Metals and Alloys 2

Question 1

what is an alloy?

Answer 1

an alloy is a combination of:
- two or more metals
OR
- a metal with a metalloid

Question 2

what are some advantages of alloys > metals?

Answer 2

• higher elastic limit stress
• high fracture strength
• more rigid
• corrosion resistance
• lower melting point than individual metal

Question 3

what are some dental uses for alloys?

Answer 3

STEEL = burrs, instruments
GOLD ALLOY = inlays, crowns, bridges, partial dentures, wires
COBALT CHROMIUM = partial dentures
AMALGAM = restorative material

Question 4

what is meant by ‘phase’ in reference to alloys?

Answer 4

a physically distinct homogenous structure (can have more than one component)

Question 5

what is meant by solution when in reference to alloys?

Answer 5

homogeneous mixture at an atomic scale

Question 6

on crystallisation of alloys, the two metals may take what structures?

Answer 6

• be insoluble (exist as two phases)
• form an inter metallic compound with a specific chemical formulation
• be soluble & form a solid solution (form a common lattice)

Question 7

what is a substantial solid solution? what are the classifications of this type?

Answer 7

atoms of one metal replace the other metal in the crystal/lattice grain:

• Random
• Ordered

Question 8

what are the different types of solid solutions?

Answer 8

1. substitutional

2. interstitial

Question 9

how can the grain structure of a random substitutional solid solution be described?

Answer 9

unpredictable/no order

- the metal atoms are randomly positioned within the grain

Question 10

how can the grain of an ordered substitutional solid solution be described?

Answer 10

order is predictable

Question 11

what is an interstitial solid solution?

Answer 11

solid solution in which the atoms are markedly different in size

Question 12

how can the grain structure of an interstitial solid solution be described?

Answer 12

smaller atoms are located in spaces in the lattice/grain structure of larger atom

Question 13

how does the cooling curve of a pure metal differ to that of an alloy?

Answer 13

pure metal = when crystallisation happens, the temperature steadies and doesn’t change (crystallises at one temp)
alloy = crystallisation takes place while the temperature changes (over a temperature range)

Question 14

what does the liquidise line on a phase diagram represent (in reference to alloys)?

Answer 14

line representing the temperatures which different alloy compositions begin to crystallise

Question 15

what does the solidus line represent on a phase diagram (in reference to alloys)?

Answer 15

line representing the temperatures which different alloy compositions have completely crystallised

Question 16

what occurs if an alloy is cooled slowly?

Answer 16

• allows metal grains to diffuse through lattices
• ensures grain composition is homogeneous
• BUT this causes large grains (undesirable)

Question 17

what is coring in reference to alloys?

Answer 17

a defect in an alloy caused by cooling the alloy too fast

Question 18

what does rapid cooling of a molten alloy result in?

Answer 18

• prevents atoms diffusing through lattice

- causes coring

Question 19

what can coring in an alloy lead to?

Answer 19

reduced corrosion resistance of solid alloy

Question 20

how is coring in alloys resolved?

Answer 20

homogenising annealing!

Question 21

what is homogenising annealing (alloys)?

Answer 21

• once solid cored alloy is formed REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous

Question 22

when annealing, what is essential about the temperature used? why is this?

Answer 22

must be BELOW recrystallisation temperature

- grains altered if it goes above

Question 23

why is an alloy more fracture resistant than a metal?

Answer 23

• alloys have 2 types of metals of different atomic size
• defects don’t ‘roll’ over the lattice plane when a force is applied like in metals
• more energy/force is needed for the defect to overcome the different sized atoms and move along the lattice to the grain boundary
• hence, requiring greater stress to move dislocations in a solid solution

Question 24

what are the properties of a Eutectic alloy?

Answer 24

• metals are soluble in liquid state
• metals insoluble in solid state (so 2 phases)
• each metal forms physically distinct grains

Question 25

what are the property disadvantages of a eutectic alloy?

Answer 25

• hard but brittle

- poor corrosion resistance

Question 26

what occurs in partially soluble alloys on annealing?

Answer 26

precipitation hardening

Question 27

what is precipitation hardening?

Answer 27

alloy becomes stronger & the surface is harder

- atoms are pushed to the grain boundaries