Metals and Alloys 1

Question 1

what are metals (& alloys) used for in dentistry?

Answer 1

• partial denture framework
• crowns
• denture base
• orthodontic appliance
• restorations

Question 2

why are metals used?

Answer 2

they have superior mechanical properties:

• strength
• rigidity
• elastic limit
• ductility
• hardness

Question 3

what is meant by good ductility?

Answer 3

shape is able to be adjusted chairside

Question 4

what is a negative of metals?

Answer 4

poor aesthetics

Question 5

what is the definition of a metal?

Answer 5

aggregate of atoms in a crystalline structure

Question 6

what is meant by an alloy?

Answer 6

combination of metal atoms in a crystalline structure

Question 7

what metal factors affect its mechanical properties?

Answer 7

• crystalline structure
• grain size
• grain imperfections

Question 8

what type of structures can metals take?

Answer 8

• crystal

- lattice

Question 9

when a PURE METAL cools, what occurs (in terms of the cooling curve)?

Answer 9

• molten
• temperature drops to melting point
• atoms begin to crystallise (straight line on graph)
• all atoms are crystallised
• solid metal cools to room temp

Question 10

explain the growth of metal crystals?

Answer 10

• atoms act as nuclei of crystallisation
• crystals grow to form dendrites
• crystals (or grains) grow until they impinge on other crystals
• region where grains make contact is called GRAIN BOUNDARY

Question 11

what is meant by equi-axed grains?

Answer 11

occurs when crystal growth is of equal dimension in each direct

Question 12

when would a radical grain structure occur?

Answer 12

molten metal cooled quickly in cylindrical mould

Question 13

when would a fibrous grain structure occur?

Answer 13

wire pulled through die

Question 14

in terms of crystal growth, what happens when the metal undergoes fast cooling (quenching)?

Answer 14

• more nuclei

- small fine grains

Question 15

in terms of crystal growth, what occurs when a metal undergoes slow cooling?

Answer 15

• few nuclei

- large coarse grains

Question 16

what are some nucleating agents?

Answer 16

impurities or additives act as foci for crystal growth

more impurities = more small grains

Question 17

what is meant by a ‘grain’ ?

Answer 17

each grain is a single crystal (lattice) with atoms orientated in given directions (dendrites)

Question 18

what is meant by a ‘grain boundary’?

Answer 18

• where grains collect with one another

- impurities concentrate here

Question 19

what are some advantages of small grains?

Answer 19

• high elastic limit (deformation requires a large stress)

- increased FS & hardness

Question 20

what is a disadvantage of small fine grains?

Answer 20

decreased ductility

Question 21

what is meant by the dislocation area of a lattice?

Answer 21

where the lattice structure is most likely to fracture

Question 22

what occurs if a force is applied to the dislocation area?

Answer 22

the forces moves the dislocation along the lattice until it reaches the grain boundary

Question 23

what is the propagation of dislocations through a lattice known as?

Answer 23

slip

Question 24

what are dislocations?

Answer 24

imperfections/defects in the crystal lattice

Question 25

what role do grain boundaries play in the movement of dislocations? why is this important?

Answer 25

• grain boundaries stop the propagation of defects spreading to another grain
• therefore in metals/alloys made up of many fine grains… the movement of dislocations are impeded leading to SPECIFIC PROPERTIES

Question 26

what specific properties does impeding movement of dislocations in metals/alloys cause?

Answer 26

INCREASES:

• elastic limit
• fracture stress
• hardness

DECREASES:

• ductility
• impact resistance

Question 27

what is meant by cold work?

Answer 27

• applying forces to metals/alloys alongside a LOW TEMP

Question 28

what is the purpose of cold work?

Answer 28

• causes SLIP
• therefor dislocations collect at grain boundaries
• hence stronger, harder material

Question 29

what is a negative aspect of cold work?

Answer 29

• lower ductility
• lower impact strength
• lower corrosion resistance**

Question 30

what is a residual stress?

Answer 30

stresses that form within the metal/alloy itself (UNDESIRABLE)

Question 31

what does residual stress lead to?

Answer 31

• instability in lattice

- distortion over time

Question 32

how do residual stresses occur?

Answer 32

residual stresses increase as cold work increases

Question 33

how can residual stresses be relieved?

Answer 33

ANNEALING PROCESS

Question 34

what is meant by annealing?

Answer 34

heating metal (or alloy) so that greater thermal vibrations allow migration of atoms (any instability in the lattice is reversed)

Question 35

does annealing alter the grain structure and mechanical properties?

Answer 35

NO

Question 36

what occurs when metal/alloy is heated?

Answer 36

recrystallisation of new smaller equiaxed grains

Question 37

the greater the amount of cold work….

Answer 37

the lower the recrystallisation temperature

Question 38

what would occur if there was an excessive temperature rise of a metal?

Answer 38

large grains replace the smaller coarse grains yielding poorer mechanical properties