Metals and Alloys 2 Flashcards Preview

BDS2 Dental Material Science > Metals and Alloys 2 > Flashcards

Flashcards in Metals and Alloys 2 Deck (27)
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1
Q

what is an alloy?

A

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

2
Q

what are some advantages of alloys > metals?

A
  • higher elastic limit stress
  • high fracture strength
  • more rigid
  • corrosion resistance
  • lower melting point than individual metal
3
Q

what are some dental uses for alloys?

A

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

4
Q

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

A

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

5
Q

what is meant by solution when in reference to alloys?

A

homogeneous mixture at an atomic scale

6
Q

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

A
  • 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)
7
Q

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

A

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

  • Random
  • Ordered
8
Q

what are the different types of solid solutions?

A
  1. substitutional

2. interstitial

9
Q

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

A

unpredictable/no order

- the metal atoms are randomly positioned within the grain

10
Q

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

A

order is predictable

11
Q

what is an interstitial solid solution?

A

solid solution in which the atoms are markedly different in size

12
Q

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

A

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

13
Q

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

A

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)

14
Q

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

A

line representing the temperatures which different alloy compositions begin to crystallise

15
Q

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

A

line representing the temperatures which different alloy compositions have completely crystallised

16
Q

what occurs if an alloy is cooled slowly?

A
  • allows metal grains to diffuse through lattices
  • ensures grain composition is homogeneous
  • BUT this causes large grains (undesirable)
17
Q

what is coring in reference to alloys?

A

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

18
Q

what does rapid cooling of a molten alloy result in?

A
  • prevents atoms diffusing through lattice

- causes coring

19
Q

what can coring in an alloy lead to?

A

reduced corrosion resistance of solid alloy

20
Q

how is coring in alloys resolved?

A

homogenising annealing!

21
Q

what is homogenising annealing (alloys)?

A
  • once solid cored alloy is formed REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous
22
Q

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

A

must be BELOW recrystallisation temperature

- grains altered if it goes above

23
Q

why is an alloy more fracture resistant than a metal?

A
  • 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
24
Q

what are the properties of a Eutectic alloy?

A
  • metals are soluble in liquid state
  • metals insoluble in solid state (so 2 phases)
  • each metal forms physically distinct grains
25
Q

what are the property disadvantages of a eutectic alloy?

A
  • hard but brittle

- poor corrosion resistance

26
Q

what occurs in partially soluble alloys on annealing?

A

precipitation hardening

27
Q

what is precipitation hardening?

A

alloy becomes stronger & the surface is harder

- atoms are pushed to the grain boundaries