In a paper published in the Science and Technology of Advanced Materials, Bhadeshia introduces the world’s first bulk nanostructured metal in commercial production. The nanostructure-controlled high-strength bainitic steel, where the thickness of bainitic ferrite platelets is controlled between 20 and 50 nm is shown in the figures below.
The review paper explains why nanostructure plays an important role in strengthening materials, and the conditions required to design and develop such “nanostructured” materials. In particular, the biggest challenge is to keep the production cost as low as that of bottled water. So, what magic is needed to produce low-cost nanostructured bulk steel? The answer is simple – keep the bulk at 200 degrees Celsius for 10 days, which will lead to the formation of plate-like bainitic structure. One deficiency of the material is that it is yet difficult to weld, but the author lays out possible solutions to overcome this.
The strength can be as high as 2.5 GPa with excellent combinations of strength, ductility, toughness, fatigue resistance and wear resistance; some of the properties are displayed in figure 6. There have even been powder metallurgical variants where a strength of some 1800 MPa and a ductility of about 6% is obtained in spite of an oxygen content of 0.23 wt%
A class of iron alloys has been discovered in which a high density of strong interfaces can be created by heat-treatment alone. The packing of interfaces is so large, and the fact that there is an intrinsic work hardening mechanism in the structure, leads to remarkable properties. The genesis of this structure, its commercialization, the new science associated with the discovery, and its limitations are all explored in this short review.
The world’s first bulk nanostructured metal is now in production; its essence lies in detailed solid-state phase transformation theory, meticulously researched and argued over many decades. There is now a framework of knowledge which allows such materials to be developed systematically, beginning with calculations that define experiments.
The weakness of the nanostructured bainite is that it cannot at the moment be satisfactorily welded. This limits its applications to objects which do not require joining, such as armour, shafts and bearings. It would be good to focus technological research in this area to develop practical and cheap methods which might turn out to be generically helpful for strong steels.
It has not been possible in this short review to address the fact that combinations of properties need to be optimized in order for a steel concept to be successful. Strength alone is obviously not enough. It is worth in this context pointing out an exciting development where delamination is exploited to obtain remarkable fracture toughness at low temperatures
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