Microstructure And Properties Of Nanocrystalline Refractory High-Entropy Alloys And High-Entropy Alloy Nitrides

As a hot research direction in recent years,high-entropy alloys have many excellent properties compared with traditional alloys,such as high hardness,good wear resistance,good corrosion resistance and high work hardening ability.As a new type of material with high performance,it shows a broad application prospect.However,the research on the field of high-entropy alloys is still in the initial stage,and at this stage,the melting method is mostly used to prepare the high-entropy alloys,and the grain size is usually relatively large,which leads to the limitation of its mechanical properties.In this paper,Nanocrystalline high-entropy alloys powder was prepared by mechanical alloying,and Nanocrystalline refractory high-entropy alloys and high-entropy alloys nitride were further synthesized by high-temperature and high-pressure curing technology.The microstructure and properties of the alloy powders and blocks were tested and analyzed.The effect of sintering process on the microstructure and properties of the alloy was discussed in order to obtain the best technological parameters and improve the properties of the alloys.Single-phase nanocrystalline BCC VNbMoTaW HEAs powders with particle size of about 140 nm were obtained by mechanical alloying for 30 h.The average grain size of the mechanically alloyed VNbMoTaW HEAs nanocrystals is 5.9 ± 2.8 nm,and the lattice constant of the nanocrystals in the TEM photographs is 3.153.Bulk NC VNbMoTaW HEAs,was prepared by sintering nanocrystalline powders by high temperature and high pressure technology.The experimental pressure was 5 GPa,and the temperature range was800 ? ~ 1500 ?.When the sintering temperature is between 800 ? and 1150 ?,the results of XRD show that the average grain size of single-phase NC VNbMoTaW HEAs,is about 6 nm,and the grain size does not change much with the increase of temperature.Lattice strain and dislocation density ratio sintered at 1150 ? ~ 1500 ? About one order of magnitude higher.When the sintering temperature is above 1150 ?,the grain size increases rapidly from single-phase NC VNbMoTaW HEAs to two-phase mixture,and a small amount of metal carbides with hexagonal structure are observed.The hardness of samples sintered at 5 GPa at high temperature and high pressure,due to the interaction ofgrain size and densification,increases first and then decreases with the increase of temperature,reaching a peak value of 11.4 ± 0.04 GPa,at 1150 ?.It is more than twice that of the corresponding CG VNbMoTaW HEAs,and its high hardness is analyzed.It is found that the main strengthening mechanism of the sample is solution strengthening and fine grain strengthening.High-entropy alloys nitride powder with single-phase nanocrystalline BCC structure was obtained by mechanically alloying VNbMoTaW with N element in nitrogen atmosphere for 30 h.Bulk high entropy alloys nitride was prepared by sintering nanocrystalline powder by high temperature and high pressure technology.The experimental pressure was 5 GPa,the temperature range was 800 ? ~ 1300 ?.When the temperature was 800 ? ~ 900 ?,The results of XRD show that the sintered blocks are still single-phase BCC structure.When the sintering temperature is higher than 1000 ?,phase separation begins to occur,XRD results show that VN precipitates.The average grain size of the high-entropy alloys nitride sintered at 5 GPa,1000 ? is 33.1 ± 8.1 nm,and the lattice constant of the high-entropy alloys nitride at 5 GPa,1000 ? is 3.143 ?.After high-entropy alloys nitride sintering at 5 GPa,1000 ?,there are three kinds of MoTaW metal phase,precipitated phase VN,nano-twinning phase Nb,after high temperature and high pressure sintering of high-entropy alloys nitride under high temperature and high pressure.After sintering at high temperature and high pressure,the bulk high entropy nitride mainly has solid solution strengthening,second phase strengthening and fine grain strengthening,which makes the high-entropy alloys nitride have very high hardness.Starting at 800 ?,with the increase of temperature,Vickers hardness gradually increased,reaching the peak value of 1939 HV,at 1000 ?,which was63% higher than that of VNbMoTaW HEAs.When the temperature exceeded 1000 ?,the hardness decreased with the increase of temperature,and reached 1748 HV.at1300 ?.