Microstructure And Mechanical Properties Of BCC-based Al-TM High Entropy Alloys With Coherent Precipitation Strengthening

Body-centered cubic（BCC）based Al-TM（transition metal）high-entropy alloys（HEAs）have attracted extensive attention in the application of structural materials due to their excellent properties such as high strength,high hardness and wear resistance,which are mainly attributed to precipitation strengthening by ordered second-phase particles.However,it is difficult to obtain cuboidal or spherical shapes of coherent B2 precipitates in BCC-based 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 in Al-contained HEAs,leading to a serious brittleness.In recent years,it has been widely-reported that the lattice mismatch between matrix and ordered pahse is the most critical factor to determine the shape and size of precipitated particles.Therefore,the lattice mismatch between BCC matrix and second-phase particles can be achieved by adjusting TM elements in Al-TM high-entropy alloys,and then the desired coherent structure can be obtained.In this paper,a series of BCC-based Al-TM high-entropy alloys with the guide of the cluster formula approach were prepared by suction-cast processing.And then microstructures and mechanical properties were characterized comprehensively by X-ray diffraction,optical microscopy,scanning electron microscopy,transmission electron microscopy,microhardness test and compression test,and so on.The conclusions are as follows:（1）In two series of Al-LTM HEAs（LTM,Late Transition Metals）,Al_0.7NiCoFe_1.5Cr_1.5.5 and Al_0.7NiCoFeCr₂,a certain amount of Ti（Al/Ti=3/1,2/1,1/1）substituting of Al can change the coherent-precipitated ordered B2-AlNi in BCC matrix to a highly-ordered L2₁-Ni₂AlTi phase.When Ti content is too high（Al/Ti=1/1）,the matrix of（AlTi）_0.7NiCoFeCr₂ HEA transforms into?-FeCr phase,rather than BCC structure.（2）The morphology of L2₁-Ni₂AlTi precipitates is closely related to the lattice misfit（?）between the ordered L2₁ and BCC matrix.The coherent cuboidal L2₁ precipitates with particle size of 55⁹0 nm inner-dendrites are attributed to the moderate lattice misfit（?=0.4⁰.5%）between BCC and L2₁ phases.However,a large lattice misfit（?>1.0%）can alter the particle shape,leading to a lath-like microstructure composed by L2₁ and BCC matrix in the inter-dendrites of HEAs.（3）The two series of BCC-based Al₂（Ni,Co,Fe,Cr）₅ HEAs exhibit high strength due to the precipitation strengthening of coherent cuboidal B2 or L2₁ nanoparticles,i.e.the hardness HV=540⁶40,and their compressive yield strength?_y=1560¹800 MPa.Additionally,when the matrix is?phase,the（AlTi）_0.7NiCoFeCr₂ HEA shows a high brittleness.Furthermore,it was found that the high strength of the two series of alloys is closely related to the size of nanoparticles according to the strengthening mechanism.When the particle size reaches a critical value,the maximum strength increment of the alloys can be achieved by coherent precipitation strengthening.（4）In Al-ETM HEAs（ETM,Early Transition Metals）,the Al-Ti-Zr-Nb-Ta-Mo refractory high entropy alloys designed by cluster formula approach exhibit dual-phase BCC structure.Therein,when the TM=Ti₄Zr₄Nb₃Ta₃,there are a large number of Al-and Zr-rich coherent B2 and D8₈-Al₃Zr₅ nanoparticles precipitated on the BCC matrix of the Al₂Ti₄Zr₄Nb₃Ta₃refractory high-entropy alloy after solution（1300°C/2h）and aging（600°C/24 h）at high temperature.The size of the particles is about 30⁵0 nm,which makes the alloy obtain high strength,and the hardness is HV=660.In addition,the transformation from B2 to D8₈-Al₃Zr₅is discussed,i.e.,the D8₈-Al₃Zr₅ is transformed from B2 and there is no significant difference in morphology and composition.