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Metals and Alloys4 lectures

1. Introduction to metals:

definition, general properties

importance of metallurgy within materials science alloys: composition and microstructural development

microstructure / property relationships: strengtheningmechanisms

2 4. Metals in practice: ferrous, non-ferrous

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MetalsDefinition:

Composed of one or more metallic elementscontain large numbers of free electrons

good conductors of heat and electricity shiny

strong yet deformable

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Components of metallurgy Chemical metallurgy

Extraction, processing, corrosion

Mechanical and physical metallurgy optimise mechanical properties by

manipulating composition andmicrostructure

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Pure metals are very soft and therefore rarelyused in engineering applications

alloys: metal mixed with one or more otherelements (metallic or not)

Describing alloys

Composition

Microstructure

Alloys

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MicrostructureAlloys with the same composition can have

very different properties importance ofmicrostructure

Microstructure of an alloy is determined by theprocessing techniques used

The microstructure of an alloy describes thesize and shape of the grains of the differentphases, their orientation and distribution

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RF Cochrane, University of Leeds DoITPoMS micrograph library, University of Cambridge

Bronze:

Copper + 15 wt% Tin

Cast

Cast and annealed

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Factors determining microstructure

What are the stable phases? phase

diagram What processing route was used? In

particular: Was there enough time forthe stable phases to form? diffusion

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Sugar-water system

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Salt-water system

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Lead-tin system

100% Pb 100% Sn

T

+ + + +

+ + + + L

+ + + + L

Liquid

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100% Cu

Copper-Zinc system

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DiffusionProcess by which atoms move around the

crystal lattice

Phase diagram gives stable phases

BUT

Diffusion rate determines how fast new phasesform (if at all)

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How does diffusion happen?

substitutional atomse.g. Zn atom in brass

interstitial atomse.g. C atom in steel

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Diffusion rates

Depend on two factors:

is there somewhere for the atom to move to? does the atom have enough energy to jump to its

new position?

As a result:

Diffusion rates for interstitial solute atoms are 10-109

times greater than diffusion rates for substitutional

solute atoms

Diffusion rates increase with increasing temperature

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Microstructure / property relationships

Theory: strengthening mechanisms

Practiceferrous alloys (iron, steel)

non-ferrous (e.g. aluminium

magnesium

nickel)

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Strengthening mechanisms

Yield strength of metals can be tailored:

Yield is brought about through dislocation motion

Strength can be tailored by creating or removingobstacles to dislocation motion.

4 mechanisms:

1. Grain refinement

2. Work hardening

3. Solid solution strengthening

4. Precipitation strengthening

stress

strain

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Strengthening mechanisms:1. Grain boundaries

grain size strength

Summarised in Hall-Petch equation:

y = 0 + k.d-1/2

y = yield stress

o, k = material constants

d = grain size

Grain boundaries act

as barriers todislocation motion.

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Strain fields around dislocations

Compressive strain field

Tensile strain field

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Strengthening mechanisms:

2. Work hardening

Each dislocation creates a strain field in the lattice around it.This strain field interacts with the strain field around otherdislocations, creating a barrier to their motion.

Principle behind work hardening / strainhardening / cold work:

Plastic deformation results in increaseddislocation density increased interactionbetween dislocations strength

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Strengthening mechanisms:

3. Solid solution strengthening

Solute atoms introduce strain field into the lattice.This interacts with strain field around dislocationslows dislocation motion strength

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Strengthening mechanisms:

3. Solid solution strengthening stress strain curves

strain

stress

upper yield point: dislocations have to bepulled away from solute atoms

lower yield point

e.g. steel

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Strengthening mechanisms:

4. Precipitation strengthening

Precipitation hardening requires a fine distribution of secondphase particles. Two possibilities:

1. Particles coherent with matrix:dislocation can cut through particles, butinteraction between strain fields meansdislocation motion is hindered

2. Incoherent particles: dislocation isforced to bow round particlesincreased stress required to producelonger dislocation line