Microstructure and mechanical properties

topic 4

Polycrystaline

A metal solid made up of many small crystals.

Process: Baby crystals/nuclei —→ nuclei grows —→ becomes crystal —→ grain ——> solid metal

equiaxed grain

grains that are roughly equal size

Heterogeneous Nucleation

solid crystal starts forming on pre existing surfaces. More likely to occur than Homogeneous Nucleation

Homogeneous Nucleation

solid crystal forms spontaneously within bulk of liquid

Grain Boundaries properties

High energy regions - due to free atomic bonds

Grain Structure development diagram (Chill, Columnar Crystals)

Types of middle grain structure development

Equiaxed Grain, Columnar Grains meet, Hole/Pore, Dendrite Formation

How to get an equiaxed grain structure (lab 1)

1) use grain refiners

2) Annealing (heat treatment)

Metallography

A process used in order to reveal grain boundaries of a metal

Metallography processes

1) Grinding - removes big scratches

2) Polishing - removes small scratches. Done with diamond paste

3) Etching - reveals grain boundaries through etching grooves. Done with acid/alkali

How does grain size affect mechanical properties of metals?

Big grains - Fewer Grain boundaries, lower Grain boundaries density, materials weaker, materials more ductile

Small grains - More Grain boundaries, higher Grain boundaries density, materials stronger, materials less ductile

Hall Patch Effect

Describes how the strength of a metal/ceramic increases as its grain size decreases.

yield Stress = Friction stress + Hall patch constant x (Grain size ^-0.5)

Grain Deformation

When a metal is plastically deformed the individual grains are permanently deformed. Eg equiaxed grains changes into elongated grains.

Working Hardening

When a metal is plastically deformed it becomes stronger.

Work Hardening and the process with dislocations

As dislocations move —→ dislocations multiply —→ dislocations density increase —→ causes traffic jam of dislocations —→ material becomes stronger

Cold-worked

Metal is plastically deformed at a temperature below its recrystallisation temperature.

Cold-worked Rolling process

%CW = (initial thickness - final thickness)/(initial thickness) x 100%

Assumption = width does Not change

Cold-worked Extrusion process

%CW = (initial area - final area)/(initial area) x 100%

Hardness property

The materials resistance to localised plastic deformation

How to test hardness property

Indentation test: harder material = smaller indent