Metals

Question 1

when are metals used in dentistry?

5

Answer 1

• partial denture framework (CoCr, type IV gold)
• crowns (stainless steel)
• denture base (stainless steel)
• orthodontic appliance (NiTi)
• restorations (amalgam)

Question 2

why are metals used in dentistry?

Answer 2

mechanical properties
* high strength
* rigidity
* surface hardness
* degree of ductility

Question 3

what is the definition of a metal?

Answer 3

aggregate of atoms in a crystaline structure

Question 4

what is the definition of an alloy?

Answer 4

combination of two or more types of metal in a crystalline structure

Question 5

what factors affect mechanical properties?

Answer 5

• crystalline structure
• grain size
• grain imperfections

Question 6

describe the cooling curve of a metal

Answer 6

• in molten form, the metal atoms flow readily and are arranged randomly
• as the temperature drops to its melting point, some metal atoms begin to crystalise and temperature remains constant
• when crystallisation is complete, all metal atoms have formed a crystalline structure and the temperature drops gradually to room temperature

Question 7

describe how crystals grow

Answer 7

• molten metal atoms reach their melting point and crystallise and form nuclei of crystallisation
• the crystals grow as dendrites
• crystals/grains continue developing until they impinge on a neighbouring crystal - grain boundary (stops each other from further growth)
• grains are of different size and shape

Question 8

what are equi-axed grains?

Answer 8

when crystal growth is of equal dimension in each direction

Question 9

what is a radical structure?

Answer 9

when molten metal is cooled quickly in a cylindrical mould

Question 10

what is a fibrous structure?

Answer 10

when molten wire is pulled through a die

Question 11

what is the difference between fast and slow cooling in crystal growth?

Answer 11

fast cooling (quenching)
* more nuclei
* small fine grains
slow cooling
* few nuclei
* large coarse grains

Question 12

what are nucleating agents?

Answer 12

impurities or additives that act as foci for crystal growth
* the more added, the greater the number of small grains

Question 13

what is a grain?

Answer 13

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

Question 14

what is a grain boundary?

Answer 14

formed whenever there is a change in orientation of the crystal planes so the grains collide with each other

impurities and defects concentrate at the grain

Question 15

what factors are required for rapid cooling (quenching)?

Answer 15

• small quantity of metal
• heat metal just above melting point before cooling
• store the metal in a mould that readily conducts heat

Question 16

what is dislocation?

Answer 16

• when there is an imperfection/defect in a grain it is a dislocation (dislocation in the lattice)
• the dislocation area is most likely to be fractured when a force is applied
• the force pushes the dislocation along the lattice plane
• the defects propagates along the crystal plane until a grain boundary is reached
• the shape of the grain has changed and no dislocation is present

Question 17

what mechanical properties does impeding movement of dislocations in metals and alloys increase?

Answer 17

• elastic limin stress
• fracture strength and UTS
• surface hardness

Question 18

what mechanical properties does impeding movement of dislocations in metals and alloys decrease?

Answer 18

• ductility
• impact resistance

Question 19

what factors impede dislocation movement?

Answer 19

• grain boundaries
• alloys - atoms of different sizes
• cold working - pushes dislocations towards teh grain boundaries

Question 20

what is cold working?

Answer 20

• work done/pressure on a metal or alloy to change its shape by pushing defects towards the grain boundary
• done at low temperatures
• creates a harder and stronger material

Question 21

what mechanical properties does cold working increase?

Answer 21

• elastic limit
• fracture stress
• hardness

Question 22

what mechanical properties does cold working decrease?

Answer 22

• ductility
• impact strength
• lower corrosion strength

Question 23

what is residual stress? how do you relive it?

Answer 23

• develop in metals/alloys where atoms become oriented out of equilibrium and grow with cold working
• undesirable - causes instability of the lattice and results in distortion of the metal/alloy over time
• relieved by annealing process

Question 24

what is annealing?

Answer 24

a process whereby heating a metal/alloy so that greater thermal vibration allows migration of atoms so they can rearrange themselves - distortions are eliminated

Question 25

what is stress-relief annealing?

Answer 25

* when **annealing** (heating) is carried out on a metal/alloy that has been **cold worked** * eliminates the **residual stresses** developed from cold working by allowing atoms to **rearrange themselves within grains** * **grain structure** is unchanged, **mechanical properties** are unchanged and **more cold work** can be done if required

Question 26

what is recrystallisation?

Answer 26

* if **cold working does not work out** as planned, the metal/alloy can be recrystallised * **re-sets** the metal/alloy and cold work can be **repeated** * spoils the benefits of cold work

Question 27

what does recrystallisation do to mechanical properties?

Answer 27

* **changes grain structure** (smaller equiaxed grains) * lowers elastic limit * lowers UTS * lowers hardness * increases ductility

Question 28

what does the temperature of recrstallisation depend on?

Answer 28

depends on the **amount of cold work** the metal/alloy has been subjected to * the **greater** the amount of cold working, the **lower** the recrystallisation temperature

Question 29

what can happen if the temperature when annealing is too high?

Answer 29

**grain growth** can occur * **large** grains replace **smaller** coarse grains * results in **poorer mechanical properties** - strength and hardness