Phase Diagrams and Alloys

Define a solution (slide 3).

• A single phase (e.g. a solid solution).

Define a mixture (slide 3).

• More than one phase.

Define the solubility limit.

• The maximum concentration for which only a single-phase solution occurs.

Sugar–water: what is the solubility limit at 20 °C (slide 3)?

65 wt% sugar.

Sugar–water at 20 °C: what do you get if C₀ < 65 wt% sugar? If C₀ > 65 wt%?

• Below 65: syrup only (one phase, L). Above 65: syrup + solid sugar (two phases, L + S).

How does sugar solubility change with temperature?

• It increases: the solubility limit line moves to higher wt% sugar as T rises.

Slide 6: going from A (20 °C, 70 wt%) to B (100 °C, 70 wt%) changes the phases how?

• 2 phases → 1 phase. Heating raises the solubility limit past 70 wt%, so all the sugar dissolves.

Two ways to change the number of phases (slide 6)?

• Change temperature T, or change composition C₀.

Define a component (slide 4).

• The elements or compounds present in the mixture (e.g. Al and Cu).

Define a phase (slide 4)

• The physically and chemically distinct material regions that result (e.g. α and β).

Slide 4 Al–Cu micrograph: which phase is darker, which lighter?

α is the darker phase; β is the lighter phase.

What does it mean that a phase is homogeneous (slide 5)?

• It has uniform physical and chemical properties throughout.

What happens to properties across a phase boundary?

• Physical and/or chemical characteristics change abruptly.

Microstructure is determined by which three things (slide 5)?

• The number of phases present;

• the proportions of the phases;

• the distribution and arrangement of the phases.

What does microstructure determine (slide 5)?

• Mechanical properties.

State the equilibrium condition (slide 7).

• The Gibbs free energy is at a minimum.

Define metastable (slide 7).

• A non-equilibrium system whose rate of approach to equilibrium is extremely slow.

Phase diagrams show phases as functions of which variables (slide 7)?

• Temperature, composition and pressure (T, C, P).

What are the axes of a one-component phase diagram such as water (slide 8)?

• Pressure (atm, log scale) vs temperature (°C). Composition is fixed so it isn't an axis.

What is the triple point?

• The point where solid, liquid and vapour all coexist in equilibrium (where the three phase-boundary lines meet).

Why are Cu and Ni highly soluble in each other (Hume-Rothery)?

• Same crystal structure (FCC),

• similar electronegativities and

• similar atomic radii: the Hume-Rothery rules.

For this course, how many components do phase diagrams have, and what are the independent variables (slide 11)?

• Binary: 2 components.

• Independent variables T and C₀; P = 1 atm almost always.

What is the liquidus?

• The line above which the alloy is entirely liquid.

Where do the liquidus and solidus meet on the Cu–Ni diagram?

• At the melting points of the pure components (0 and 100 wt% Ni, circled on the diagram).

How are solid solutions labelled on phase diagrams?

• With lower-case Greek letters (α, β, γ …).

What does complete mutual solubility mean for the α field?

• The α phase field stretches from 0 to 100 wt% Ni. Due to Cu and Ni having complete solubility, they showcase isomorphism.

How do you find phase compositions in a two-phase field?

• Draw the tie line at T; read C_L where it meets the liquidus and C_α where it meets the solidus.

Formulae for the lever rule in terms of compositions:

Write the lever rule using R and S.

What does S and R represent?

• R: Length from edge of liquidus to C(o)

• S: Length from edge of solidus to C(o)

• R+S = whole line

Quick sense check for any lever rule answer?

• The phase whose composition is closer to C₀ is the one you have more of.

Why might you use volume fraction instead of mass fraction (slide 16)?

• Phase volume fractions can be measured directly from the microstructure (micrographs).

Define V_α in terms of phase volumes.

Convert mass fractions to V_α.

Convert volume fractions to W_α.

What are metal alloys (slide 18)?

• Mixtures of metal types along with other elements, e.g. Fe with carbon.

What two variables define an alloy's microstructure via a phase diagram (slide 18)?

• Weight % (composition) and temperature.

Define cast iron (slide 19)

• Ferrous alloys with > 2.1 wt% C,

• more commonly 3–4.5 wt% C.

• low melting (also brittle) so easiest to cast

Two traditional ways to fabricate metals (slide 20)?

• Blacksmith hammer (forged) and moulding (cast).

What is a forming operation?

• Rough stock formed to final shape.

Hot working: temperature and deformation?

• T high enough for recrystallisation; larger deformations.

Cold working: temperature and effects?

• Well below T_m; work hardening; smaller deformations.

Three families of fabrication methods on slides 21–26?

• Forming, casting, joining.

Forging: how and example products?

• Hammering/stamping a blank between dies (often at elevated T). Wrenches, crankshafts.

Rolling: products and conditions?

• Hot or cold rolling between rolls.

• I-beams, rails, sheet & plate.

Drawing: how and products?

• Pulling (tensile force) through a die. Rods, wire, tubing. Die must be well lubricated and clean.

Extrusion: how and products?

• A ram pushes a billet through a die. Rods, tubing. Ductile metals, e.g. Cu, Al (hot).

Which forming method uses a tensile force?

• Drawing.

Advantages of casting (slide 22)?

• Most common,

• cheapest method;

• good production of shapes;

• good option for brittle materials.

Disadvantages of casting (slide 22)?

• Weaker products;

• internal defects.

Why sand for sand casting (slide 23)?

• It withstands > 1600 °C, is cheap and is easy to mould around a pattern.

Sand casting is used for?

• Large parts, e.g. auto engine blocks.

Describe investment casting.

• Make a paraffin (wax) pattern, encase it in plaster of Paris, melt out the wax, pour metal into the hollow mould.

Investment casting is used for?

• Low volume, complex shapes, e.g. jewellery, turbine blades.

Die casting is used for?

• High volume, low-T alloys.

Continuous casting is used for?

• Simple slab shapes.

Powder metallurgy: which materials and how?

• Materials with low ductility.

When is welding used (slide 26)?

• When one large part is impractical.

Define the heat-affected zone.

• The region in which the microstructure has been changed (by welding heat, without melting).