Effects Of W On Microstructure And Properties Of CoCrNiTiAl Series High-Entropy Alloys

High-entropy alloys have attracted wide attention due to their excellent comprehensive mechanical properties,thermal stability,corrosion resistance and radiation resistance.They are regarded as potential replacements for traditional alloys in aerospace,turbine engines,automobiles,defense weapons and nuclear industries.Due to the lack of phase diagram data of high-entropy alloys with four and more elements and the prediction of alloy properties by alloying elements,many researchers and businesses are concerned about the successful development and effective application of high-entropy alloys.Co Cr Ni Ti Al-based high-entropy alloys are expected to replace traditional high-entropy alloys in high-temperature environments.However,the research data on the phase diagrams and phase stability of high-entropy alloys are still insufficient.The effects of alloying elements on the mechanical properties and high-temperature oxidation resistance of high-entropy alloys disciplines and mechanisms are still unclear,leading to the fact that high-entropy alloys cannot completely replace traditional nickel-based super alloys.Therefore,it is urgent to make breakthroughs in the selection of alloying elements,the regulation of composition,and the preparation process.The study found that the W element has a significant effect on the improvement of the mechanical properties of the alloy,but more research on the effect of its composition on its high temperature performance is needed.Therefore,this paper will study the phase diagram of Co Cr Ni Ti Al-based high-entropy alloys and the influence of W element on the microstructure and properties of Co Cr Ni Ti Al-based high-entropy alloys,so as to realize the improvement of alloy properties by regulating the composition and proportion of alloying elements.In this paper,the phase diagrams of（Fe Co Ni Cr）₉₄Al₂Ti₄and Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W_xhigh entropy alloy in the temperature range of 500-1500℃are calculated by Thermo-Calc software and TCNI-5 database,and compared with the prediction performance,the Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W_xhigh entropy alloy is selected as the research object in this paper;Co_33.5Cr_33.5Ni₂₀Ti₃Al₅W_x（x=0,3,5,7,9）high-entropy alloys with different W contents were successfully prepared by vacuum arc melting.The effect of W content on the microstructure of Co_33.5Cr_33.5Ni₂₀Ti₃Al₅W_x（x=0,3,5,7,9）high-entropy alloys was discussed by optical microscope,SEM,XRD and EDS characterization;the effects of W content on the high-temperature oxidation resistance and mechanical properties of Co_33.5Cr_33.5Ni₂₀Ti₃Al₅W_x（x=0,3,5,7,9）high-entropy alloys were investigated by high-temperature oxidation experiment and room temperature tensile experiment.The main results of this paper are listed as following:（1）Through Thermo-Calc software and TCNI-5 database,the alloy phase diagrams with different phase ratios of（Fe Co Ni Cr）₉₄Al₂Ti₄between 500-1500℃are calculated.Compared with the previous experimental results,the calculated results are consistent with the actual results,which proves that the phase diagram calculation results have high reliability;calculate and predict Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₅alloy phase diagram,comparing Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₅alloy phase diagram and（Fe Co Ni Cr）₉₄Al₂Ti₄alloy phase diagram.The results show that Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₅alloy has a smaller range of brittle phases in the phase diagram structure.Finally,Co Ni Cr Al Ti W high-entropy alloy is selected as the experimental object in this paper;the phase diagrams of Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₃alloy and Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₁₂alloy are obtained by phase diagram calculation.The phase diagrams of Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₃,Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₅and Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₁₂alloys with different W content are compared.The results show that Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W₅alloy is better in microstructure due to smaller brittle phase temperature range and longer two-phase region.（2）Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W_x（x=0,3,5,7,9）high-entropy alloys with different W contents were successfully prepared by vacuum induction with melting furnace and pure metal block raw materials.XRD,optical microscope analysis,SEM and EDS characterization show that 0W alloy is mainly composed of two-phase solid solution phase,the base phase is gamma phase with relatively large proportion of Co and W elements,and the dendritic phase is BCC phase;3W,5W and 7W alloys are mainly composed of two-phase solid solution phase and granular precipitates.The granular phase is HCP phase with a relatively large proportion of Cr and Ti elements.With the increase of W content,the microstructure grain size first decreased and then increased,and the number of granular HCP precipitates also showed a trend to increase first and then decrease.Among them,5W grain size is the smallest and the number of HCP particles is the largest;The microstructure of 9W alloy is single-phase gamma phase solid solution phase.（3）The oxidation weight gain of Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W_x（x=0,3,5,7,9）high-entropy alloy at 900℃for 4 hours was measured by thermal analyzer,and the oxide layer was characterized by SEM and EDS.The results show that with the addition of W content,the oxidation resistance of Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W_x（x=0,3,5,7,9）high-entropy alloy showed a trend of decreasing first,then strengthening and then decreasing,among which 0W alloy and 7W alloy have better oxidation resistance.The mechanical properties of Co_33.5Ni_33.5Cr₂₀Al₅Ti₃W_x（x=0,3,5,7,9）high-entropy alloy at room temperature were measured by micro hardness tester and universal testing machine,and the tensile fracture morphology was characterized by SEM.The results show that the fracture strength,plastic strain and micro hardness of the alloy all increase first and then decrease with the increase of W content.Among them,the mechanical properties of 5W alloy are the best due to the joint effect of solid solution strengthening caused by the addition of W element and fine grain strengthening caused by grain reduction.