Steels: nanostructured alloys
Channel: bhadeshia123
Date: 2022-03-16

A nanostructured material is here defined as one containing an exceptionally large density of strong interfaces, rather than one which simply contains a minor fraction of features such as precipitates, which are small in size. The desire for such materials in the engineering context comes from the expectation of novel mechanical properties, particularly the stress that can safely be  tolerated in service. It is difficult to invent such materials  because any design must address three basic issues: 

(i)  it should ideally be possible to make samples which are large in all dimensions, not simply wires or thin sheets;

(ii) there are commercially available steels in which the distance between interfaces is of the order of 250--100~nm. The novelty is in approaching a structural scale in polycrystalline metals which is an order of magnitude smaller.

(iii) The material concerned must be cheap to produce if it is not to be limited to niche applications. A good standard for an affordable material is that its cost must be similar to that of bottled water when considering weight or volume.



An alloy system based on iron is described in which it has been possible to create a high density of interfaces by heat--treatment alone. The resulting structure consists of a mixture of slender platelets of bainitic ferrite, just 20--40\,nm in thickness, embedded in a matrix of carbon--enriched austenite. The rate at which this structure evolves is slow by conventional standards, but this permits components to be made which are large in all three dimensions, with uniform properties throughout. The fundamental mechanisms behind this novel nanostructured steel are reviewed, along with the factors determining its strength, ductility and fracture toughness. It is argued that although reasonable toughness can be achieved in the context of strength levels exceeding 2000\,MPa, the impact toughness remains poor and that it may not be possible to improve this particular parameter.


Associated teaching materials can be found on:
http://www.phase-trans.msm.cam.ac.uk/teaching.html
H. K. D. H. Bhadeshia