ENGR 151: Materials of Engineering LECTURE #15: PHASE DIAGRAMS

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1 ENGR 151: Materials of Engineering LECTURE #15: PHASE DIAGRAMS

2 TENSILE TESTING VIDEO

3 PROPERTIES OF ISOMORPHOUS ALLOYS Solid solution strengthening For Ni-Cu alloy tensile strength increases with increasing Ni wt% However, ductility decreases

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5 Example: Copper-silver system Three single-phase regions (α,β,l) Alpha(α): copper as solvent, FCC Beta(β): silver as solvent, FCC Pure copper and pure silver are considered to be α and β phases respectively

6 Below BEG line: only a limited amount of metal will dissolve in other metal for α, β phases Solubility limit for α phase corresponds to boundary line CBA Notice maximum amount of silver possible for α phase. Increases to a certain temperature, then decreases to zero at the melting point of pure copper

7 Solubility limit line separating α and α+β phases is solvus line Solubility limit line separating α and α+l phases is solidus line Solubility limit line separating β and α+β phases is solvus line Solubility limit line separating β and β+l phases is solidus line

8 Horizontal line BEG can also be considered a solidus line (lowest temperature at which liquid exists for alloy at equilibrium)

9 Three two-phase regions: α+l, β+l, α+β Tie-lines and lever rule stills apply to these regions

10 COPPER-SILVER PHASE DIAGRAM As silver is added to copper, temperature decreases at which alloy becomes liquid (melting point lowered by addition of silver) Also works the other way around (liquidus lines meet at point E) Invariant point: associated with composition (C E ) and temperature (T E ), 71.9 wt% Ag and 779 C

11 As temperature passes through invariant point (T E ), reaction occurs: cooling L( CE ) ( C E ) ( C E ) heating Liquid is transformed into two solid phases at T E (opposite reaction upon heating) Eutectic reaction (easily melted) C αe, C βe are compositions of α and β phases at T E (tieline) cooling L(71.9 wt% Ag) (8.0 wt% Ag) (91.2 wt% Ag) heating

12 General rules: At most two phases may be in equilibrium within a phase field (no α+β+l, only at equilibrium line) Single phase regions are separated by two-phase regions Horizontal solidus line BEG at T E is called a eutectic isotherm

13 If a binary eutectic solution is cooled through the invariant point, direct solidification occurs No intermediate L phase For binary phase system, no more than two phases may be in equilibrium within a phase field At points along a eutectic isotherm, three phases may be in equilibrium (e.g. point B)

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15 Lead-Tin (Pb-Sn) system: Notice that Sn-Pb melts at 185 C (365 F), attractive for soldering.

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21 These values add up to 1.0

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24 These values add up to 1.0

25 EUTECTIC ALLOY DEVELOPMENT For Lead-Tin alloy at 1 wt% Sn decreasing in temperature from L phase: Remains liquid until crossing of liquidus line at 330 C Continued cooling creates more α Solidification is completed at solidus line

26 EUTECTIC ALLOY DEVELOPMENT For Lead-Tin alloy at 15 wt% Sn decreasing in temperature from L phase: Past the solidus line, small β-phase particles form Continued cooling slightly increases the presence of β

27 EUTECTIC MICROSTRUCTURE For Lead-Tin alloy at 61.9 wt% Sn decreasing in temperature from L phase (invariant point, C E ): No change until T E is reached Liquid transforms into two phases α, β cooling L(61.9 wt% Sn) (18.3 wt% Sn) (97.8 wt% Sn) heating

28 EUTECTIC MICROSTRUCTURE

29 EUTECTIC MICROSTRUCTURE For Lead-Tin alloy at 61.9 wt% Sn: Distribution of α & β phases are accomplished by atomic diffusion (alternating layers of α & β, lamellae) Eutectic Structure Lead atoms diffuse towards α-phase Tin diffuses towards β-phase

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31 EUTECTIC MICROSTRUCTURE For Lead-Tin alloy at 40 wt% Sn decreasing in temperature from L phase: α-phase is present both in a eutectic structure and α+l region α-phase in eutectic structure is called eutectic α α-phase primary to eutectic isotherm is called primary α

32 HOMEWORK HW (Due Monday, April 17 th ) 9.5, 9.6, 9.10, 9.13, 9.14

33 HOMEWORK HW (Due Monday, April 24 th ) 9.21, 9.27, 9.34, 9.37, 9.44

34 EUTECTIC MICROSTRUCTURE Microconstituent: an element of the microstructure having an identifiable and characteristic structure In 40 wt% Sn, there exist two microconstituents in the α+β phase (primary α and eutectic structure)

35 EUTECTIC MICROSTRUCTURE Computing the amounts of eutectic and primary α microconstituents: Use lever rule from solvus line to eutectic composition W e, fraction of eutectic microconstituent is equal to fraction of liquid W L from which it transforms W α, fraction of primary α is equal to fraction of α phase in existence prior to transformation

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37 EUTECTIC MICROSTRUCTURE Eutectic α Primary α Total α (w.r.t entire solution)

38 EUTECTIC MICROSTRUCTURE Total α (w.r.t entire solution) Total β (w.r.t entire solution)