Introduction
The strengthening of high entropy alloys (HEAs) can be persued through a variety of different mechanisms, including grain refinement, twinning, transformation induced plasticity, and second-phase hardening. In 2016, a variant of the Cantor alloy, which is based on the substitution of Cr with Cu, was introduced. The CoCuFeMnNi is characterized by an unstable FCC phase that promotes the formation of second phases at high temperatures [1]: both a BCC phase rich in Fe and Co, and a Cu-rich FCC second phase could be obtained through appropriate thermal treatments. The present work aims at enhancing mechanical properties of the CoCuFeMnNi HEA through a properly designed heat treatment schedule, aimed at inducing athe formation of second phases through a controlled decomposition of the parent solid solution. Moreover, a modification of this alloy was persued in order to allow the precipitation of further second phases.

Results
The CoCuFeMnNi alloy was subjected to a conventional thermal treatment schedule, based on solution treatment and ageing. Nanometric rounded Cu-rich clusters in the solution-treated alloy and coherent, regularly oriented Cu-rich discs in the peak-aged condition were possibly produced by spinodal decomposition [2]. An almost 100 % enhancement of mechanical  strength was obtained thanks to the modulation of composition. Moreover, mechanical behaviour at cryogenic temperature was improved by ageing, both in terms of strength and ductility.
Thereafter, a modification of the base alloy was designed in order to allow the formation of Ni3Ti, precipitates, which are known to effectively strengthen several types of alloys. CALPHAD method was used to determine an the optimal composition of the modified alloy, which was then synthesised and atudied. Ageing treatments were found able to improve the mechanical behaviour of the CoCuFeMnNiTi alloy over its unmodified counterpart. Finally, the role of different phases in strengthening both alloys, and the mechanisms involved, were addressed.

Conclusions
The CoCuFeMnNi and CoCuFeMnNiTi alloys were strengthened by second phases formed by either precipitation or spinodal decomposition. Microstructural features and mechanical behaviour were assessed.