Metals & Alloys 1

Question 1

Where are metals and alloys used in dentistry?

Answer 1

•Partial denture framework (CoCr, Type IV gold)
•Crowns (stainless steel)
•Denture base (stainless steel)
•Orthodontic appliance (NiTi)
•Restorations (amalgam)

Question 2

What is a metal and what is an alloy?

Answer 2

metals - aggregate of atoms arranged in a crystalline structure

alloys - combinations of different metal atoms in a crystalline structure

Question 3

What is the crystalline structure made of?

Answer 3

made up of grains, which are regions of atoms arranged in a specific pattern or lattice

Question 4

What factors in metals affect mechanical properties?

Answer 4

• crystalline structure

• grain size

• grain imperfections/defect

Question 5

What are examples of crystal/ lattice structure?

Answer 5

simple cubic
face-centered cubic (FCC)
body-centered cubic (BCC)

Question 6

How do metals form?

what are regions where grains make contact called?

Answer 6

When molten cools down

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 7

What is a dendrite?

Answer 7

3D branched lattice network

Question 8

What do grains vary in?

Answer 8

Size
Shape

Question 9

fast cooling VS slow cooling

Answer 9

faster cooling (quenching) - smaller, finer grains, more nuclei

slower cooling - larger, coarse grains, less nuclei

Question 10

What are nucleating agents and where do they concentrate?

Answer 10

Impurities or addictives act as Foci for crystal growth

at the grain boundaries

Question 11

What is a grain?

Answer 11

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

Question 12

What happens at the grain boundary?

Answer 12

•change in orientation of the crystal planes
•(impurites concentrate here)

Question 13

advantages/disadvantages of small fine grains?

Answer 13

advantages - increased FS, high EL, increased hardness, increased UTS

disadvantages - decreased ductility (brittle)

Question 14

What are factors required for rapid cooling?

Answer 14

*small bulk
*heat metal/alloy just above melting temperature
*mould - high thermal conduction
*quench

Question 15

What are dislocations and what is slip?

Answer 15

Dislocations are imperfections/defects in the crystal lattice

Slip is due to Propagation of Dislocations and involves rupture of only a few bonds at a time

Question 16

Why are moving dislocations bad?

Answer 16

When a metal is subjected to stress or deformation, dislocations can move relatively freely through the crystal lattice, allowing the metal to deform plastically (permanently change shape). This dislocation movement is responsible for the ductility and malleability of metals.

Question 17

What happens if you impede movement of dislocations in metals/ alloys?

Answer 17

INCREASES
*elastic limit
*fracture stress /UTS
*hardness
Brittle

DECREASES
*ductility
*impact resistance

Question 18

What are factors impeding dislocation movement?

Answer 18

•Grain boundaries (hence fine grains)

•Alloys: different atom sizes

•Cold working - dislocations stopped at grain boundaries

Question 19

What is cold work, what does it cause?

Answer 19

deforming the material at low temperatures

causes dislocations due to slip to collect at grain boundary, resulting in strain hardening and increased strength and hardness.

Question 20

What does cold work do to grain structure and what properties does it cause?

Answer 20

modifies grain structure:
higher
•Elastic Limit
•Fracture stress/UTS
•hardness

lower
•ductility
•impact strength
•lower corrosion resistance

Question 21

What stress increases with cold work?

how is it relieved?

Answer 21

Internal stress (causes instability/brittleness in lattice)

annealing process

Question 22

What is annealing?

what does it benefit?

Answer 22

heating the cold-worked metal to a specific temperature range, holding it at that temperature for a certain time, and then cooling it at a controlled rate. (does not cause metal to remelt but causes recrystallisation - formation of new smaller grains)

can relieve internal stresses caused by cold working by allowing atoms to rearrange within the grains

Question 23

What does recrystallisation result in?

Answer 23

reduces strength, elastic limit, and hardness (eliminates dislocations) but increases ductility.

Question 24

What does recrystallisation temperature depend on?

Answer 24

•depends on amount of cold work

•greater the amount of cold work the lower the recrystallisation temperature

Question 25

What does excessive temperature rise in annealing cause?

Answer 25

large grains to replace smaller coarse grains reducing grain boundaries yielding poorer mechanical properties

              CAREFUL when annealing