Metals and Alloys 2

Question 1

What is an alloy

Answer 1

An alloy is a combination of two or more metals or a metal with a metalloid

Question 2

What is the advantage of an alloy

Answer 2

improved properties

lower melting point than the individual metal

Question 3

What are the improved properties of an alloy

Answer 3

→ Mechanical (EL, UTS, hardness)

Corrosion resistance

Question 4

What is a phase

Answer 4

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

Question 5

What is a solution

Answer 5

homogenous mixture at an atomic scale

Question 6

Are metals usually soluble or insoluble when molten?

Answer 6

soluble

Question 7

What can happen to the metals regarding solubility on crystallization

Answer 7

they may be insoluble
they may form an inter metallic compound with specific chemical formulation
they may be soluble

Question 8

If the metals are soluble on crystallization what will they form

Answer 8

a solid solution

Question 9

What happens if the metals are insoluble on crystallization

Answer 9

there is no common lattice network and there is two phases

Question 10

What are the 2 types of solid solution

Answer 10

substitutional

interstitial

Question 11

What is a substitutional solid solution

Answer 11

atoms of one metal replace the other metal in the crystal lattice

Question 12

What are the 2 types of substitutional solid solutions

Answer 12

random

ordered

Question 13

What is a random substitutional solid solution

Answer 13

metal atoms similar in size, valency and crystal structure and are randomly positioned in the lattice

Question 14

What is a ordered substitutional solid solution

Answer 14

metal atoms in a regular lattice arrangement. The metals must be similar in size, valency and crystal structure

Question 15

What is an interstitial solid solution

Answer 15

□ Atoms markedly different in size (DIFFERENCE FROM SUBSTITIONAL)
Smaller atoms located in spaces in lattice/grain structure of larger atom

Question 16

What is a metal cooling curve like

Answer 16

In a metal cooling curve it shows that after all the metal has crystallised then temperature of the metal will drop resulting in a ‘plateau phase’

Question 17

Why is the cooling curve for alloys different from metals

Answer 17

METALS CRYSTALLISE AT ONE TEMPERATURE BUT ALLOYS CRYSTALLISE OVER A TEMPERATURE RANGE

Question 18

What is the liquidus

Answer 18

represents the temperature which different alloy compositions begin to crystallise

Question 19

What is the solidus

Answer 19

represents the temperature which different alloy compositions have completely crystallised

Question 20

What is the cooling slowly result in

Answer 20

metal atoms to diffuse through the lattice which ensures that grain composition is homogenous but results in large grains

Question 21

What does cooling rapidly result in

Answer 21

Rapid cooling prevents atoms diffusing through lattice and causes coring as the composition varies throughout the grain

Question 22

For coring to occur what needs to happen

Answer 22

→ Fast cooling of liquid state

Liquidus and solidus must be separated and determines the extent of coring

Question 23

What is the effect of coring

Answer 23

may reduce the corrosion resistance of the solid form of an alloy

Question 24

What can coring be resolved

Answer 24

through homogenising annealing

Question 25

What happens in homogenizing annealing

Answer 25

Once a solid cored alloy is formed it should be reheated to allow atoms to diffuse and so cause grain composition to become homogenous

Question 26

Why do alloys forming a solid solution and consisting of metals of different atomic size have improved mechanical properties

Answer 26

has a distorted grain structure which impedes dislocation movement
instead falls into the larger space existing between large and small atoms meaning more energy is needed for the defect to overcome the different sized atoms meaning greater stress it required to move dislocations and so alloys are inherently more fracture resistant and so stronger than metals

Question 27

What are the properties of a eutectic alloy

Answer 27

→ Metals are soluble in liquid state
→ Metals are insoluble in solid state
Each metal forms physically distinct grains

Question 28

What is the eutectic composition

Answer 28

When the liquidus and solidus coincide (where the crystallisation process occurs at a single temperature) and where grains of individual metals formed simultaneously

Question 29

What is the lowest melting point at eutectic composition used for

Answer 29

solder

Question 30

What are those alloys of eutectic composition like

Answer 30

hard but brittle

poor corrosion resistance

Question 31

What is the non-eutectic composition

Answer 31

The excess metal crystallises first then the liquid reaches eutectic composition and then BOTH the metals crystallise (forming separate grains)

Question 32

What are solubility limit lines

Answer 32

ndicates that a range of compositions are not possible

Question 33

What are the two grain structures possible

Answer 33

alpha and beta solid solution

Question 34

What do partially soluble alloys undergo on annealing

Answer 34

a supersaturated alloy will undergo precipitation hardening

Question 35

What is a solid solution

Answer 35

A solid solution is a mixture of elements at the atomic level, and is analogous to a mixture of liquids which are soluble in one another