  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.
