Phase formation and mechanical features in (AlCoCrFeNi)_100-xHf_x high-entropy alloys: The role of Hf

Tailoring the alloy composition, which thus adjusts the phase selection and the generation of intermetallic phases to obtain the desired mechanical properties, is a suitable approach in structural applications. To modify the mechanical properties of equimolar AlCoCrFeNi high entropy alloy (HEA), we design a few (AlCoCrFeNi)_100-xHf_x HEAs (x = 0, 2, 4 and 6, at.%) and evaluate the effect of Hf additions on the phase evolution and mechanical properties. The additions of Hf transform the microstructures of HEA series from a single-phase BCC solid solution to a hypoeutectic microstructure, in which the hypoeutectic microstructure is composed of primary BCC phase and the eutectic structure with alternately growing BCC and Laves phases. Meanwhile, several criteria (δ-Δχ and Δχ) are employed to predict the formation possibility of intermetallics in the present HEA system. When raising Hf additions, the volume fraction of Laves phase increases from 0 to 33.8 vol.%, thus resulting in an increment in compressive yield strength from about 1273 MPa to ∼2023 MPa with degrading the plasticities of HEA series to some extent. This resultes from solid solution strengthening, second phase strengthening and fine grain strengthening. This work will open up other doors for HEA design with superior properties.