Abstract: Although tension-compression (T-C) asymmetry in yield strength was rarely documented in coarse-grained face centered cubic (FCC) metals as critical resolved shear stress (CRSS) for dislocation slip differs little between tension and compression, the T-C asymmetry in strength, i.e., higher strength when loaded in compression than in tension, was reported in some FCC high entropy alloys (HEAs) due to twinning and phase transitions activated at high strain regimes in compression. In this paper, we demonstrate a reversed and atypical tension-compression asymmetry (tensile strength markedly exceeds compressive strength) in a non-equiatomic FCC Ni₂CoFeV_0.5Mo_0.2 medium entropy alloy (MEA) under dynamic loading, wherein dislocation slip governs dynamic deformation without twins or phase transitions. The asymmetry can be primarily interpreted as higher CRSS and more hard slip modes (lower average Schmid factor) activated in grains under dynamic tension than compression. Besides, larger strain rate sensitivity in dynamic tension overwhelmingly contributes to the higher flow stress, thanks to the occurrence of more immobile Lomer-locks, narrower spacing of planar slip bands and higher dislocation density. This finding may provide some insights into designing MEAs/HEAs with desired properties under extreme conditions such as blast, impact and crash.

Key words: Medium entropy alloy, Dynamic deformation, Tension-compression asymmetry, Slip trace analysis, EBSD and TEM