Chapter 4: Imperfections In Solids Flashcards
Is there a perfect crystal?
No, many important properties are due to the presence of imperfections.
What are the types of imperfections
Point defects which include:
Vacancy atoms
Interstitial atoms
Substitutional
Line defects: dislocations
Grain boundaries: Area defects
Describe types of point defects.
Vacancies are Vacant atomic sites in a structure which cause distortion of planes
Equilibrium concentration varies with temperate due to atoms vibrations around equilibrium position and high temperatures can cause atoms to jump out therefore higher temperatures have higher vacancis
How can activation energy be calculated
Nv/N = exp (-Qv/kT)
Swhere Nf = number of defects
N = number of potential sites
-QV = Activation energy
K = Boltzmann constant
T = temperature
Nv/N = defect concentration
-Qv/k = slope of on Nv/N
What are self interstitials?
Extra atoms positioned between atomic sites, another for of point defects which causes a bigger distortion than vacancies
Appears less
Are pure metals possible why?
Pure metals of one atom are impossible]
Most commonly metals are alloys and impurities are added intentional
What are the solutions to point defects in alloys
1 substitutional solution of atoms are of the same size, the secondary atom will fill in the gaps where vacancies are present in the primary
2 interstitial solution if the secondary atom is much smaller than the primary, it will settle in the gaps between the original lattice structure
Causes different composition and structure
What is the Hume - Rotherham rule and what are the conditions?
Conditions for substitutional solid solution
– 1. ∆r (atomic radius) < 15%
– 2. Proximity in periodic table
• i.e., similar electronegativities
– 3. Same crystal structure for pure metals
– 4. Valency
• All else being equal, a metal will have a greater tendency
to dissolve a metal of higher valency than one of lower
valency
If the electronegativity difference is too great, the metals will
tend to form intermetallic compounds instead of solid solutions
How to calculate specification of composition ? And how do you convert from weight percentage to atom percentage?
Weight percentage C1 = m1/m1 + m2 x 100
Where m1 = mass of component 1
Atom percentage = C1 = nm1/nm1 + nm2 x 100
Where nm1 = number of moles of component 1
To convert weight to atom to atom percentage?
Check slides
Mass per unit volume = C1” = (C1/(c1/p1)+(c2/p2)) x 10^3
Is the total alloy volume exactly equal to the sum of the volume of the individual elements?
It is assumed so however this is only valid for dilute solutions and compositions where solid solutions exist
What are dislocations? List of types
These are line defects that occur from slip between crystal planes result when dislocations move. They produce permanent plastic deformation they are one dimensional defects in which atoms are misaligned
Edge dislocation: extra half plane of atoms inserted in a crystal structure the edge of which terminates within the crystal. Atoms above dislocation are squeezed together the ones below are pulled apart and the magnitude of distortion decreases with distance from dislocation line
Screw dislocation
Parallel planar ramp resulting from shear deformation Burgers vector (b) is perpendicular to dislocation line
What is the burger vector
The magnitude of the amount of deformation or steps to get back to the original point. A measure of deformation
Describe the motion of edge dislocation
• Dislocations move in response to an external stress σ.
• Dislocation motion requires the successive bumping of a half plane of
atoms (from right to left here).
• Bonds across the slipping planes are broken and remade in succession.
• As soon as a critical shear stress is reached, the dislocation starts
moving and deformation is no longer elastic but plastic, because the
dislocation will not move back when the stress is removed.
How many planes and directions/direction and how does it relate to dislocations
FCC- many close packed planes/directions
HCP - only one plane 3 directions
ACP - One plane
BCC- None
Dislocations prefer close packed planes and directions therefore FCC has a higher chance of
What are planar or interfacial defects
Interfacial defects are two-dimensional boundaries that depart regions with different crystal structures and or crystallographic orientations
External surface: reconstruction of surfaces to reduce surface energy due to dangling bonds.
Grain boundaries: Boundaries separating two grains with different crystallographic orientations
