Effects of Al and Ti additions on precipitation behavior and mechanical properties of Co35Cr25Fe40-xNix TRIP high entropy alloys

Precipitation behavior and mechanical properties of Ti and Al containing transformation-induced plasticity (TRIP) high entropy alloys (HEAs) are investigated. Plastic deformation mechanism during uniaxial tensile tests are systematically studied by electron backscatter diffraction and transmission electron microscope. The results indicate the thermally induced HCP phase is absent with alloying additions of Ti and Al elements in dual phase TRIP Co35Cr25Fe40-xNix (x = 5, 14) alloy. With the decrease of Ni content, the volume fraction of Heusler phase is high in a wider temperature range, inversely, L1(2) phase are not formed when the Ni content is 5 at.%. Deformation induced HCP phase are also not observed owing to the additions of Ti and Al, replaced by deformation twinning or stacking faults for different Ni content, which indicate the change of plastic deformation mechanism. The variation of hardness and yield strength of (Co35Cr25Fe40-xNix)(94)Al3Ti3 are discussed. The growth of Heusler particles is the main reason for the drop of ductility with the increase of aging time, resulting from the increasing difficulty for the dislocation movements across the grain boundaries. This work is beneficial to comprehensively understand the precipitation-hardened HEAs.