Phase and microstructures in sputter deposited nanocrystalline Fe-Cr thin films

A series of FexCr1-x thin films (where X = 0 to 1) were magnetron sputter deposited. Their phase and microstructures were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). The elemental films adopted the BCC structure with in-plane grain sizes (in the tens of nanometers). The Fe-49Cr (at.%) film grew as the high temperature sigma-phase, with similarly shaped but even more refined grain sizes. Films of Fe-16Cr, Fe-24Cr, and Fe-72Cr deposited as the A2 structure. The Fe-24Cr and Fe-72Cr films revealed abnormal grain sizes that ranged from tens of nanometers to hundreds of nanometers. It is proposed that the thermodynamic driving force to phase separation contributed to the increased adatom mobility that resulted in varied grain sizes. These films also revealed a peculiar 'star-burst' contrast pattern within their grains which has been revealed to be from a modest crystallographic orientation shift contributed to the early onset of phase separation evident in the APT analysis. In the lower solute limit, the Fe-5Cr film short-range ordering of Cr (confirmed by APT) was noted and confirms prior modeling and diffuse neutron diffraction results of its presence at this composition. Upon depositing this film at 673 K, the short-range ordering was lost.