Deformation mechanisms in crystalline-amorphous high-entropy composite multilayers

Magnetron sputtering at room temperature was used to synthesize high-entropy composite multilayers (HECMs) with alternating layers of nominal composition CrMnFeCoNi and TiZrNbHfTa. HECMs with individual layer thickness of 50 nm exhibited an amorphous structure in the TiZrNbHfTa layers, and face-centered cubic (FCC) nanocrystalline structure with stacking faults and nanotwins in the CrMnFeCoNi layers. However, the HECMs with 5 nm individual layer thickness exhibited completely amorphous structures in both layers. Nanoindentation, followed by electron microscopy imaging of the indent plastic zone, was used to experimentally characterize the hardness and deformability. Molecular dynamics simulations were used to elucidate the deformation mechanisms. The high hardness of 7.8 GPa in the 5 nm layer thickness HECM is attributed to the amorphous structures in both layers with multiple shear bands around the indents. The 50 nm layer thickness HECM also exhibits high hardness of 5.6 GPa but with a more homogeneous spread of plasticity resulting in a reduced density of shear bands around indents. Strengthening in the 50 nm HECM results from a combined effect from the high density of stacking faults and nanotwins in CoCrFeMnNi, and the amorphous structure of the TiZrNbHfTa layers. The amorphous 50 nm TiZrNbHfTa nanolayers exhibit a heterogeneous nano-glass-type structure where the reorientation and agglomeration of amorphous zones seem to provide channels for plastic flow enabling enhanced deformability. Composite thin films of high entropy alloys exhibit a variety of fine structures, nanotwinned or homogeneous amorphous in CrMnFeCoNi, and heterogeneous amorphous in TiZrNbHfTa layers that enable new approaches to tune the strength and deformability.