Metals & Alloys

Question 1

The potential for coring is _____ when there is a wide separation of solidus and liquidus lines

Answer 1

greater

Question 2

Ductility

Answer 2

Ductility is measured by the maximum degree of extension in response to an applied tensile force

Question 3

Malleability

Answer 3

Malleability is measured by the maximum degree of compression in response to an applied compressive force

Question 4

What is an eutectic alloy?

Answer 4

An alloy that crystallizes at a specific temperature, behaving in a similar way to a pure metal (instead of over a range of temperature).

Question 5

What is the hardening effect?

Answer 5

Mechanical resistance to the movement of dislocations along slip planes formed by atoms of different atomic radii within the same lattice

Question 6

What is a cooling curve

Answer 6

A plot of temperature against time when the material is heated till molten then allowed to cool

Question 7

The degree of ______ is given by the separation of the solidus and liquidus lines on the phase diagram

Answer 7

coring

Question 8

How can metals and alloys be shaped? (x3)

Answer 8

1. Casting 2. Cold working 3. Amalgamation

Question 9

Risk of alloys completely insoluble in each other?

Answer 9

Electrolytic corrosion

Question 10

What promotes a small grain size? (x3)

Answer 10

1. Rapid solidification 2. Provision of ‘extra’ nucleation sites 3. Different temperature of mould in which wax is placed to melting temperature - i.e. rapid cooling

Question 11

Name one of the uses of eutectic alloy in dentistry

Answer 11

construction of bridges

Question 12

What are the 4 possibilities of reaction when an alloy mixture is cooled to below the melting point?

Answer 12

1. Remain soluble in each other - solid solution 2. Completely insoluble in each other 3. Partially soluble in each other 4. Form intermetallic compounds if the metals have affinity for each other (Ag3Sn)

Question 13

List 4 properties of metals at ambient temperatures

Answer 13

1. Hard and lustrous 2. Closely packed crystalline structures, in which atoms are closely packed together 3. Opaque (also radio-opaque) 4. Good conductors of heat and electricity

Question 14

Benefits of small grain size

Answer 14

Generally harder and have higher values of yield stress

Question 15

Define alloys

Answer 15

An alloy refers to a mixture of 2 or more metals (2: binary; 3: ternary)

Question 16

For a material with a cored structure: i. The ______ melting point being concentrated close to the nucleus ii. The ______ melting point being close to the grain boundaries

Answer 16

Higher; Lower

Question 17

Alloys with composition close to the eutectic composition have ______ melting ranges and melting points considerably ______ than those of the pure metal component

Answer 17

narrow; lower

Question 18

What are the 3 types of solid solution?

Answer 18

1. Ordered solid solution: Component metal atoms occupy specific sites within a common crystal lattice 2. Random solid solution: Random sites in common crystal lattice 3. Interstitial solid solution: Atoms of one metal lie within the primary lattice sites of the other; normally found where the atomic radius of one component is much smaller than the other

Question 19

Define a grain boundary

Answer 19

It refers to the area between the grains in contact, forming a natural barrier to the movement of dislocations

Question 20

What is a phase diagram?

Answer 20

A plot of temperature range over which an alloy crystallizes against its percentage composition