2. Materials, transformation temperatures & strength

3. Olson and Hartman, 1982

4. Olson and Hartman, 1982

5. Olson and Hartman, 1982

6. Diffusionless transformation?
Martensite can form at very low temperatures.
Martensite can grow very rapidly.
No composition change during transformation.

9. Irrational: why?

11. Orientation relationships: irrational

12. athermal transformation

13. Glissile interface

15. Glissile interface cannot contain more than one set of dislocations.
Martensitic transformation only possible if the deformation which changes the parent into the product leaves one line undistorted and unrotated, i.e. an invariant-line.
Deformation is an invariant-line strain.

18. 50 mm

19. general invariant-plane straind s 1
uniaxial dilatation
simple shear
general invariant-plane strain
s=0.26
d=0.03

21. s d c r 1
Christian, 1957

23. body-centred cubic
cubic close-packed

24. b a a b b a 3 3 2 1 2 1 (a) (b) BAIN STRAIN (c) (d) Body-centered
tetragonal
cubic martensite
austenite

26. [001] b b' o
[100] a' a b
(a) o b'
(b)
a,a'

28. RB P P 1 2 (c) x w z y Martensite (wrong shape) Austenite (a) w x y z
Observed
shape,
wrong
structure P
(b) w x z y 1
Twinned
Martensite
Twin
Boundary
Correct macroscopic shape, correct structure x w z y z
Slipped
Martensite
LATTICE
-INVARIANT
DEFORMATION x w y

30. transformation twins (Wayman)

31. hexagonal close-packed
cubic close-packed

32. P 1 Austenite (a) w x y z Observed shape, correct structure (b) w x z

33. Brooks, Loretto and Smallman, 1979

35. a
+ strain a
Free Energy g a ag ga x x x
Carbon Concentration