Research On The Microstructure And Mechanical Properties Of The CrFeNi(CuCo,MnAl) High-entropy Alloys

High-entropy alloys?HEAs?are one of the major discoveries in the field of metal materials in recent years.Unlike the design concept of a single principal element of traditional metal materials,HEAs usually contain at least four similar alloying elements.The collective effects of various components are reflected in the high entropy effect,the slow diffusion effect,the larger lattice distortion and the cocktail effect.HEAs tend to have novel structures and excellent performance and have received extensive attention in key areas such as automotive,marine,nuclear,electronics,and aerospace.Recently,the cubic structural HEAs are widely studied as new alloy systems,but there are still many problems to be solved.Firstly,the single-phase face-centered cubic?FCC?HEAs are often treated as the excellent base for further tuning the microstructure and the mechanical properties.However,few researches on the duel-phase FCC structural HEAs are available in the literature.Secondly,the body-centered cubic?BCC?structural HEAs usually have high strength and poor plasticity,which greatly limits the application of these alloys.There are still few reports on improving the room temperature plasticity of the BCC structural HEAs.The CrFeNi HEA system was selected as the base alloy.We tuned the microstructure and mechanical properties of the FCC and BCC structural HEAs,using differnent methods.In this paper,the dual-phase FCC structural CrFeNiCuCo HEA is treated as the base alloy.Firstly,the alloying effects of Gd on the microstructure and the mechanical properties of CrFeNiCuCo alloy were systematically studied.It was found that the addition of Gd induced the phase structure transition from the dual-phase FCC structure to HS phase?CaCu₅ type?+FCC structures.The hardness of HS phase was about three times of the FCC phase.The formation of HS phase increased the strength and hardness and decreased the plasticity of the alloy.Secondly,the effects of cold rolling and annealing on the microstructure and mechanical properties of CrFeNiCuCo alloy were also studied.It was found that the recrystallization nucleation was preferred in the FCC1 phase region.The fully recrystallization microstructure was obtained after annealing at 1000°C.Annealing can decrease the dislocation density and eliminate the lattice distortion.The hardness,yield strength and tensile strength of the alloy decrease with increasing the annealing temperature,while the plasticity increases with the increase of temperature.Thirdly,the cell structural CrFeNiCuCo semi-solid slurries were successfully fabricated by the RAP process.The interdendritic regions of these alloys were the Cu-riched FCC1phase and the globular grains were FCC2 phase.The grain size and shape factor of the globular grains can be tuned by controlling the semi-solid temperature and soaking time.The coarsening kinetics of the globular grains during the semi-solid processing was also analyzed.The coarsening activation energy of the globular grains was further calculated to be 206.9 kJ/mol.The relationship between the Vickers hardness and average grain size agree well with the Hall-Petch relationship.In addition,a series of novel BCC-based CrFeNiMnAl_x?x=0.5-0.8?HEAs with trimodal structure was fabricated.These HEAs possess microstructures consisting of a BCC matrix and the trimodal distributions of coherent B2 precipitates with three orders of magnitudes in size distributions(i.e.,10⁰?m for the p-B2 precipitates,10^-1?m for the s-B2 precipitates,and 10^-2?m for the t-B2 precipitates).These trimodal HEAs exhibit a good combination of the high compressive yield strength?1091–1200 MPa?and large plasticity?⁴0%?,which are superior to the recently-reported BCC/B2 HEAs.The TEM observations of the alloys at different strains were carried out.The deformation mechanisms of the trimodal alloys were analyzed in detail.The coherent interfaces and the trimodal distribution structure were the main reasons for its excellent performance.The strengthening mechanisms of the designed HEAs are quantitatively analyzed in terms of the solid-solution strengthening,precipitation strengthening,and grain-boundary strengthening.Precipitation and grain-boundary strengthening mechanisms play major roles in strengthening the HEAs.