  Traditional metallic alloys are mixtures of elements where the atoms of
minority species tend to distribute randomly if they are below their solubility
limit, or lead to the formation of secondary phases if they are above it.
Recently, the concept of medium/high entropy alloys (MEA/HEA) has expanded this
view, as these materials are single-phase solid solutions of generally
equiatomic mixtures of metallic elements that have been shown to display
enhanced mechanical properties. However, the question has remained as to how
random these solid solutions actually are, with the influence of chemical
short-range order (SRO) suggested in computational simulations but not seen
experimentally. Here we report the first direct observation of SRO in the
CrCoNi MEA using high resolution and energy-filtered transmission electron
microscopy. Increasing amounts of SRO give rise to both higher stacking fault
energy and hardness. These discoveries suggest that the degree of chemical
ordering at the nanometer scale can be tailored through thermomechanical
processing, providing a new avenue for tuning the mechanical properties of
MEA/HEAs.
