Metastable phase evolution and grain growth in annealed nanocrystalline Cr-Fe-Ni films

Nanocrystalline Cr-Fe-Ni thin films libraries are prepared by radio frequency magnetron sputter codeposition on Al2O3 substrates. The variation in distance from different points on the substrate to the three sputter sources produces a spread in composition in the films. Synchrotron radiation is used to analyze composition, crystallographic phase, and grain size as a function of sample position, both as- deposited and after annealing. In addition to three equilibrium phases (bcc, fcc and σ-FeCr), a fourth phase is observed which does not correspond to any equilibrium phase in the Cr-Fe-Ni system, but which is consistent with the α-Mn structure. The films are initially nanocrystalline; both equilibrium and nonequilibrium phases are observed. Ternary phase diagrams measured at annealing temperatures of 200-800 °C show that the stability of nanocrystalline material against thermal grain growth varies widely with composition, and that the crystallographic phase can be selected to be an appropriate heat treatment. Thus, the approach used here is useful for technologies such as magnetic recording media where understanding the stability of the nanograin structure is critical.