An unsolved mystery: The composition of bcc Cu alloy precipitates in bcc Fe and steels

Copper precipitation strengthened steels are of considerable technical importance. An application to a new bridge in Illinois is described. The Cu precipitates are body centered cubic (bcc) and a few nanometer in diameter at maximum strength. While recently published ab initio calculations of the mixing energy at 0 K predict that bcc Cu precipitates in bcc Fe should be pure Cu, literature data show that they contain almost 50% Fe according to atom probe tomography whereas combined small angle neutron and magnetic scattering predicts the composition is almost pure Cu. Possible sources of these differences are discussed.