Steel has a lot of good things going for it. It is relatively cheap, extremely versatile and easily processed by just about all methods. The problem is that it’s relatively heavy. Other metals can be much lighter, but their cost tends to weigh them down, so to speak. New research may help engineers get the best of both worlds.
Getting both high strength and low density is something materials researchers have been working on for decades, centuries, or even millennia, really. Equally challenging is getting high strength and high ductility in a single material. Work to overcome these challenges has led to many breakthroughs in materials, but there is still room for improvement, so they’ll keep improving.
The newest improvement, described in the journal Science, uses a brittle phase to make “low-density” steel both strong and ductile. Low-density steel is created by alloying iron with lighter elements such as aluminum, but iron can react with aluminum to form a brittle intermetallic phase, FeAl. An intermetallic phase is an ordered structuring of elements, and in the most recent work it is responsible for enhancing strength.
In metals, one way to increase the specific strength is by heat treatment and the formation of a second phase. In steels this second phase is often martensite produced during quenching from high temperature, but it can also be created using precipitation hardening or sometimes a dispersion of oxide or carbide particles.
All of these methods seek to achieve a composite-like, rule-of-mixtures balance of properties. The more ductile you want a material, the more of the ductile phase you keep. Want it stronger? Add more of the high-strength phase. It’s not always so simple in practice, and that’s what makes the new alloy different.
The researchers added nickel to help push the FeAl intermetallic formation. This way the intermetallics nucleate rapidly and form at the nanoscale. Instead of embrittling the steel, they strengthen it by pinning dislocation motion; the mechanism by which most materials deform.
The resulting material is exceptionally strong while maintaining a high ductility. In fact, it outperforms titanium in regard to yield strength and total elongation during tension tests. To be clear, the low-density steel is still heavier than titanium (by a little over 50%), but its specific strength is even better for the same ductility. This doesn’t indicate its corrosion or high-temperature service properties for which titanium is often chosen.
The research was conducted with 40 kg of material, but the processes used are representative of commercial equivalents, and scaling the process seems promising. While it certainly could be cheaper than titanium alloys, we’ll have to wait for that outcome.
Image: BMG Metals