Effects of Ni and Al on precipitation behavior and mechanical properties of precipitation-hardened CoCrFeNi high-entropy alloys

Morphology and volume fraction of precipitates are crucial for mechanical properties of precipitation-hardened high-entropy alloys (HEAs). In this study, we systematically examined the effects of Ni and Al on precipitation behavior and mechanical properties of (FeCoCr) 98-x-y NixTi2Aly (x = 25, 30, 35; y = 4, 6, 8) HEAs. It was found that an increase of Ni and Al can effectively promote the formation of coherent γ′ precipitates, but an incoherent Heusler phase (Ni2AlTi) also inevitably appeared once the Al content exceeds 6%. Both γ′ and the Heusler phases could strengthen the materials, however, since they shared the same constituent elements, the two phases produced synergistic effects. For instance, excessive Heusler phase not only restrains the formation of γ′, but also reduces the ductility. Our analysis indicated that the ratio of Ni over Al played a pivotal role on the Heusler phase formation, and the Heusler phase could be completely eliminated when the Ni/Al ratio is larger than 5. In such a case, the maximum volume fraction of γ′ in (FeCoCr)57Ni35Ti2Al6 reaches 33% and the alloy exhibits tensile strength over 1 GPa with a decent ductility of about 35%. Our findings provide a useful guideline for the optimal design of precipitation-hardened HEAs, it also shed light on understanding precipitation behavior in highly concentrated alloys.