Composition And Microstructure Design And Properties Of Al-TM High Entropy Alloys With BCC/B2 Coherent Precipitation

Al-TM?transition metal?high-entropy alloys?HEAs?have attracted more interests in the application of structural and functional materials due to their prominent properties such as high strength,high hardness,microstructural stability,superior resistances to oxidation at elevated temperatures,soft magnetic properties and so on,which are mainly attributed to the special coherent microstructure with second-phase particles precipitated in the solid solution matrix.Similar to Ni-based superalloys,the coherent microstructure of cuboidal B2nanoparticles precipitated in the body-centered cubic?BCC?matrix would obtain the excellent combination of strength and plasticity especially at high temperatures.Similar to Fe-based nanocrystalline alloys,the coherent microstructure of spherical magnetic BCC nanoparticles precipitated in the B2 matrix would exhibit outstanding soft magnetic properties.However,the cuboidal or spherical shapes of coherent precipitates is difficult to form in Al-TM HEAs due to a large lattice misfit between BCC and B2 phases induced by large composition difference.Thus,a weave-like microstructure of BCC and B2 is always existed,destroying the properties severely.It has been confirmed that the lattice mismatch between matrix and ordered phase could determine the shape and size of precipitated particles.Therefore,the moderate lattice mismatch between matrix and second-phase particles can be achieved by adjusting TM ratios and the phase compositions in Al-TM high-entropy alloys,and then the desired coherent microstructure can be obtained.In this work,BCC and B2-based Al-TM high-entropy alloys with the guide of the cluster formula approach were prepared by suction-cast processing and heat treatment.And then microstructures,mechanical and soft magnetic properties were characterized comprehensively by various experimental methods.The main conclusions are as follows:?1?In the Al_0.7NiCoFe_1.5Cr_1.5.5 HEA,the coherent microstructure with cuboidal B2nanoprecipitates in BCC matrix can be formed in the as-cast state and be stabilized up to 823K with a particle size of 60¹20 nm.Even after being heat-treated at 773 K for 1080 h,the cuboidal B2 nanoprecipitates were still stabilized without any coarsening.In light of the precipitation strengthening mechanism,the HEA obtained high strength as the compressive yield strength at room temperature was 1523 MPa.?2?The series of Al_1.5?Co,Fe,Cr?₇ HEAs all exhibited excellent soft magnetic properties.Especially,the 773 K/24h-treated Al_1.5Co₃Fe₃Cr HEA with 2⁴ nm magnetic BCC coherent nanoprecipitates performed outstanding soft magnetic properties with saturation magnetization?M_S?of 146.3 emu/g,coercivity?H_C?of 111 A/m?1.4 Oe?,Curie temperature?T_C?of 1059 K,and the room-temperature electrical resistivity?7??)of 276??·cm.The service temperature is as high as 873 K since even after heat treatment at 873 K for 480 h,the soft magnetic properties of the 773 K-treated Al_1.5Co₃Fe₃Cr HEA were still stable.?3?For Al-TM high entropy alloy systems,the lattice mismatch?determines the shape and size of precipitated particles.The low lattice misfit??<0.2%?that favored the formation of fine coherent cuboidal BCC nanoprecipitates.Besides,it was due to a moderate lattice misfit?of 0.4⁰.5%that favored the formation of coherent cuboidal B2 nanoprecipitates.?4?The absent micro-scaled cells and fine nanoparticles which probably influenced by the heat-treated conditions are important for achieving the lower H_C in the Al-TM HEAs.The chemical composition of the soft magnets should contain enough number of ferromagnetic elements?Fe,and Co?and properly Co substitution for partial Fe could improve B_S obviously in the HEAs.