We have carried out two beamtimes to elucidate the deformation mechanisms of FeCoCrNi based high entropy alloys(HEAs) with single FCC phase using neutron diffraction, first at room temperature then at 77 K, determining the stacking fault energy (SFE) and confirming the occurrence of twinning-induced plasticity. Now we propose to continue to study deformation mechanisms in HEAs using in situ tensile tests. First we want to carry out the tests at ultra-low temperatures (10 and 40 K) on the FCC single phase HEAs, which will allow us to measure SFE across different temperatures. Secondly, we want to perform the tests on three new FeCoCrNi HEAs micro-alloyed with C, Al and Mo with a duplex structure (FCC+BCC), in order to determine the stress partitioning between the two phases. The results will be of significance for HEA design making full use of micro-alloying effects.