Materials: Topic 2, Crystals and Imperfections Flashcards
What is a crystal?
A material in which atoms are situated in repeating or periodic array over large atomic distances. Crystals form usually on solidification from the molten state.
What is a lattice?
An infinite array of points in space, in which each point has identical surroundings to all others. This can coincide with the atomic positions but does not necessarily.
What is a unit cell?
The smallest component of the crystal, which when stacked together with pure translational repetition reproduces the whole crystal.
What are lattice constants or parameters?
The lengths of the sides of the unit cell and the angles between them.
Which of the following have usually have crystalline structures? Metals, ceramics, glasses, polymers, semiconductors.
Metal - Usually crystalline (simple) Ceramics - Crystalline (can be complex) Glasses - Non crystalline (amorphous) Polymers - Non crystalline or a mixture Semi Conductors - Crystalline (can be complex)
State the seven unique cell shapes
Cubic, tetragonal, othorhombic, hexagonal, monoclinic, triclic, trigonal. (See slide 9 for images)
How do we know about different crystal structures?
X-Ray diffraction. The 3D crystals act as a diffraction grating. nλ= 2d sin θ (Braggs Law)
Name four causes of imperfect crystals
The solid solution, point defects, linear defects, planar defects.
What is solid solution?
It is impossible to avoid impurities, materials usually contain a number of different components. Solid solution allows two or more species to ‘share’ the lattice. E.g. Brass = Tin (Solute) dissolved in Copper (solvent).
What is a point defect?
Point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension. Typically these defects involve at most a few extra or missing atoms.
Obey me glass of water
CURSE YOU GLASS OF WATER
What is a line defect?
A dislocation may be defined as a disturbed region between two substantially perfect parts of a crystal. A
dislocation is a linear defect around which some of the atoms are misaligned.
How do defects affect properties?
• They make it easier to plastically deform
the lattice structure. Without dislocations, metals would be much closer to their theoretical strength.
Why is learning about defects really important n shit?
Understanding them and being able to hinder their movement allows us to strengthen materials.
What are planar defects?
Grain Boundaries: Grain Boundaries are areas
of atomic disorder
State 4 strengthening mechanisms
Solid solution strengthening, grain size strengthening, work hardening, precipitation hardening.
What is solid solution strengthening?
• The addition of a solute species will strengthen a material • The solute introduces lattice strain • The larger the size difference the greater the strain • The lattice strain hinders the movement of lattice defects: dislocations.
What is grain size strengthening?
• An alloy will be stronger with a smaller grain size
• Yield strength proportional to d-1/2 where d = grain size
• Hall Petch relationship
• Dislocations have less distance to travel
• Grain Size can be manipulated.
– High cooling rates from liquid
– Grain refiners
– Thermo-mechanical
treatments in the solid state
How does work hardening strengthen a material?
1) Introduce material strain
2) Dislocations occur
3) Dislocations ‘tangle’
4) Materials becomes stronger
What is precipitation hardening?
- Alloys can form precipitates
- 2nd phase particle
- Act as barrier to dislocation movement
- Size, crystal structure and distribution
Rising up
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