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: strengthening

mechanisms

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

MetalsDefinition:Composed of one or more metallic elements �

contain large numbers of free electrons

� good conductors of heat and electricity� shiny

� strong yet deformable

Why spend 4 lectures talking about metals?

• Tonnage• Mature science: huge amount of knowledge built up

over centuries• Properties:

– Strong, yet ductile– Easy to form

– Possible to vary properties over large range and tailor to application

Components of metallurgy

• Chemical metallurgy� Extraction, processing, corrosion

• Mechanical and physical metallurgy� optimise mechanical properties by

manipulating composition and microstructure

Pure metals are very soft and therefore rarely used in engineering applications� alloys: metal mixed with one or more other elements (metallic or not)

Describing alloys� Composition� Microstructure

Alloys

Microstructure

Alloys with the same composition can have very different properties � importance of microstructure

Microstructure of an alloy is determined by the processing techniques used

The microstructure of an alloy describes the size and shape of the grains of the different phases, their orientation and distribution

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

Bronze:Copper + 15 wt% Tin

Cast

Cast and annealed

Factors determining microstructure

• What are the stable phases? � phase diagram

• What processing route was used? In particular: Was there enough time for the stable phases to form? � diffusion

Sugar-water system

Salt-water system

Lead-tin system

100% Pb 100% Sn

T

α + βα + βα + βα + β

α + α + α + α + Lαααα

β + β + β + β + L

Liquid

ββββ

100% Cu

Copper-Zinc system

Diffusion

Process by which atoms move around the crystal lattice

Phase diagram gives stable phasesBUTDiffusion rate determines how fast new phases form (if at all)

How does diffusion happen?

substitutional atoms e.g. Zn atom in brass

interstitial atoms e.g. C atom in steel

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 substitutionalsolute atoms

• Diffusion rates increase with increasing temperature

Microstructure / property relationships

• Theory: strengthening mechanisms• Practice

ferrous alloys (iron, steel)

non-ferrous (e.g. aluminiummagnesiumnickel)

Strengthening mechanisms

Yield strength of metals can be tailored:Yield is brought about through dislocation motion �Strength can be tailored by creating or removing obstacles to dislocation motion.

4 mechanisms:1. Grain refinement2. Work hardening3. Solid solution strengthening4. Precipitation strengthening

stress

strain

Strengthening mechanisms: 1. Grain boundaries

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grain size

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strength

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Summarised in Hall-Petch equation:

σσσσy = σσσσ0 + k.d-1/2

σσσσy = yield stressσσσσo, k = material constantsd = grain size

Grain boundaries act as barriers to dislocation motion.

Strain fields around dislocations

Compressive strain field

Tensile strain field

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 other dislocations, creating a barrier to their motion.

Principle behind work hardening / strain hardening / cold work:

Plastic deformation results in increased dislocation density

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increased interaction between dislocations

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strength

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σσσσ

εεεε

Strengthening mechanisms: 3. Solid solution strengthening

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

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slows dislocation motion

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strength

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Strengthening mechanisms: 3. Solid solution strengthening – stress strain curves

strain

stress

upper yield point: dislocations have to be pulled away from solute atoms

lower yield point

e.g. steel

Strengthening mechanisms: 4. Precipitation strengthening

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

1. Particles coherent with matrix:dislocation can cut through particles, but interaction between strain fields means dislocation motion is hindered

2. Incoherent particles: dislocation is forced to bow round particles

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increased stress required to produce longer dislocation line