Two‐Dimensional Alloying at the Iron‐Copper Interface in Steel Driven by Magnetic Friedel Oscillations

Abstract Copper precipitation is widely used in steel engineering, but the nanoscale size of early‐stage Cu precipitates makes determining their composition experimentally challenging. The late Professor Morris Fine highlighted the puzzling discrepancy between the low solubility of Fe in bulk Cu and the surprisingly high Fe content in Cu precipitates revealed by atom probe tomography. Using rigorous first‐principles density functional theory calculations, stable Fe/Cu interfacial structures are systematically searched for and find a single diffuse layer at the (001) interface, up to two diffuse layers in the [111] direction, and no diffuse atomic layer in the [110] orientation. Magnetic Friedel oscillations drive 2D alloying at the (001) interface, while configurational entropy governs alloying at (111). These interfacial alloying effects explain the significant Fe content observed in nanoscale Cu precipitates. Further calculations show that this alloying substantially alters impurity segregation at the (001) interface. In particular, enhanced hydrogen trapping at the interface suggests that Cu precipitates may help mitigate hydrogen embrittlement in steel. These findings reveal how quantum interference effects can drive interfacial mixing and open new avenues for atomic‐scale alloy design guided by electronic structure.