Steel has a high tensile strength and is relatively cheap. It is therefore employed in a host of engineered structures. However, the problem is that it is likely to crack as it undergoes repeated stress.

Now, a team of researchers led by Cemal Cem Tasan of MIT has identified two types of steel (ferrite-cementite pearlitic steel and martensite-austenite transformation-induced plasticity steel) with structures that resemble those found in bone, which has superior crack resistance thanks to its hierarchical and laminated substructure. Both types of steel are much more resistant to cracking than the type of steel typically employed in automotive systems.

Repeated cycles of high and low stress

The researchers tested their steels by subjecting them to repeated cycles of stress at both high and low levels, and found that microscopic cracks did not grow rapidly, even at high stress levels. This resistance to cracking comes from “roughness-induced crack termination”, they say. The materials also contain metastable areas, which means that they contain tiny areas poised between different stable states, some of which are more flexible than others. Transitions between these (soft and hard) phases help absorb the energy of spreading cracks, when they occur, and even stop the crack growth totally.

The crack roughness enhances the resistance of the steels at high stress fatigue and the phase transformations that occur enhance it at low stress fatigue, explains team member Motomichi Koyama of Kyushu University.

“Our work deals with fatigue, which is regarded as the most important phenomenon for structured materials,” he tells nanotechweb.org. “In fact, more than 70% of failure accidents in structures are caused by fatigue.

“At the moment, engineers are obliged to make specific materials for specific operating conditions – for example, structural components exposed to high stress are designed very differently to those exposed to relatively low stress. We have now succeeded in providing a design concept that allows for high crack resistance under both low and high stress amplitude fatigue conditions. This concept will allow us to create very safe, robust materials that could operate under a variety of different conditions.”

Zig-zag cracks are rough

A material with a high resistance to fatigue crack growth has a rough crack surface, he continues. “We therefore looked for microstructures in which cracks propagate in a hierarchical zig-zag fashion – which is what happens when bones crack. We believe that this zig-zag crack shape produces a large roughness and to realize this crack shape, we selected hierarchically-structured steel microstructures including nanolaminates. Indeed, we found that we could control their structure using a heat treatment that we hope to further optimize in the future.”

The team, reporting its work in Science DOI: 10.1126/science.aal2766, says that it will now attempt to apply its design strategy to other structural materials that contain hierarchically metastable microstructures, beginning with other types of steels.

Visit the Nanotechnology focus collection for more on hierarchical nanostructures