University:
McMaster University
Course:
MATLS 2H04 | Introduction to Materials Engineering
Academic year:

2023
Views:

323

Pages:

6
Author:

Sabrina S.

Phase Diagrams 8.3 Phase Diagrams. Eutectic eutectic regions on phase diagrams have low melling temperatures & they form unique microstructures a eutectic transformation is when, on cooling, a liquid transforms directly into 2 solids L(C) -> α(C) + β(C) eutectic alloy needs a particular composition & temp. o in this alloy the eutectic melling temp is much lower than the individual melting temps o for Sn & Pb, α phase contains 18.3 wt %. Sn β phase contains 97.8 wt %. Sn 619 °C 581 °C Lever rule at eutectic temp wt % α = (619-18.3) / (97.8-18.3) * 100 = 54.9 wt % wt % β = (97.8-61.9) / (97.8-18.3) * 100 = 45.1 wt % Development of Microstructure when the eutectic develops, two solid phases form this leaves a lamellae-like structure w layers of alternating composition cureche will grow w an even growth front into the liquid phase 三 growth of layers within this solid is diffusion controlled Pb diffuses to the α phase which contains a high concentration of Sn Sn diffuses to the β phase the short diffussion lengths make the final lamellar structure eutectic point Diagram: A phase diagram is present, showing temperature on the y-axis and composition (wt% Sn) on the x-axis. There are two solid phases (α and β) and a liquid phase (L). A eutectic point is indicated. Applications of Eutectics Casting lower melting points, easy casting/high fluidity for filling complex shaped moulds develops useful microstructures: "micro-composite structures" of phases w diff properties Soldering fluidity of metal solder flows into gap between components on solidification, it joins components eutectic allows for better fluidity to lower temperatures 8.4 Phase Diagrams: Hypo- and Hyper-Eutectic Alloys hypo-eutectic alloy: composition below the eutectic composition hyper-eutectic alloy: composition above the eutectic composition there is little change in microstructures at low temps due to very slow diffusion rates Hypo-Eutectic eg. 50% Sn & 50% Pb α phase is in 2 forms: primary α: forms between liquidus and eutectic temp (600-327°C) eutectic α: forms at eutectic temp (327°C) as eutectic composition treat with lever rule wt% α (primary) = 27.3 wt% wt% liquid (eutectic) = 72.7 wt% Hyper-Eutectic β phase is in 2 forms: primary β: forms between liquidus and eutectic temp (310-327°C) eutectic β: forms at eutectic temp (327°C) Diagrams: Two phase diagrams are present, showing temperature on the y-axis and composition (wt% Sn) on the x-axis. Both diagrams illustrate the formation of primary and eutectic phases in hypo-eutectic and hyper-eutectic alloys.