Metals & Alloys 2

Question 1

What are the two definitions of alloy?

Answer 1

a combination/mixture of

1. two or more metals
2. a metal/s with a metallloid (Si, C)

Question 2

What is the advantages of alloys and where are they used in dentistry?

Answer 2

Advantages: improved mechanical (FS, EL, hardness) properties, corrosion resistance, lower melting point

Dental uses: steels, gold, cobalt-chromium, nickel-titanium, nickel-chromium, amalgam

Question 3

What is phase and what is solution?

Answer 3

Phase: physically distinct, homogeneous structure that can be composed of one or more components.

Solution: homogeneous mixture at atomic scale

Question 4

If metals A and B are in a homogeneous mixture, how many phases are present?

Answer 4

Homogeneous mixture of metals A and B in each grain = one phase (solid solution)

Question 5

If metals A and B are situated in a lattice as seperate grains, how many phases are present?

Answer 5

two phases

Question 6

On crystallisation of an alloy, what are the solubilities of metals (3)?

Answer 6

Solubility refers to the ability of metallic elements in an alloy to dissolve into each other in the solid state. There are three cases:

Complete solid solubility - Forming a single phase solid solution (metals can completely dissolve into each other in the solid state)

No solid solubility - The metals are completely insoluble in each other in the solid state. Each metal forms its own distinct grains/phases (euctetic)

Partial solid solubility - Two solid solution phases form, with solubility limits in solid state (amalgam)

Question 7

When are metals soluble?

Answer 7

when they are molten

Question 8

What are the two types of substitutional solid solutions?

Answer 8

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

Can be:
a) Random - Metal atoms are similar in size, valency, crystal structure (FCC) but not in consistent locations
b) Ordered - Metal atoms are arranged in a regular pattern in the lattice, with uniform gaps

Question 9

What is interstitial solid solution?

Answer 9

Smaller atoms fit into the spaces/interstices in the lattice of larger atoms
Requires metals with significantly different atomic sizes

Question 10

Explain the cooling curve of a pure metal?

Answer 10

When a pure molten metal is cooled, it crystallizes at a single temperature

The cooling curve shows a flat arrest during crystallization as heat is released

Question 11

Explain the cooling curve of an alloy?

Answer 11

For alloys, crystallization occurs over a temperature range rather than a single point

The cooling curve shows two arrests between which both liquid and solid phases coexist

Question 12

What do insoluble and soluble alloy solutions crystallise to form?

What does the proximity of TL and TS depend on?

Answer 12

For a solid solution alloy, both metals crystallize together forming a homogeneous solid solution phase

For an insoluble alloy system like a eutectic, each metal forms separate solid grains as it crystallizes
The proximity of TL and TS depends on the solid solubility - they converge for insoluble systems like eutectics

Question 13

What are the two arrests called?

Answer 13

Liquidus temperature (TL) - The temperature at which crystallization begins

Solidus temperature (TS) - The temperature at which crystallization completes

Question 14

What does slow cooling of molten alloy allow?

Answer 14

allows metal atoms to diffuse through lattice
ENSURES grain composition is homogeneous

BUT this results in LARGE GRAINS

Question 15

What does rapid cooling of molten alloy cause?

Answer 15

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

prevents atoms diffusing through lattice causing CORING (composition varies throughout grain.)

Question 16

What happens for coring to occur and why is it disadvantageous?

Answer 16

fast cooling of liquid state;
LIQUIDUS and SOLIDUS must be separated (see Phase Diagram), larger separation = more coring

reduces corrosion resistance of solid alloy

Question 17

What is homogenising anneal and what is used to resolve?

Answer 17

once solid cored alloy formed
REHEAT to allow atoms to diffuse and so cause grain
composition to become homogeneous
NOTE: keep below recrystallisation temperature, otherwise grains altered

resolves coring

Question 18

What is an advantage of interstitial solid solution (different atomic sizes)?

Answer 18

distorted grain structure which IMPEDES dislocation movement and so improves mechanical properties (EL, UTS, hardness)

defect does not “roll” over the lattice plane. Instead it falls into the spaces between the large & small atom.
More energy/force is needed to move any dislocations present in a solid solution
making alloys inherently more fracture resistant (ie stronger) than a metal with better mechanical properties

Question 19

What happens to defects (dislocations) in metals?

Answer 19

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

easier to move dislocation (less mechanical properties)

Question 20

Why are ordered solutions better at impeding than random?

Answer 20

distorted grain structure

atoms are arranged without any particular order. Dislocations can move more easily through this random arrangement because there are no consistent barriers to their motion. Dislocations can glide more freely between the atoms without needing to overcome specific obstacles.

Question 21

What are euctetic alloys and what are their qualities?

Answer 21

Metals insoluble in solid state, form separate grain phases. Crysalisation process occurs at single temperatue

Eutectic composition has lowest melting point
Hard but brittle, poor corrosion resistance

Question 22

What are partial soluble alloys?

how are they strengthed

Answer 22

• Partially soluble Alloys always produce two distinct grains, however, both grains will have both metals, but one grain will be richer in one metal and vice versa for the other grain.
• Annealing of a Partially soluble alloy will make the alloy undergo Precipitation Hardening - which makes the alloy stronger and its surface harder.