1. Nanocrystalline Materials
Bulk
Nanocrystalline Materials
Francisca Caballero
Carlos Garcia Mateo
Mohamed Sherif

2. Problem: to design a bulk nanocrystalline steel which is very strong, tough, cheap ….

3. Brenner, 1956

4. Scifer, 5.5 GPa and ductile
Kobe Steel

6. 50-10 Denier Scifer is 9 Denier
1 Denier: weight in grams, of 9 km of fibre
50-10 Denier
Scifer is 9 Denier

7. Morinobu Endo, 2004

8. Claimed strength of carbon nanotube is 130 GPa
Edwards, Acta Astronautica, 2000
Claimed modulus is 1.2 TPa
Terrones et al., Phil. Trans. Roy. Soc., 2004

9. Equilibrium number of defects (1020)
Strength of a nanotube rope 2 mm long is less than 2000 MPa

11. Summary
Strength produced by deformation limits shape: wires, sheets...
Strength in small particles relies on perfection. Doomed as size increases.

12. Smallest size possible in polycrystalline substance?

14. Yokota & Bhadeshia, 2004

15. Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893.
Howe, Materials Science and Technology 16 (2000) 1264.

16. Interstitial-free steel
Courtesy of Tsuji, Ito, Saito, Minamino, Scripta Mater. 47 (2002) 893.

19. Introduce work-hardening capacity Need to store the heat Reduce rate
Nanocrystalline steel by transformation
Introduce work-hardening capacity
Need to store the heat
Reduce rate
Transform at low temperature

21. Cementite suppressed using silicon

22. Fe-2Si-3Mn-C wt% 800 B 600 Temperature / K 400 M 200 0.2 0.4 0.6 0.8 1
0.2
0.4
0.6
0.8 1
1.2
1.4
Carbon / wt%

23. Fe-2Si-3Mn-C wt% 1.E+08 1 month Time / s 1.E+04 1.E+00 0.5 1 1.5
1 year
1 month
Time / s
1.E+04
1.E+00
0.5 1
1.5
Carbon / wt%

24. wt% Low transformation temperature Bainitic hardenability
Reasonable transformation time
Elimination of cementite
Austenite grain size control
Avoidance of temper embrittlement
wt%

25. Homogenisation Austenitisation Isothermal transformation Temperature
1200 o C
2 days
1000 o C
15 min
Temperature
Air
125 o C -
325 o C
slow
cooling
hours -
months
cooling
Quench
Time

26. Temperature/ oC Time / s 700 600 500 400 300 200 100 1.E+00 1.E+02B
~ 350 o C
Temperature/ oC S
300
200 M
= 120 o C S
100
1.E+00
1.E+02
1.E+04
1.E+06
1.E+08
Time / s

27. retained austenite bainitic ferrite Percentage of phase Temperature/ C
100
retained austenite 80
X-ray diffraction results 60
Percentage of phase 40
bainitic ferrite 20
200
250
300
325
Temperature/ o C

29. g g a a a
Caballero, Mateo, Bhadeshia
200 Å

30. Low temperature transformation: 0.25 T/Tm
Fine microstructure: nm thick plates
Harder than most martensites (710 HV)
Carbide-free
Designed using theory alone

31. Caballero, Mateo, Bhadeshia

32. Caballero, Mateo, Bhadeshia

36. Sherif, 2005, Ph.D. thesis, Cambridge

37. Sherif, 2005, Ph.D. thesis, Cambridge

38. Sherif, 2005, Ph.D. thesis, Cambridge

39. Below percolation threshold
Above percolation threshold

40. Geometrical percolation threshold of overlapping ellipsoids

42. Faster Transformation
Cobalt (1.5 wt%) and aluminium (1 wt%) increase the stability of ferrite relative to austenite
Refine austenite grain size

43. 200oC
250oC
300oC

44. Stress / GPa
Velocity km s-1
Hammond and Cross, 2004

45. “more serious battlefield threats”

46. ballistic mass efficiency
consider unit area of armour

47. g g a a a Very strong Huge uniform ductility No deformation
No rapid cooling
No residual stresses a
Cheap
Uniform in very large sections a
200 Å

48. Fe-2Si-3Mn-C wt% 1.E+08 1 month Time / s 1.E+04 1.E+00 0.5 1 1.5
1 year
1 month
Time / s
1.E+04
1.E+00
0.5 1
1.5
Carbon / wt%

49. Fe-1.75C-Si-Mn wt%
Chatterjee & Bhadeshia, 2004

50. Thank you