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Microstructures in Eutectic Systems: I

Microstructures in Eutectic Systems: I. T (°C). L: C o wt% Sn. 400. L. a. L. 300. L. a. +. a. 200. (Pb-Sn. a : C o wt% Sn. T E. System). 100. b. +. a. 0. 10. 20. 30. C o. ,. wt% Sn. C o. 2. (room T solubility limit). • C o < 2 wt% Sn • Result:

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Microstructures in Eutectic Systems: I

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  1. Microstructures in Eutectic Systems: I T(°C) L: Cowt% Sn 400 L a L 300 L a + a 200 (Pb-Sn a: Cowt% Sn TE System) 100 b + a 0 10 20 30 Co , wt% Sn Co 2 (room T solubility limit) • Co < 2 wt% Sn • Result: --at extreme ends --polycrystal of a grains i.e., only one solid phase.

  2. Microstructures in Eutectic Systems: II L: Co wt% Sn T(°C) 400 L L 300 a L + a a: Cowt% Sn a 200 TE a b 100 b + a Pb-Sn system 0 10 20 30 Co , wt% Sn Co 2 (sol. limit at T ) 18.3 room (sol. limit at TE) • • 2 wt% Sn < Co < 18.3 wt% Sn • • Result: • Initially liquid +  • then  alone • finally two phases • a polycrystal • fine -phase inclusions

  3. Microstructures in Eutectic Systems: III Micrograph of Pb-Sn T(°C) eutectic L: Co wt% Sn microstructure 300 L Pb-Sn system a L + a b L 200 183°C TE 100 160m a : 97.8 wt% Sn Adapted from Fig. 9.14, Callister 7e. : 18.3 wt%Sn 0 20 40 60 80 100 97.8 18.3 CE C, wt% Sn 61.9 • Co = CE • Result: Eutectic microstructure (lamellar structure) --alternating layers (lamellae) of a and b crystals.

  4. Microstructures in Eutectic Systems: IV L T(°C) L: Co wt% Sn a L a 300 L Pb-Sn system a L + a b b L + R S 200 TE S R a b 100 + a primary a eutectic b eutectic 0 20 40 60 80 100 18.3 61.9 97.8 Co, wt% Sn • 18.3 wt% Sn < Co < 61.9 wt% Sn • Result:a crystals and a eutectic microstructure

  5. Hypoeutectic & Hypereutectic hypoeutectic: Co = 50 wt% Sn hypereutectic: (illustration only) a b a b a a b b a b a b 175 mm 300 L T(°C) a L + a b b L + (Pb-Sn 200 TE System) a + b 100 Co, wt% Sn 0 20 40 60 80 100 eutectic 61.9 eutectic: Co=61.9wt% Sn 160 mm eutectic micro-constituent

  6. Intermetallic Compounds Mg2Pb Note: intermetallic compound forms a line - not an area - because stoichiometry (i.e. composition) is exact.

  7. Cu-Zn Phase diagram

  8. T(°C) 1600 d L 1400 g +L g A 1200 L+Fe3C 1148°C (austenite) R S 1000 g +Fe3C a Fe3C (cementite) + 800 B g a 727°C = T eutectoid 600 a +Fe3C 400 0 1 2 3 4 5 6 6.7 4.30 0.76 Co, wt% C (Fe) eutectoid C Iron-Carbon Phase Diagram Extract

  9. Result: Pearlite = alternating layers of a (ferrite-soft) a andFe3C phases 120 mm Pearlite Fe3C (cementite-hard)

  10. T(°C) 1600 d L 1400 (Fe-C g +L g g g System) 1200 L+Fe3C 1148°C (austenite) g g 1000 g +Fe3C Fe3C (cementite) 800 727°C a 600 a +Fe3C 400 0 1 2 3 4 5 6 6.7 Co, wt% C (Fe) C0 0.76 Hypoeutectoid Steel

  11. a pearlite w = w g pearlite w = S /( R + S ) a w = (1- w ) a Fe3C Hypoeutectoid Steel 100 mm pearlite Proeutectoidferrite

  12. T(°C) 1600 d L 1400 (Fe-C g +L g g g System) 1200 L+Fe3C 1148°C g g (austenite) 1000 g +Fe3C Fe3C (cementite) 800 a 600 a +Fe3C 400 0 1 2 3 4 5 6 6.7 Co, wt%C (Fe) Hypereutectoid Steel Co 0.76

  13. pearlite w = w g pearlite w = S /( R + S ) a w = (1- w ) a Fe3C Hypereutectoid Steel 60mm proeutectoid Fe3C pearlite

  14. • Teutectoid changes: • Ceutectoid changes: Ti Si Mo Ni (°C) W (wt%C) Cr Cr Si Eutectoid Mn W eutectoid Mn Ti Mo T Ni C wt. % of alloying elements wt. % of alloying elements Alloying Steel with More Elements

  15. Taxonomy of Metals Steels Cast Irons <1.4wt%C 3-4.5wt%C microstructure: T(°C) ferrite, graphite 1600 d cementite L 1400 g +L g L+Fe3C 1200 1148°C austenite Eutectic: 4.30 1000 g a +Fe3C + Fe C 800 a g 3 727°C ferrite Eutectoid: cementite a +Fe3C 600 0.76 400 0 1 2 3 4 5 6 6.7 (Fe) Co, wt% C Metal Alloys Ferrous Nonferrous Cu Al Mg Ti

  16. Steels

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