Dissemination of IT for the Promotion of Materials Science (DoITPoMS)

DoITPoMS Teaching & Learning Packages Ternary Phase Diagrams Solidification of ternary alloys – complete solid solution
Ternary Phase Diagrams AimsBefore you startIntroductionRepresenting ternary phase diagramsSections through ternary phase diagramsTie lines and tie trianglesApplying the Gibbs phase rule to ternary systemsSolidification of ternary alloys – complete solid solutionSolidification of ternary alloys – one eutectic valleySolidification of ternary alloys – Class I reactionsSolidification of ternary alloys – Class II reactionsSolidification of ternary alloys – Class III reactionsCase Study: Sn-Ag-Cu systemUses of ternary alloysSummaryQuestionsGoing further
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Solidification of ternary alloys – complete solid solution

The ternary system below shows complete solid solubility. Each of its bounding binaries show complete solid solubility.

space model of ternary

Figure 7: Space model for a ternary system with complete solid solubility. Adapted from https://commons.wikimedia.org/wiki/File:Space_diagram_of_a_three-component_system.jpg

This system has a single-phase liquid region (at high temperatures); a single-phase solid region (at low temperatures, spanning the whole composition triangle) and a two-phase (liquid + solid) region.

All alloys in this system cool through all three regions.

Initially they cool through the liquid phase field. When they reach the liquidus, alloys enter the two-phase field where the solid phase begins to form and the proportion of liquid in the system decreases. Once all the liquid has been exhausted, the alloy passes across the solidus into the single-phase solid field.

When in the single-phase fields (the liquid at high temperatures and the solid at low temperatures), the alloy is homogenous, and its composition is the bulk composition.

When the alloy is in the two-phase field, it is heterogenous. The compositions of the solid and liquid at each temperature are given by tie lines which span the two-phase region. These tie lines change with decreasing temperature so the equilibrium compositions of the solid and liquid also change with decreasing temperature.

Consider alloy W in the diagram below:

composition of alloy W in a ternary system with complete solid solubility

Figure 8: The composition of alloy W is plotted in a ternary system showing complete solid solubility. Adapted from https://commons.wikimedia.org/wiki/File:Space_diagram_of_a_three-component_system.jpg

The solidification behaviour of alloy W is described below:

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