chem584.phasediagrams - Phase Diagrams Multiple variables...

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1 Phase Diagrams Multiple variables can be plotted: Chemists are used to Pressure vs. Temperature. Metallurgists: Temperature vs. Binary Composition, Nobody: Pressure vs. Binary Composition Phase Diagrams (don’t panic) Cu-Zn (Brass/Bronze) Al-Cu
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2 ISSUES TO ADDRESS. .. • When we combine two elements. .. what equilibrium state do we get? • In particular, if we specify. .. a composition (e.g., wt% Cu - wt% Ni), and a temperature ( T ) then. .. How many phases do we get? What is the composition of each phase? How much of each phase do we get? Phase Diagrams Phase B Phase A Nickel atom Copper atom nice tutorial: http://www.soton.ac.uk/~pasr1/ Phase Diagrams For a phase diagram with temperature on the vertical axis, a solidus is a line below which the substance is stable in the solid state. A liquidus is a line above which the substance is stable in a liquid state. There may be a gap between the solidus and liquidus; within the gap, the substance is in equilibrium only with solid and liquid both present. The triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance can coexist .
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3 Solid Solutions A solid solution is a solid-state solution of one or more solutes in a solvent. Solid Solution and not a compound IFF crystal structure of remains unchanged by addition of the solutes and the mixture remains in a single homogeneous phase. Solute incorporation either substitutionally or interstitially Solid solutions may form if the solute and solvent have: Similar atomic radii (15% or less difference) Same crystal structure Similar electronegativities Similar coordination number (a.k.a. valency) Eutectics eutectic: the composition of a mixture that has the lowest melting point where the phases simultaneously crystallize from molten solution at this temperature. From the Greek 'eutektos', meaning ‘easily melted’. When a non-eutectic alloy freezes, one component of the alloy crystallizes at one T and the other at a different T. With a eutectic alloy, the mixture freezes as one at a single T. A eutectic alloy therefore has a sharp melting point, and a non-eutectic alloy exhibits a plastic melting range.
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4 Uses of Eutectic Phases soldering: Sn and Pb and sometimes Ag or Au. casting alloys: Al-Si, cast iron (austenite-cementite Fe-C) brazing: diffusion removes alloying elements from the joint. temp. response: Wood's (Bi-Pb-Sn-Cd, mp 70 C) & Field's (Bi-In-Sn, mp 62 C) metals for fire sprinklers, prototype casting, repairs non-toxic mercury replacements: galinstan (Ga-In-Sn) NaK alloys: liquid at room temperature, used as coolant in fast neutron nuclear reactors. Phase Equilibria: Solubility Limit Introduction – Solutions – solid solutions, single phase –M ix tu res – more than one phase Solubility Limit : Max concentration for which only a single phase solution occurs.
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This note was uploaded on 12/04/2011 for the course CHEM 584 taught by Professor Suslick,k during the Fall '08 term at University of Illinois, Urbana Champaign.

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chem584.phasediagrams - Phase Diagrams Multiple variables...

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