L4: Crystal Structures And Defects

Question 1

Describe the two main structures materials can adopt

Answer 1

Crystalline - arranged in regular repeating pattern throughout material. Fixed bond lengths and angles. Metals and many ceramics are like this under normal conditions

Amorphous- no regular arrangement, lengths and angles distorted, no long range order through material

Question 2

In practise what are the 4 structures of materials?

Answer 2

Single crystal- whole material has one arrangement of atoms (eg Si)
Amorphous - whole material amorphous (eg glass, some polymers)
Semi- crystalline- has crystalline regions surrounded by amorphous regions (important for some polymers)
Polycrystalline- many small single crystal regions, oreintated at random to eachother (eg most metals in manufacture)

Question 3

In a polycrystalline material, what is a rough grain size?

Answer 3

10-60 micro m

Question 4

What determines the arrangement of atoms within the solids?

Answer 4

Bond type,
Bond strength,
Number of bonds between neighbours,
Aim is to minimise E of system

Question 5

What is the unit cell?

Answer 5

The smallest segment that can be repeated in 3D

• Usually only a few atoms but maybe thousands
• defined by lengths of 3 edges (a,b,c) and the angles between them (alpha, beta, gamma)

Question 6

Define polymorphic

Answer 6

Can adopt more than 1 crystal structure. Which one depends on temp and pressure.
- under diff circumstances one is more thermodynamically favourable than another

Question 7

Define allotropic

Answer 7

Pure elements which adopt diff structures at diff temps and pressures

Question 8

What are the 3 common metallic crystal structures? What are their coordination numbers and some egs?

Answer 8

Face centred cubic- 12, eg Cu, silver, gold, Al, Fe 912C- 1394C

Body centred cubic- 8, eg molybdenum, tungsten, Fe below 912C

Hexagonal close packed,12, eg Zn, Mg

Question 9

Why do macroscopic properties of materials differ considerably to those predicted by bonding?

Answer 9

Defects In arrangement of atoms in crystal lattice- ie vacancies, dislocations, impurities

Weak interactions at grain boundaries in polycrystalline materials

Question 10

Why do grain boundaries have higher E than the bulk of he material?

Answer 10

Irregular bond lengths and coordination numbers of atoms in these regions

Crystal planes do not match so there’s a transition region a few atomic layers wide

Question 11

Define a vacancy?

What can it also be called?

Answer 11

A hole formed at a specific location in the crystal structure due to the absence of an atom
Aka point defect

Question 12

What does the number of vacancies depend on?

Answer 12

Temperature (energy given into crystal)

Higher temp= more vacancies

Question 13

What is an advantage (2) and disadvantage (1) of vacancies?

Answer 13

Increases disorder- favourable

Costs energy as breaks bonds and stretches others- unfavourable

Enable relatively easy diffusion of atoms through solid lattice- atoms move into hole then another fills its hole etc
Energy allows atom to be ejected from system

Question 14

What are the 2 types of dislocations?

And what is the typical quantity of dislocations in a material?

Answer 14

Screw dislocations
Edge dislocations- extra, incomplete plane of atoms in the crystal structure

Typical quantity- 10m per mm3

Question 15

Define elastic deformation and explain the 2 egs?

Answer 15

A materials bonds stretch but return to original shape when the load is removed

Tensile load- 2 forces pull out

Shear load- 2 forces applied, one at top, one at bottom, diff directions

Question 16

Define plastic deformation

Answer 16

Atoms mover permanently in relation to eachother usually due to shear stresses (t) cause by applied load

Shear in perfect crystal very difficult- have to break many bonds at once

Question 17

Describe edge dislocation slip and the property it allows

Answer 17

Enables ductility

Shear stress causes A to break with B and form new bond with C- dislocation moves along a row.
Can continue until dislocation is at grain boundary

Question 18

What does controlling the number and movement of dislocations enable?

Answer 18

Controlling mechanical properties of materials - basis of metal strengthening programmes

Question 19

In terms of slip which bond types are brittle and which are ductile?

Answer 19

Metallic- arranged in rows which are easy to move- ductile

Ionic- ions with same charge must pass eachother for slip to occur- requires too much energy do material breaks- brittle

Covalent- bonds are strong and directional- hard for one plane to slide past another- brittle