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BDS2 Dental Materials Sciences > metals and alloys 2 > Flashcards

metals and alloys 2 Flashcards

1
Q

alloy

A

a combination or mixture of 2 or more metals, or metals with a metalloid (Si, C)

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2
Q

3 advantages of alloys

A

Mechanical (EL, UTS, hardness) than metals

Corrosion resistance (potentially)

lower melting point than individual metal

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3
Q

4 dental examples of alloy uses

A

STEEL - burs, instruments

AMALGAM - filling material

GOLD ALLOY - inlays, crowns, bridges, partial dentures, wires

NICKEL CHROMIUM - crowns, bridges, wires
etc……

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4
Q

phase

A

Physically distinct homogenous structure (can have more than one component)

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5
Q

solution

A

Homogenous mixture at an atomic scale

Lattice arrangement of 2 or more metals
- Metals coexist in lattice structure called a solution

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6
Q

how many phases are there in a metal formed of one metal atom only

A

1 phase

1 atom homogenous

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7
Q

how many phases are there in a metal made of individual grains of 2 phases situated in a lattice network

A

2 phases

don’t exist in same grain - so 2 phases

number of individual grains of different metals

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8
Q

how many phases are there in a metal made of 2 metals in a homogenous mix

A

1 phase

2 metal atoms coexisting in same lattice structure on atomic scale

  • Homogenous
  • Single phase

Solid solution

  • Several grains of 2 different metal atoms
  • Grains of varying shape and size
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9
Q

what is the physical property of metals when molten

A

soluble usually

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10
Q

what are the 3 types of solid solution that can occur on crystallisation of molten alloy

A

insoluble (no common lattice; 2 phases)

form inter-metallic compound with specific chemical formulation

be soluble and form a solid solution

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11
Q

properties of solid solution

A

form a common lattice, co-exist in solid-solution

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12
Q

when an alloy is in a molten state

A

all metal components are soluble in one another

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13
Q

what is substitutional solid solution

A

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

RANDOM:- metal atoms similar in:- SIZE, VALENCY, CRYSTAL STRUCTURE (e.g. fcc)
eg AuAg, AuCu

ORDERED:- metal atoms in regular lattice arrangement, conditions as above
Regular fashioned arrangement, form ordered solid solution

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14
Q

what is random substitutional solid solution

A

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

metal atoms similar in:- SIZE, VALENCY, CRYSTAL STRUCTURE (e.g. fcc)
eg AuAg, AuCu

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15
Q

what is ordered substitutional solid solution

A

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

metal atoms in regular lattice arrangement, conditions as above
- Regular fashioned arrangement, form ordered solid solution

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16
Q

what are the 2 types of substitutional solid solution

A

random

ordered

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17
Q

what is interstitial solid solution

A

atoms markedly different in size

smaller atoms located in spaces in lattice/grain structure of larger atom (cannot predict where smaller atoms are)
e.g Fe-C stainless steel BDS3

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18
Q

what can vary between the cooling curves of different compositions of alloys

A

Each composition have to measure the cooling curve

- TL and TS

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19
Q

cooling curve of metal

A

crystallises at one temperature

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20
Q

cooling curve of alloy

A

crystallises over TEMPERATURE RANGE

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21
Q

state of soluble solid solution

A

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

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22
Q

state of insoluble solid solution

A

grains of individual metals formed

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23
Q

phase diagram

A

Can plot on phase diagram the varying Tl and Ts

Temperature Vs alloy composition

Top curve TL values is liquidous

Lower curve TS values is solidus

24
Q

top curve on phase diagram is

A

liquidous

25
Q

lower curve on phase diagram is

A

solidus

26
Q

what state is the alloy if the temperature is above the lilquidous line

A

the 2 metals will be molten

27
Q

what happens to the alloy as its cooled to the liquidous line

A

Some will crystallise to grains

Part liquid part solid

28
Q

what happens to the alloy as its cooled to the solidus line

A

All solid

Crystallisation is complete - completely solid

29
Q

LIQUIDUS

A

line representing the temperatures which different alloy compositions begin to crystallise

30
Q

SOLIDUS

A

line representing the temperatures which different alloy compositions have completely crystallised

31
Q

slow cooling of alloy

A

allows metal atoms to diffuse through lattice

ENSURES grain composition is homogeneous

BUT this results in LARGE GRAINS
- Undesirable

Do not want to cool slowly

32
Q

what does rapid cooling of alloy create

A

coring

only way to determine structure/compositions of grains of final grain is from drawing tie lines to solidus

33
Q

rapid cooling cooling of alloy make grains of

A

Different concentrations/ compositions of grains

Not homogenous (cored structure)

  • Different % within each grain
  • more likely to corrode
  • Not desirable
34
Q

rapid cooling of molten alloy leads to

A

prevents atoms diffusing through lattice

causes CORING
as composition varies throughout grain.

NOTE: initial grain composition IS NOT the same as the molten alloy.

35
Q

what are conditions needed for coring

A

fast cooling of liquid state;

Liquidus and Solidus must be separated (far apart on Phase Diagram) and determines extent of coring

  • eg Au-Pt
  • if close together then not a large amount of coring
36
Q

what is the main disadvantage of coring/rapid alloy cooling

A

reduce corrosion resistance

37
Q

fast cooling of alloy advantages and disadvantage

A

generates MANY SMALL grains which impede dislocation movement, improving its MECHANICAL PROPERTIES

BUT causes coring, which is undesirable

38
Q

what is homogenising anneal

A

once solid cored alloy formed REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous
- get rid of cored structure

NOTE: keep below recrystallisation temperature, otherwise grains altered
- Allows grains to move around and eliminate cored structure

39
Q

why must the temperature be kept below recrystallisation temperature for homogenising annealing

A

higher will alter grains structure

- Allows grains to move around and eliminate cored structure

40
Q

how are defects eliminate in metal lattices

A

Metal lattice is perfect with planes as all atoms the same size

  • defect slides along the lattice plane when a force applied
  • Until reach grain boundary

Defect “rolls” over the atoms in the lattice plane.
- Little energy/force is needed for defect to move along slip plane

41
Q

solid solution atoms sizes

A

Alloys forming a SOLID SOLUTION and consisting of metals of different atomic size have a distorted grain structure

Not perfect as metal lattice
- Big, small, big, small

42
Q

benefit of solid solution impeding dislocation movement

A

improves mechanical properties (EL, UTS, hardness, more fracture resistant)

more likely to prevent defects from moving in the structure

43
Q

how does solid solution impede dislocation movement

A

Defect does not “roll” over the lattice plane.

Instead it falls into the larger space existing between large & small atom.

More energy/force is needed for the defect to overcome the different-sized atoms, and move along lattice to the grain boundary.

  • requires greater stress to move dislocations in a solid solution
  • making alloys inherently more fracture resistant (i.e. stronger) than metals.
44
Q

what is benefit of 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 resits dislocation movement and so improves mechanical properties (EL, UTS, hardness)

benefited by improved mechanical properties due to fact atoms are in ordered fashion

45
Q

ordered solid solution

A

atoms distributed at specific lattice sites have a distorted grain structure (eg Au-Cu)

which IMPEDES resits dislocation movement and so improves mechanical properties (EL, UTS, hardness)

46
Q

eutectic alloys properties

A

metals are soluble in liquid state

metals INSOLUBLE in solid state (so 2 PHASES) not soluble in each other

  • i.e. each metal forms physically distinct grains
  • CONTRAST this with Au-Pt alloy

not used much in dentistry

47
Q

what is the unique quality of the melting point of binary eutectic alloys

A

Less than point A (mp of A) and point E (mp of B)

Less than individual metals

48
Q

eutectic alloy compositor

A

where Liquidus and Solidus coincide
(i.e. where crystallisation process occurs at a single temperature)

where grains of individual metals formed simultaneously

lowest melting point - at eutectic composition: used for solder dental technicians

hard but brittle relatively easy to fracture

poor corrosion resistance

49
Q

physical properties of eutectic alloys

A

lowest melting point - at eutectic composition: used for solder dental technicians

hard but brittle relatively easy to fracture

poor corrosion resistance

50
Q

non-eutectic alloy compositon

A

excess metal crystallises first

then liquid reaches eutectic composition

and BOTH metals crystallise (forming separate grains)

51
Q

solid solution forming alloy

A

alloy of 2 metals coexist in same lattic structure

52
Q

eutectic alloy

A

2 metals exist in separate grain

53
Q

solubility limit line

A

indicates that a range of compositions of Ag and Cu (corresponding to the horizontal section of the solidus (H1 to H2) ) ARE NOT POSSIBLE
- cool down rapidly get grains of both no matter starting composition

Hence molten alloy of composition Z
- DOES NOT cool rapidly to produce a 50:50 grain comprising Ag and Cu;
- instead grains of alpha and beta are formed
only extremes exists

54
Q

what happens to partially soluble alloys on annealing

A

in partially soluble alloys on annealing, a supersaturated alloy will undergo PRECIPITATION HARDENING

One of the atoms (silver or cooper) pushed to grain boundary and makes the alloy stronger and harder
- Enhances the mechanical properties

55
Q

precipitation hardening

A

annealing partially soluble alloys

One of the atoms (silver or cooper) pushed to grain boundary and makes the alloy stronger and harder
Enhances the mechanical properties

e.g. Type IV gold 
 Has Ag and Cu present 
Benefit when anneal the alloy
- Cool to room temp 
- Get rid of grain structure
56
Q

positive and negative of alloys

A

Have better mechanical properties than metals (e.g. fracture strength, rigidity, elastic limit surface hardness) due to :
- solution, order & precipitation hardening

But Cored structure must be removed by annealing

BDS2 Dental Materials Sciences (13 decks)

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