The military technology of A Song of Ice and Fire, George R. R. Martin's series of fantasy novels, is medieval with an admixture of the supernatural. Dragons aside, among the most prized weapons are swords made from Valyrian steel, which are lighter, stronger, and sharper than ordinary steel swords.
Like many of the features in the rich world of the novels and their TV adaptation, Game of Thrones, Valyrian steel has a historical inspiration. Sometime before 300 BC, metalworkers in Southern India discovered a way to make small cakes of high-carbon steel known as wootz. Thanks to black wavy bands of Fe3C particles that pervade the metal, wootz steel was already strong. When forged by the metallurgists of Damascus, it became stronger still.
Eddard Stark, lord of Winterfell and warden of the North, brandishes his Valyrian steel greatsword, Ice. CREDIT: HBO
Perhaps because the properties of wootz and Damascus steels depended, in part, on a particular kind of iron ore, the ability of metallurgists to make the alloys was lost sometime in the 18th century. In A Song of Ice and Fire, the plot plays out during an era in which making Valyrian steel is a long-lost art.
Martin's knowledge of metallurgy is perhaps shaky. The first episode of the fourth season of Game of Thrones begins with the melting down, in air, of one ancient Valyrian steel greatsword—Eddard Stark's Ice—to make two new Valyrian steel longswords. Even when it contains just two elements, iron and carbon, steel is a complex material whose strength-endowing microstructure depends on how it is processed. Melting Damascus steel might preserve its chemical composition, but you could reconstruct the material only if you knew the processing recipe.
Valyrian and Damascus steels were on my mind earlier this week when I attended a session at TechConnect World on the use of nanotechnology in the nuclear power industry.
Scott Anderson of Lockheed Martin gave the introductory talk. Before the Fukushima disaster, Anderson pointed out, the principal materials science challenge in the nuclear industry lay in extending the lifetime of fuel rods. Now the focus has shifted to accident-tolerant fuels and safer, more durable equipment.
Among the other speakers was MIT's Ju Li, who described his group's experiments with incorporating carbon nanotubes (CNTs) in aluminum to boost the metal's resistance to radiation damage. In a reactor core, neutrons and other ionizing particles penetrate vessels, walls, and other structures, where they knock atoms off lattice sites. The cumulative effect of those displacements is to create voids and other defects that weaken the structures.
Using helium ions as a proxy for the ionizing particles found in a nuclear reactor, Li and his collaborators have discovered that a 0.5% admixture by weight of CNTs not only boosts aluminum's tensile strength by 20%, but also forestalls the formation of voids in the material: For the same integrated flux of He ions, the CNT-containing aluminum sample had just 1% of the total volume of voids as the unadulterated sample had.
Li isn't sure yet how the CNTs resist irradiation and toughen the aluminum, but at the end of his talk he recalled their appearance in another metal, steel.
In 2006 Peter Paufler of Dresden University of Technology and his collaborators used high-resolution transmission electron microscopy (TEM) to examine the physical and chemical microstructure of a sample of Damascus steel from the 17th century.
The saber from which the sample was taken was forged in Isfahan, Persia, by the famed blacksmith Assad Ullah. As part of their experiment, Paufler and his colleagues washed the sample in hydrochloric acid to remove Fe3C particles. A second look with TEM revealed the presence of CNTs.
Because of its high carbon content, Damascus steel would be brittle without extensive thermal and mechanical processing to modify the microstructure. In their 2006 paper, Paufler and his colleagues speculated that the processing caused trace catalysts present in the ore and furnace to induce the formation of CNTs. Those CNTs, in turn, induced the formation of strength-bestowing nanowires and particles of Fe3C.