Introduction to Materials Science, Chapter 9, Phase Diagrams

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Development of microstructure in isomorphous alloys

Equilibrium (very slow) cooling

Upon cooling from the liquidus line (in the solid + liquid phase region) formation of the solid occurs gradually.

Compositions of the solid and the liquid change gradually during cooling

(as determined by the tie-line method.)

At the solidus line, nuclei grow to consume all the liquid

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Non-equilibrium cooling

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Development of microstructure in isomorphous alloys

Non-equilibrium cooling• Compositional change diffusion

SOLID

• Diffusion is very slow Tie-line invalid

New layers formed on top of existing grains have the equilibrium composition at that T Formation of layered (cored) grains.

LIQUID

• Diffusion is fast Tie-line method works

Lever rule greater proportion of liquid phase as compared to equilibrium at the same T Solidus line is shifted to the right (higher Ni content), solidification is complete at lower T, outer parts of grains are richer in the low-melting component (Cu).

• Upon heating grain boundaries will melt first. This can lead to premature mechanical failure.

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Mechanical properties of isomorphous alloys

Solid solution strengthening

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Binary Eutectic Systems (I)

alloys with limited solubility

silver (Ag) / copper (Cu)radii differ

The melting point of eutectic alloyis lower than that of the components (eutectic = easy to melt in Greek).

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Three single phase regions = solid solution Ag in Cu matrix, = solid solution of Cu in Ag matrix, L = liquid Three two-phase regions ( + L, +L, +)

Solvus limit of solubility

Separates one solid solution from the mixture

Copper – Silver phase diagram

Binary Eutectic System

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Binary Eutectic System

Eutectic or invariant point - Liquid + two solid phases co-exist at eutectic composition CE and eutectic temperature TE Eutectic isotherm - horizontal solidus line at TE

Lead – Tin phase diagram

Invariant or eutectic point

Eutectic isotherm

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Binary Eutectic System

Eutectic reaction – transition from liquid to mixture of two solid phases, + at eutectic concentration CE.

Two phases in equilibrium except: Three phases (L, , ) in equilibrium only at a few points along the eutectic isotherm. Single-phase regions are separated by

2-phase regions.

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Binary Eutectic System

Compositions and relative amounts of phases are determined from the same tie lines and lever rule, as for isomorphous alloys--demonstrate

C

B

A

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Microstructure in eutectic alloys (I)

Several types of microstructure formed in slow cooling an different compositions. Cooling of liquid lead/tin system at different compositions.

In this case of lead-rich alloy (0-2 wt. % of tin) solidification proceeds in the same manner as for isomorphous alloys (e.g. Cu-Ni) that we discussed earlier.

L +L

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Microstructure in eutectic alloys (II)At compositions between room temperature solubility limit and the maximum solid solubility at the eutectic temperature, phase nucleates as the solid solubility is exceeded at solvus line.

L

+L

+

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No changes above eutectic temperature, TE. At TE

liquid transforms to and phases (eutectic reaction).

L +

Microstructure in eutectic alloys (III)Solidification at the eutectic composition (I)

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Microstructure in eutectic alloys (IV)Solidification at the eutectic composition (II)

Compositions of and very different eutectic reaction involves redistribution of Pb and Sn atoms by atomic diffusion. Simultaneous formation of and phases results in a layered (lamellar) microstructure:called eutectic structure.

Formation of eutectic structure in lead-tin system.

Dark layers are lead-rich phase.

Light layers are the tin-rich phase.

Introduction to Materials Science, Chapter 9, Phase Diagrams

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Microstructure in eutectic alloys (V) Compositions other than eutectic

Primary phase is formed in the + L region, and the eutectic structure that includes layers of and phases (called eutectic and eutectic phases) is formed upon crossing the eutectic isotherm.

L + L +

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Microstructure in eutectic alloys (VI)

Microconstituent – element of microstructure having a distinctive structure. For case described on previous page, microstructure consists of two microconstituents, primary phase and the eutectic structure.

Although the eutectic structure consists of two phases, it is a microconstituent with distinct lamellar structure and fixed ratio of the two phases.

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Relative amounts of microconstituents?

Eutectic microconstituent forms from liquid having eutectic composition (61.9 wt% Sn)

Treat eutectic as separate phase and apply lever rule to find relative fractions of primary phase (18.3 wt% Sn) and eutectic structure (61.9 wt% Sn):

We = P / (P+Q) (eutectic) W’ = Q / (P+Q) (primary)