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2. Chapter Learning Objectives
Explain dispersion strengthening effects on the mechanical properties of metallic materials.
Draw a eutectic phase diagram indicating:
Melting temperature of each pure element
The eutectic temperature
The phases present in each region
Calculate amounts & compositions for each phase for any point on eutectic phase diagram.
Sketch the likely microstructure of a eutectic system given the composition & cooling treatment history.
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3. Chapter Learning Objectives
Determine an alloy’s composition based on its cooling curve.
Describe the mechanism of vapor-liquid-solid (VLS) nanowire growth.
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4. Chapter Outline Sections
11-1 Principles & Examples of Dispersion Strengthening
11-2 Intermetallic Compounds
11-3 Phase Diagrams Containing 3-Phase Reactions
11-4 The Eutectic Phase Diagram
11-5 Strength of Eutectic Alloys
11-6 Eutectics & Materials Processing
11-7 Nonequilibrium Freezing in Eutectic Systems
11-8 Nanowires & the Eutectic Phase Diagram
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5. 11-1 Principles & Examples of Dispersion Strengthening
In dispersion-strengthened alloys, small , strong & hard particles of one phase (precipitate) are introduced into weaker but ductile second phase (matrix).
Dispersion-strengthened alloys may be produced by a eutectic reaction (e.g., cast iron, aluminum alloys), in which a liquid solidifies into 2 solid phases.
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6. 11-1 Principles & Examples of Dispersion Strengthening
If the goal is increased strength and toughness, the following guidelines should be followed:
Matrix should be ductile, and dispersed phase should be hard and strong
Dispersed phase should be discontinuous, while matrix should be continuous
Dispersed phase particles should be small numerous to better prevent slip
Dispersed particles should be round, i.e. less likely to initiate crack propagation
Higher concentrations of dispersed phase increase strength
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7. 11-2 Intermetallic Compounds
Intermetallic compounds contain 2 or more metals in a single phase.
They are often hard & brittle, so they are commonly used as the dispersed phase.
Stoichiometric intermetallic compounds have a fixed composition and are often used to strengthen steels
Non-stoichiometric intermetallic compounds have a range of compositions.
Intermetallic compounds have structural, aerospace, and microelectronic applications.
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8. 11-2 Intermetallic Compounds
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9. 11-3 Phase Diagrams w/ 3-Phase Reactions
Binary systems may produce complicated phase diagrams, such as those involving 3 phases.
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10. 11-3 Phase Diagrams w/ 3-Phase Reactions
Eutectic, peritectic & monotectic reactions are part of the solidification process
Eutectic reactions occur in alloys used for soldering or casting
The monotectic reaction improves the machinability of copper alloy
Peritectic reactions lead to nonequilibrium solidification and segregation
Eutectoid and peritectoid reactions are completely solid-state. Only eutectic & eutectoid reactions can lead to dispersion-strengthening.
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11. 11-4 The Eutectic Phase Diagram
The lead-tin system contains only a simple eutectic reaction with 4 classes of alloys.
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12. 11-4 The Eutectic Phase Diagram
Solid-Solution Alloys:
Alloys containing < 2% Sn are single phase, and are strengthened by strain hardening, controlling grain structure & solid-solution strengthening
Alloys Exceeding Solubility Limit:
Alloys with 2% < Sn < 19% also produce a single phase solid solution, but after additional cooling a second solid phase precipitates out of the first one.
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13. 11-4 The Eutectic Phase Diagram
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14. 11-4 The Eutectic Phase Diagram
Eutectic Alloys:
The eutectic composition is exactly 61.9% Sn. This composition has the lowest melting temperature, and solidification occurs at a single temperature, not a range. A lamellar microstructure is seen.
Hypo/Hypereutectic Alloys:
Hypoeutectic reactions occur between the left-end of the eutectic reaction tie line and eutectic composition.
In hypoeutectic reactions, the liquid begins to solidify at the liquidus temperature producing one solid phase. Solidification is completed by going through the eutectic reaction, which precipitates the second phase to form a lamellar structure.
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15. 11-4 The Eutectic Phase Diagram
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16. 11-4 The Eutectic Phase Diagram
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17. 11-5 Strength of Eutectic Alloys
Are solid-solution strengthened
May be cold-worked in some cases
Can have controlled grain size
Eutectic Colony Size:
Colonies nucleate & grow independently
Within a colony, lamellae have same orientation
Interlamellar Spacing:
Small spacing increases strength of the eutectic
Interlamellar spacing is determined by growth rate
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18. 11-5 Strength of Eutectic Alloys
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19. 11-5 Strength of Eutectic Alloys
Amount of Eutectic:
Increasing the stronger eutectic microconstituent increases the alloy’s strength.
Microstructure of the Eutectic:
Eutectics can have microstructures besides lamellae, e.g. needle-like flat plates in Al-Si alloys
Addition of alloying elements can cause modification of the microstructure, e.g. needle-like platelets can be converted by dendritic growth into thin interconnected rods. This increases tensile strength & percent elongations
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20. 11-5 Strength of Eutectic Alloys
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21. 11-6 Eutectics and Materials Processing
Manufacturing processes often take advantage of the lower melting point associated with the eutectic reaction.
Casting alloys such as cast iron and aluminum alloys are based on eutectic alloys.
Eutectic alloys such as Pb-Sn are used for soldering during assembling parts.
Eutectics are also important in many ceramic systems, such as silica and alumina.
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22. 11-6 Eutectics and Materials Processing
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23. 11-7 Nonequilibrium Freezing in Eutectic System
Alloys that normally solidify as a solid solution normally freeze well above the eutectic.
However, if rapidly cooled, a nonequilibrium eutectic microconstituent may form.
When heat treating such alloys, the maximum temperature must be kept below the eutectic temperature to prevent partial melting.
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24. 11-7 Nonequilibrium Freezing in Eutectic System
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25. 11-8 Nanowires & Eutectic Phase Diagram
Nanowires are cylinders of material with diameters on the order of nm.
One method of producing them is vapor-liquid-solid (VLS) growth.
As an example, silicon nanowires may be fabricated using gold and silane (SiH4)
A layer of pure gold is deposited on a substrate and then heated to form gold nanoparticles.
Silane vapor (the V of VLS) is introduced, which decomposes into pure silicon and hydrogen.
The silicon diffuses through the gold nanoparticles, making them molten (the L of VLS).
As the silicon content increases in the gold, silicon solidifies, making solid (the S of VLS) silicon nanowire.
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26. 11-8 Nanowires & Eutectic Phase Diagram
In the previous example, it is essential to heat the substrate above the Si-Au eutectic temperature, but below the Au melting point.
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27. 11-8 Nanowires & Eutectic Phase Diagram
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28. Summary
Dispersion strengthening is obtained by producing a material containing 2 or more phases.
In metallic materials, phase boundaries impede dislocation movement and increases strength.
In ceramics & polymers, introduction of multiple phases may improve fracture toughness.
For optimum dispersion strengthening, particularly in metallic materials, a large number of small, hard, discontinuous particles should form in a soft, ductile matrix to provide effective obstacles for dislocations.
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29. Summary
Round dispersed phase particles minimize stress concentrations, and final alloy properties can be controlled by relative amounts of these & the matrix.
Intermetallic compounds, which are often strong but brittle, are often used as dispersed phases.
Phase diagrams for materials containing multiple phases often contain 1 or more 3-phase reactions.
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30. Summary
The eutectic reaction allows liquid to solidify as a mixture of 2 solid phases. A wide range of properties can be achieved by controlling the solidification process.
Some factors that can be controlled are
Grain size/secondary dendrite arm spacings of primary microconstituents
Colony size of the eutectic microconstituent
Interlamellar spacing within eutectic microconstituent
Amount of formed eutectic microconstituent
Shape of phases in the eutectic microconstituent
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31. Summary
The eutectoid reaction causes a solid to transform to a mixture of 2 other solids. Heat treatments to control the eutectoid reaction provide an excellent basis for dispersion strengthening.
Nanowires can be grown from eutectic systems by the process of vapor-liquid-solid (VLS) growth. In one example of VLS silicon nanowire growth:
Silane gas is passed over gold catalysts.
Silicon deposits on gold, forming a binary system.
Increase in silicon concentration melts the catalyst.
Further increase forms a solid phase, the nanowire.
Nanowire length is controlled by growth time.
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