Ch 3 and 4

Question 1

Short-range order

Answer 1

When the special arrangement of the atoms extends only to the atom’s nearest neighbours (molecules have no special arrangement with respect to each other’s position)

Question 2

Long-range order

Answer 2

When the special atomic arrangement extends over length scales greater than/equal 10 nm

Question 3

Crystalline materials

Answer 3

When atoms are arranged in a regular repetitive, grid-like pattern

Question 4

Polycrystalline material

Answer 4

Composed of many small crystals with varying orientations in space

Question 5

Grain boundaries

Answer 5

The borders between crystals

Question 6

Liquid crystals

Answer 6

Polymeric materials that have a special type of order

Question 7

Amorphous material

Answer 7

A solid that exhibits only a short-ranger order (that is, a noncrystalline solid)

Question 8

What type of material are glasses?

Answer 8

Amorphous

Question 9

Unit cell

Answer 9

A subdivision of the lattice that still retains the overall characteristics of the entire lattice

Question 10

A lattice

Answer 10

A collection of points, called lattice points, which are arranged in a periodic pattern so the surroundings of each point in the lattice are identical

Question 11

Basis or motif

Answer 11

A group of atoms located in a particular way with respect to each other and associated with a lattice point

Question 12

How we obtain a crystal structure

Answer 12

By placing the atoms of the basis on every lattice point (ie, crystal structure = lattice + basis)

Question 13

Lattice parameters

Answer 13

The lengths of the sides of the unit cell and the angles between those sides

Question 14

Close-packed directions

Answer 14

Directions in a crystal along which atoms are in contact

Question 15

Coordination number

Answer 15

Number of atoms touching a particular atom

Question 16

Coordination number in SC structure

Answer 16

6

Question 17

Coordination number in BCC structure

Answer 17

8

Question 18

Coordination number in FCC structure

Answer 18

12

Question 19

Packing factor

Answer 19

Fraction of space occupied by atoms - (number of atoms per cell times volume of each atom)/volume of unit cell

Question 20

Close-packed structure

Answer 20

Structure showing a packing fraction of 0.74 (FCC and HCP)

Question 21

Allotropy

Answer 21

The characteristic of an element being able to exist in more than one crystal structure, depending on temperature and pressure

Question 22

Polymorphism

Answer 22

Compounds exhibiting more than one type of crystal structure, depending on temperature and pressure

Question 23

Anisotropic

Answer 23

Material properties can vary with direction

Question 24

Isotopic

Answer 24

Material’s properties are identical in all directions

Question 25

Many polycrystalline materials are effectively what?

Answer 25

Effectively isotropic, because the random orientation of the grains cancels out the anisotropy of individual grains

Question 26

Vacancies do what to entropy?

Answer 26

Increase entropy, and thus increase the thermodynamic stability of crytalline materials

Question 27

Interstitial defect

Answer 27

When an extra atom is inserted into the crystal structure at a normally unoccupied position

Question 28

What is the effect of an interstitial defect?

Answer 28

• distort the surrounding lattice, thus generating localised stresses and strains
• this stress often strengthens the material

Question 29

Edge dislocation

Answer 29

What happens when you slice partway through a perfect crystal, spread the crystal apart, and fill the cut with an extra half plane of atoms

Question 30

Screw dislocation

Answer 30

When you cut partway through a perfect crystal and then skew the crystal by one atomic spacing

Question 31

Mixed dislocation

Answer 31

Has both an edge and a screw component

Question 32

Slip

Answer 32

Sliding of planes of atoms over another

Question 33

Slips systems?

Answer 33

• Always in the directions with the shortest distance between like atoms
• Usually the planes are the most densely packed planes

Question 34

Strain hardening

Answer 34

Process of strengthening a material by deforming it, which increases dislocation density

Question 35

Solid-solution strengthening

Answer 35

Atoms/ions of a guest material are incorporated into a host crystal structure