1. CHAPTER 11: PHASE TRANSFORMATIONS
ISSUES TO ADDRESS...
• Transforming one phase into another takes time.
• How does the rate of transformation depend on
time and T?
• How can we slow down the transformation so that
we can engineering non-equilibrium structures?
• Are the mechanical properties of non-equilibrium
structures better? 1

2. FRACTION OF TRANSFORMATION
• Fraction transformed depends on time.
• Transformation rate depends on T.
• r often small: equil not possible! 2

3. TRANSFORMATIONS & UNDERCOOLING3 3

4. EUTECTOID TRANSFORMATION RATE ~ DT
• Growth of pearlite from austenite:
• Reaction rate
increases with
DT. 4

5. NUCLEATION AND GROWTH
• Reaction rate is a result of nucleation and growth
of crystals.
Adapted from
Fig. 10.1, Callister 6e.
• Examples: 5

6. ISOTHERMAL TRANSFORMATION DIAGRAMS
• Fe-C system, Co = 0.77wt%C
• Transformation at T = 675C. 6

7. EX: COOLING HISTORY Fe-C SYSTEM
• Eutectoid composition, Co = 0.77wt%C
• Begin at T > 727C
• Rapidly cool to 625C and hold isothermally. 7

8. PEARLITE MORPHOLOGY Two cases: • Ttransf just below TE
--Larger T: diffusion is faster
--Pearlite is coarser.
• Ttransf well below TE
--Smaller T: diffusion is slower
--Pearlite is finer. 8

9. NON-EQUIL TRANSFORMATION PRODUCTS: Fe-C
• Bainite:
--a lathes (strips) with long
rods of Fe3C
--diffusion controlled.
• Isothermal Transf. Diagram 9

10. OTHER PRODUCTS: Fe-C SYSTEM (1)
• Spheroidite:
--a crystals with spherical Fe3C
--diffusion dependent.
--heat bainite or pearlite for long times
--reduces interfacial area (driving force)
• Isothermal Transf. Diagram 10

11. OTHER PRODUCTS: Fe-C SYSTEM (2)
• Martensite:
--g(FCC) to Martensite (BCT)
• Isothermal Transf. Diagram
• g to M transformation..
-- is rapid!
-- % transf. depends on T only. 11

12. COOLING EX: Fe-C SYSTEM (1)
Adapted from Fig , Callister 6e. 12

13. COOLING EX: Fe-C SYSTEM (2)
Adapted from Fig , Callister 6e. 13

14. COOLING EX: Fe-C SYSTEM (3)
Adapted from Fig , Callister 6e. 14

15. MECHANICAL PROP: Fe-C SYSTEM (1)15

16. MECHANICAL PROP: Fe-C SYSTEM (2)16

17. MECHANICAL PROP: Fe-C SYSTEM (3)
• Fine Pearlite vs Martensite:
• Hardness: fine pearlite << martensite. 17

18. TEMPERING MARTENSITE • reduces brittleness of martensite,
• reduces internal stress caused by quenching. 18

19. SUMMARY: PROCESSING OPTIONS19

20. PRECIPITATION HARDENING
• Particles impede dislocations.
• Ex: Al-Cu system
• Procedure:
--Pt A: solution heat treat
(get a solid solution)
--Pt B: quench to room temp.
--Pt C: reheat to nucleate
small q crystals within
a crystals.
• Other precipitation
systems:
• Cu-Be
• Cu-Sn
• Mg-Al 20

21. PRECIPITATE EFFECT ON TS, %EL
• 2014 Al Alloy:
• TS peaks with
precipitation time.
• Increasing T accelerates
process.
• %EL reaches minimum
with precipitation time. 21

22. STRENGTHENING STRATEGY 3: PRECIPITATION STRENGTHENING
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
• Result: 24