Metals and Alloys 2 Flashcards Preview

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Flashcards in Metals and Alloys 2 Deck (35)
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1

What is an alloy

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

2

What is the advantage of an alloy

improved properties
lower melting point than the individual metal

3

What are the improved properties of an alloy

→ Mechanical (EL, UTS, hardness)
Corrosion resistance

4

What is a phase

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

5

What is a solution

homogenous mixture at an atomic scale

6

Are metals usually soluble or insoluble when molten?

soluble

7

What can happen to the metals regarding solubility on crystallization

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

8

If the metals are soluble on crystallization what will they form

a solid solution

9

What happens if the metals are insoluble on crystallization

there is no common lattice network and there is two phases

10

What are the 2 types of solid solution

substitutional
interstitial

11

What is a substitutional solid solution

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

12

What are the 2 types of substitutional solid solutions

random
ordered

13

What is a random substitutional solid solution

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

14

What is a ordered substitutional solid solution

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

15

What is an interstitial solid solution

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

16

What is a metal cooling curve like

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'

17

Why is the cooling curve for alloys different from metals

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

18

What is the liquidus

represents the temperature which different alloy compositions begin to crystallise

19

What is the solidus

represents the temperature which different alloy compositions have completely crystallised

20

What is the cooling slowly result in

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

21

What does cooling rapidly result in

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

22

For coring to occur what needs to happen

→ Fast cooling of liquid state
Liquidus and solidus must be separated and determines the extent of coring

23

What is the effect of coring

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

24

What can coring be resolved

through homogenising annealing

25

What happens in homogenizing annealing

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

26

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

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

27

What are the properties of a eutectic alloy

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

28

What is the eutectic composition

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

29

What is the lowest melting point at eutectic composition used for

solder

30

What are those alloys of eutectic composition like

hard but brittle
poor corrosion resistance