Materials- Metals: Defects Flashcards
What are defects?
Imperfections in the crystal lattice
Are properties of materials influenced by the presence of defects?
Yes. They can change with the number of particular defects
What are point defects and give 4 examples?
Defects associated with one or two atomic positions.
E.g vacancies, self-interstitial atoms, interstitial solute atoms, substitutional atoms
What is a vacancy?
A lattice site that would normally be occupied from which an atom is missing.
Formula for number of vacancies in a given quantity of material
Nv=Ne^(-Qv/kT) v is subscript Nv is equilibrium number of vacancies N is total number of atomic sites Qv is energy required for the formation of a vacancy (J or eV) T is absolute temperature (K) k is Boltzmann’s constant (J/atom/K)
What is a self-interstitial?
An atom from the crystal that is crowded into an interstitial site (a small void space that under ordinary circumstances is not occupied)
Are vacancies or self-interstitials more common?
Vacancies
What is a solid solution?
A homogeneous crystalline phase that contains two or more chemical species
What is a substitutional solid solution?
A solid solution where the solute atoms replace or substitute for the host atoms. The two types of atoms are of a similar size
What is an interstitial solid solution?
A solid solution where relatively small solute atoms occupy interstitial positions between the solvent or host atoms.
How to calculate the size of the interstitial site at the (1/4,1/2,0) location of a BCC material
Worked out using right angle triangles. It is the gap above and between the two bottom atoms of the cube in a unit cell. The atomic radius plus interstitial radius squared is equal to have the unit cell length squared plus a quarter of the unit cell length squared.
How to calculate the size of the interstitial site at the (1/2, 0, 0) location in FCC materials
It is between the nearest left two atoms where one is above the other. The interstitial radius doubled plus the atomic radius doubled equals the unit cell length.
Give 4 examples of bulk defects
Bubbles, precipitates, pores, cracks
Give an example of a line defect
Dislocation
What is a dislocation.
A linear or one-dimensional defect around which some of the atoms are misaligned
What is dislocation density in a material?
The total dislocation length per unit volume. Or equivalently, the number of dislocations that intersect a unit area of a random section. Units mm^-2
How does number of dislocations present within the crystal vary with increasing plastic strain?
It increases
What is an edge dislocation?
An extra half plane of atoms, the edge of which terminates within the crystal. It is a linear defect that centres on the line that is defined along the end of the extra half plane of atoms- the dislocation line.
How does the dislocation line for an edge dislocation look on paper?
If the extra half plane of atoms appears as a vertical line going into a square plane of atoms, the dislocation line is perpendicular to the page so appears as an upside down T at the end of the half plane. Symbol is T either way up (not the letter but that shape)
What is a screw dislocation?
A dislocation formed by a shear stress resulting in the upper front region of the crystal being shifted one atomic distance to the right relative to the bottom portion. The dislocation line passes through the centre of a spiral. Symbol of circular arrow
What is a Burgess vector?
The magnitude and direction of the lattice distortion associated with a dislocation. Denoted b. The nature of a dislocation is defined by the relative orientations of the dislocation line and Burgess vector.
What are the relative orientations of the dislocation line and Burgess vector for edge and screw dislocations?
Edge: they are perpendicular
Screw: they are parallel
Describe the motion of an edge dislocation
Moves in response to a shear stress applied in a direction perpendicular to its line. If the extra half plane comes down from the top and the top part is pushed right and the bottom part of the material left: dislocation moves one atomic distance right as the EHP links up with the bottom part of the vertical plane to its right, the upper portion of this plane becomes the EHP, this process repeats such that the EHP moves right by successive and repeated breaking of bonds and shifting by interatomic distances of upper half-planes. The dislocation moves parallel to the shear stress direction.
What is slip?
The process by which plastic deformation is produced by dislocation motion
What is a slip plane?
The crystallographic plane along which the dislocation line traverses
What is the slip direction?
The direction of movement a slip plane follows.
What is a slip system?
The combination of the slip plane and slip direction
What does the slip system depend on?
The crystal structure of the metal. It is such that the atomic distortion that accompanies the motion of a dislocation is a minimum.
In which direction does slip occur?
Along a number of equivalent and most favourably orientated planes and directions at various positions along the specimen length.
Describe the appearance of slip deformation of a cylindrical sample with diagonal slip planes under tension
Forms small diagonal steps that are parallel to each other and loop around the specimen. The steps appear as lines called slip lines. This is for a single crystal specimen.
How and why do two edge dislocations close to each other with the same sign and identical slip plane interact?
They repel each other. The compressive and tensile strain fields both lie on the same side of the slip plane. The strain field interaction is that there exists between these two isolated dislocations a mutual repulsive force that tends to move them apart.
Which side of the dislocation line of edge dislocations experience compressive strain and which experience tensile strain?
The atoms immediately on the side of the dislocation line with the extra half plane are squeezed together so experience a compressive strain relative to atoms positioned in the perfect crystal. The atoms immediately on the other side of the dislocation line to the extra half plane sustain an imposed tensile strain. The cross of the T shows the side in tension.
What is a strain field?
The lattice distortions that radiate from the dislocation line so the strain extends into the surrounding atoms. The magnitude of strain decreases with radial distance from the dislocation.
How and why do two edge dislocations close to each other with the opposite sign and identical slip plane interact?
They are attracted to one another. Dislocation annihilation occurs when they meet. The two extra half planes of atoms will align and become a complete plane.
What is the direction of motion for a screw dislocation relative to the applied shear stress?
Perpendicular to the shear stress direction. Like tearing paper.
How does the net plastic deformation for motion of screw and edge dislocations compare?
They are the same
