Metals and Alloys

Question 1

What are some properties of metals?

Answer 1

At average temperature

– Hard and lustrous

– Closely packed crystalline structures

– Opaque

– Conductors of heat and electricity

• One exception is Hg (mercury) used in Amalgams

Question 2

What is the mixure of 2 metals called?

What is the mixure of 3 metals called?

What is and Alloy system

What do metals display in molten state?

Answer 2

Mixture of two or more metals

– Eg two metals – Binary Alloy,

three metals – Ternary Alloy

• Alloy system – All possible combinations of an alloy
• In molten state metals display mutual solubility

Question 3

In metals what do we mean when we say Regular crystalline structure?

Answer 3

• Regular crystalline structure – Composed of many small crystals
• Form from molten metal – As cools
• Impurities enable metal atoms to deposit upon them
• These nucleate crystal growth

Question 4

In metals what are crytals known as ?

Answer 4

Crystals are known as GRAINS.

• Grains grow until all melt metal consumed
• Where crystals abut (lean) their neighbours termed a Grain Boundary

Question 5

What are the 2 possible Nucleations of metals?

Answer 5

Nucleation may be

– HETEROGENEOUS

• Many sites of nucleation
• Majority of dental applications

– HOMOGENEOUS

• Single site
• Metal is pure so requires 4 metal atoms to come together
• Requires specialised equipment

Question 6

What does grain size influence and who can control this?

Answer 6

– Influences physical properties

– Controlled by manufacturer

• Rapid solidification eg. Casting dental gold alloy by lost wax technique

Question 7

What are some other factors affecting grain size?

Answer 7

– Temperature of mould

• If similar to melt, slow cooling and large grain size
• If different to melt, rapid cooling, small grain size

– Shape of mould

• Walls of mould initiate growth and therefore influence direction of crystal growth

Question 8

How do we visulise Grain Bounderies?

Answer 8

• Polished metal surface, then apply suitable (etching) chemical

Then use Light reflecting microscope

Question 9

How can u shape metals and Alloys?

Answer 9

Casting

– Molten Investment mould

Amalgamation

– Mix with Hg (mercury)

• Plastic mass
• Hardening chemical reaction
• Crystallisation

Question 10

What is the definition of:

DUCTILITY

MALLEABILITY

Answer 10

• DUCTILITY – the maximum degree of extension in response to an applied tensile force.
• MALLEABILITY – the maximum degree of compression in response to an applied compressive force.

Question 11

On cooling from molten component metals may do what?

Answer 11

– Remain soluble in each other – SOLID SOLUTION

– Be completely insoluble in each other

– Be partially soluble in each other

– Form intermettalic compounds if the metals have affininty for each other e.g. Ag3Sn.

Question 12

What 3 types of solid solutions are there?

Answer 12

– Ordered solid solution

• Random Solid solution

– Interstitial solid solution

Question 13

What is Ordered solid solution?

Answer 13

– Ordered solid solution

• Component metal atoms have specific sites in common lattice

Question 14

What is Random Solid solution ?

Answer 14

• Random Solid solution – Random sites in common crystal lattice

Question 15

What is Interstitial solid solution?

Answer 15

• Interstitial solid solution – Atoms of one lie within the primary lattice sites of the other

Question 16

What do solid solutions have compared to pure metals?

Answer 16

Cf. pure metals are

– Harder

– Stronger

– Higher elastic limits

• Hardening effect – solution hardening

Question 17

What do cooling curves indicated and how are they plotted?

Answer 17

– Record the Melt of metal/alloy

– Record and plot temperature versus time

Question 18

In Phase Diagrams what does a large separation between solidus and liquidus mean?

Answer 18

Phase diagrams • The larger the separation between solidus and liquidus the greater the coring