Microstructure Evolution And Mechanical Property Control Of Al_xCrFeNi High Entropy Alloys

High entropy alloys（HEAs）are known as the most promising alloys due to their high strength plasticity,good oxidation resistance,corrosion resistance and high temperature stability.Since the advent of HEAs,the series of Al Co CrFeNi HEAs are widely studied HEAs,and their crystal structure mainly includes solid solution parent phase and intermetallic compounds.However,it found that the cost of the first generation Al Co CrFeNi HEAs was high,which hindered its industrial application to some extent.Therefore,the expensive Co element is removed without damaging the core skeleton of Al Co CrFeNi based on the method of phase formation theory,and a Co-free Al_xCrFeNi（x=0,0.5,1.0,1.25,1.5,1.75）and AlCrFeNi-M quaternary HEAs are designed and prepared.The main conclusions of the study are as follows:（1）The paper studied the change of microstructure and mechanical properties with as-cast and heat treatment Al_xCrFeNi.With the increase of Al content,the phase composition changes from FCC phase to FCC+BCC dual phases.The morphology changes from uniform speckled to large dendritic and interdendritic composite structure.Heat treatment of equal atomic ratio AlCrFeNi are found that the phase composition is transformed into BCC+B2 phases.Its morphology also changes into uniform interlaced network structure and short stem cutting structure.Finally,AlCrFeNi HEAs has excellent mechanical properties when the heat treatment reached 600℃,the compressive strength is 1.9 GPa,and the compressive strain exceed 18%.（2）The evolution of microstructure and mechanical properties of high Al content and heat treatment Al_xCrFeNi are also investigated.The alloys are composed of BCC+B2 phases.When Al content is 1.25,its morphology changes to mixing structure composed of sunflower core structure and local eutectic structure.The structure continues to Al_1.5CrFeNi.When the Al content is 1.75,Al_1.75CrFeNi has a complete eutectic structure.The phase composition and structure of the alloy are not significantly different after heat treatment at 1000℃,but the mechanical properties are improved.Al_1.75CrFeNi with complete and dense eutectic structure has the most excellent mechanical properties under both as-cast and heat treatment.After heat treatment,the compressive strength of Al_1.75CrFeNi reaches 1.9GPa,and the compressive strain is about 19%.（3）The effect of different elements on the structure and mechanical properties of equiatomic ratio AlCrFeNi is also investigated.（AlCrFeNi）_88%M_12%（M=Ti,V,50%Ti+50%V）HEAs are designed.It found that the alloys precipitates new phases without damaging the original phase composition after adding different elements.Add a single Ti element to precipitate a new Ni₂Ti Al phase,and add a single V element to precipitate a new V-Fe phase.For（AlCrFeNi）_88%Ti_6%V_6%with two elements,both phases are precipitated.When the two elements are added to the matrix,the structure shows obvious two-phase segregation.After heat treatment at 1000℃,the alloys also have no obvious changes in structure,but the grain size is obviously refined.The mechanical properties of the alloys after heat treatment are also significantly improved compared with the as-cast alloys.After heat treatment at 1000℃,（AlCrFeNi）_88%Ti_6%V_6%has the excellent mechanical properties,the ultimate compressive strength reaches 1.7 GPa,and the engineering strain reaches 27.1%.