Strain and stress are two critical

Strain and stress are two critical extrinsic factors for themartensite phase transformation [9,18]. The strain provides the nucleation sites and the stress offers the drivingforce [38], both of which are essential for martensitephase transformation. In current study, at the samestrain, the fraction of martensite is higher under compression than tension (see Fig. 4), suggesting that thetension-compression asymmetry results from the different applied stresses. Much higher stress in compression(see Table 1) provides stronger driving forces to overcome the martensite nucleation barrier, thus accountingfor the formation of much more HCP phases. Especiallyfor current HEA with a relative high SFE compared withother metastable HEAs, more driving force is required totrigger the martensite phase transformation. For instance,the stress of the compressive samples at the later deformation stage (strain of 0.6) is ~500 MPa higher thanthat of the tensile samples, resulting in the increment of30% HCP phase. Similar phenomenon has been observedin literatures. Liu et al. [18] found martensite phasetransformation in the FeCoCrNi HEA at low temperaturetension (e.g., 4.2 K) due to the higher applied stress, whilethe martensite was absence at room temperature. Wang etal. [21] reported that more martensites were formed atliquid nitrogen temperature than at room temperatureunder the same strain in the tensile tested Fe49.5Mn30Co10Cr10C0.5 HEA. An important reason is the very highflow stress under the cryogenic conditions. In addition,when comparing the T3 and C2 samples with very closestress level (~1250 MPa), the fractions of HCP phase arenearly the same. This suggests that the martensite phasetransformation is more sensitive to the applied stress inthis HEA at relatively high strain levels. It is reasonablesince sufficient nucleation sites were provided for phasetransformation at very high strains. The stress became thedominating factor for martensite transformation. Inshort, a higher extent of martensite phase transformationduring compression than tension in this HEA originatesfrom the higher applied stress, which provides the largerdriving force.