Phase Stability and Microhardness of the AlxCr2-xCoFeNi High-Entropy Alloys

The phase constituent, phase stability, microstructure, and hardness of seven arc melted AlxCr2-xCoFeNi (x = 0.25 similar to 1.75) high-entropy alloys, in as-cast and 800 or 1000 degrees C annealed states, were investigated using XRD, SEM, EDS and DSC. With the increase of Al content, the FCC phase disappeared in the as-cast AlxCr2-xCoFeNi alloys. As for the as-cast alloys with 25 similar to 30 at.%Al, the composition difference between the former and the later solidified B2 phase can be clearly identified. As for the AlxCr2-xCoFeNi (x = 0.25 similar to 1.0) alloys, the sigma phase formed at above 576 degrees C and decomposed at 912 similar to 1033 degrees C. Fine FCC, BCC, B2 and sigma phases existed in the 800 degrees C annealed AlxCr2-xCoFeNi (x = 0.25 similar to 1.25) alloys. And the hardness became higher than that of the as-cast alloys. When the Al content was 25 at.%, the hardness of the alloy decreased significantly with the disappearance of the sigma phase. After annealing at 1000 degrees C, the sigma phase only existed in the Al0.25Cr1.75CoFeNi and Al0.5Cr1.5CoFeNi alloys. With the increase of Al content, the alloy hardness also increased. Prolonging the annealing time, from 120 h to 480 h, can decrease the Al content in precipitated FCC or BCC phases and coarsen the phase grains, which will decrease of the alloy hardness. In all, the change of hardness after annealing depends on the contributions of formation of the harder sigma phase or the softer FCC/BCC phases and the grain size of the precipitates. The results will help design the composition and treatment process of Al-Co-Cr-Fe-Ni high-entropy alloys.