Alloys

Question 1

what is an alloy?

Answer 1

a combination of two or more metals or a metal(s) with a metalloid (Si, C)

Question 2

what are the advantages of alloys?

Answer 2

• improved mechanical properties (EL, FS, hardness)
• improved corrosion resistance
• lower melting point than individual metal

Question 3

what are some dental uses of alloys?

Answer 3

• steel - burs, instruments
• gold alloy - inlays, crowns, bridges, partial dentures, wires
• cobalt chrome - partial dentures
• nickel-titanium - wires
• nickel chromium - crowns, bridges, wires
• amalgam - restorative material

Question 4

what is the definition of a phase?

Answer 4

physically distinct homogenous structure (can have more than one somponent)

individual grains of metal A and B - two phases

Question 5

what is the definition of a solution?

Answer 5

a homogenous mixture at an atomic scale

metals A and B in a homogenous mixture - one phase - solid solution

Question 6

what can happen on crystallisation of two metals?

Answer 6

1. be insoluble - no common lattice and exists as two separate phases
2. form an intermetallic compound with specific chemical formulation (e.g. Ag3Sn)
3. be soluble and form a solid solution - common lattice and exist as one phase

Question 7

what is a substantial solid solution?

Answer 7

atoms of one metal replace the other metal in the crystal lattice/grain in a:
* random way - requires atoms to be similar in size, valency and crystal structure
* ordered way - in a regular lattice arrangement with similar size, valency and crystal structure

Question 8

what is a interstitial solid solution?

Answer 8

atoms of remarkedly different sizes with smaller atoms located in spaces in the lattice/grain structure of larger atoms

Question 9

describe the cooling curve of an alloy

Answer 9

• the alloy is cooled from its molten state where both metals are not in a lattice
• crystallisation begins at TL
• the first few metal atoms are cooled to form nuclei of crystallisation
• the two metals have different melting points and crystallise when these are reached
• there is a change of temperature when the alloy crystallises from TL to TS

Question 10

what is the difference in the crystallisation of a metal and alloy?

Answer 10

• metals crystallise at one temperature
• alloys crystallise over a temperatre range

Question 11

describe the cooling curve of AuPt

Answer 11

• the TL and TS values for each alloy are different
• if you change the ratio of alloys, it produces different cooling curves due to their different melting points
• platinum has a higher melting point than gold

Question 12

what happens when slow cooling of molten alloys occurs?

Answer 12

• allows metal atoms to diffuse through lattice and find their equilibrium position
• ensures the grain composition is homogenous
• BUT results in large grains forming - undesirable

Question 13

a complete solid solubilty diagram is a phase diagram joined together. describe the complete solubility diagram of AuPt

Answer 13

• each point on the liquidus represents the temperature that crystallisation started for the alloy
• each point on the solidus represents the temperature that crystallisation ends for the alloy
• liquidus: alloy in liquid state
• solidus: alloy in solid state
• between the curves, the solid is partly liquid and partly solid

Question 14

what happens when rapid cooling (coring) of molten alloys occurs?

Answer 14

• ensures grains are small and that there are many grain boundaries which will impede dislocations
• alloy with composition X begins to crystalise at TL and a tie line is drawn horizontally and vertically to find the first grain composition (85%A, 15%B)
• when the temperature is dropped, the grain formed is composed of 75%A and 25%B
• when the temperature is dropped again, the grain formed is composed of 65%A and 35%B
• the temperature of the alloy X cools to the temperature of the solidus and the last grain to form is composed of 20%A and 80%B
• rapid cooling/quenching results in non-homogeneous grains

Question 15

what does rapid cooling (coring) of a molten alloy result in?

Answer 15

• prevents atoms diffusing through lattice
• causes coring as composition varies throughout grain
• improves mechanical properties
• may reduce corrosion resistance of solid form of alloy

Question 16

what are the conditions required for coring?

Answer 16

• fast cooling of liquid state (drop into cold water)
• liquidus and solidus must be separated and determines extent of coring

Question 17

how would you overcome the non-homogenous grains formed from rapid cooling?

Answer 17

**homogenising anneal **
* reheat the non-homogenous solid to allow the metal atoms to diffuse and cause the grain composition to overcome the concentration gradient and homogenise
* keep the alloy below its recrystallisation temperature, otherwise its grains will be altered

Question 18

what is solution hardening?

Answer 18

• when alloys that form a solid solution consist of two or more metals of different atomic sizes arrange in a lattice, they have a distorted grain
• the distorted lattice impedes dislocation movement so improves the mechanical properties (EL, UTS, hardness)
• heat alloy below its recrystallisation temperature and cool slowly

Question 19

what is the difference between dislocation movement in a metal and an alloy?

Answer 19

• metal - defects roll over atoms with little energy to move along the slip plane until grain boundary
• alloy - defects fall into spaces between large and small atom, requiring more energy to overcome different sized atoms until grain boundary (more fracture resistant as more energy needed)

Question 20

what is order hardening?

Answer 20

• alloys that form a ordered solid solution have metal atoms distributed in a regular lattice but still have a distorted grain structure as lattice impedes dislocation movement
• the resistance to dislocation gives improved mechanical properties (EL, UTS, hardness)
• heat alloy below its recrystallisation temperature and cool slowly

Question 21

describe this graph showing the complete solid solubility of AuCu and order hardening

Answer 21

• the liquidus and solidus are quite close together so the extent of the coring is limited
• not much of a concentration gradient when rapidly cooled (quenched)
• the area under the graph shows that there are two possible ordered solutions - AuCu3 and AuCu
• these ordered solutions need to be annealed to remove coring
• heat alloy below its recrystallisation temperature (424 degrees) and cool slowly

Question 22

what is an eutectic alloy?

Answer 22

a homogeneous mixture of two or more substances, that melts or solidifies at a single, sharp temperature, lower than the melting point of any individual constituent

Question 23

describe the phase diagram of a eutectic alloy

Answer 23

• liquidus is defined by point a, c, e
• solidus is defined by point b, c, d
• liquidus and solidus coincide at point c which is the eutectic point/composition where the crystallisation process occurs at a single temperature and grains of individual metals are formed simultaneously
• on cooling rapidly, a eutectic alloy will form grains of individual metals (non-homogenous) in 2 phases

Question 24

what is the lowest melting point used for in eutectic alloys?

Answer 24

soldering parts together

Question 25

what are the properties of a eutectic alloy?

Answer 25

* hard but brittle * poor corrosion resistance

Question 26

what happens when non-eutectic alloys crystallise?

Answer 26

* non-eutectic alloys crystallise over a **temperature range** * one metal will crystallise **before** the other * but both metals crystallise to form **separate grains** (non-homogenous)

Question 27

what is a partially soluble alloy?

Answer 27

an allow that is **inbetween** a solid solution alloy which is completely soluble and a eutectic alloy which is insoluble with two separate grains

Question 28

describe this phase diagram for a partially soluble alloy (AgCu)

Answer 28

* liquidus and solidus are **separate** * the dashed lines from points H1 and H2 are **solubility limit lines** * solubility limit lines show that alloys of AgCu **cannot form grains** with a composition **between** H1 and H2 * the alloy can form grains which are **less than H1%** (Ag rich or α-grains) or **more than H2%** (Cu rich or β-grains) * at X, the **initial grains are α** and on completion, α and β grains are present but **more α-grains** (α-solid solution) * at Y, the **initial grains are β** and on completion, α and β grains are present but **more β-grains** (β-solid solution) * at Z, the **initial grains are α and β** and on completion, **α and β grains are still present**

Question 28

what is precipitation hardening?

Answer 28

* alloys that form a **partially solid solution** have metal atoms that are more or less abundant than the other * annealing **pushes the less abundant atoms to the grain boundary**, resisting movements of dislocations within the grain * the resistance to dislocation gives **improved mechanical properties** (EL, UTS, hardness) * **heat** alloy below its recrystallisation temperature and **cool slowly**