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DMS > Metals and Alloys > Flashcards

Metals and Alloys Flashcards

1
Q

What is an alloy?

A

Combination of metal atoms in a crystalline structure.
- 2 or more types of metal atoms.

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2
Q

What is ductility?

A

The amount a material will deform to stretch before it fractures.
- amount of plastic deformation prior to fracture.

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3
Q

What does it mean if a material has a high YM?

A

The material is rigid
- it will undergo less strain when a tensile force is applied.

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4
Q

In terms of a metal- what factors affect it’s mechanical properties?

A

Crystalline structure
Grain size
Grain imperfections

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5
Q

What stages occur in crystal growth within a metal?

A

Atoms within a metal act as nuclei of crystallisation.
Crystals grow to form dendrites.
They grow until they impinge on other crystals
- grain boundary.

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6
Q

What is the difference between fast cooling (quenching) and slow cooling?

A

Fast cooling- smaller grains, more nuclei
Slow cooling- large grains, few nuclei.

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7
Q

What is a grain?

A

Single crystal lattice with atoms orientated in given directions.

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8
Q

What is a grain boundary?

A

Change in orientation of the crystal planes- when grains collide.

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9
Q

Why are small grains advantageous?

A

High elastic limit
Increased fracture stress and UTS, hardness

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10
Q

What is a dislocation?

A

Imperfection or defect in the crystal structure.
When a force is applied, the defect moves along the lattice until it reaches a grain boundary.

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11
Q

What is Slip?

A

Propagation of dislocations and involves rupture of only a few bonds at a time.

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12
Q

What factors impede movement of dislocations?

A

Grain boundaries
Alloys
Cold working.

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13
Q

What is cold working?

A

Work done on a metal or alloy at a low temperature.
Causes lip, dislocations collect at grain boundaries.
End up with a stronger, harder material.

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14
Q

What is an advantage of cold working?

A

Causes the metal to be harder, increased fracture stress and elastic limit.

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15
Q

What is a disadvantage of cold working?

A

low ductility.
low Impact strength
Low corrosion resistance.
residual stress- cause instability within the attic and distortion over time.

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16
Q

How do we reduce the risk of residual stress?

A

Stress relief annealing.
- heating the metal so that greater thermal vibrations allows migration of atoms.

17
Q

What is the definition of hardness?

A

Resistance of surface to indentation of abrasion.

18
Q

What is the purpose of a metal in a MCC?

A

Helps eliminate defect/cracks on the porcelain surface.
- limits the strain that porcelain can undergo because it is more ductile and less rigid.

19
Q

Why do you need a supporting metal element in an MCC?

A

Porcelain has low tensile strength, forms surface defects which leads to fracture under low stress.

20
Q

What metals can be bonded to porcelain for a MCC?

A

High or low gold alloy
Silver Palladium
Nickel chromium
Cobalt chromium

21
Q

What properties are required of a metal alloy to bond to porcelain?

A

Similar thermal expansion co-efficients
Porcelain forms bonds with metallic oxides on surface.
Avoid discolouration of the porcelain
High elastic modulus
Hardness
Recrystallisation temperature of alloy higeher than the fusion temperature of porcelain- creep can occur if not.

22
Q

What is creep?

A

Gradual increase in permanent strain experience under prolonged application of stress.

23
Q

What is the bonding mechanism found in porcelain-metal?

A

Mechanical- interlock through surface irregularities
Stressed skin effect- slight differences in thermal contraction co-efficients lead to compressive forces.
Chemical- electron sharing in oxides.

24
Q

What is stainless steel used for?

A

Orthodontic wires
Denture base

25
Q

What is the composition of stainless steel?

A

72% iron
18% chromium
8% nickel
1.7% titanium
0.3% carbon

26
Q

What is the purpose of Chromium in stainless steel?

A

Lowers austenite to martensite temperatures and rate.
decreases % carbon at which eutectoid formed.

Also corrosion resistant.

27
Q

What is the purpose of nickel in stainless steel?

A

Helps achieve the austenite structure at room temperature by lowering the critical temperatures at which the austenite structure breaks Down on cooling.

28
Q

What type of stainless steel is used in orthodontic wires and partial dentures?

A

18-8 stainless steel.
Austenite stainless steel.

28
Q

When Austenite is quenched, what is formed?

A

Martensite.

29
Q

What components of stainless steel suppress the transition of austenite to martensite?

A

Chromium and Nickel.

30
Q

Why is martensite not desirable to use in dentistry?

A

Very hard and brittle.
Distorted lattice.

31
Q

What is a wrought alloy?

A

Manipulated/shaped by cold working.

32
Q

What is springiness?

A

EL/YM

Ability of a material to undergo large deflections without permanent deformation.

33
Q

What are the requirements of an orthodontic wire?

A

High springiness
High ductility
Easily joined
corrosion resistant.

34
Q

What properties are desirable for a denture base?

A

High YM and high EL.

35
Q

What is weld decay?

A

Occurs between 500-900 degrees.
Chromium carbide’s precipitate at grain boundaries.
This makes the metal very brittle.

36
Q

What properties are desirable for a denture clasp?

A

Lower YM- to allow it to flex over a tooth.
High EL.

37
Q

What is the composition of cobalt chromium (in a denture)?

A

54% cobalt
25% chromium
15% nickel
5% molybdenum
0.4% carbon

DMS (13 decks)

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