Our study again proves the excellen

Our study again proves the excellent mechanical performance of metastable HEAs, of which the true tensilestrain is close to 0.7 with a very high tensile true stress of1300 MPa. Such high strength and large ductility are ascribed to the activation of multiple strain hardeningmechanisms, as shown in Figs 5–8. Fig. 9 summarizes thedeformation microstructure evolution of the HEA withthe increase of strain via schematic illustration and TEMimages. The HEA has a single FCC structure in the undeformed state with the lack of HCP phase. Upon deformation, the planar dislocations dominated the earlydeformation stage, while the martensite phase transformation occurred later. Besides, multiple widely extendedSFs and deformation twinning also took place to coordinate the local strain during deformation. In a moremicro level, the planar dislocations generated and slippedat the early deformation stage. With further deformation,these dislocations were tangles and dissociated into partial dislocations to form SFs. The partial dislocationsregularly glided on every other {111} planes and eventually changed the atom sequences to form HCP lamellae.These lamellae combined with each other to form theHCP block.Dislocation slip, as the most common deformationmechanism, is usually stored and arranged in orderedpatterns during deformation. The motions of dislocationsare patterned with two modes: planar slip and wave slip[29]. The wave slip is caused by frequent cross-slip, whilethe planar slip is promoted by low SFE, or high internalfriction stress via short range ordering or solute content[10,30]. In our HEA, the planar slip along {111} planesare seen in Figs 5a and 8a under both tension and compression. The HEAs with five major elements are knownfor the very high lattice friction stress due to the masssolid solution strengthening [4,30]. In addition, the current HEA has a relatively low SFE of ~20 mJ m−2, asshown in Fig. 2. The observation of many SFs and thefollowed martensite phase transformation are also theevidence of low SFE. Thus, planar slip is facilitated by thelow SFE and the large internal friction stress in this HEA.Deformation twinning has been widely reported inFCC materials with SFE in the range of 18–35 mJ m−2[31]. The SFE of our HEA, as noted earlier, is ~20 mJ m−2,very close to the critical point, which explains that onlyrare mechanical twins can be seen. Though very rare, twodifferent types of twinning are detected (Fig. 7c and e).The first type of twinning is formed in a coarse grain(Fig. 7c), while the second one is formed in a nano-sizeband structure (Fig. 7e). Once a dislocation source isobtained in the grain interior of coarse grain, twins can beformed via partial dislocations with the same Burgersvector gliding on every {111} plane [32]. The nano twins