Human lives are a wonderful series of accidents. Walter Rosenhain was born in Germany, grew up in Australia, and did the work for which he is remembered at England’s National Physical Laboratory. After training as a civil engineer, then devoting himself to problems of optical glass, he took a professor’s advice and applied himself to the question of how metals yield under applied stress.

Rosenhain built apparatus that allowed him to observe the polished and chemically etched surface of a test specimen under high magnification as stress was applied to it. When that stress became great enough to cause plastic (permanent) deformation, Rosenhain saw mysterious parallel lines appear permanently on his test specimens. After further experiment and thought, he concluded that these were “slip bands,” visible lines formed as planes of atoms in metal crystals slipped over one another, rather like planes of magnetized metal “marbles” might act. The metaphor he used was that of a deck of cards slid sideways, making the edges of the cards stand out as slip bands.

This was 1906. When other investigators used the known bond strengths of atoms in ideal crystals to compute how much force it should take to drive slip of this kind, it was fantastically greater than the force Rosenhain had seen producing slip in his metal specimens.

As the foundry arts evolved into metallurgical science, the reason for this difference was discovered: Real crystals were far from ideal in that they contained some incomplete rows of atoms. Under much less force than required to slip a whole plane of perfectly arrayed atoms, such crystal defects could be made to move one step at a time. The presence of such defects allowed shifts of a single row or a partial row of atoms to occur at much lower stress.

JE Pro Series pistons
Forged pistons are manufactured from alloy 2618 (JE Pro Series shown) or 4032. The former is 94 percent aluminum mixed with copper, magnesium, iron, nickel, silicon, and titanium. The latter is 85 percent aluminum and 12 percent silicon, plus single-digit percentages of magnesium, copper, and nickel.JE Pistons

Such crystal defects—imperfections in their otherwise orderly atomic array—were given the name “dislocations.” A new science of plastic deformation, filled with daunting mathematics, quickly came into being. This at last made it possible to understand how French steel makers in 1888 increased the strength of steel 40 percent by adding five percent nickel. The US Navy in 1891 adopted such steel as armor plate for its ships. Nickel steel was applied to making bicycle chains more durable in 1898 and a year later to doing the same for seamless bicycle tubing.

Adding atoms of a different size, in this case nickel, created local regions of increased strain in the otherwise orderly crystal lattice around each one. As applied stress tried to pop dislocations from one row of atoms to the next in the stepwise process of slip, when the dislocation approached a nickel atom it encountered increased resistance to further movement. In other words, the steel was made stronger.

Henry Ford understood that to use less material, making the car lighter and cheaper, required that the material be stronger.

In 1882, Robert Hadfield had discovered that steel’s resistance to shattering on impact could be greatly increased by adding 12–13 percent manganese to it. This arrived just in time to produce the rock-breaking bits that made possible Europe’s 1880s achievements in driving miles-long rail tunnels through mountains.

Henry Ford, wanting to sell his cars not to a few millionaires but to everyone—the largest possible market—understood that to use less material, making the car lighter and cheaper, required that the material be stronger. Metallurgist J. Kent Smith interested Ford in the effects of vanadium in steel: It produced the ultra-fine-grain structure and high strength normally found in quenched carbon steels but without their brittleness. A Ford Model T driveshaft I found in a barn has served me well as a strong pry bar for 40 years.

Alloying combined with careful heat treatment made possible another means of stopping the movement of dislocations: precipitation hardening. Alloy components that are soluble in a given metal when melted are often less so as it cools. This can lead to the precipitation of tiny but very hard particles of another phase as the metal solidifies. An investigator named Alfred Wilm in 1906 discovered this effect in aluminum to which 5 to 8 percent copper had been added.

Doug Brauneck and Dr. John Wittner
Dr. John Wittner (right) poses at Willow Springs International Raceway with rider Doug Brauneck, who won the 1987 AMA Pro Twins title on Wittner’s Moto Guzzi. Wittner later moved to Italy to join the factory’s design and development team working on new models.David Dewhurst

Rosenhain himself in 1916 contributed an aluminum alloy of remarkable hot strength, Y alloy, whose modified descendant alloy 2618—the English call it RR58—continues to be used in the production of high-performance aluminum pistons to this day. When dentist-turned-race-engineer John Wittner was developing Moto Guzzi’s four-valve cylinder heads in the 1990s, the material he chose from the Italian alloy system was Y alloy.

Yet a further metal-strengthening method, dispersion hardening, has been used to make small quantities of piston material far stronger than 2618. Instead of precipitating hard particles out of solution as melted metal alloy solidifies, the particles are prepared separately—as ultra-fine aluminum oxide dust, for example—and mixed into the aluminum by the intense shearing action of extrusion through hard dies.

If you look forward to self-driving cars because they’ll give you more time on social media, don’t look as closely at reality as Rosenhain did. He spent the rest of his life exploring a widening circle of fascinating phenomena.