Cluster-formula Design Of BCC/B2 Coherent Al-Ni-Co-Fe-Cr High-Entropy Alloys And Their Properties

High-entropy alloys(HEAs)generally contain multiple principal elements in equimolar or near-equimolar proportions,and often have simple crystal structures,e.g.,face-centered cubic(FCC),body-centered cubic(BCC),or ordered derivatives(such as cP4-AuCu3(L12)and cP2CsCl(B2)).And the microstructure of Al-TMs(TMs,transition metals)HEAs is sensitive to the content of Al.Therefore,the weave-like spinodal decomposition microstructure of BCC and B2 is always observed,and strongly destroyed the mechanical properties.It is found that the alloys,such as Ni-based superalloys,with coherent cuboidal nanoprecipitates have good high-temperature mechanical properties.In addition,the alloys,such as Fe-based nanocrystalline alloys,with spherical magnetic nanoprecipitates have good soft-magnetic properties.However,there is still a lack of systematic study on the coherent morphology of BCC and B2 phases.The key that determines the coherent morphology of BCC and B2 phases in Al-TMs HEAs are unknown.And the stabilities of BCC/B2 coherent microstructure are unknown.The present work will characterize the chemical short-range orders(CSROs)in Al-Ni-CoFe-Cr HEAs based on cluster-plus-glue-atom model,then adjust the chemical composition of BCC and B2 phases simultaneously,and realize the control of BCC/B2 coherent microstructure to obtain HEAs with cuboidal or spherical nanoprecipitates.Meanwhile,the structural stabilities and mechanical/magnetic properties of these new HEAs will be characterized by theory and experiments.The relationships among cluster-formula composition,coherent structure,and mechanical/magnetic properties will be established finally.The details are as follows.1.The existence of CSROs in Al-Ni-Co-Fe-Cr HEAs was verified based on cluster-plusglue-atom model.Two HEAs with a composition of Al2M14(M=Ni4Co4Fe3Cr3 or Ni1Co1Fe2Cr1)were designed,which were derived from the cluster model of[Al-M12](M2Al1)in FCC(face-centered-cubic)structure or[Al-M14]Al1 in BCC structure.It was found from the results of neutron diffraction that the cluster models can describe CSROs and the induced phase transformation(FCC to BCC)better.2.The shape of coherent B2 precipitates is determined primarily by the lattice misfit (?)between the BCC matrix and B2 phase.A small ε(ε<0.2%)results in spherical/ellipsoidal precipitates.At a large ε(ε>0.6%),however,a weave-like microstructure is developed,which causes serious ductility loss.At a moderate ε(ε～0.4%),a microstructure with uniform distribution of coherent cuboidal B2 nanoprecipitates is obtained.Even after heat treatment at 773 K for 1080 h,the cuboidal B2 nanoprecipitates were still stabilized with 107～120 nm,and without any coarsening comparing with as-cast Al0.7NiCoFeCr2 alloy(100 nm).3.The strengthening effect is calculated by the particle shearing and Orowan bowing mechanisms,and the calculations are compared favorably with experiments.The lattice misfitε appears to be the most effective factor to control the strength of the alloy,and specifically an intermediate value of ε～0.4%produces the strength maximum.For example,the calculated critical size of the B2 cuboids in Al0.7NiCoFe2Cr HEA is r0=60 nm,which is actually quite consistent with the experimental average value of r=70 nm.And the ideal largest strength increment is 1187 MPa,which is consistent with the experimental yield strength of 1085 MPa.4.The coherent HEAs has high thermo-mechanical stability.It found that the coherent microstructure with cuboidal B2 nanoprecipitates in BCC based Al0.7NiCoFeCr2 HEAs can be maintained up to a high temperature of 873 K with a high compressive yield strength of σYS=1727～2190 MPa.After a series of thermo-mechanical processing,including coldrolling,homogenizing at 1573 K,and aging at 923 K,nano-sized ellipsoidal ordered Ll2 particles are formed in the FCC dendrites,and the BCC particles are still in B2 inter-dendrites.The ultimate tensile strength of FCC-based AlNi2Co2Fe1.5Cr1.5 HEAs can be enhanced up to 1240 MPa,with a good elongation of 12.4%.5.A soft-magnetic HEA is obtained.The "reversal" coherent microstructure of ultrafine ferromagnetic BCC nanoprecipitates(3～7 nm)uniformly distributed in a B2-phase matrix was obtained,in which the BCC nanoparticles contained Fe,Co,and Cr.and the B2 matrix contains Al and Co.The HEA with "reversal" coherent microstructure exhibited high saturation magnetization(Ms=135.3 emu/g),low coercivity(HC=127.3 A/m),high structure stability and the induced high Curie temperature(Tc=1061 K).These prominent soft-magnetic properties were observed to retain even after the alloy being thermally exposed at 873 K for 555 h(Ms=126.1 emu/g,He=214.9 A/m),apparently attributable to the excellent stability of the coherent BCC particles.The highest electrical resistivity(p=244 μΩ·cm),which is 3～5 times to the resistivity of existing soft magnetic alloys,was obtained due to multi-element alloying in HEAs.The high ρ reduces the eddy current loss of the alloy,and provides a guarantee for its service in high frequency and high temperature.The present work realizes the control of BCC/B2 coherent microstructure,and provides a new design concept for the development of high-performance superalloys and soft-magnetic alloys based on cluster model and multi-element alloying in HEAs.