Ultra-strong and thermally stable nanocrystalline CrCoNi alloy

Grain refinement to the nanocrystalline regime is the most effective way to strengthen materials but this often deteriorates the grain-size thermal stability and plasticity. Here we manufactured a nanocrystalline face centred cubic CrCoNi medium entropy alloy with columnar grains via magnetron sputtering. Compression of CrCoNi pillars with diameters of ∼ 1 µm revealed a record high yield strength of ∼5 GPa for pillars with face centred cubic structures and engineering plastic strain of > 30%. The alloy possessed an outstanding grain-size thermal stability even at 1073 K. Both nanocrystalline grain size and a high density of nanotwins/stacking faults are critical to the exceptional yield strength. Deformation twinning, grains refinement during deformation, grain boundary sliding and random grain orientation all contribute to the large plasticity. The outstanding thermal stability is attributed to the sluggish diffusion effect and the low energy of twin boundaries.