Steel-Aluminum alloy? Any metallurgist would tag you as a n00b for bringing it up. “Can’t do it. Incompatible. It’s the metallurgical equivalent of dogs and cats lying down together. Winds up with crystallized, embrittled aluminum weakening the steel.” Three Korean scientists, all stereotypically named Kim, from the Pohang University of Science and Technology have almost managed to pull it off — after many years of theory, trial, and error, and standing on the shoulders of previous researchers, as always. The lead investigator is Hansoo Kim.
The resulting material has properties that sound like Ayn Rand’s fictional Rearden Metal — light, strong, potentially cheap. It exceeds titanium alloys, hitherto the lightest and strongest alloys known, for lightness and strength. Yet the aluminum that lightens the alloy doesn’t embrittle it — it leaves it ductile, or workable. That’s why this research has potential outside of the metallurgist’s lab.
The secret appears to be accepting that Fe-Al “intermetallic compound” inclusions (they call this compound B2) within the metal will be somewhat brittle, and managing their size and dispersion so that they lighten the resulting steel without embrittling it. They did this by adding nickel, which “catalyses the precipitation of nanometre-sized B2 particles in the face-centred cubic matrix of high-aluminium low-density steel during heat treatment of cold-rolled sheet steel.” In much the way that windows don’t break and make a skyscraper fall because they’re not load-bearing structures, these fracture-prone B2 particles are individually so small and so widely and evenly dispersed that a crack has no pathway to propagate. Think of it as rip-stop steel at the nanometer scale.
This work is evolutionary as much as it is revolutionary. It builds on previous work on TRIPLEX steels, which are steels with significant amounts of manganese, aluminum and carbon serving to modify iron’s physical properties (and that in turn builds on 1970s research in the USSR). Previous TRIPLEX research by Springer and Raabe (details linked below) found that while holding manganese and carbon content constant at 30% and 1.2% respectively, strength went up as up to 8% aluminum displaced some of the iron in the balance.
Springer and Raabe, and others, built on Soviet work that developed high-strength but very brittle iron-aluminum steels.
How can a material be strong and brittle? They’re separate properties. Strong suggests how far you have to go to make the metal fail. Brittle suggests a material that then fails abruptly by breaking. It doesn’t deform. (Imagine a car that, crashed into a tree, shattered into shards rather than got dented). But that’s not just a problem for designers: it’s a hell of a problem for manufacturers, for many of our steel-processing approaches expect steel to be ductile. We bend it on anvils or stamp it in dies; we shear it with cutting tools; we curve pipes around; we hydroform it. All of those processes depend on the ductility of the metal.
The tables and graphs in the paper in Nature (one of the two most prestigious peer-reviewed journals in the world) suggests that this novel aluminum-bearing steel alloy not only has superior balance of strength and ductility to TRIPLEX, but also offers real ductility advantages over typical titanium-aluminum-vanadium alloy. (If you’ve ever worked with titanium, you know ductility is not its strong point).
What do alloys like this mean for firearms? The three Dr Kims are excited about automotive and aviation applications, because those are the primary users of large quantities of lightweight alloys (and have been turning increasingly to more exotic materials, like carbon fiber, carbon-carbon, and lithium alloys, in pursuit of lighter strong materials). But the technology that shows up on the auto line and in the aerospace factory does make it to firearms, especially as every firearms designer now alive is alert to how 1940s aviation technology enabled Stoner, Sullivan et. al. to revolutionize firearms design in the 1950s and early 60s. If nobody in your engineering shop is getting SAE’s Aerospace Engineering, you’re either committed to traditional materials and processes, or a me-too design shop.
While the material itself is of great interest, the scientists think that the process will be, in the long run, far more important because it will allow the invention of entire classes of previously “impossible” alloys.
The process has one major hurdle before it can be commercialized: a method must be found to prevent the oxidation of the steel, in-process. Technologies used on conventional steels won’t work, and building the foundry on an airless asteroid solves the oxidation problem, but leaves you with the steel somewhere other than the planet where it’s required.
And right about now, perhaps in some unexpected corner of the world, a grad student is mulling this problem, and sometime soon a light will go on in his head….
For more information
Popular Mechanics article for laymen: http://www.popularmechanics.com/technology/news/a13919/new-steel-alloy-titanium/?1443670416676=1
Nature article (abstract, references and tables only w/o subscription): http://www.nature.com/nature/journal/v518/n7537/full/nature14144.html
Information on TRIPLEX steel-AL alloys (Forerunner of this research): http://www.dierk-raabe.com/triplex-steels/