Microstructure And Properties Of AlFeNiCoCr System High Entropy Alloys Prepared By Mechanical Alloying

Multi-principal-component high entropy alloys are defined as a new class of alloy consisting of at least four principal-elements with 5-35 at.% concentrations for each element. Due to the “high-entropy effect”, solid-solution phases are usually formed in HEAs. These novel alloys can possess high hardness, high strength, excellent wear resistance, high thermal stability, distinctive electrical and magnetic properties and excellent mechanical performance at cryogenic temperatures, etc. To date, HEAs is one of the hot topics in the field of materials science.In this paper, Alx Fe Ni Co Cr high entropy alloy system were fabricated by mechanical alloying(MA) and spark plasma sintering(SPS). The phase composition, microstructure characteristic and mechanical properties of Alx Fe Ni Co Cr alloys were studied. The effects of adding Ti, Mn and different contents of Co and Ni on microstructure and properties of Alx Fe Ni Co Cr high entropy alloys were studied. In addition, the influence of different parameters for the MA process and sintering temperature, and the effect of introducing reinforcements on microstructure and mechanical properties of Alx Fe Ni Co Cr high entropy alloys were investigated. The main conclusions are as follows:(1) Increasing Al content can promote the formation of BCC phases in poweders during MA. Following SPS, Fe Ni Co Cr alloy consists of mostly FCC solid-solution and a trace amount of Cr-rich phase; Al0.25 Fe Ni Co Cr alloy consists of only a single FCC phase; however, Al0.5Fe Ni Co Cr、Al0.75 Fe Ni Co Cr and Al Fe Ni Co Cr alloys consist of FCC+BCC phases. In addition, FCC phases are Fe-Co-Cr-rich phases while BCC phases are Al-Ni-rich phases. TEM analysis indicated that FCC phases of Fe Ni Co Cr and Al0.75 Fe Ni Co Cr alloys contain some nanoscale twins.(2) With the increase of Al element, the compressive strength and hardness of Alx Fe Ni Co Cr alloys increase, whereas ductility decrease. Al0.75 Fe Ni Co Cr alloy shows the best compressive properties, its compressive yield strength, compressive strength and strain-to-failure are 1938 MPa and 2221 MPa and 7.6%, respectively. Fe Ni Co Cr alloy displayed the best ductility, its tensile yield strength, tensile strength and elongation are 1295 MPa, 1315 MPa and 1.7%, respectively.(3) By adding 5at.% Ti element into Al0.75 Fe Ni Co Cr alloy, Al-Ti-Ni-rich phases exhitbing BCC sturctrue are formed in the alloy, and the solid-solution strengthening effect is enhanced. Therefore, the strength of alloy increase while ductility decrease. The addition of Mn would change the intrinsic nature of the BCC and FCC phases, and accordingly both strength and ductility of the investigated alloys are decreased. Increasing Co and Ni contents are in favor of forming FCC phases in the alloys. Furthermore,the nanocrystalline in the powder forms could grow slowly during the sintering process. As a result, the bulk alloys exhibit mostly nanoscale grains, which would lead to higher strength and lower ductility.(4) Compared with the 45 h dry miling + 5h wet miling process, the SPSed Al0.75 Fe Ni Co Cr alloy fabricated by 50 h wet milling shows higher strength and hardness, while lower ductily. The grains of Al0.75 Fe Ni Co Cr alloy sintered at 1200 ℃ are coarser than those of Al0.75 Fe Ni Co Cr alloy sintered at 1000 ℃. Therefore, Al0.75 Fe Ni Co Cr alloy sintered at 1200 ℃ shows a higher yield strength and a lower ductility, and displays good work hardening capacity. The Ti C or WC reinforcements are distributed primarily in the FCC phase of Ti C and WC/Al0.75 Fe Ni Co Cr composites. Obviously, adding Ti C or WC reinforcements can promote the strength of Al0.75 Fe Ni Co Cr composites, while reduce ductility.