  High-entropy alloys, near-equiatomic solid solutions of five or more
elements, represent a new strategy for the design of materials with properties
superior to those of conventional alloys. However, their phase space remains
constrained, with transition metal high-entropy alloys exhibiting only face- or
body-centered cubic structures. Here, we report the high-pressure synthesis of
a hexagonal close-packed phase of the prototypical high-entropy alloy
CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed
to suppression of the local magnetic moments, destabilizing the initial fcc
structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the
transformation is sluggish, occurring over a range of >40 GPa. However, the
behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following
decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This
demonstrates a means of tuning the structures and properties of high-entropy
alloys in a manner not achievable by conventional processing techniques.
