alloys

Question 1

what are alloys

Answer 1

a combination of two or more metals

or a metal with a metalloid

Question 2

Benefits of alloys

Answer 2

improved properties
o EL
o FS
o UTS
o Hardness
o More resistant to corrosion
o Lower melting point than individual metal

Question 3

uses of alloys

Answer 3

steel
 burs and instruments

gold alloy
 inlays and crowns etc

cobalt chromium
 dentures

nickel titanium
 ortho wires

nickel chromium
 crowns and bridge

amalgam
 restoration

Question 4

phase definition

Answer 4

Physically distinct homogenous structure
Can have more than one component

Question 5

solution definition

Answer 5

Homogenous mixture at an atomic scale

Question 6

pure metal has how many phases

Answer 6

1

Question 7

an alloy with 2 insoluble metals has how many phases

Answer 7

2 phases

Question 8

solid solution has how many phases

Answer 8

1

Question 9

solid solution def

Answer 9

2 metals coexisting in a lattice network; both metals in a homogenous mixture

Question 10

3 categories of alloys

Answer 10

1. 2 phases (insoluble)
2. intermetallic compound (partially soluble)
3. 1 phase (solid solution)

Question 11

solid solution has 3 types what r they

Answer 11

1. substitutional random
2. substitutional ordered
3. interstitial

Question 12

diff between cooling curve of metal and alloy?

Answer 12

pure metal Crystalises at one temperature

alloy Crystallizes over a range of temperatures (between TL and Ts)

Question 13

liquidus def

Answer 13

line representing the temperature which different alloy composition begin to crystallise

Question 14

solidus def

Answer 14

line representing the temperature which different alloy composition have completely crystallised

Question 15

do you want fast or slow cooling

Answer 15

fast

Question 15

benefits and disadvantages of fast cool

Answer 15

small grains good but coring occurs, can overcome with annealing

Question 16

benefits and disadvantages of slow cool

Answer 16

homogenous good but large grains with poor properties

Question 17

what determines the extent of coring?

Answer 17

 the larger the difference between the liquidus and solidus line, means that there will be MORE CORING during rapid cooling

Question 18

when does coring occur

Answer 18

fast cooling, and a difference between the liquidus and solidus state

Question 19

why is coring bad

Answer 19

alloy becomes less resistant to corrosion

Question 20

Benefits of annealing after coring

Answer 20

• makes the grains homogenous
• allows atoms to diffuse and rearrange

Question 21

Benefits of small grains over large ones

Answer 21

• impede dislocation movement
• improve mechanical properties

Question 22

why is there less dislocation movement in alloys compared to metals

Answer 22

• because solid solution has metals of different atomic size which means there is a distorted grain structure.
• distorted grain structure impedes dislocation movement and improves mechanical properties such as (EL UTS FS hardness)

-> defects have to “climb” over each atom and settle in between the metal atoms. it requires a lot of energy to climb over the next atom in the lattice plane. hence it takes more force to move the defect to the grain boundary. greater stress to move any dislocations, harder to distort, more fracture resistant and stronger than metal

Question 23

what sort of alloy is Au-Cu

Answer 23

solid solution, completely soluble

Question 24

what sort of alloy is Au-Pt

Answer 24

solid solution

Question 25

what is eutectic alloy

Answer 25

an alloy with 2 metals that are completely insoluble so the metals exist in seprate grains DO NOT FORM solid solution, that is for soluble metals

Question 26

what sort of alloy is Ag-Cu

Answer 26

partially soluble

Question 27

when does crystallisation occur for eutectic alloys

Answer 27

at a single temperature, where the liquidus line meets the solidus line, the grains of each metal form simultaneously

Question 28

function of eutectic alloys

Answer 28

soldering alloys tgt since it has the lowest melting point of all alloys

Question 29

properties of eutectic alloys

Answer 29

hard brittle poor corrosion resistance

Question 30

How do you improve the mechanical properties of a partially soluble alloy

Answer 30

precipitation hardening = pushes the metals to the grain boundaries to make the alloy stronger and harder

Question 31

what is the key point of partially soluble alloy graph with the solid solubility limits?

Answer 31

solid Solubility limit lines means alloys cannot form grains with the composition between H1 and H2 % they can only form grains with composition below H1 or above H2 producing alpha or beta grains which are either silver rich or copper rich grains