Microstructure and Properties of Al0.3CoCrFeNi High Entropy Alloy with Different Nb Contents

High entropy alloys have broken the traditional alloy design concept and expanded the research fields. Due to the interaction between the multi-principal component atoms，high entropy alloys exhibit excellent properties，such as high strength，hardness and wear resistance，good corrosion resistance and oxidation resistance，which has a diversified industrial application prospect. Compared with conventional alloys，the microstructure of high entropy alloys exhibits single face-centered cubic（fcc）phase，single body-centered cubic（bcc）phase or dual phases（fcc+bcc）. Al-Co-Cr-Fe-Ni system is a relatively mature system for researchers at home and abroad. The phase composition of AlxCoCrFeNi alloy changes from single phase fcc，dual phase and single-phase bcc with increasing Al content. Among them，Al0.3CoCrFeNi alloy with single phase fcc exhibit good plasticity and corrosion resistance，but low hardness. The negative mixing enthalpy between Nb and other elements and maximum atomic size in Al0.3CoCrFeNiNb can result in strong precipitation strengthening or eutectic structure. Therefore，Al0.3CoCrFeNiNbx（mole ratio，x=0，0.1，0.3，0.5，0.7，1.0）high entropy alloy rods were prepared by vacuum arc melting. The microstructures were analyzed by X-ray diffraction（XRD），optical microscope （OM）and scanning electron microscope（SEM），and meanwhile the micro Vickers hardness and compression property at room temperature were tested. The effect of Nb content on the microstructure and properties of Al0.3CoCrFeNi alloy were studied. Based on the research results，the optimal composition was obtained. The results showed that the microstructure of Al0.3CoCrFeNi alloy was single phase fcc，and then transformed into eutectic structure composed of fcc and Laves phases after adding Nb. With increasing Nb content，the volume fraction of Laves phase increased. The lattice constant results showed that the diffraction peak the diffraction peak fcc（111）moved first to small angle，and then to large angle，and the lattice constant also first increased and then decreased. Because Nb atom exhibited significantly larger radius than other atoms，it would inevitably lead to lattice distortion when Nb atoms entered fcc solid solution lattice. With increasing Nb content，Nb atoms would be precipitated from fcc solid solution to form Laves phase，so the lattice constant decreased. In addition，Al0.3CoCrFeNi Nb0.7 alloy and Al0.3CoCrFeNiNb1.0 alloy exhibited significantly lower diffraction peaks intensity than Al0.3CoCrFeNi alloy and Al0.3CoCrFeNi Nb0.1 alloy. The results of OM and SEM images showed that the microstructure of Al0.3CoCrFeNiNbx alloy were dendritic，and the microstructure changed from hypoeutectic to hypereutectic. When x=0.5，the microstructure of the alloy changed obviously，and most of the regions were typical eutectic structure composed of fcc and Laves. The two-phase mixed structure was evenly distributed in the cell，and the strip eutectic structure was radial to the periphery. Notably，the primary phase changed from fcc to Laves. EDS analysis showed that Nb was enriched in Laves region，while Al and Ni were enriched in fcc region. Ni and Al atoms exhibited similar fcc structure and were easy to replace each other in the lattice position. While，Nb atom exhibited the largest atomic radius and most negative mixing enthalpy with other atoms，and tended to form Laves phase. Addition of Nb could effectively improve the strength and hardness. With increasing Nb content，the hardness linearly increased from HV 152 for Al0.3CoCrFeNi alloy to HV 745 for Al0.3CoCrFeNiNb1.0 alloy. The compressive strength first increased，and then decreased. Al0.3CoCrFeNiNb0.5 alloy exhibited the best comprehensive mechanical properties，and its compressive strength and Vickers hardness were 2397 MPa and HV 613，respectively. © 2024 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.