Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

BDS2 - dental material science > metals and alloys 2 > Flashcards

metals and alloys 2 Flashcards

1
Q

what is an alloy

A

A combination (or mixture) of two or more metals
Or
Metal(s) with a metalloid (silicone, carbon)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what are the advantages of alloys

A

• Improved properties
○ Mechanical (EL, UTS, hardness)
○ Corrosion resistance
○ Lower melting point than an individual metal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what are the dental uses of alloys

A

• Steel
○ Burs
○ Instruments

• Amalgam
○ Filling material
○ Special case

• Gold alloy
○ Inlays
○ Crowns
○ Bridges
○ Partial dentures
○ Wires

• Nickel chromium
○ Crowns
○ Bridges
Wires

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what does phase mean

A

Phase = physically distinct homogenous structure (can have more than one component

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what is a solution

A

homogenous mixture at an atomic scale

2 metals co-exist in a lattice structure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

if a lattice structure only has one type of metal in it how many phases are there

A

one phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

if a lattice structure has individual grains of metal A and metal B, how many phases are there

A

2 phases

2 grains, 2 metals but they do not co-exist in the same grain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

if a lattice structure has 2 metals in a homogenous mixture how many phases are there

A

one phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

when are metals normally soluble

A

when molten

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what are the 3 options for metals on crystallisation

A

On crystallisation, metals may:

A) Be insoluble, no common lattice - 2 phases

B) Form metallic compound with specific chemical formulation (eg Ag3Sn)
- This is the case with amalgam

C) Be soluble and form a solid solution, ie form common lattice

  • 3 types of solid solution
  • 2 metals are soluble in one another
  • 2 types of metals co-exist in the same lattice arrangement
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what are the types of solid solution

A

substitutional
> atoms of one metal replace the other metal in the crystal lattice

interstitial
> atoms markedly different in size

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what are the types of substitutional solid solution

A

> random

  • metal atoms similar in size valency, crystal structure
  • can’t tell what the next atom in a row will be

> ordered

  • metal atoms in regular lattice arrangement
  • can predict what the next atom in a row will be because there is an order to the lattice arrangement
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what does interstitial solid solution consist of

A

atoms of different sizes
smaller atoms are located in spaces in lattice / grain structure or larger atoms

cannot predict where the small atom will be
just know it wont be in the lattice structure position

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

how does the cooling curve of an alloy differ from the cooling curve of a metal

A

metal: crystallises at one temperature
alloy: crystallises over a temperature range

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is TL on the alloy cooling curve

A

the temperature where crystallisation of the alloy begins

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is TS on the alloy cooling curve

A

the temperature when the crystallisation process is complete

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

what happens to soluble metals during the alloy cooling curve

A

solid solution formed (homogenous mixture of metals in each grain)

18
Q

what happens to insoluble metals during the alloy cooling curve

A

grains of individual metals formed

19
Q

what is a phase diagram

A

shows temperature Vs alloy composition

20
Q

what is complete solid solubility

A

can have any combination of the metals

different compositions of the 2 metals works

21
Q

what is the liquidus

A

Line representing the temperature which different alloy compositions begin to crystallise

22
Q

what is the solidus

A

Line representing the temperature which different alloy compositions have completely crystallised

23
Q

what does slow cooling of molten alloy result in

A

> allows metal atoms to diffuse through lattice
ensure grain composition is homogenous
results in large grains

24
Q

what does rapid cooling result in

A

> small grains
= good mechanical properties

> concentration gradient
= more likely to corrode

> causes coring - composition varies throughout the grain

> prevents atoms diffusing through lattice

> Generates many small grains which impede dislocation movement, improving its mechanical properties

25
Q

what does coring result in

A

grains structures of different concentrations

26
Q

what are coring conditions

A

• Fast cooling of liquid state

• Liquidus and solidus must be separated (see phase diagram) and determines extent of coring
○ Eg Au-Pt

• May reduce corrosion resistance of the solid form of the alloy

27
Q

how is coring resolved

A

homogenising anneal

28
Q

what is homogenising anneal

A

• Once solid cored alloy formed then reheat to allow atoms to diffuse and so cause grain composition to become homogenous

• Reheat alloy grains (but not to melting temperature) and this allows the atoms to move around and eliminate the core structure
○ Crucial process

• Note: keep below recrystallisation temperature, otherwise grains are altered

29
Q

what is solution hardening

A

Alloys forming a solid solution and consisting of metals of different atomic size have a distorted grain structure

Which impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)

30
Q

explain dislocation movement in a metal lattice

A

defect rolls over the atoms in the lattice plane until it settles at the grain boundary

little energy / force is needed for the defect to move along the slip plane

31
Q

explain dislocation movement in a solid solution

A

defect does not roll over the lattice plane, it has to climb over each atom before it will settle by the grain boundary

instead it falls into the larger space existing between large and small atom

more energy / force is needed for the defect to overcome the different sized atoms and move along lattice to the grain boundary

as greater stress is needed to move dislocations in solid solutions - alloys are more fracture resistant / stronger than metals

32
Q

explain order hardening

A

Alloys forming an ordered solid solution (atoms distributed at specific lattice sites) have a distorted grain structure (eg Au-Cu)
Which impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)

33
Q

what are the properties of eutectic alloys

A

• Metals are soluble in liquid state

• Metals insoluble in solid state (so 2 phases)
Ie each metal forms physically distinct grains
2 metals not soluble in one another form physically distinct grains

34
Q

what is eutectic compostion

A

• Where liquidus and solidus coincide
○ Ie where crystallisation process occurs at a single temperature

• Where grains of individual metals formed simultaneously

35
Q

what is binary eutectic

A

complete solid insolubility

becomes melting point of the alloy?

36
Q

what are the advantages and disadvantages of eutectic alloys

A

Lowest melting point - at eutectic composition: used for solder

Hard but brittle

Poor corrosion resistance

37
Q

what is non-eutectic composition

A
  • Excess metal crystallises first
  • Then liquid reaches eutectic composition
  • And both metals crystallise (forming separate grains)
38
Q

what happens in a partially soluble alloy

A

has a solubility limit line which indicates the range of compositions of metal a and metal B (corresponding to the horizontal section of the solidus [H1 to H2]) are not possible

has alpha and beta solid solutions
extremes - the alloy is usually very rich in one metal or the other

39
Q

what is precipitation hardening

A

heat treatment process that helps make metals stronger. The process does this by producing uniformly dispersed particles within a metal’s grain structure that help hinder motion and thereby strengthen it

happens in partial soluble alloys

on annealing a super saturated alloy will undergo precipitation hardening

40
Q

what has better mechanical properties - alloys or metals?

A 
alloys 
- better fracture strength, ridgidity, elastic limit, surface hardness
- due to
> solution
> order
> precipitation hardening
41
Q

how should cored structures be dealt with

A

removed by annealing

BDS2 - dental material science (13 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions