The tension-compression asymmetry of martensite phase transformation in a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy

The microstructural evolution of a metastable face centered cubic (FCC) Fe40Co20Cr20Mn10Ni10 high-entropy alloy (HEA) under both tension and compression is systemically investigated. The results show much higher level of martensite phase transformation from FCC structure to hexagonal closed packed (HCP) structure under compression than tension, indicating a distinct tension-compression asymmetry. The compressive tests underwent higher true stresses, which further provided stronger driving forces to trigger the phase transformation than those in tensile tests. Except for the martensite phase transformation, dislocation planar slip prevails in both tension and compression, along with the occasional formation of mechanical twins. Dislocation slip dominates the whole tensile deformation, while both dislocation motions and martensite phase transformation play critical roles in the compressive deformation. The martensite phase transformation is preferred to nucleate at grain or subgrain boundaries due to a medium stacking fault energy (SFE) of similar to 20 mJ m(-2). The formation of HCP phase via partial dislocation emission from low angle grain boundaries offers additional pathways for martensite phase transformation. Our study thus remarkably benefits the understanding of the de formation mechanisms of metastable HEAs.