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2nd year > DMS-Metals and alloys > Flashcards

DMS-Metals and alloys Flashcards

1
Q

What is ductility?

A

Ability to undergo plastic deformation before fracture.

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2
Q

What is an alloy?

A

A mix of 2 or more metals in a crystalline structure.

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3
Q

How does the crystalline structure form?

A
  1. Nuceli of crystalisation.
  2. Dendrites are produced
  3. the crystals grow until they impinge on other crystals.
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4
Q

What a grain?

A

A single crystal lattice

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5
Q

What is the grain boundary?

A

Where grains come into contact with each other

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6
Q

Compare fast cooling and slow cooling?

A

Fast cooling- many nuclei/ small fine grains

Slow cooling- few nuclei- large coarse grains

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7
Q

Which type of grains are desirable and why?

A

Small fine grains.

Increases

  • Tensile strength
  • hardness
  • elastic limit.
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8
Q

What are dislocations?

A

These are defects and imperfections in the crystaline structure due to misalignment of the atoms.

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9
Q

How do we deal with dislocations and why?

A

We put pressure on the lattice in order to move the dislocations to the grain boundary and remove them.

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10
Q

What factors affect dislocation movement?

A

Grain boundaries (more grain boundaries = more places to push the defects)

Alloys- as there are different sizes of atoms (can prevent dislocation)

Cold working- you build up the dislocations at the grain boundaries.

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11
Q

What is cold working?

A

When you put pressure on the metal at a lower temperature in order to move the dislocations to the grain boundary. (improving properties

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12
Q

What are the difficulties with cold working and how do we solve them?

A

This increases the residual stress which causes an instability in the lattice that would result in distortion over time.

This is solved by Stress relief annealing.

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13
Q

What is stress relief annealing?

A

Heating up the metal so that the atoms can re-arrange and eliminate stress.

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14
Q

What is a solid solution?

A

When 2 metals are soluble in each other and form a lattice structure.

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15
Q

Compare the types of solid solution?

A

You can have random/ ordered and interstitial (small particles between large particles)

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16
Q

Compare cooling metals and alloys?

A

Metals crystalise at the one temperature.

Alloys crystalise over a series of temperatures.

17
Q

What lines do you find on a phase diagram?

A

Liquidus - temp at which the alloy starts to crystalises

Solidus- temp at which the alloy completes crystalisation

Tie lines - used to estimate composition

18
Q

what is coring?

A

When a structure’s composition varies throughout the grain.

19
Q

How do we fix coring?

A

We use homogenous annealing.

The alloy is reheated allowing the atoms to move around and eliminate the cored structure.

20
Q

What is solution hardening?

A

When alloys have a distorted grain structure causing improved mechanical properties by impeding dislocation movement.

A greater force is needed to move the disloctions making the alloys more fracture resistant.

21
Q

What is an Eutectic alloy?

A

An alloy where the melting point is lower than that of the original metals.

22
Q

Compare eutectic and non-eutectic composition?

A

Eutectic composition- the alloy crystalises at once.

Non eutectic composition- The excess metal crystalises first, then the liquid reaches euteric compostion & crystalises.

23
Q

What is a partially soluble alloy ?

A

The metals in the alloy are not soluble enough to produce a full series of solid solutions (only certain alloy compositions can exist)

24
Q

What is precipitation hardening?

A

When the defects are pushed to the grain boundary in a partially soluble alloy- improving properties.

25
Q

Why do we want a low density denture material?

A

As a low density material is lighter- this would make the denture more comfortable for the patient.

26
Q

Discuss the ideal properties of a base and the clasp?

A

You want the base to be rigid.

High YM - maintaining shape

High elastic limit- avoid deformation

You want the clasp to be flexible

Low YM - to flex over the tooth.

High elastic limit -maintain elasticity over a wide range of movements

27
Q

Discuss the components of Type 4 gold?

A

Cu- causes corrosion

Au- makes partially soluble alloy with copper

Pt- coring can occur.

Pd- grains are bigger & absorbs gas when moutlen (causing a weakened denture frame)

28
Q

Discuss the components of Co-Cr?

A

Co- chance of coring

Cr- forms passive oxide layer to reduce corrosion

Ni- improves ductility but reduces strength

Mo-makes particles smaller

C- undesirable- carbide makes it brittle.

29
Q

Compare Co-Cr and Type 4 Gold

A

Co-Cr is harder.

Co-Cr greater rigidity

Co-cr greater shrinkage (-ve)

Gold- greater elongation

Gold- Greater strength

Gold- more dense.

30
Q

Discuss Titanium as partial denture alloy?

A
  • Greater biocompatability
  • Components can be welded together
  • more expensive
  • Good corrosion resistance.

2nd year (63 decks)

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