Microstructure evolution and orientation relationships of CoCrFeNi thin films grown on (0001) alpha-Al2O3
Résumé
The study of HEA in the form of thin films aims at providing missing data on HEA phase stability. Indeed, the small bulk volume of a film, combined with the presence of grain boundaries and interfaces, helps in achieving thermodynamic equilibration of these complex alloys. In addition, polycrystalline films can be used to test the role of grain boundaries on the mechanical properties of HEAs.
Here we present a study of the stability of polycrystalline CoCrFeNi thin films, grown on (0001) α-Al2O3 (c-sapphire), with respect to annealing treatments. It has been found that extremely large grains can grow in the film depending on their orientation relationship to the c-sapphire substrate. Despite their complex chemistry, the grains in the 200 - 670 nm thick CoCrFeNi FCC films investigated, adopt the same orientation relationships as those of pure FCC metal films on c-sapphire. However, the grains in CoCrFeNi grow much larger at homologous annealing temperatures.
The films have been synthesized at room temperature, using magnetron sputtering from four pure element targets in a chamber with UHV base vacuum. They consist of 30-100 nm wide columnar FCC grains with a <111> texture. Upon annealing for 1 hour in the range 973 K to 1423 K under an Ar-H2 atmosphere, grain growth and grain boundary grooving compete to either stabilize or break-up the film. The microstructure evolves into larger grains and/or islands, of four different orientation relationships: OR1 (Me(111)[1-10]//α-Al2O3(0001)[1-100] and OR2 (Me(111)[1-10]//α-Al2O3(0001)[11-20] and their twins (OR1t and OR2t).
Thinner films and higher temperatures favor the dewetting of the film to form single-crystalline islands. Dewetting initiates from holes which nucleate at grain boundary triple junction grooves, and which have deepened sufficiently to reach the substrate. At the highest temperatures, the OR2 and OR2t grains grow to sizes exceeding 1000 times the film thickness. The migration of the grain boundaries of these grains is fast enough to overcome both grooving and the nucleation of dewetting holes.