Structural stability of higher-energy phases in Cu and Cu-Fe alloy revealed by ab initio calculations

The structural stability of bcc, fcc and orthorhombic 9R Cu and Cu-50 at.% Fe alloy phases were investigated by ab initio method. The results show that the equilibrium fcc structure is the most stable, followed by orthorhombic 9R and bcc structures in order for Cu, whereas for Cu-50Fe alloy the orthorhombic 9R is the most stable, followed by 3R, fcc and bcc structures in order. The predicted elastic constants show that orthorhombic 9R Cu and fcc/orthorhombic 9R Cu-50Fe alloy phases satisfy Born's criterion and thus exist as metastable phases, but bcc Cu and Cu-50Fe alloy phases do not. It is confirmed that transformation from the unstable bcc to orthorhombic 9R structure is energetically favorable for Cu alloy precipitates with a high content of Fe, for example 50 at.% Fe, and the intermediate orthorhombic 9R structure eventually transforms into the fcc structure for Cu-Fe alloy precipitates with a higher content of Cu in ferritic steels.