Chapter 7

Question 1

What are equiaxed grains?

Answer 1

Grains that have approximately the same dimension in all directions

Question 2

What are the two types of deformations?

Answer 2

Elastic and plastic

Question 3

What properties measure a materials’ resistance to plastic deformation?

Answer 3

Strength and hardness

Question 4

Can you think of an edge or screw dislocation as a result from a shear distortion?

Answer 4

Screw– its dislocation line passes through the center of the spiral

Question 5

What does an edge dislocation move in response to?

Answer 5

A shear stress applied in a direction perpendicular to its line

Question 6

When is the lattice structure disrupted through the process of edge dislocation?

Answer 6

Before and after the movement, the atomic arrangement or ordered and perfect. During the passage the extra half-plane of atoms is disrupted.`

Question 7

What is a slip?

Answer 7

The process by which plastic deformation is produced by dislocation motion

Question 8

For an edge dislocation, what direction does the dislocation line move in relation to the shear stress?

Answer 8

in the direction of the applied shear stress

Question 9

For a screw dislocation, what direction does the dislocation line move in relation to the stress direction?

Answer 9

Perpendicular to the stress direction

Question 10

What is the direction of motion of a mixed dislocation line?

Answer 10

Neither parallel or perpendicular to the applied stress

Question 11

What are the 3 causes of dislocations in metals?

Answer 11

1. Solidification
2. Plastic Deformation
3. Consequence of thermal stresses that result from rapid cooling

Question 12

What is dislocation density?

Answer 12

The number of dislocations in a material which is expressed as the total dislocation length per unit volume

Question 13

What characteristics of dislocations are important with regard to the mechanical properties of metals?

Answer 13

Strain fields determine the mobility of the dislocations and their ability to multiply

Question 14

When metals are plastically deformed, where is the retained energy stored?

Answer 14

As strain energy associated with dislocations

Question 15

For a screw dislocation, what are the kinds of strain?

Answer 15

Pure shear strain only

Question 16

For edge dislocation, what are the kinds of strain?

Answer 16

Tensile and shear strains

Question 17

What is the relationship between radius and strain magnitude?

Answer 17

Magnitude decreases with radial distance from the dislocation

Question 18

What is the relationship between two dislocations that have the same sign and identical slip planes?

Answer 18

The strain field interaction is such that there exists a mutual repulsive force between the two isolated dislocations

Question 19

What is the relationship between two dislocations of opposite sign and having the same slip plane?

Answer 19

They are attracted on one another and dislocation annihilation (forming a perfect crystal)

Question 20

What are three important sources of dislocations?

Answer 20

Existing dislocations which multiply

Gain boundaries

internal defects and surface irregularities which act as stress concentrations

Question 21

What is a slip plane?

Answer 21

The plane along which dislocation motion occurs

Question 22

What is the slip direction?

Answer 22

The direction of movement of the dislocation

Question 23

What is a slip system?

Answer 23

The combination of the slip plane and the slip direction is termed slip system

Question 24

What magnitude of atomic distortion does the slip system work with?

Answer 24

The slip system is such that the atomic distortion that accompanies the motion of a dislocation is a minimum

Question 25

For a particular crystal structure, where is the slip plane?

Answer 25

Plane that has the densest atomic packing– greater planar density

Question 26

For a particular crystal structure, where is the slip direction?

Answer 26

To the direction in the plane with the most closely packed atoms– highest linear density

Question 27

What ()[]{}<> occurs to what?

Answer 27

(plane) {plane family}

[direction] <direction></direction>

Question 28

How do metals with FCC or BCC structures compare to HCP?

Answer 28

FCC and BCC have a large # of slip systems and are ductile because extensive plastic deformation is normally possible

HCP metals are brittle because they have few active slip systems

Question 29

What are 5 FCC metals? What is there slip plane and slip direction?

Answer 29

Cu, Al, Ni, Ag, Au

{111}

<110>

12 slip systems

Question 30

What are 4 BCC metals? What are there slip plane and slip directions

Answer 30

a-Fe, W, Mo, K

a-Fe, W, Mo: {110} <111> 12 systems

a-Fe, W: {211} <111> 12

a-Fe, K: {321} <111> 24

Question 31

What is the magnitude of the Burgers vector equal to?

Answer 31

The unit slip distance (interatomic separation in this direction)

Question 32

How do you calculate the Burgers vector for the different crystal structures?

Answer 32

b(FCC) = (a/2)<110>

b(BCC) = (a/2)<111>

b(HPC) = (a/2)<11-20>

Question 33

At what angles do shear components of applied stress exist?

Answer 33

At all but parallel or perpendicular alignments to the stress direction

Question 34

What are resolved shear stresses? What do they depend on?

Answer 34

Shear components that exist at all but parallel or perpendicular alignments to the stress direction

Depend on the applied stress and the ORIENTATION of both the slip plane and the direction in the plane

Question 35

What is critical resolved shear stress?

Answer 35

The minimum shear stress required to initiate slip and is a property of the material that determines when yielding occurs

Question 36

When does a single crystal plastically deform or yield?

Answer 36

When Tr(max)= Tcrss and the magnitude of the applied stress required to initiate yielding

Question 37

What is the yield strength of a single crystal depend on?

Answer 37

The critical resolved shear stress and the orientation of the most favorably oriented slip system

Question 38

What does a step result from?

Answer 38

The movement of a large number of dislocations along the same slip plane.

Question 39

What is the direction of slip for polycrystalline materials?

Answer 39

Direction of slip varies from one grain to another. Dislocation motion occurs along the slip system that has the most favorable orientation.

Question 39

What does gross plastic deformation of a polycrystalline specimen correspond to?

Answer 39

The comparable distortion of the individual grains by means of slip

Question 39

What is a slip line?

Answer 39

What steps appear as on a polished single-crystal

Question 39

For FCC and BCC metals, where will slip begin? Why?

Answer 39

The second slip system since it’s the most favorably oriented with the tensile axis

Question 40

During slip, are the mechanical integrity and coherency maintained in a polycrystalline material?

Answer 40

Yes– basically the grain boundaries usually don’t come apart of open up

Question 41

Are polycrystalline metals or single-crystal metals stronger?

Answer 41

Polycrystalline metals are stronger, meaning that a greater stress is required to initiate slip

Question 42

What is twinning?

Answer 42

A shear force produces atomic displacements such that one side of a plane (the twin boundary), atoms are located with mirror-image positions of atoms on the other side. Basically metals create twins and that’s a deformation

Question 43

For BCC metals?

Answer 43

What are the twin plane and direction?

(112)

{111}

Question 44

For twinning, is the slip deformation homogenous or heterogeneous?

Answer 44

Homogenous

Question 45

What are the differences between twinning and slip?

Answer 45

1. Crystallographic orientation
   - for twinning, there’s a reorientation across the twin plane
   - for shear, the orientation is the same above and below
2. slip occurs in distinct steps and twinning is less than atomic separation

Question 46

What are the conditions that have to be met for mechanical twinning?

Answer 46

1. BCC HPC crystal structure
2. Low temperature
3. High rates of loading

Question 47

What is the importance of twinning?

Answer 47

Twinning may place new slip systems in orientations that are favorable relative to the stress axis such that the slip process can now take place?