Microstructure and small-scale size effects in plasticity of individual phases of Al0.7CoCrFeNi High Entropy alloy

High Entropy alloys (HEAs) are solid solution alloys containing five or more principal elements in equal or near equal atomic percent (at %). We synthesized Al0.7CoCrFeNi HEA by vacuum arc melting and homogenized it at 1250 °C for 50 h. The microstructure shows the presence of two phases: the Body-Centered Cubic (BCC: A2+B2) and the Face-Centered Cubic (FCC). Using the Focused Ion Beam, we fabricated single-crystalline cylindrical nano-pillars from each phase within individual grains in the Al0.7CoCrFeNi HEA. These nano-pillars had diameters ranging from 400 nm to 2 μm and were oriented in the [324] direction for the FCC phase and in the [001] direction for the BCC phase. Uniaxial compression experiments revealed that the yield strength is 2.2 GPa for the 400 nm diameter samples in the BCC phase and 1.2 GPa for the equivalent diameter samples in the FCC phase. We observed the presence of a size-effect in both phases, with smaller pillars having substantially greater strengths compared with bulk and with larger-sized samples. The size-effect power exponent for the BCC phase was −0.28, which is lower than that of most pure BCC metals, and the FCC phase had the exponent of −0.66, equivalent to most pure FCC metals. We discuss these results in the framework of nano-scale plasticity and the intrinsic lattice resistance through the interplay of the internal (microstructural) and external (dimensional) size effects.