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Unformatted text preview: VK250 Lecture 06 S TRUCTURES OF SOLUTIONS AND COMPOUNDS Few elements used in pure state (usually alloyed) E.g. Steel (Fe & C), 2xxx Al (Al +4%Cu+Mg, Si, Mn) Titanium (Ti-6%Al-4%V) Brass (Cu = 20% Zn) Bronze (Cu + 10-30%Sn) Solid solutions Alloy element (or solute) dissolves in solvent element Wide range of solubilities (<0.01% to 100%) Fe can only dissolve 0.007 wt% C Sn can dissolve only 0.3 wt% Pb at room temperature 2.5wt% at 183 C Pb can dissolve 2wt% Sn at room temperature 19wt% at 183 C Cu and Ni are completely soluble in each other Two ways for atoms to dissolve: Interstitial solid solutions (if solute has small atoms such as C, B, N) E.g. , C in Fe Substitutional solid solutions E.g., Cu Sn If there is an excess of alloying element - a precipitate is formed E.g. , sugar in water, or moisture in air) The precipitates may be a saturated solid solution of host in alloy element E.g. Dissolve more than 2% Sn in Pb at room temperature Excess precipitates out as Sn with 0.3% Pb dissolved in it Precipitate may be a chemical compound E.g. Fe 3 C in steels; CuAl 2 in Al/Cu systems Region of material with uniform physical and chemical properties is called a phase E.g., Ice, water, vapor are 3 different phases of H 2 O 1 VK250 Lecture 06 At 0 C ice/water co-exist as a 2-phase mixture Fe 3 C is a phase that is different from Fe(0.007wt%C) Fe(0.007wt%C) is the same single phase as Fe (0.003wt%C) Analogy: 20% sugar water cannot exist in equilibrium with 10% sugar water or with pure water all three are considered to be the same phase. When sugar precipitates from solution, a two-phase solution is formed: (i) water (fully saturated with dissolved sugar) + (ii) solid sugar (fully saturated with water) In solids there are different types of interphase boundaries Coherent: Lattices continuous across boundary Coherent with strain: Lattices continuous across boundary but with induced strains Semi-coherent: Incoherent : Lattices continuous across boundary but with dislocations at boundary No relationship between lattices of the two phases Each of these is important in its own way for strengthening materials....
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- Fall '08