Study On BCC/B2 Phase Coherent Precipitation Of Al_xCrFe_yNi High Entropy Alloy

Al-TM（Transition Metal）high entropy alloys show excellent properties in terms of strength,hardness,wear resistance and high temperature stability,and have shown great potential for applications in the fields of structural and functional materials.Alloy properties are directly related to the precipitation phase morphology,and the super-coherent precipitation of nanocubic B2 particles can effectively enhance the mechanical properties and high-temperature performance of the alloy.However,it is not easy to achieve super-coherent lattice precipitation of nanoscale B2 particles in Al-TM high-entropy alloys,and in most cases the second phase is dendritic or woven morphology,which can lead to deterioration of alloy properties.The influence of lattice mismatch on the B2 phase morphology is dominant,which includes the second phase size and volume fraction.Therefore,by changing the content of Al or Fe elements in Al_xCr_1.5Fe_1.5Ni high-entropy alloy,the lattice mismatch between the matrix phase and B2 phase in the alloy can be adjusted to obtain nano-cubic B2 phase with super-coherent lattice structure with the matrix phase,which is important to obtain the design law of high performance alloy.In this work,Al_xCr_1.5Fe_1.5Ni（x=0,0.25,0.5,0.75,1.0）and Al_0.5Cr_1.5Fe_yNi（y=0.5,1.0,1.5,2.0,2.5）series high entropy alloys were prepared by vacuum arc melting furnace,and the microstructure and mechanical properties of the alloys were characterized using optical microscope,X-ray diffractometer,scanning electron microscope,transmission electron microscope,Vickers hardness tester,and universal testing machine.hardness tester,universal testing machine and other experimental equipment to characterize the microstructure and mechanical properties of the alloy,the study concluded as follows:（1）In Al_xCr_1.5Fe_1.5Ni（x=0,0.25,0.5,0.75,1.0）high-entropy alloys,the alloy has a two-phase FCC+BCC structure when x=0,0.25;the alloy phase structure is BCC+B2 phase when x=0.5,0.75;and the alloy has a single-phase BCC solid solution structure when x=1.（2）Al_0.5Cr_1.5Fe_yNi（y=0.5,1.0,1.5,2.0,2.5）high entropy alloy,with the variation of Fe content,when 0.5≤y≤1.5,the alloy keeps BCC+B2 dual-phase structure;when y≥2,the alloy is single-phase BCC structure.（3）The lattice mismatch between the B2 phase of the Al_0.5Cr_1.5Fe_1.5Ni alloy and the BCC matrix phase isε=0.34%,and the B2 phase precipitates in a super-eutectic structure with a B2phase size of 564±118nm and a volume fraction of about 43%.The lattice mismatch between the matrix BCC and B2 phases in the Ni high-entropy alloy is 0.45%,with the eutectic precipitation of cubic nano-B2 phase accompanied by the precipitation of daisy-like eutectic phase.（4）The super-eutectic and eutectic structures of the nano-B2 phase and the matrix phase BCC phase in the Al_0.5Cr_1.5Fe_1.5Ni and Al_0.5Cr_1.5Fe_0.5Ni alloys give the alloys high strength and hardness with a compressive yield strength of 1554MPa and 2004MPa,elongation of 27%and19%,respectively,and hardness of 530HV and 656HV.The high strength of the Al_0.5Cr_1.5Fe_0.5Ni alloy comes from the combined effect of the daisy-like eutectic phase and the nano-B2 phase.（5）The as-cast Al_0.5Cr_1.5Fe_1.5Ni had a B2 phase size of 475±124nm and a volume fraction of about 33%after heat treatment at 773K for 2h.After 32h aging,the compressive strength of the alloy was stabilized at 1750MPa,elongation at about 15%,and Vickers hardness at 630HV.