Modifications of microstructures and mechanical properties of CoCrFeMnNi high entropy alloy films by adding Ti element

The phase evolution, microstructure and mechanical properties of CoCrFeMnNiTix (x = 0, 0.2, 0.4, 0.6, and 0.8 in molar ratio, denoted as Ti-x hereafter) high entropy alloy films (HEAFs) were studied in this work. The films were prepared by co-sputtering of CoCrFeMnNi alloy and Ti targets. Abundant nanotwins were observed in the Ti-0 and Ti-0.(2) films and the crystalline structures of both films were identified to be a single FCC phase. In Ti-0.4 and Ti-0.(6), the matrix exhibited an amorphous structure, and both the nanocrystalline grains and nanotwins were embedded in the amorphous matrix. With further addition of Ti, the Ti-0.(8) film transformed to an amorphous structure. As a result, there was a phase transition from a single FCC to an amorphous structure with the increasing Ti content. Furthermore, nanoindentation tests showed that the hardness of the films increased from 6.62 GPa in Ti-0 to 8.99 GPa in Ti-0.(8). Micropillar compression tests showed that the fracture strain decreased from > 19.42% (no fracture) to 7.78% with the increasing Ti content. The best mechanical properties in compression were observed in Ti-0.(4), and the compressive yield and fracture strengths were 2.82 and 4.44 GPa, respectively, with the fracture strain of 12.4%, which was achieved by a balanced amount of nanocrystalline grains in the amorphous matrix.