Phase Diagrams. Today s Topics

Transcription

1 MME 291: Lecture 05 Eutectic Type Phase Diagrams Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics Binary eutectic systems with partial miscibility Nomenclatures and invariant reaction Determining composition and relative amounts Development of microstructures Binary eutectic systems with complete immiscibility Reference: 1. WD Callister, Jr. Materials Science and Engineering: An Introduction, 5 th Ed., Ch SH Avner. Introduction to physical metallurgy, 2 nd Ed., Ch. 6. Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 02 1

2 with Partial Solubility Lead - Tin Binary Phase Diagram with Partial Solubility The Eutectic Point E An invariant point where two liquidus lines meet, and three phase are in equilibrium. And through which the horizontal isotherm line BEG passes through. Point E is designated by the composition, C E and the temperature, T E of the alloy. Lead - Tin Binary Phase Diagram Melting point of eutectic alloy is lower than that of the components (eutectic = easy to melt ) 2

3 with Partial Solubility The Eutectic Reaction Eutectic reaction is the transition between liquid and mixture of two solid phases, α + β, at eutectic concentration C E. cooling L (C E ) α (C α E) + β (C βe ) heating eutectic mixture For Pb-Sn system, the eutectic reaction may be written as: cooling L 61.9 Sn α 19.0 Sn + β heating 97.5 Sn Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 05 with Partial Solubility Which alloy to choose for : electrician s solder plumber s solder Pb Hypoeutectic Alloys EUTECTIC ALLOY Hypereutectic Lead Tin Binary Phase Diagram Sn Eutectic alloy Short-freezing-range alloy No mushy zone Hypo/hypereutectic alloy Long-freezing-range alloys Wide mushy zone 3

4 In the case of lead-rich alloy (0-2 wt. % of tin) solidification proceeds in the same manner as for isomorphous alloys (e.g. Cu-Ni) that we discussed earlier. L L + α α Alloys with no Eutectic and forming Unsaturated Solid Solutions At compositions bt between the minimum ii and the maximum solid solubility limit, β phase nucleates as the α solid solubility is exceeded upon crossing the solvus line. L L + α α α + β Precipitates in Al-Si alloy Alloys with no Eutectic and forming Supersaturated Solid Solutions 4

5 alternate layers of α and β crystals The Eutectic Alloy How does eutectic microstructure form? Compositions of α and β phases are very different. The eutectic reaction involves redistribution of Pb and Sn atoms by atomic diffusion. This simultaneous formation of α and β phases result in a layered (lamellar) microstructure that is called the eutectic structure. Micrograph of Pb-Sn eutectic, containing alternate layers of Pb-rich α phase (dark layers), and Sn-rich β phase (light layers) Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 10 5

6 Eutectic Pro eutectic also known as primary α or proeutectic α The Hypoeutectic Alloy Microstructural Development in Problem For the Pb-Sn alloy system, (a) What is the compositions and relative amounts of the phases that constitute the eutectic microstructure? (b) For an alloy containing 30 % Sn, (i) What are the relative amounts of α and β phases present at the eutectic temperature? (ii) What are the relative amounts of the eutectic and the proeutectic α phase present at the same temperature? Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 12 6

7 Solutions [a] Eutectic structure consists of α and β phases C α = 18.3 wt% Sn C β = 97.8 wt% Sn W α = 100R/(P+Q+R) = 45.2 % W β = 100(P+Q)/(P+Q+R) = 54.8 % [b] (i) W α = 100(Q+R)/(P+Q+R) = 85.3 % W β = 100P/(P+Q+R) = 14.7 % [b] (ii) W α = 100(Q)/(P+Q) = 73.2 % W E = 100P/(P+Q) = 26.8 % The Cooling Curves in L L Temperature α L + α Temperature L L + (α+β) (α + β) Temperature L + α (α+β) + α α + β Time Time Time alloys with no eutectic eutectic alloy hypo/hyper eutectic alloy Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 14 7

8 Changes in Mechanical Properties in Effect of the composition and strengthening mechanism on the tensile strength of lead-tin alloys Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 15 Temperature, C with Complete Insolubility 1600 The Eutectic Reaction L cooling 1200 L 12 Si (Al + Si) heating Al + L L + Si C Al + Si Al Wt.% Si 100 % Eute ectic Pha se Aluminium Silicon Phase Diagram Primary Al Primary Si Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 16 Si 0 Eutectic Mixture 12 Wt.% Si 8

9 with Complete Insolubility Primary Aluminium (white areas) Al-Si eutectic mixture % Al Al - 2.8%Si Al - 12%Si Primary Silicon (black particles) Al - 20%Si Al - 50%Si Pure Si Microstructures of Al-Si Alloys Rashid, DMME, BUET MME 291, Lec 05: Interpretation of the phase diagrams P 17 Next Class MME 291: Lecture 06 Other Complex Phase Diagrams 9