Alloys and Strengthening

What is an alloy?

A mixture of a metal with one or more other metals or non-metals.

What is a phase?

A region of material with uniform physical and chemical characteristics

What is a solid solution?

• A single phase region

• solute atoms mix with solvent atoms

• homogeneous composition.

hat does equilibrium mean in phase diagrams?

It means enough time is allowed for phase changes to occur fully at a given temperature and composition.

Why are alloys used instead of pure metals?

• pure metals often have low strength and high ductility

• alloying can improve strength, corrosion resistance and wear resistance.

What can alloying reduce?

ductility, toughness

What is substitutional solid solution?

A solid solution where solute atoms replace solvent atoms in the crystal lattice.

What is interstitial solid solution?

A solid solution where small solute atoms fit into spaces between solvent atoms in the lattice

What are interstitial sites?

Spaces between atoms in a crystal lattice where small solute atoms can fit.

Give an example of an interstitial solid solution

Carbon atoms in the iron lattice in steel.

Give examples of atoms that can form interstitial solid solutions

,B, C, Cl, H, N, O, P and S.

Why are gases usually avoided as interstitial solutes?

Because they can cause problems such as hydrogen embrittlement.

What are the Hume-Rothery rules?

Rules that predict when substitutional solid solutions form

when does substitutional solid solution form? (Hume-Rothery)

solute and solvent atoms have similar:

• size

• crystal structure

• valence electrons

• electronegativity

What atomic size difference is usually allowed for substitutional solid solution?

Atomic diameters should be within about 15%.

Why must solute and solvent atoms be similar in size for substitutional solid solution?

large size mismatch causes too much lattice distortion.

What happens if the Hume-Rothery rules are not satisfied?

A different lower-energy structure or second phase may form instead.

What is complete solid solubility?

When solute atoms are soluble in solvent atoms at all compositions without forming a second phase

example of complete solid solubility

Copper-nickel alloys

What is limited solid solubility?

When only a limited amount of solute can dissolve before a second phase forms.

What happens when the solubility limit is exceeded?

A second phase forms.

What is a eutectic point?

A point where one liquid phase is in equilibrium with two solid phases

What is the eutectic reaction on cooling?

Liquid transforms into two solid phases at the eutectic temperature

What does the liquidus line show?

above which the alloy is completely liquid

solidus line show?

below which the alloy is completely solid

What does the solvus line show?

The boundary showing the limit of solid solubility.

In the Pb-Sn system, what does α represent?

A Pb-rich solid solution.

In the Pb-Sn system, what does β represent?

An Sn-rich solid solution.

Why do many alloys freeze over a temperature range?

Because solid and liquid phases with different compositions coexist during solidification.

What happens when an alloy is slowly cooled?

Equilibrium phase changes occur because atoms have time to diffuse.

What happens during eutectic solidification?

Liquid transforms into a fine mixture of two solid phases.

Why does alloying strengthen metals?

Solute atoms distort the lattice and can obstruct dislocation motion.

How does a substitutional solute strengthen a metal?

replaces solvent atoms and creates lattice strain that makes dislocation movement harder.

Why can alloying reduce ductility?

Because obstructing dislocations makes plastic deformation harder

Why can a phase diagram not directly describe rapid cooling?

rapid cooling may produce non-equilibrium structures where atoms do not have time to diffuse.