Strengthening and Non-Equilibrium Conditions

What is solid solution hardening?

adding solute atoms into the lattice,

making it harder for dislocations to move.

Why are solute atoms attracted to dislocations?

• They reduce lattice strain energy

• by sitting in regions where their size mismatch helps cancel the dislocation strain field.

Where do smaller solute atoms tend to segregate around a dislocation?

compressive strain field

Why do smaller solute atoms go to the compressive region?

impose tensile strain on the lattice, so reduce distortion

Where do larger solute atoms tend to segregate around a dislocation?

tensile strain field

Why do larger solute atoms go to the tensile region?

impose compressive strain on the lattice

How does solute segregation strengthen the material

It pins or obstructs dislocations

Give an example of solid solution hardening in copper alloys.

Adding zinc to copper forms brass

What is bronze?

copper-tin alloy

Why is 70% Cu–30% Zn called cartridge brass?

It is ductile and easily cold worked

What is Monel?

A copper-nickel alloy

75% Cu and 25% Ni

used for coins

What is precipitation hardening?

small second-phase particles form inside a metal and block dislocation motion.

What are precipitates?

Small particles of a second solid phase that form inside the original phase.

Why do precipitates strengthen alloys?

They obstruct dislocation movement

What type of process is precipitation hardening?

A non-equilibrium heat treatment

What does non-equilibrium mean in this context?

The alloy is cooled or treated too quickly for the normal equilibrium phase changes to fully occur.

What is a supersaturated solid solution?

A solid solution containing more solute than it should under equilibrium conditions at that temperature.

How is a supersaturated solid solution formed?

By heating into a single-phase region and then quenching rapidly so solute atoms do not have time to diffuse out.

What are the main steps in precipitation hardening?

Solution treatment, quenching, ageing, then cooling to room temperature.

What happens during solution treatment?

alloy is heated so all solute atoms dissolve into a single solid solution phase.

What happens during quenching?

The alloy is rapidly cooled to trap solute atoms in a supersaturated solid solution.

What happens during ageing?

The alloy is reheated to an intermediate temperature so fine precipitates can form.

Why is ageing needed after quenching?

Quenching traps solute atoms, but ageing allows controlled diffusion so fine strengthening precipitates form.

Why is precipitation hardening useful?

increasing strength and hardness.

In Al-Cu precipitation hardening, what is the aluminium-rich phase called?

α

In Al-Cu alloys, what is the θ phase?

CuAl₂.

What happens if a 96% Al 4% Cu alloy is cooled slowly from the α region?

Coarse θ precipitates form, mainly at grain boundaries, giving a relatively soft structure (fewer obstacles for dislocation)

Why does slow cooling produce coarse precipitates?

There is enough time for copper atoms to diffuse and form large θ particles.

What happens if the same alloy is quenched from the α region?

A supersaturated solid solution of Cu in Al forms.

Is the as-quenched supersaturated Al-Cu alloy stable?

unstable and CuAl₂ will eventually precipitate if given enough time.

hat temperature range is used for ageing the Al-Cu alloy

About 150–200°C.

What happens during ageing of the quenched Al-Cu alloy

Fine CuAl₂ precipitates form throughout the α phase.

What condition is needed for precipitation hardening to be possible?

The second phase must be more soluble in the first phase at high temperature than at low temperature.

What two processes are needed to form the second phase during ageing?

Nucleation (The initial formation of a new phase)

diffusion

How does nucleation depend on temperature?

It occurs faster when the temperature is further below the equilibrium temperature where the second phase disappears.

Why is precipitate formation a balance between nucleation and diffusion?

Low temperatures favour nucleation but slow diffusion, while high temperatures favour diffusion but reduce nucleation driving force.

What axes are used on a TTT diagram?

Temperature on the vertical axis and time on the horizontal axis.