislocation line and Burgers vector.2. For an edge, they are perpendicular(Figure 4.3),

whereas for a screw, they are parallel (Figure

4.4); they are neither perpendicular nor parallel

for a mixed dislocation.

3. Virtually all crystalline materials contain some

dislocations that were introduced during

solidification, during plastic deformation, and asa consequence of thermal stresses that result

from rapid cooling.


18/32

18

Figure 4.3 The atom positions around an edge dislocation; extra half-plane of

atoms shown in perspective.


19/32

19

Figure 4.4 (a) A

screw dislocation

within a crystal.


20/32

Edge Dislocations

The edge defect can be easily visualized as an

extra half-plane of atoms in a lattice. The dislocation is called a line defect because

the locus of defective points produced in the

lattice by the dislocation lie along a line. This line runs along the top of the extra half-

plane.

The inter-atomic bonds are significantlydistorted only in the immediate vicinity of the

dislocation line.

20


21/32

21


22/32

22

Figure7.3 Representation of the analogy

between caterpillar and dislocation motion.


23/32

23

Figure 7.2 The

formation of a

step on

the surface of

a crystal by the

motion of (a)

an edge

dislocation and

(b) a screw

dislocation.

Note that for

an edge, the

dislocation line

moves in the

direction of

the applied

shear stress for

a screw, the

dislocation line

motion is

perpendicular

to the stress

direction.


24/32

7.4 SLIP SYSTEMS Dislocations do not move with the same degree of ease

on all crystallographic planes of atoms and in all

crystallographic directions.

Ordinarily there is a preferred plane, and in that plane

there are specific directions along which dislocation

motion occurs. This plane is called the slip plane; it follows that the

direction of movement is called the slip direction.

This combination of the slip plane and the slip direction is

termed the slip system.

The slip system depends on the crystal structure of the

metal and is such that the atomic distortion that

accompanies the motion of a dislocation is a minimum.24


25/32

SLIP SYSTEMS For a particular crystal structure, the slip plane is

the plane that has the most dense atomic

packingthat is, has the greatest planar density. The slip direction corresponds to the direction, in

this plane, that is most closely packed with atomsthat is, has the highest linear density.

Consider, for example, the FCC crystal structure, aunit cell of which is shown in Figure 7.6a.

There is a set of planes, the {111} family, all ofwhich are closely packed.

A (111)-type plane is indicated in the unit cell; inFigure 7.6b, this plane is positioned within theplane of the page, in which atoms are nowrepresented as touching nearest neighbors.

25


26/32

26

Figure 7.6 (a) A {111}slip system shown within an FCC

unit cell. (b) The (111) plane from (a) and three

slip directions (as indicated by arrows) within that plane

comprise possible slip systems.


27/32

SLIP SYSTEMS Slip occurs along-type directions within the

{111} planes, as indicated by arrows in Figure 7.6.

Hence, {111}represents the slip plane anddirection combination, or the slip system for FCC.

Figure 7.6b demonstrates that a given slip plane maycontain more than a single slip direction.

Thus, several slip systems may exist for a particularcrystal structure; the number of independent slipsystems represents the different possiblecombinations of slip planes and directions.

For example, for face-centered cubic, there are 12 slipsystems: four unique {111} planes and, within eachplane, three independentdirections.

27


28/32

SLIP SYSTEMS The possible slip systems for BCC and HCP crystal

structures are listed in Table 7.1. For each of these structures, slip is possible on

more than one family of planes (e.g., {110}, {211},

and {321} for BCC). For metals having these two crystal structures,

some slip systems are often operable only at

elevated temperatures. Metals with FCC or BCC crystal structures have a

relatively large number of slip systems (at least

12).

28


29/32

SLIP SYSTEMS These metals are quite ductile because extensive

plastic deformation is normally possible along thevarious systems.

Conversely, HCP metals, having few active slipsystems, are normally quite brittle.

With regard to the process of slip, a Burgers vectorsdirection corresponds to a dislocations slip direction,whereas its magnitude is equal to the unit slipdistance (or interatomic separation in this direction).

Of course, both the direction and the magnitude ofbwill depend on crystal structure, and it is convenientto specify a Burgers vector in terms of unit cell edgelength (a) and crystallographic direction indices.

29


30/32

30


31/32

31


32/32

Lec3-Dislocation, Slip Systems and Twining

Documents

Transcript of Lec3-Dislocation, Slip Systems and Twining