100-Year-Old Metallurgy Puzzle Solved, Reveals How Folded Metal Hardens

Scientists were long puzzled over what happens when, say, a blacksmith folds metal into swords.
Brad Bergan

Researchers solved a 100-year puzzle in the field of metallurgy regarding how single crystals exhibit staged hardening, while others don't — according to a recent study published in Nature Materials.


100-year-old puzzle, how metal hardens when folded

For thousands of years, people have enjoyed the natural property of metals to harden into stronger materials when mechanically deformed. Linked to the motion of dislocations, metal hardening mechanisms have remained an unknown of physical metallurgists for more than a century, reports Phys.org.

The team of researchers from the Lawrence Livermore National Laboratory (LLNL) — under the leadership of materials scientist Vasily Bulatov — carried out atomistic simulations of a large enough size to statistically represent macroscopic crystal plasticity — which pushed the limits of supercomputing.

Observing metal hardening, dislocations, crystal plasticity

However, these simulations also had to be fully-resolved so the researchers could study the origins of metal hardening at the most basic and fundamental level of atomic motion.

The simulations happened thanks to the Mira supercomputer at Argonne Laboratory Computational Facility, and the Vulcan and Lassen supercomputers at Livermore.

The fundamental causes of metal hardening escaped scientific explanation until 86 years ago, when dislocations — technically, curvilinear crystal defects created via lattice disorder — were suggested as a possible cause for crystal plasticity. While direct causal connections between dislocations and crystal plasticity had a strongly-established theoretical basis, no one saw this happen in media res — within the bulk material itself.

Dislocations through all phases of metal hardening

"We relied on a supercomputer to clarify what causes metal hardening," said Bulatov. "Instead of trying to derive hardening from the underlying mechanisms of dislocation behavior, which has been the aspiration of dislocation theory for decades, we performed ultra-large-scale computer simulations at a still more basic level — the motion of atoms that the crystal is made of."

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The team showed how the notorious staged (or, inflection) hardening of metals happens because of a crystal rotation under what's called uni-axial straining. In contrast to the widely-varying and contradictory views on the subject in scientific literature, the team found the fundamental mechanisms of dislocation processes are the same throughout every stage of metal hardening.

"In our simulations we saw exactly how the motion of individual atoms translates into the motion of dislocations that combine to produce metal hardening," said Bulatov.

Depicted endlessly in medieval tales of knights, valor, and modern-day streaming shows like "Game of Thrones," the iconic figure of a blacksmith making swords via folding and hammering metal has finally found a materialistic explanation — not just in theory, but in scientific fact.