Study On The Preparation And Mechanical Properties Of Rare Metals Nb/Mo/Ta/W Based Ultra-high-temperature High-entropy Alloys

New high-performance high-temperature structural materials which are superior to traditional superalloys are in high demand for the refractory fields such as aerospace engines.The traditional superalloys have been unable to meet the demand for thermal structural materials in these fields.Recently,owing to the unique high-temperature properties,the emerging refractory high-entropy alloys(RHEAs)have attracted increasing attention from scientific researchers.The NbMoTaW RHEA still exhibits a yield strength of more than 400 MPa at 1600°C,which makes it as a promising structural material in the refractory fields.However,NbMoTaW alloys exhibit low room temperature plasticity,limiting the application.Therefore,it is very essential to study how to improve the ductility of RHEAs at room temperature under the premise of satisfying high-temperature strength.Based on the NbMoTaW RHEA,the present work designed new pure metal elements RHEAs and ceramic-reinforced RHEAs by adding refractory metal elements or non-metal elements.The RHEAs were prepared by vacuum arc melting.This study also discussed the room temperature strengthening mechanisms,explored the high temperature strengthening mechanisms of metal/non-metal elements in the alloy,realized the design of high-strength alloys at1800?,and completed the study on the fabrication and properties of ultra-high-temperature high-entropy alloys,providing theoretical guidance and research ideas for the design of ultra-high-temperature high-entropy alloys.The present work mainly included the following contents:The effects of equimolar metal elements on the structure and properties of NbMoTaW RHEA as well as the corresponding strengthening mechanisms were investigated through alloying the NbMoTaW alloy with Re,Hf,V,Cr,Zr,Ti elements.It is found that adding equimolar Ti,Zr and Hf elements can effectively improve the strength and plasticity of the alloy at the same time.The compressive strength of NbMoTaWHf and NbMoTaWTi at 1600°C was 232 MPa and 218 MPa,respectively,which were slightly less than that of NbMoTaW alloy(243 MPa),showing good high-temperature strength.Due to the high melting point helped the alloy maintain high strength at 1600°C,so a Mo Ta WRe alloy with the high calculated melting point was designed.The alloy displayed strong resistance to high-temperature softening and a compressive strength of 244 MPa at 1600°C.The Zr element with close-packed hexagonal structure played an important role in improving the plasticity of NbMoTaW alloy.The compressive strength and plasticity of NbMoTaWZr_1.5 alloy were 2470MPa and 21.1%,respectively,which were 2.3 times and 12.4 times that of NbMoTaW alloy.The alloy was strengthened by the combination of solid solution strengthening,fine grain strengthening,and second phase strengthening.The improvement of plasticity was caused by the addition of Zr that regulated the plasticity of the interdendritic zone and changed the fracture mechanism.By adding non-metallic C and N elements to the NbMoTaWM(M=Ti,Zr,Hf or None)alloy,the structure and properties of the equimolar NbMoTaWMC alloy and NbMoTaWMN alloy were studied,and the effects of equimolar non-metallic C and N elements on the NbMoTaWM alloys were also discussed.After adding equimolar C element,the FCC phase formed in the alloy with the refined flake structure of alternating light and dark areas,and the strength and plasticity of the alloy were significantly improved.The room temperature compressive strength of NbMoTaWC was about 2706 MPa,which was 2.5 times that of NbMoTaW alloy.Nitride phase formed in the alloy via adding N element.The grain size was much smaller than that of NbMoTaW alloy,and the strength and plasticity of the alloy were significantly improved.The room temperature compressive strength of NbMoTaWHfN was 2099MPa,which was 1.9 times that of NbMoTaW alloy.By adjusting the C content in the(NbMoTaW)_100-xC_x alloys,the effects of the C content on the phase distribution,microstructure and mechanical properties of the alloys were studied,the comprehensive room temperature mechanical properties of the alloy were adjusted,and the strengthening mechanisms of the alloys were discussed.The mechanical properties and strengthening mechanisms of NbMoTaWC alloy at high temperature were investigated.The alloy exhibited the best compressive strength and plasticity with the C content of 10%-20%.The alloy was strengthened by fine grain strengthening and second phase strengthening at room temperature,and the increase of plasticity was mainly caused by grain refinement.The BCC phase in the NbMoTaWC alloy was removed by acid corrosion,and the microstructure of the residual FCC phase skeleton was studied separately.It is found that the FCC phase was a high-entropy carbide ceramic phase(HEC),which proved that(NbMoTaW)_100-xC_x alloy was a new high-entropy composite material composed of a HEA phase and a HEC phase.The yield strength and compressive strength of the NbMoTaWC alloy at 1600°C were 554 MPa and 585 MPa,respectively,which were2.4 times that of the NbMoTaW alloy under the same condition.The yield strength and compressive strength of the NbMoTaWC alloy at 1800°C were 262 MPa and 291MPa,respectively.The high strength of NbMoTaWC alloy at high temperature can be attributed to the combining of high soft point,fine-grain strengthening,and second phase strengthening.By adjusting the content of Hf and N elements in the NbMoTaW(HfN)_x alloy,the effects of nitrides on the phase distribution,microstructure and mechanical properties of the alloy were studied,the comprehensive room temperature mechanical properties of the alloy were adjusted,and the strengthening mechanisms of the alloy were discussed.The mechanical properties and strengthening mechanisms of NbMoTaWHfN alloy at high temperature were investigated.The compressive strength of NbMoTaWHfN alloy was 1.9 times that of NbMoTaW alloy.The alloy was strengthened by fine-grain strengthening,dispersion strengthening,and solid solution strengthening at room temperature.The yield strength and compressive strength of the NbMoTaWHfN alloy at 1800°C were 288 MPa and 316 MPa,respectively,and the strength was slightly higher than that of the NbMoTaWC alloy.The high strength of NbMoTaWHfN alloy at high temperature can be attributed to the combined influences of high soft point,fine-grain strengthening,and second phase strengthening.The reasons for high strength of NbMoTaWC and NbMoTaWHfN at ultra-high temperatures can be attributed to the stably existing of high-entropy carbide ceramic phase and HfN ceramic phase at high temperature and fine grains.NbMoTaWC and NbMoTaWHfN RHEAs display good strength and strong plasticity at room temperature as well as high strength at high temperature,which exceeds that of the RHEAs studied currently,making them become the very promising structural materials for applications in the field of ultra-high temperature.