ch10.2-S06 - Chapter 10: Phase Diagrams Containing Eutectic...

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MY2100 – Ch 10, Part 2, Slide 1 Chapter 10: Phase Diagrams Containing Eutectic Reactions structure processing properties Performance ISSUES TO ADDRESS. .. What do phase diagrams look like when 2 or more solid phases exist in the system? What is a microconstituent and why is it important?
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MY2100 – Ch 10, Part 2, Slide 2 More complex phase diagrams When you look in the book, you will find that most phase diagrams are much more complicated than the isomorphous Cu/ Ni system because they contain several solid phases (solids with different crystal structures). Example: Cu and Fe have different crystal structures, so there cannot be a region of complete solid solubility. Example: Rocks on the beach (e.g., at McLain state park) often have mixtures of quartz and other minerals; these are different phases . Almost all structural metals, as well as many ceramics and polymers, contain more than one solid phase, just like rocks do, but the phase sizes are at the micrometer scale and cannot be seen without special preparation of a sample and a microscope.
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MY2100 – Ch 10, Part 2, Slide 3 Eutectic Systems The simplest kind of system with two solid phases is called a eutectic system. A eutectic system contains two solid phases at low temperature. These phases may have different crystal structures, or the same crystal structure with different lattice parameters. Examples: Pb (FCC) and Sn (tetragonal) - solder systems Fe (BCC) and C (graphite - hexagonal) - cast irons Al (FCC) and Si (diamond cubic) - cast aluminum alloys Cu (FCC) and Ag (FCC) - brazing systems
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MY2100 – Ch 10, Part 2, Slide 4 Cu/Ag system Copper and Silver are both FCC, but their lattice parameters and atomic radii are very different, so there is almost zero solid solubility at low temperature. This means that when we add one to the other, the atoms are not dissolved, but they form a second phase. The phase diagram looks like this: at low temperatures, two FCC solid phases exist ( α and β in the diagram), and for some alloy compositions these phases are in equilibrium with each other. At high temperatures, there is a new type of equilibrium called a eutectic in which three phases (two solids and a liquid) are simultaneously in equilibrium.
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MY2100 – Ch 10, Part 2, Slide 5 Cu/Ag system α phase, mostly Cu β phase, mostly Ag Eutectic invariant C e Eutectic temperature
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MY2100 – Ch 10, Part 2, Slide 6 Eutectic reaction At the eutectic invariant, there are three phases in equilibrium (L, α , β ). The reaction which occurs on cooling (solidification) is called the eutectic reaction.
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ch10.2-S06 - Chapter 10: Phase Diagrams Containing Eutectic...

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