TU researcher demonstrates self-healing steel

“TU Delft has a reputation in developing self-healing materials,” said Dr. Niels Van Dijk (Faculty of Applied Sciences). “Self-healing properties have previously been demonstrated in concrete, composites and ceramics. Now we have shown that steel can be made self-healing as well.”

“To be honest, we had already demonstrated self-healing properties in steel eighteen months earlier. But we worked with gold at the time, which is too expensive to have any practical significance for the steel sector. In our latest article, we show that adding molybdenum to steel makes it self-healing as well. Naturally, that sparked a much larger response,” said Van Dijk.

Damage called creep

Van Dijk is the lead author of the article Autonomous Filling of Grain-Boundary Cavities during Creep Loading in Fe-Mo Alloys. The article, written by Van Dijk and eleven co-authors, appeared in the periodical Metallurgical and Materials Transactions A in October 2016.

The type of damage Van Dijk targets is called ‘creep’, which occurs in steel under stress at temperatures of over 450 degrees Celsius. Imagine the blades of a steam turbine. Metallurgy expert Dr. Erik Offerman (3mE Faculty) explained: “Energy plants work most efficiently at high temperatures and pressures. But under these conditions plastic deformation or ‘creep’ occurs, which eventually leads to material failures.”

Fill the micro fractures

Under high temperatures and forces, crystals in the steel may start to detach, forming microscopic fractures. If the steel contains a foreign element whose atom size prevents it from fitting in the steel crystal structure well, it will migrate out and fill the micro fractures. Van Dijk initially demonstrated this principle with gold (atom size 13% larger than iron), and found that the principle applies to molybdenum (9% larger) as well.

“As a filler, you need an element that does not mix with iron that well, yet at the same time is trapped. The additive will remain frozen in the material until a crack between crystals triggers it to migrate and fill the gap,” Van Dijk explained.

Still, Van Dijk expects it will take some time before self-healing metals can be used. In practice, steel is an alloy of many materials giving it specific properties for strength, ease of production, hardness, welding, or anti-corrosion. It remains to be seen if 6% molybdenum (in weight) will have a similar self-healing effect in practical alloys as it has shown in pure iron.

‘An important step’

Offerman commented: “Dr. Van Dijk’s work on iron-molybdenum alloys is an important step in understanding the role of molybdenum in alloys that are more creep-resitant. Future research should reveal the interaction between other alloys metals and molybdenum. But this article has established an essential mechanism in the development of creep-resistant alloys.”

Meanwhile, Van Dijk has shifted his attention to tungsten. As tungsten is one row below molybdenum in the periodic table, the atoms are larger and may offer an alternative in making steel self-healing.

• The ASM International Henry Marion Howe Medal will be awarded to Dr. Van Dijk and co-authors during the Awards Dinner in Pittsburgh on October 10, 2017.