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1

How to increase strength

make it difficult for dislocations to move

2

Microstructure features

Grains
Precipitates

3

Grains

-Small crystals
-Within each grain lattice of atoms same, but orientation changes from one grain to another
-typical grain size 10-100 microm

4

Precipitates

-Small particles, typically 1microm or less in size
-lattice structure of atoms in precipitate diff from that in rest of material

5

Grains and dislocations

-Dislocation can't cross grain boundary
-refine grain size - yield strength goes up

6

Hall-Petch equation
(effect of grain size)

τᵧ = τₒ + k/√d

τᵧ = yield strength in shear
τₒ = yield strength of lattice
k = constant
d = grain size

7

Precipitates and dislocations

-Dislocation can't enter particle bc has diff lattice structure
-Dislocations can overcome precipitates by breaking them or looping around
-happens within each grain, think of it as changing value of τₒ
-reduce spacing between precipitate particles, yield strength goes up

8

Equation describing changing the value of τₒv

τₒ = Gb/L

G = shear modulus
b = spacing between atoms
L = spacing between precipitates

9

Work Hardening

-in stress/strain curve, after σᵧ line continues to rise, need more stress to get more strain

10

Why Work Hardening

-σᵧ stress at which dislocations able to move
-need lot of dislocations moving to get lots of plastic strain
-more dislocations formed as test goes on

11

Annealing/Tempering

Heating the metal, making grains larger

12

Microstructures - Simple single-element metals

eg. aluminium, titanium, have simple microstructures

13

Cold working

Deforming metal at room temp

-produce distorted grains
-inc sctrength

14

Recrystallisation

Heating cold-worked metal causes new grains to form & grow

15

Way to get small grain size

Annealing after cold working (recrystallisation)

16

Precipitation Strengthening

-To get precipitates need metal with more than one element in it
-Bc chem composition of precipitate particles must be diff to that of rest of material (called the matrix)
-atype of strengthening only possible w/ alloys

17

eg of alloys

-Steel (Fe + C + other elements)
-Titanium Alloys (eg. Ti + 6% Al + 4%V)
-Brasses (eg. Cu + 40%Zn)
-Aluminium

18

Amount of carbon in steel

-less than 1%, often less than 0.1%

19

Tricks steel uses

1. Sometimes C "dissolves" in the Fe
2. Sometimes C combines w/ some Fe to form new material
3. Fe atoms have diff lattice structure at high temp & low temp

20

C dissolves in Fe

-a solid solution
-C can fit into gaps between larger atoms (intersitial)
-In other alloys one atom can substitute for another

21

A solid solution

when atoms of one element dissolve in the other

22

Fe atoms and temp

-High temp: Fe has fcc lattice
-Room temp: bcc
-Change is reversible, happens instantaneously on heating and cooling

23

Carbon dissolving and temp

-Can dissolve in high-temp lattice (holes bigger)
-Cant dissolve in low-temp lattice
-At high temp: material like single-element metal, has grains but no precip, yield strength low as all C in solid soln
-Cooler: C precipitates out of soln, forming Fe3C particles + raising yield strength

24

High-temp form of Fe

Austenite fcc

25

Low temp form of Fe

Ferrite bcc

26

Alloys vs Single-element metals

Alloys better bc they can use precipitation strengthening, creating lots of small particles to impede dislocation motion

27

At low temps, C

Instead of dissolving, forms separate phase, Fe3C "cementite"

28

Pearlite

-high yield strength, v high if layers are v thin
-consists of thin, flat layers of ferrite alternating w/ layers of cementite
-0.76% Carbon content

29

Ferrite and pearlite

soft, lower σᵧ than pearlite alone

30

Normalising

Natural cooling, when you take steel out of furnace and let it cool naturally in air