Bulk Nanocrystalline Materials www msm cam ac ukphasetrans

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

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.

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

Cementite suppressed using silicon 1 µm

Temperature / K Fe-2 Si-3 Mn-C wt% 800 BS 600 400 MS 200 0 0 0. 2 0. 4 0. 6 0. 8 1 Carbon / wt% 1. 2 1. 4

Fe-2 Si-3 Mn-C wt% 1. E+08 1 year Time / s 1 month 1. E+04 1. E+00 0 0. 5 Carbon / wt% 1 1. 5

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

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

700 Temperature/ o. C 600 500 400 BS ~ 350 o. C 300 200 MS = 120 o. C 100 0 1. E+02 1. E+04 Time / s 1. E+06 1. E+08

Percentage of phase 100 80 retained austenite X-ray diffraction results 60 40 bainitic ferrite 20 0 200 250 300 Temperature/o. C 325

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

Above Below 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

200 o. C 250 o. C 300 o. C

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

“more serious battlefield threats”

ballistic mass efficiency consider unit area of armour

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

Fe-2 Si-3 Mn-C wt% 1. E+08 1 year Time / s 1 month 1. E+04 1. E+00 0 0. 5 Carbon / wt% 1 1. 5

Fe-1. 75 C-Si-Mn wt% Chatterjee & Bhadeshia, 2004

Thank you