1. Dispersion Strengthening and Eutectic Phase Diagrams
Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Chapter 11:
Dispersion Strengthening
and Eutectic Phase Diagrams
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Learning Objectives
Principles and examples of dispersion strengthening
Intermetallic compounds
Phase diagrams containing three-phase reactions
The eutectic phase diagram
Strength of eutectic alloys
Eutectics and materials processing
Nonequilibrium freezing in the eutectic system
Nanowires and the eutectic phase diagram
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3. Principles and Examples of Dispersion
Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Principles and Examples of Dispersion
Strengthening
Dispersion strengthening
Increasing the strength of a material by forming more than one phase
Matrix
The continuous solid phase in a complex microstructure
Dispersed phase or precipitate
A solid phase that forms from the original matrix phase when the solubility limit is exceeded
Microconstituent
A phase or mixture of phases in an alloy that has a distinct appearance
Eutectic
A three-phase invariant reaction in which one liquid phase solidifies to produce two solid phases.
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4. Principles and Examples of Dispersion
Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Principles and Examples of Dispersion
Strengthening
Eutectoid
A three-phase invariant reaction in which one solid phase transforms to two different solid phases
Age hardening
A strengthening mechanism that relies on a sequence of solid-state phase transformations
in generating a dispersion of ultrafine particles of a second phase
Interphase interface
The boundary between two phases in a microstructure
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5. Principles and Examples of Dispersion
Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Principles and Examples of Dispersion
Strengthening
(a) The aluminum-antimony phase diagram includes a stoichiometric intermetallic compound γ.
(b) The molybdenum-rhodium phase diagram includes a nonstoichiometric intermetallic compound γ.
An intermetallic compound contains two or more metallic elements.
Stoichiometric intermetallic compound: A phase formed by the combination of two components into a compound having a structure and properties different from either component.
Nonstoichiometric intermetallic compound or intermediate solid solutions: A phase formed by the combination of two components into a compound having a structure and properties different from either component.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.3
The strength and ductility of the intermetallic compound Ti3Al compared with that of a conventional nickel superalloy. The Ti3Al maintains its strength to higher temperatures than does the nickel superalloy.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.4
The five most important three-phase reactions in binary phase diagrams.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.6
The lead–tin equilibrium phase diagram.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.7
Solidification and microstructure of a Pb-2% Sn alloy. The alloy is a single-phase solid solution.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.8
Solidification, precipitation, and microstructure of a Pb-10% Sn alloy. Some dispersion strengthening occurs as the β solid precipitates.
Isopleth: A line on a phase diagram that shows constant chemical composition.
Isoplethal study: Determination of reactions and microstructural changes that are expected while studying a particular chemical composition in a system.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.9
Solidification and microstructure of the eutectic alloy Pb-61.9% Sn.
L61.9%Sn→ α19%Sn+ β97.5%Sn
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.11
Atom redistribution during lamellar growth of a lead–tin eutectic. Tin atoms from the liquid preferentially diffuse to the β plates and lead atoms diffuse to the α plates.
(b) Photomicrograph of the lead-tin eutectic microconstituent (× 400).
Lamella - A thin plate of a phase that forms during certain three-phase reactions, such as the eutectic or eutectoid.
Eutectic microconstituent - A characteristic mixture of two phases formed as a result of the eutectic reaction.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.13
The solidification and microstructure of a hypoeutectic alloy (Pb-30% Sn).
Hypereutectic alloy - An alloy composition between that of the right-hand end of the tie line defining the eutectic reaction and the eutectic composition.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.14
A hypoeutectic lead-tin alloy.
(b) A hypereutectic lead-tin alloy. The dark constituent is the lead-rich solid α, the light constituent is the tin-rich solid β, and the fine plate structure is the eutectic (×400)
Hypoeutectic alloy - An alloy composition between that of the left-hand end of the tie line defining the eutectic reaction and the eutectic composition.
Primary or proeutectic microconstituent - The microconstituent that forms before the start of a three-phase reaction.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.15
The cooling curve for a hypoeutectic Pb-30% Sn alloy.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.16
The cooling curve for a hypoeutectic Pb-30% Sn alloy.
Colonies in the lead–tin eutectic (× 300).
(b) The interlamellar spacing in a eutectic microstructure.
Interlamellar spacing of a eutectic is the distance from the center of one α lamella to the center of the next α lamella
γ = cR-1/2
where R is the growth rate (cm/s) and c is a constant
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.18
The effect of the composition and strengthening mechanism on the tensile strength of lead-tin alloys.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.20
Typical eutectic microstructures: (a) needle-like silicon plates in the aluminum–silicon eutectic (× 100) and (b) rounded silicon rods in the modified aluminum–silicon eutectic (× 100).
Modification - Addition of alloying elements, such as sodium or strontium, which change the microstructure of the eutectic microconstituent in aluminum-silicon alloys.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.23
A Pb-Sn eutectic alloy is often used during soldering to assemble parts. A heat source, such as a gas flame, heats both the parts and the filler material. The filler is drawn into the joint and solidifies.
(b) A phase diagram for Al2O3-SiO2.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure (Contd.)
(c) A phase diagram for the CaO-SiO2 system.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.24
Nonequilibrium solidification and microstructure of a Pb-15% Sn alloy. A nonequilibrium eutectic microconstituent can form if the solidification is too rapid.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Figure 11.25
The Au–Si phase diagram.
(b) A schematic diagram of Au nanoparticles on a substrate. The drawing is not to scale.
(c) A scanning electron micrograph of silicon nanowires.
(d) Gold catalysts atop silicon nanowires are visible in this high magnification image.
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Key Terms
Dispersion-strengthened
Matrix
Dispersed phase or the precipitate
Microconstituent
Eutectic
Eutectoid
Precipitation or age hardening
Intermetallic compound
Stoichiometric intermetallic compounds
Nonstoichiometric intermetallic compounds or intermediate solid solutions
Peritectic
Monotectic
Miscibility gap
Metastable miscibility gap
Peritectoid
Isopleth
Isoplethal study
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Chapter 11: Dispersion Strengthening and Eutectic Phase Diagrams
Key Terms
Lamellar
Eutectic microconstituent
Hypoeutectic alloy
Hypereutectic alloy
Primary or proeutectic microconstituent
Interlamellar spacing
Modification
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