Microstructure Toughening and Properties of Selective Laser Melted NbMoTaW Refractory High-Entropy Alloys(Invited)
Objective NbMoTaW refractory high-entropy alloy(RHEA)exhibits excellent mechanical properties at ultra-high temperatures,making it especially suitable for high-temperature heat-resistant load-bearing component applications;however,its room temperature brittleness limits its application in aerospace and other fields.Moreover,the high hardness and brittleness of RHEA at room temperature make it extremely difficult to process,and hence the formation of precision and complex parts of RHEA is particularly difficult,further limiting its application.Therefore,improving the formation and plasticity of NbMoTaW RHEA is an important research topic.Methods In this study,two RHEAs,NbMoTaW100-xC,and NbMoTaWTix,are prepared via selective laser melting(SLM).A tungsten plate is used as the base material,which is preheated to 180 ℃ to reduce stress.The optimal SLM process parameters are determined through multiple orthogonal tests to prepare samples with good surface formation and no macroscopic cracks.The relative density of the samples is determined via industrial computed tomography(CT)analysis,and the phase composition of the samples is analyzed using X-ray diffraction in the range of 10°-100°.A scanning electron microscope is used to analyze the surface topography and grain distribution of the specimen[electron backscattered diffraction(EBSD)analysis test].Prior to EBSD characterization,the surfaces of the RHEAs are ground with different SiC sandpapers,finely ground with alumina(Al2O3),vibro-polished,rinsed with ethanol,and air-dried.The compressive properties of the RHEAs are measured using a microcomputer-controlled universal testing machine with a strain of 10 3s 1 and a specimen size of φ2 mmX4 mm.At least five samples are selected for compression performance testing,and the average of the test results is calculated to ensure accuracy of compression performance.Results and Discussions The(NbMoTaW)99 5C0.5 RHEA prepared via SLM has a higher dislocation density,and with the continuous thermal cycling of SLM,a large number of NbC particles are precipitated at grain boundaries and dislocations(Fig.7),which produces a significant pinning effect,thus limiting the coarsening of the NbMoTaW RHEA grains during thermal cycling.The changes in the microstructure of the NbMoTaW RHEA caused by microalloying of C improve its mechanical properties.A large number of NbC particles can produce a significant precipitation strengthening effect,which significantly increases the strength of the(NbMoTaW)99 5C05 RHEA.In addition,the plasticity of the NbMoTaW RHEA also significantly improves after C microalloying.This is because the microalloying of C atoms inhibits O segregation at grain boundaries,thus ensuring stronger bonding of the matrix and inhibiting intergranular cracks.The microalloying of C with atomic fraction of 0.5％significantly improves the strength and plasticity of SLMed NbMoTaW,and the compressive yield and fracture strengths of the NbMoTaW RHEA increase significantly from 1183 MPa and 1214 MPa to 1695 MPa and 1751 MPa,respectively,an increase of 43.3％and 44.2％,respectively.Compared with those of NbMoTaW,the yield strength,compressive strength,and strain of the NbMoTaWTi0.5 RHEA increase by 20.7％,30.7％,and 117.9％,respectively.The variation in yield strength and compressibility with Ti content is shown in Fig.12(b).The yield strength of the NbMoTaWTi,RHEA increases rapidly after the addition of a small amount of Ti and gradually decreases with the subsequent increase in Ti,whereas the compressive strain of the NbMoTaWTi,RHEA increases approximately linearly with an increase in Ti content.The results show that the addition of Ti can increase the grain boundary cohesion of NbMoTaWTi,RHEAs,which can effectively inhibit the propagation of intergranular cracks and improve the plasticity of NbMoTaWTi,RHEAs.Therefore,with an increase in Ti content,the strength and plasticity of NbMoTaWTi,RHEAs prepared via SLM increase at the same time.Conclusions Combining LSM technology and the alloying method,two RHEAs,(NbMoTaW)100-xC,and NbMoTaWTi,,are successfully prepared,and the brittle resistance of the NbMoTaW RHEA at room temperature is improved using the alloying method with C and Ti.A well-formed(NbMoTaW)99 5C0.5 RHEA with a density of 99.6％is prepared using SLM technology.The results show that the addition of C with atomic fraction of 0.5％results in the refinement of the grains of the NbMoTaW RHEA and the precipitation of NbC nanoparticles.By microalloying of of C with atomic fraction of 0.5％,the yield and fracture strengths of the NbMoTaW RHEA increase by 43.3％and 44.2％,respectively,and the plastic strain increases from 3.9％to 6.9％.With an increase in Ti content,the yield strength,compressive strength,and plastic strain of NbMoTaWTi,RHEAs increase significantly by 20.7％,30.7％,and 117.9％,respectively.Moreover,a simulated part of a 100 mm×80 mm×20 mm key component of an ultra-hypersonic aircraft is successfully prepared via SLM.
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