Plastic instabilities, mechanochemistry, and a bag of (metal) chips

Soft and highly strain hardening metals like Ni, Al, Ta and stainless steels, are notoriously difficult to cut, earning them the moniker “gummy”. This difficulty is well-known for its commercial implications, yet its origins have remained largely mysterious. This talk presents high-speed in situ investigations of this problem in two parts. In the first, we unveil the occurrence of a highly unsteady plastic flow mode, termed sinuous flow, as the cause of this difficulty. Sinuous flow arises due to a surface plastic buckling instability and is characterized by repeated material folding, large local strains (>10) and energy dissipation. The physics of this flow, its dependence on material properties, and manifestation across metals are directly observed. In the second part, we demonstrate how sinuous flow can be perturbed using mechanochemistry. A suitable chemical medium applied to the metal surface causes a local ductile-to-brittle transition by coupling plastic instabilities with interface energetics. Consequently, surface buckling and sinuous flow are replaced by a periodic fracture instability in the presence of the medium, with near absence of defects on the cut surface and significantly lower energy dissipation (~80%). The transition in flow is also reflected in the morphologies of the resulting metal chips. This mechanochemical effect is controllable and largely material agnostic, with many chemical media working equally well across different metal systems. Additionally, the benign nature of the media presents exciting opportunities for fundamentally enhancing deformation processing of metals in industrial settings.