Nanoscale modulated structures by balanced distribution of atoms and mechanical/structural stabilities in CoCuFeMnNi high entropy alloys

We observed the nanoscale modulated structures in the homogenized CoCuFeMnNi and CoCu1.71FeMnNi high entropy alloys and the mechanical/nanostructural stabilities of modulated structures were studied by compressive deformation up to the true strain of 1.0. Constituent elements in CoCuFeMnNi and CoCu1.71FeMnNi alloys with dendrite structure comprised of the Cu-Mn rich interdendritic region and Co-Fe rich dendrite arms in the as-cast alloys appeared to be uniformly distributed on the microscopic scale after homogenization. One of the most interesting and unique observation in the homogenized alloys is the presence of nanoscale phase separation with no distinct separation or splitting of electron diffraction spots, suggesting two separated phases have the same structure and similar lattice constants. We propose that the reduction of lattice strain and interface energies through selective and balanced distribution of various atoms to minimize difference of lattice constants enabled nano-scale separation of the alloy into two FCC phases. The lattice strain energy decreased from 0.93 kJ mol(-1) to 0.75 kJ mol(-1) in CoCuFeMnNi and from 0.85 kJ mol(-1) to 0.38 kJ mol(-1) in CoCu1.71FeMnNi by separation into two phases with similar lattice constants. Both CoCuFeMnNi and CoCu1.71FeMnNi alloys exhibited rapid strain hardening up to the strain of 0.23 and then saturation of the flow stress. The initial rapid increase of hardening is attributed to the presence of nanoscale modulated two-phase structure.