Microstructure And Mechanical /Magnetic Properties Of High Entropy Alloys Containing Boron And Rare Earth Prepared By Microwave Sintering

High-entropy alloys?HEAs?is a multi-component alloy with simple phase structure and excellent performance.By adjusting the type and content of elements,alloys with different structures and different properties can be obtained.By using specific preparation methods and matching different post-treatment process,some new materials can be prepared with special ingredients,specific microstructure and excellent performance that cannot be obtained by conventional preparation methods.At present,the methods for preparing high-entropy alloys are generally complicated to operate,and the preparation cost is relatively high,which is not conducive to the practical application and promotion of high-entropy alloys.In this paper,the preparation route of"high energy ball milling+cold isostatic pressing+microwave sintering"was selected to prepare a series of high-entropy alloy containing boron and rare earth elements.The method is simple to operate and easy to control the process.Under the demand of lower cost,it fully utilizes the technical advantages of each stage,and the prepared alloy has excellent performance,which provides a useful reference for the future application of high-entropy alloys.The experiment utilizes a thermal analyzer?DSC?to measure the high-temperature phase structure and weight change of the alloy,and uses the scanning electron microscope?SEM?,energy disperse spectrometer?EDS?,X-ray diffractometer?XRD?,and transmission electron microscope?TEM?to observe the microstructure and phase structure,test the compressibility and microhardness,and uses the vibrating sample magnetometer?VSM?to test the magnetic properties of the high-entropy alloy.The research results show that the preparation route of"high-energy ball milling+cold isostatic pressing+microwave sintering"is helpful to obtain bulk high-entropy alloy with higher density.From the point of optimized high-energy ball milling parameters,the suitable milling parameters are:ball diameter ratio m₅:m₁₀:m₁₅ is 4:2:1,ball material ratio is 5:1 and ball milling time is 60h,at this time the degree of alloying is high and the particle shape is intergrated.The particle size is less than or equal to 2?m.As for the effect of microwave sintering parameters on the HEAs containing boron&neogram,the heating rate has a greater influence.The suitable heating rate for sintering FeCoNiCrAlB_0.1Nd_0.05 HEAs is 30?/min,the AlBO₃ precipitates helps to expulse the gas and improve the compactness and performance.The suitable heating rate for sintering FeCoNiCrCuB_0.1Nd_0.05 HEAs is 50?/min.On this condition,the amount of precipitated boride phase Cr₃B₅ is moderate and benefical to improve the strength and toughness.In addition,the compactness and mechanical properties of aluminum-containing HEAs are lower than those of copper-containing HEAs.This is because the copper-containing HEAs have better thermal conductivity and the corresponding alloys exhibit better compactness and mechanical properties.As for the effect of Y addition,the added Y increases the lattice distortion of the FeCoNi_1.5CuB matrix alloy,and the diffraction peak of the FCC base structure phase shifts to a low angle.The microstructure is composed by three parts as boride M₃B region,FCC HEA base region with high copper content and yttrium-rich BYO₃ region.As the Y increases,the entropy enthalpy ratio decreases,and YB₁₂ particles appear in the yttrium-rich precipitated phase region.The hardness,compressive strength,yield strength and maximum compression ratio of optimized FeCoNi_1.5CuBY_0.2 HEA has increased by 1.3%,23.1%,9.6%,and 47.4%respectively in comparison to the FeCoNi_1.5CuB,which is attributed to the solution strengthening,fine grain strengthening and dispersion strengthening.When the adding amount of Y is 0.5,too high Y will destroy the original HEA structure,and the effect of inhibiting the boride growth will also be reduced,so that the plasticity and soft magnetic properties will be reduced.In accordance with the effect of adding B,the addition of B is not conducive to maintaining the crystal structure during the milling process.It reduces the liquidus temperature of the alloy and is beneficial to the rapid low-temperature sintering.The added B increases the compactness and eliminates the adverse effects of Y₂O₃ on the alloy substrate,and generates a BYO₃ phase with a inferior strength to the base phase.The increase of B content is accompanied by the continuous improvement of the strength and hardness of FeCoNi_1.5CuB_x Y_0.2 HEA,but its soft magnetic properties are reduced.When the B content exceeds 0.6 molar ratio,the boride appears because of the decrease of entropy enthalpy ratio.When the adding amount of B is 0.4 molar ratio,the HEA has the largest compression ratio.Once exceeds 0.4,the compression ratio decreases,indicating that the boride is not conducive to the improvement of plasticity.In comparison to the effects of adding Y and B,it is considered that the addition of B and Y will gradually increase the mixing entropy,lower down the mixing enthalpy,and enhance the average atomic radius difference,resulting in apparent lattice distortion.It is the main reason to account for the enhancement of hardness and strength.The difference is that the entropy enthalpy ratio of Y-containing FeCoNi_1.5CuB HEA gradually decreases;while the entropy enthalpy ratio of B-containing FeCoNi_1.5CuY_0.2 HEA increases first then decreases,it is shown that the degree of solid dissolution of the alloy first increased and then decreased.FeCoNi_1.5CuB_0.2Y_0.2 alloy had the maximum solid solubility,the XRD peak strength of the alloy is the highest and the microstructure is the densiest.