Chapter 7 Flashcards
What are equiaxed grains?
Grains that have approximately the same dimension in all directions
What are the two types of deformations?
Elastic and plastic
What properties measure a materials’ resistance to plastic deformation?
Strength and hardness
Can you think of an edge or screw dislocation as a result from a shear distortion?
Screw– its dislocation line passes through the center of the spiral
What does an edge dislocation move in response to?
A shear stress applied in a direction perpendicular to its line
When is the lattice structure disrupted through the process of edge dislocation?
Before and after the movement, the atomic arrangement or ordered and perfect. During the passage the extra half-plane of atoms is disrupted.`
What is a slip?
The process by which plastic deformation is produced by dislocation motion
For an edge dislocation, what direction does the dislocation line move in relation to the shear stress?
in the direction of the applied shear stress
For a screw dislocation, what direction does the dislocation line move in relation to the stress direction?
Perpendicular to the stress direction
What is the direction of motion of a mixed dislocation line?
Neither parallel or perpendicular to the applied stress
What are the 3 causes of dislocations in metals?
- Solidification
- Plastic Deformation
- Consequence of thermal stresses that result from rapid cooling
What is dislocation density?
The number of dislocations in a material which is expressed as the total dislocation length per unit volume
What characteristics of dislocations are important with regard to the mechanical properties of metals?
Strain fields determine the mobility of the dislocations and their ability to multiply
When metals are plastically deformed, where is the retained energy stored?
As strain energy associated with dislocations
For a screw dislocation, what are the kinds of strain?
Pure shear strain only
For edge dislocation, what are the kinds of strain?
Tensile and shear strains
What is the relationship between radius and strain magnitude?
Magnitude decreases with radial distance from the dislocation
What is the relationship between two dislocations that have the same sign and identical slip planes?
The strain field interaction is such that there exists a mutual repulsive force between the two isolated dislocations
What is the relationship between two dislocations of opposite sign and having the same slip plane?
They are attracted on one another and dislocation annihilation (forming a perfect crystal)
What are three important sources of dislocations?
Existing dislocations which multiply
Gain boundaries
internal defects and surface irregularities which act as stress concentrations
What is a slip plane?
The plane along which dislocation motion occurs
What is the slip direction?
The direction of movement of the dislocation
What is a slip system?
The combination of the slip plane and the slip direction is termed slip system
What magnitude of atomic distortion does the slip system work with?
The slip system is such that the atomic distortion that accompanies the motion of a dislocation is a minimum
For a particular crystal structure, where is the slip plane?
Plane that has the densest atomic packing– greater planar density
For a particular crystal structure, where is the slip direction?
To the direction in the plane with the most closely packed atoms– highest linear density
What ()[]{}<> occurs to what?
(plane) {plane family}
[direction] <direction></direction>
How do metals with FCC or BCC structures compare to HCP?
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
What are 5 FCC metals? What is there slip plane and slip direction?
Cu, Al, Ni, Ag, Au
{111}
<110>
12 slip systems
What are 4 BCC metals? What are there slip plane and slip directions
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
What is the magnitude of the Burgers vector equal to?
The unit slip distance (interatomic separation in this direction)
How do you calculate the Burgers vector for the different crystal structures?
b(FCC) = (a/2)<110>
b(BCC) = (a/2)<111>
b(HPC) = (a/2)<11-20>
At what angles do shear components of applied stress exist?
At all but parallel or perpendicular alignments to the stress direction
What are resolved shear stresses? What do they depend on?
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
What is critical resolved shear stress?
The minimum shear stress required to initiate slip and is a property of the material that determines when yielding occurs
When does a single crystal plastically deform or yield?
When Tr(max)= Tcrss and the magnitude of the applied stress required to initiate yielding
What is the yield strength of a single crystal depend on?
The critical resolved shear stress and the orientation of the most favorably oriented slip system
What does a step result from?
The movement of a large number of dislocations along the same slip plane.
What is the direction of slip for polycrystalline materials?
Direction of slip varies from one grain to another. Dislocation motion occurs along the slip system that has the most favorable orientation.
What does gross plastic deformation of a polycrystalline specimen correspond to?
The comparable distortion of the individual grains by means of slip
What is a slip line?
What steps appear as on a polished single-crystal
For FCC and BCC metals, where will slip begin? Why?
The second slip system since it’s the most favorably oriented with the tensile axis
During slip, are the mechanical integrity and coherency maintained in a polycrystalline material?
Yes– basically the grain boundaries usually don’t come apart of open up
Are polycrystalline metals or single-crystal metals stronger?
Polycrystalline metals are stronger, meaning that a greater stress is required to initiate slip
What is twinning?
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
For BCC metals?
What are the twin plane and direction?
(112)
{111}
For twinning, is the slip deformation homogenous or heterogeneous?
Homogenous
What are the differences between twinning and slip?
- Crystallographic orientation
- for twinning, there’s a reorientation across the twin plane
- for shear, the orientation is the same above and below - slip occurs in distinct steps and twinning is less than atomic separation
What are the conditions that have to be met for mechanical twinning?
- BCC HPC crystal structure
- Low temperature
- High rates of loading
What is the importance of twinning?
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?
