Nanocrystalline Materials Bulk Nanocrystalline Materials www.msm.cam.ac.uk/phase-trans Francisca Caballero Carlos Garcia Mateo Mohamed Sherif

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

Brenner, 1956

Scifer, 5.5 GPa and ductile Kobe Steel

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

Morinobu Endo, 2004

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

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

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

Smallest size possible in polycrystalline substance?

Yokota & Bhadeshia, 2004

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

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

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

Cementite suppressed using silicon

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%

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%

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

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

Temperature/ oC Time / s 700 600 500 400 300 200 100 1.E+00 1.E+02 B ~ 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

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

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

Low temperature transformation: 0.25 T/Tm Fine microstructure: 20-40 nm thick plates Harder than most martensites (710 HV) Carbide-free Designed using theory alone

Caballero, Mateo, Bhadeshia

Caballero, Mateo, Bhadeshia

Sherif, 2005, Ph.D. thesis, Cambridge

Sherif, 2005, Ph.D. thesis, Cambridge

Sherif, 2005, Ph.D. thesis, Cambridge

Below percolation threshold Above percolation threshold

Geometrical percolation threshold of overlapping ellipsoids

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

200oC 250oC 300oC

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

“more serious battlefield threats”

ballistic mass efficiency consider unit area of armour

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

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%

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

Thank you