Materials - D Hutt

Question 1

Define Valency

Answer 1

How easy it is for an electron to achieve a complete outer shell by losing sharing or gaining electrons.

Eg Lithium has a Valency of 1

Question 2

Which elements are electropositive and what does it mean?

Answer 2

Elements to the left of the periodic table are electropositive.

Electropositive means that an element wants to lose electrons to have a complete outer shell.

Question 3

Which elements are electronegative and what does it mean?

Answer 3

Elements to the right of the periodic table are electropositive. Apart from noble gases.

Electronegative means that an element wants to gain electrons to have a complete outer shell.

Question 4

What are Primary Bonds? Give examples

Answer 4

Primary Bonds: Strong chemical interactions between atoms (electrons shared or transferred between atoms)

Eg: Ionic, Covalent, Metallic

Question 5

What are secondary bonds?
Give examples

Answer 5

Secondary bonds: weak interactions between atoms and / or molecules (electrons remain with the parent atoms)

There is no transfer or sharing of electrons

Eg: Van der Waals, Hydrogen Bonding

Question 6

Ionic Bonding

Answer 6

Transfer of electrons from one atom to another creating ions.
Ions are held together by electrostatic attraction between the positive and negative charges

Question 7

Covalent Bonding

Which are strong and which are weak?

Answer 7

Atoms sharing electrons between them to form molecules or solids

Bonds within molecules are covalent (strong)

Bonds between molecules may be secondary (weak)

Question 8

If a material has all covalent bonds what properties does it have?

Answer 8

High Young’s Modulus - often brittle

High melting and Boiling Point

Question 9

Metallic Bonding ‘definition’

Answer 9

In the metallic bonding model, electrons are considered to be completely disconnected from the atoms and free to move anywhere. This leads to electrical conductivity

Electrostatic attraction between electron cloud and metal ions binds the metal together

Question 10

Metallic bonding properties

Answer 10

Good tensile and compressive strength
Electrical conductivity
High ductility - how much a material can be plastically deformed

Question 11

Van Der Waals Forces

Answer 11

As atoms or molecules approach, the negative electrons and positive nuclei repel their respective selves and an induced dipole is formed.

Easily distributed by heating

Overall the attraction forces outweigh the repulsion forces (barely) and a very small force is produced which develops that holds the two atoms together.

Question 12

Hydrogen Bonding

Answer 12

Weak forces but stronger than Van Der Waals forces

Electrostatic attraction between the small positive and negative charged holding the molecules together.

Takes place between molecules that have H-F, H-O or H-N bonds within them

Question 13

What is Enthalpy Change
When is it positive?
When is it negative?

Answer 13

Change in energy

If energy is put into the system to make the process happen then enthalpy change is positive this is thermodynamically unfavourable

If energy is given out enthalpy change is negative - this is favourable

Question 14

On a small scale, what are the two material structures a solid can have?

Answer 14

Crystalline or amorphous

Question 15

Crystalline

Answer 15

The atoms / molecules are arranged in a regular array that repeats.

In the solid state, metals and many ceramics have crystalline structures under normal conditions

Question 16

Amourphous

Answer 16

The atoms / molecules have an irregular arrangement to each other and there is no long range order

Question 17

Single Crystal (materials on a larger scale)

Answer 17

Whole material has one arrangement of atoms

Question 18

Semi-crystalline (materials on a larger scale)

Answer 18

Material has crystalline and amorphous regions

Question 19

Polycrystalline (materials on a larger scale)

Answer 19

Material is composed of many small single crystal regions (grains) that are oriented at random to each other.

Question 20

What is the aim of arranging atoms in a certain way?

Answer 20

To minimise the energy of the system. This is why disorder is favoured.

(It takes energy to keep the atoms separate so its favourable to ‘let them mix’)

Question 21

Allotropic

Answer 21

When pure elements can adopt different crystal structures at different temperatures and pressures.

Question 22

Polymorphic materials

Answer 22

Can adopt more than one crystal structure

Question 23

Unit cell

Answer 23

Smallest segment that can be repeated in 3 dimensions

Question 24

You can’t just predict the strength of materials from bonding considerations alone. You should also consider:

Answer 24

Grain boundaries in polycrystalline materials

Defects in the arrangement of atoms in the crystal lattice such as:
- Impurities
-Vacancies
-Dislocations

Question 25

What is a Vacancy (Think about temperature, a pro and a con)

Answer 25

A vacancy is a hole formed at a specific location in the crystal structure for to the absence of an atom - also known as a point defect All crystalline materials contain vacancies as a result of solidification or deformation. Higher temperature - more vacancies Introducing vacancies: - Increases disorder (entropy) of the system - favourable - Costs energy due to breaking some and stretching other bonds - unfavourable

Question 26

Dislocations / Dislocation slip

Answer 26

Defects that involve planes of atoms The slip movement of dislocations enables materials to be ductile By controlling the number and difficulty of movement of dislocations it is possible to control the mechanical properties of materials

Question 27

Plastic Deformation

Answer 27

Material is **permanently deformed,** doesn’t return to its origins shape when the load is removed Usually due to shear stresses caused by the applied load

Question 28

Dislocation slip in metals

Answer 28

In **metals** metallic bind means dislocations slip relatively easily - **ductile**

Question 29

Dislocation slip in ionic materials

Answer 29

In **ionic materials** - ions with same charge are forced past each other - requires too much energy - material tends to break before slip occurs - **brittle**

Question 30

Dislocation slip in Covalent materials

Answer 30

In **covalent** materials, the bonds are strong and very directional - it’s difficult for one plane of atoms to slide past another - tend to be **brittle**

Question 31

What is tensile testing designed to do?

Answer 31

Designed to determine: - tensile strength - yield strength - Young’s Modulus -Ductility (Of materials)

Question 32

Why is the standard sample geometry of a testing piece such a shape?

Answer 32

To avoid regions of high or non-uniform stress being generated

Question 33

Engineering Stress Formula and units

Answer 33

Force / Area Units: N/m^2 or Pa

Question 34

Engineering Strain Formula and units

Answer 34

Extension / original length mm/mm or no units

Question 35

Poisson’s Ratio, v

Answer 35

When a material is stretched one way, it usually contracts in the perpendicular directions v = -strain(x)/strain(z) Or v = -strain(y)/strain(z)

Question 36

Yield strength

Answer 36

Also known as yield stress The stress at the point where an F-e graph stops being proportional

Question 37

Tensile Strength

Answer 37

Also known as UTS (ultimate tensile strength) The maximum on the curve - neck forms in the test piece

Question 38

When does dislocation slip start

Answer 38

Dislocation slip begins when the metal starts to yield

Question 39

What is true stress?

Answer 39

The true stress shows the force divided by the actual cross-sectional area of the test piece at each stage in the test. True strain = ln(instantaneous length/original length)

Question 40

Hardness definition

Answer 40

The ability of a material to resist localised plastic deformation (surface indentation or scratching) - Often related to tensile (and yield) strength - Not a material property, its value varies depending on the test method used to measure it

Question 41

What are the 4 major methods of hardness testing of materials?

Answer 41

- Brinell test - Vickers test - Rockwell test - Knoop test All tests push a hard object into the material under a known load or size of depth of the indentation determines the hardness

Question 42

How does work (/ strain) hardening work?

Answer 42

1. Material soft & ductile, tiny grains in crystal structure easily slide past each other; easy to plastically deform 2. Mechanical forces cause dislocations in the lattice structure - make it harder for grains to move and requires more energy for them to do so 3. As more force is applied, dislocations get tangled and it makes it even harder for the grains to move past each other

Question 43

Annealing

Answer 43

- A heat treatment of metals or alloys - Increases ductility and decreases hardness - Works by raising a metal to above its recrystallisation temp but below melting point. - This higher temperature gives enough energy to the metal to allow migration within the metal’s microstructures.

Question 44

Toughness

Answer 44

The ability of a material to absorb energy during deformation and fracture Metals with *hight strength * **and** *ductility* are tough Brittle materials have *low toughness* The toughness of a material depends on which way the load is applied: - Slowly - material shows increased toughness - With sudden impacts - toughness decreases

Question 45

Impact testing What does it investigate? Examples How is it done?

Answer 45

Used to investigate the toughness of materials under sudden loading Charpy and Izod methods Both methods use a swinging pendulum to measure the amount of energy absorbed by the test piece during fracture

Question 46

Typical effect of increasing temperature of a material

Answer 46

- Reduced Young’s Modulus - Reduced yield strength (if there is yielding) - Reduced tensile strength - Reduced hardness - Reduced brittleness - (Increased ductility)

Question 47

What are the two steps of fracture?

Answer 47

Crack initiation and propagation

Question 48

Diffusion

Answer 48

The movement of atoms, ions, or molecules within a solid, liquid or gas Largely driven by the desire to increase the entropy (disorder) of the system. Can also reduce the energy of the system eg spreading out of large impurity atoms to reduce stress. Much slower in solids than liquids and gases

Question 49

What are the two bulk diffusion mechanisms?

Answer 49

Vacancy diffusion Interstitial diffusion