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Intermetallic Compounds

Intermetallic Compounds. Antifluorite Structure : FCC Unit cell with Anions occupying FCC sites Cations occupying 8 octahedral interstitial sites. Mg 2 Pb. Intermetallic compounds form lines - not areas - because stoichiometry (i.e. composition) is exact. Intermetallic Compounds.

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Intermetallic Compounds

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  1. Intermetallic Compounds • Antifluorite Structure: • FCC Unit cell with Anions occupying FCC sites • Cations occupying 8 octahedral interstitial sites Mg2Pb Intermetallic compounds form lines - not areas - because stoichiometry (i.e. composition) is exact.

  2. Intermetallic Compounds Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

  3. cool heat 26.8% Monotectic Monotectic Reaction: L L1 + Solid • Pb and Zn do not mix in solid state: • RT: Cu in Pb < 0.007% • RT: Pb in Cu ~ 0.002 – 0.005% Cu + L1 Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

  4. cool cool heat heat Eutectoid & Peritectic Eutectoid Reaction: 1 solid phase  2 solid phases  + Fe3C (727ºC) intermetallic compound - cementite Peritectic Reaction: liquid + solid 1  solid 2  + L (1493ºC)

  5. Eutectoid and Peritectic Copper-Zinc Binary Equilibrium Phase Diagram:

  6. Peritectic transition  + L Eutectoid transition  +  Eutectoid & Peritectic d + L e Cu-Zn Phase diagram

  7. Congruent vs Incongruent Congruent phase transformation: no compositional change associated with transformation • Examples: • Allotropic phase transformations • Melting points of pure metals • Congruent Melting Point Incongruent phase transformation: at least one phase will experience change in composition • Examples: • Melting in isomorphous alloys • Eutectic reactions • Pertectic Reactions • Eutectoid reactions Ni Ti

  8. Iron-Carbon System Diagram is not ever plotted past 12 wt% Cementite Hägg carbide Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

  9. Iron Carbon Phase Diagram d ferrite, BCC Formation of Ledeburite FCC A3 ACM A1 (Eutectoid Temperature) Formation of Pearlite a ferrite BCC Steel Cast Irons Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

  10. Cementite – What is it? Iron Carbide – Ceramic Compound • Cementite has an orthorhombic lattice with approximate parameters 0.45165, 0.50837 and 0.67297 nm. • There are twelve iron atoms and four carbon atoms per unit cell, corresponding to the formula Fe3C. Purple: Carbon atoms Orange: Iron atoms Source: http://www.msm.cam.ac.uk/phase-trans/2003/Lattices/cementite.html H. K. D. H. Bhadeshia

  11. Pearlite: What is it? • The eutectoid transformation: g (0.77% C) a (0.02%C) + Fe3C (6.67%C) • Alternate lamellae of ferrite and cementite w/ ferrite as the continuous phase • Diffusional Transformation • “Pearlite” name is related to the regular array of the lamellae in colonies. Etching attacks the ferrite phase more than the cementite. The raised and regularly spaced cementite lamellae act as diffraction gratings and a pearl-like luster is produced by the diffraction of light of various wavelengths from different colonies [1]

  12. Pearlite • Two phases appear in definite ratio by the lever rule: • Since the densities are same (7.86 and 7.4) lamellae widths are 7:1 • Heterogeneous nucleation and growth of pearlite colonies – but typically grows into only 1 grain Reed-Hill, Abbaschian, 1994, [5]

  13. Lamellae Nucleation Reed-Hill, Abbaschian, 1994 Reed-Hill, Abbaschian, 1994

  14. Interlamellar Spacing • Interlamellar spacing l is almost constant in pearlite formed from g at a fixed T • Temperature has a strong effect on spacing – lower T promotes smaller l • Pearlite formed at 700oC has l ~ 1 mm and Rockwell C - 15 • Pearlite formed at 600oC has l ~ 0.1 mm and Rockwell C - 40 • Zener and Hillert Eq. for spacing [1]: sa/Fe3C = Interfacial energy per unit area of a/Fe3C boundary TE = The equilibrium temperature (Ae1) DHV = The change in enthalpy per unit volume b/t g and a/Fe3C DT = The undercooling below Ae1

  15. Effect of Undercooling on l Krauss, Steels, 1995

  16. Effect of Interlamellar Spacing Stone et al, 1975

  17. T(°C) 1600 d -Eutectic (A) [4.32 %C]: L 1400 Þ g + L Fe3C g +L g A 1200 L+Fe3C 1148°C -Eutectoid (B) [0.77 %C]: (austenite) R S g Þ a + Fe3C g g 1000 g +Fe3C g g a Fe3C (cementite) + 800 B g a 727°C = T eutectoid R S 600 a +Fe3C 400 0 1 2 3 4 5 6 6.7 4.30 0.76 Co, wt% C (Fe) Fe3C (cementite-hard) eutectoid a (ferrite-soft) C Iron-Carbon (Fe-C) Phase Diagram 3 invariant points: C -Peritectic (C) [0.17%C]: L + dÞ g

  18. T(°C) 1600 d L 1400 g +L g g g 1200 L+Fe3C 1148°C (austenite) g g g 1000 g g +Fe3C g g Fe3C (cementite) r s 800 a g g 727°C a a a g g R S 600 a +Fe3C w = s /( r + s ) a w = (1- w ) g a 400 0 1 2 3 4 5 6 6.7 a Co, wt% C (Fe) C0 0.76 pearlite w = w g pearlite 100 mm w = S /( R + S ) a w = (1- w ) a Fe3C pearlite proeutectoid ferrite Hypoeutectoid Steel

  19. Proeuctectoid Ferrite – Pearlite 0.38 wt% C: Plain Carbon – Medium Carbon Steel

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