CMSE2015

The First Bulk–Nanostructured Metal
Prof. Harry Bhadeshia
Department of Materials Science & Metallurgy University of Cambridge, UK
This is a story about a most elegant structure created in steel, consisting of incredibly fine and slender crystals of ferrite permeating a matrix of austenite. The crystals are typically 20-40 nm in thickness and in the form of plates. There are so many of these crystals per unit of volume, that a material is created which has one of the highest density of interfaces known to man. And all this can be achieved in samples which are large in all three dimensions, without the use of deformation or rapid heat treatment, and at a cost which in terms of weight or volume compares with that of bottled-water. There is no new manufacturing technology required, the fabrication of the steel is conventional. But the heat-treatment is far from conventional, involving periods of up to ten days at temperatures in the vicinity of 200◦C. The end result is a hardness in excess of 700 HV, strength of the order of 2.5 GPa, uniform ductility in the range 7-27%, and toughness in the range 30-50MPam1/2. The choreography of atoms during the transformation of austenite into the crystals of bainitic- ferrite has a major role in determining the structure. I will describe how the material was discovered and the underlying phase transformations theory. Hundreds of tonnes of the material has been produced and utilised in a variety of specialised en- gineering applications such as shafts and armour. The new science associated with this material, including a remarkable new Fe-C phase diagram, will be described.
	Fig. 1: Huge samples of the steel described here undergoing heat-treatment (in this case austenitisation). For more details, see Nanostructured bainite, Proceedings of the Royal Society of London - A, Vol. 466, 2010, 3–18, and The first bulk nanostruc- tured metal, Science and Technology of Advanced Materials 14 (2013) 014202. The first bulk nanostructured metal These and other articles can be downloaded from
	www.msm.cam.ac.uk/phase-trans/2010/nano.html