Single-Layer Fe-Cu Interphase in Ferritic Steels Stabilized by Magnetic Friedel Oscillations

Copper precipitation is a technique extensively deployed in steel strengthening. Being as tiny as a few nanometers in diameter, the Cu precipitates present a real challenge to experimental techniques in determination of their composition. The late Professor Morris Fine called it a mystery when addressing the discrepancy between the fact of low solubility of Fe in bulk Cu and the remarkable content of Fe in Cu precipitates according to atom probe tomography measurement. With a thorough search using rigorous first-principles density functional theory calculations, we are surprised to find that the interface between Cu precipitate and Fe matrix is neither immiscibly sharp, nor commonly miscible over a range of atomic layers, but rather manifests itself as a single-layer mixed Fe-Cu interphase. Our detailed analysis reveals that spin polarization is a key factor in defining such an interphase. The magnetic Friedel oscillations through spin polarization is the driving force to stabilize it. When the single-layer Fe-Cu interphase is counted in, the content of Fe in Cu precipitates would be significant due to their small size. Our finding is a strong demonstration that quantum mechanical effects such as Friedel oscillations can have explicit consequence also in structural materials. The accurate knowledge of the Fe-Cu interface is crucial for the design of advanced high-strength steels.