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DMS recent > Metal & Alloys > Flashcards

Metal & Alloys Flashcards

1
Q

Which is the only alloy (in BDS2) to undergo a setting reaction.

A

Amalgam

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2
Q

What does a HIGH YOUNG’S (ELASTIC) MODULUS look like on a stress strain diagram and what does this indicate?

A

Looks like a very steep gradient.
This indicates the material is very RIGID.
So large stress => small strain (small deformation)

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3
Q

What is ductility?

A

The amount of plastic deformation prior to fracture (the extent a material can be shaped/ manipulated)

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4
Q

What are grains in metal?

A

The formation of crystals (from the nuclei of crystallisation) as the metal cools, that grow until they impinge on other crystals - called GRAIN BOUNDARY. Impurities and defects concentrate at grain boundaries.

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5
Q

Would you rather have small grains or large grains in a metal alloy and why?

A

SMALL GRAINS
Because it is will have a high elastic limit stress value – so deformation requires a large stress to be applied.
&
it results in an increased fracture strength and surface hardness (better mechanical properties)

BUT LESS DUCTILE

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6
Q

What is quenching?

A

Rapid cooling. Heat just above mpt. before cooling, use small quantity of metal.
Causes CORING.

Quenching achieves small grains which is desirable

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7
Q

What is SLIP and what role do grain boundaries play in it?

A

Slip is when a dislocation (imperfection) is propagated along the lattice when a force is applied to a metal/ alloy.
Presence of grain boundaries prevents dislocation so a metal/ alloy with small grains means dislocations are impeded.

Results in increase in elastic limit, fracture strength and surface hardness.

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8
Q

Effects of cold working (changing shape of metal below mpt/ recrystallisation temperature)

A

Higher - elastic limit, fracture stress, hardness

Lower - ductility, impact strength, lower corrosion resistance.

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9
Q

What is the purpose of annealing?

A

Heating a metal or alloy causes greater thermal vibrations allows migration of atoms - eliminates instability or distortion. DOES NOT CHANGE GRAIN STRUCTURE, used after cold work.

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10
Q

What is a substitutional solid solution?

A

Where atoms of one metal replace (or substitute for) the other metal in the crystal lattice/grain. Can be random (metal atoms must be similar in size, valency etc.), or ordered (regular lattice arrangement with same conditions size, valency etc.)
NOTE - solid solution formed when 2 metals are soluble

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11
Q

What is interstitial solid solution?

A

2 atoms to be markedly different in size - smaller atoms between larger atoms.
NOTE - solid solution formed when 2 metals are soluble

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12
Q

What is the difference between a pure metal and alloy crystallising temperature?

A

metal - crystallises at one temperature

alloy - crystallises over temperature range (Tl - crystallisation begins, Ts - crystallisation ends)

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13
Q

How is coring caused?

A

Rapid cooling of alloys

Coring = where the composition of grains vary throughout.

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14
Q

What is a negative consequence of CORING?

A

Reduces the corrosion resistance of the alloy = OVERALL UNDESIRABLE. Coring caused by the separation of the LIQUIDUS and SOLIDUS (the greater the separation the more coring)

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15
Q

Why are alloys more fracture resistant (stronger) than metals?

A

Alloys require greater stress to move dislocations than metal (greater resistance to dislocations improve mechanical properties) BECAUSE there are two types of metal in the grain.

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16
Q

What is a solid solution alloy?

A

each grain has both types of metal present

both types of metals are soluble in one another

17
Q

What is a eutectic alloy?

A

Both metals are insoluble in one another so they form separate grains.
Dental application - soldering alloy parts together.

18
Q

What are the ideal properties of a partial denture alloy?

A 
Rigid (YM)
Strong (Elastic limit stress)
Hard
Ductile
Precise casting (limited shrinkage)
19
Q

What does cold working do?

A

Pushes dislocations to the grain boundaries

20
Q

How do you change the rigidness (YM) of a material without changing the structure of it?

A

Change the thickness

Thick = high YM (base)
Thin = low YM (clasps - to flex)
21
Q

What characteristic does Nickel give in a CoCr PDA?

A

Ductility

22
Q

What does chromium add in a CoCr PDA that cobalt does not?

A

Chromium enhances CORROSION RESISTANCE.

Both increase strength, hardness, rigidity

23
Q

What is CoCr hardness?

A

370 hardness units - much harder than type IV gold.

24
Q

Components of CoCr

A 
Co (35-65%)  54%
Cr (25-30%)	25%
Ni (0-30%)	15%
Mo (5-6%)	5%
C (0.2–0.4%)   0.4%
25
Q

State some drawbacks of CoCr

A

Shrinks during casting
Is not as ductile as Type 4 Gold
More difficult to polish than Type 4 Gold

26
Q

State some reasons why CoCr is a good PDA

A

Low density - more lightweight for patient
Very rigid (high Young’s modulus)
Very hard
Fairly high abrasion resistance

DMS recent (5 decks)

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