  The CoCrFeNi alloy is widely accepted as an exemplary stable base for high
entropy alloys (HEAs). Although various investigations prove it to be stable
solid solution, its phase stability is still suspicious. Here, we identified
that the CoCrFeNi HEA was thermally metastable at intermediate temperatures,
and composition decomposition occurred after annealed at 750oC for 800 hrs. The
increased lattice distortion induced by minor addition of Al into the CoCrFeNi
base accelerated the composition decomposition and a second fcc phase with a
different lattice constant occurred in the long time annealed CoCrFeNiAl0.1
HEA. A Cr-rich {\sigma} phase also precipitated from the CoCrFeNiAl0.1 HEA. The
Al element can induce the instability of CoCrFeNi HEA. The revealed metastable
CoCrFeNi at intermediate temperatures will greatly change the way of HEAs
development.
