Two Components System

Transcription

1 Two Components System Three independent variables: T, P, compositions In general, constant pressure (fixed parameter). P+F=C+1 Simple Eutectic System The binary eutectic phase diagram explains the chemical behavior of two immiscible (unmixable) crystals form a completely miscible (mixable) melt. P+F=C+1 F=2 F=1 F=1 Eutectic temperature invariant point 1

2 Melting Crystallization Process Lever Rule Connect tie line across two phase region at the temperature of the alloy Locate overall alloy composition on tie line Calculate fraction of each phase using lever rule 2

3 Melting Crystallization Liquid fraction: YZ/XZ Process Solid B fraction: XY/XZ Eutectic Reaction X Y Z D E F G H I Liq. D + B A + B 3

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5 Liquidus Line: Maximum T at which crystals can exist. Saturation solubility curve The effect of soluable impurity on the melting point of pure compounds. 5

6 T m (Sn) = 232 C, T m (Pb) = 327 C but T m (Sn0.62Pb0.38) = 183 C, a common soldering alloy. T m (Au) = 1064 C, T m (Si) = 2550 C but T m (Au0.97Si0.03) = 363 C, so thin layer of gold is used to attach Si chip to a ceramic substrate (shock protection) 6

7 Binary system with Compound Formation AB melts congruently Simple eutectic systems A AB B Binary system with Compound Formation A+L L AB melts incongruently A+AB AB+L A, AB, Liquid Peritectic point A AB B invariant point 7

8 Binary Systems with Immiscible Liquids Immiscible dome: two liquid phase coexist L A+L 2 Liq. c a A +L B +L A +B A B Point a: Liq. a, Liq. c Crystal A P=3 C=2 F=0 Monotectic P+F=C+1 invariant point Peritectic Monotectic eutectic invariant point 8

9 Binary Isomorphous System Mutually soluble in liquid state Melting points W. Callister Chapter9 Mater. Sci. Eng. An introduction Binary system with solid solution -phase is substitutional solid consisting of both Cu and Ni and having an FCC structure F=2 F=2 atoms have similar radii; both pure materials have same crystal structure; similar electronegativity (otherwise may form a compound instead); Mutually soluble in solid State Binary Isomorphous System Mutually soluble in liquid state P+F=C+1 F=2 Phase present Point A - Point B - + L F=2 Mutually soluble in solid State 9

10 A 50%-50% composition begins melting at about 1280 o C; the amount of the liquid phase continuously increases with temperature until about 1320 o C where the alloy is completely liquid. Example: 35 wt% Ni 65 wt% Cu alloy at 1250 o C L Point B + L region Liquid phase (31.5 wt% Ni 68.5 wt % Cu) Solid phase (42.5 wt % Ni 57.5 wt % Cu) 10

11 Example T = C; + L; 35 wt % Ni 65 wt % Cu Compute % and % L C o = 35, C L = 31.5, C = W L = = 0.68 (68 wt %) W = = 0.32 (32 wt %) No microstructural changes until reach liquidus line. b composition dictated by tie line intersection of liquidus and solidus. Note: overall alloy composition remains the same but phase compositions change as cooling occurs. c from b to c -phase increases as dictated by lever rule and composition changes as dictated by tie lines and their intersection with solidus/liquidus. Equilibrium solidification requires very slow cooling! 11

12 Non-equilibrium solidification Compositional changes as defined by boundaries requires readjustment via diffusion over time non-equilibrium. Diffusion is especially low in solids much higher in liquids and decreases with temperature Decrease. Note for dashed line shown no phase change until Pt. b. Pt. c composition dictated by tie-lines as shown; but -phase has not had time to change from 46% Pb to 40% Ni reasonable average is 42%. Note similar situation at pt. d. Result is a constant liquidus line (due to much more freedom of movement and much higher diffusion; shifted solidus line (dashed) due to diffusion. Pt. d - should be complete solidification, but effects of diffusion dominate. Consequences of non-equilibrium solidification: segregation, cored structure; upon reheating grain boundaries will melt first causing loss of structural integrity. 12

13 Fractional Crystallization CaAl2Si2O8 NaAlSi3O8 Crystallization Very slow cooling rate: crystallization to the same composition Slightly faster: Fractional crystallization Very fast: no time for any crystallization, form glass 13