Chapter 5

Question 0

Amorphous

Answer 0

Disordered - as if atoms have been ‘frozen’ in their liquid state. Glass is an amorphous material.

Question 1

Atomic force microscope

Answer 1

A fine point is mounted on a cantilever arm. Forces between the point and the surface bend the cantilever. This is detected from a laser.

Question 2

Polycrystalline

Answer 2

Neither crystalline or amorphous. Crystals form at different points in the material as it cools, colliding with crystals next to it.

Question 3

Grains

Answer 3

Patchwork of tiny crystals found in polycrystalline structures.

Question 5

Grain boundary

Answer 5

Where grains (tiny crystals) meet - interference.

Question 6

Why are metals shiny?

Answer 6

Light is bounced back from the surface of metals due to the free electrons absorbing and re-emitting it.

Question 7

Colour in glass

Answer 7

Metal ions in glass absorb light of certain wavelengths, colouring the glass.

Question 8

Lattice

Answer 8

A regular arrangement of particles

Question 9

Body centered cubic

Answer 9

Atoms arranged in squares on each layer. The next layer of atoms fits in the ‘dimples’ of the first layer. The third layer is identical to the first. There is one atom in the middle of the cube, and eight others, filling the corners. Metals with this structure (e.g. gold) are harder and less malleable.

Question 10

Hexagonal close packed

Answer 10

Atoms arranged in hexagons on each layer. The next layer of atoms fits in the ‘dimples’ of the first layer. The third layer is identical to the first. Magnesium, titanium and zinc have this structure

Question 11

Face centered cubic

Answer 11

Atoms arranged in hexagons on each layer. The next layer of atoms fits in the ‘dimples’ of the first layer. The third layer of atoms again fits into the ‘dimples’, but not directly above the first layer. In a cube, there are atoms in each corner, and an atom in the centre of the cube faces. Aluminium, copper and silver have this structure

Question 12

Directional bonds

Answer 12

Bonds only ‘work’ in a specific direction, holding atoms in place, meaning the substance is not malleable

Question 13

In-directional bonds

Answer 13

Bonds ‘work’ in all directions, meaning atoms can slide past each other and substances are more malleable. E.g. metallic bonding

Question 14

Cracking in metals

Answer 14

Metals resist cracking as they are ductile. Cracks broaden and are blunted, preventing propagation

Question 15

Cracks and stress

Answer 15

Cracks deflect tensile stress, stress concentrated at the tip of crack. Bonds ‘unzip’ as next bond is stressed.

Question 16

Fracture energy =

Answer 16

Total energy used to fracture / specimen cross-sectional area. If large, material is tough

Question 17

Tensile strength =

Answer 17

Breaking force / specimen cross-sectional area. If large, material is strong

Question 18

Dislocations

Answer 18

Instead of atoms rows sliding when materials are shaped, one atom slips at a time, requiring much less energy. Dislocations can work against each other, making a material harder - material is work hardened.

Question 19

Alloys

Answer 19

Dislocations become pinned by alloy atoms, making slip much more difficult.