Preparation,Microstructure And Mechanical Properties Of Ceramic-Enhanced CrMoNbWti Refractory High-Entropy Alloys

Refractory high-entropy alloys（RHEAs）have many outstanding performance,such as high ambient/elevated-temperature mechanical properties,excellent creep,oxidation and wear resistances,due to their characteristics of extreme high melting point,effects of multi-principle solid solution,strong lattice distortion and retardation diffusion.Therefore,RHEAs have a great potential to replace Ni-based superalloys as a new generation of aerospace metallic thermal-structural materials,which is expected to expand the service temperature range of metals.At present,precipitations（B2,Laves,ω,etc.）enhanced RHEAs exhibited superior ambient/medium-temperature mechanical properties,however,when the service temperature excessed solution temperature,the precipitations generally have a strong tendency to redissolve into the solid-solution matrix,leading to the sharp drop in elevated-temperature strength.Therefore,in order to develop new metallic thermal-structural materials with better performance and higher service limiting temperature,ceramic-enhanced CrMoNbWTi RHEAs were designed and prepared by mechanically alloying and powder metallurgy techniques in this study.The nonmetallic elements of C,O,N were introduced into equimolar CrMoNbWTi powders in order to form ceramic enhancements in RHEAs by in-situ reaction between nonmetallic and metallic elements.Effects of nonmetallic elements on microstructure,phase composition and mechanical property were investigated,and the formation mechanism and strengthening mechanism of ceramic enhancements were analyzed in this study.The main research results are as follows.Trace O and N doped CrMoNbWTi RHEA was prepared by mechanically alloying and powder metallurgy techniques.Mechanical alloying process,microstructure and mechanical properties were investigated.The results indicated that CrMoNbWTi RHEA powders can be completely alloying into single BCC solid solution after 42 h positive/negative rotating high energy ball milling with rotation speed of 350 r·min^-1 and ratio of grinding to materials of 10:1.During powder metallurgy at 1450°C,the single BCC supersaturation solid solution of CrMoNbWTi with trace O and N transform to duplex-phases:BCC-CrMoNbWTi_0.75 and FCC-Ti（O,N）.The as-sintered RHEA exhibited excellent mechanical properties,i.e.,Vicker’s hardness of 7.2 GPa,yield strength of 2324MPa,fracture strain of 8.6%.The high yield strength was attributed to the solid-solution strengthening,fine-grain strengthening,Orowan strengthening and load transfer strengthening mechanisms.O and N contents in CrMoNbWTi powders were adjusted by controlling the ball milling atmosphere（vacuum,1 k Pa,2 k Pa and 4 k Pa）,and the four corresponding materials were abbreviated as ON-1,ON-2,ON-3 and ON-4 in this study.Effects of O and N content on microstructure and mechanical property were systemically investigated.The results indicated that,as the residual air increased during ball milling,O and N contents in powders increased gradually,and more nitride/oxide-ceramics were found in alloys.Phase compositions of as-sintered ON-1 to ON-4 were BCC+6%Ti（O,N）,BCC+10%Ti（O,N）,BCC+25%Cr₂Nb+19%Ti（O,N）,BCC+19%Ti₂O₃+30%（Nb,Ti）N,respectively.With the volume fraction increment of ceramic enhancements,the strengthening contribution of ceramic enhancements was improved significantly,which consequently increased hardness from 7.2 GPa to 11.1 GPa,and increased yield strength from 2324 MPa to 3546MPa,but decreased fracture strain.Moreover,excessive brittle Cr₂Nb-Laves phase were precipitated due to the entropy reduction of BCC caused by ceramic phase formation,which leads RHEAs prematurely fractured at a stress much lower than the theoretical yield strength.Decreasing Cr content and high-temperature heat-treatment can reduce the volume fraction of Cr₂Nb-Laves phase,which is helpful to increase the fracture strain and avoid premature fracture of RHEAs with excessive Laves phase.C,O and N were simultaneously introduced into CrMoNbWTi powders by adding 1wt.%C₁₈H₃₆O₂ during mechanical alloying under different ball milling atmosphere（vacuum,1 k Pa,2 k Pa and 4 k Pa）,and the four RHEAs with different amounts of C,O and N were abbreviated as CON-1,CON-2,CON-3 and CON-4.Effects of C,O and N on phase composition and mechanical property were systemically investigated.The results indicated that during powder metallurgy,nonmetallic elements were firstly reacted with metals to form ceramic enhancements,and the reaction sequence was Ti+O→Ti₂O₃,Ti+N→Ti N,Ti+C→Ti C,Nb+C→Nb C,Nb+N→Nb N,and Ti+Ti₂O₃→Ti O.Then,whether Cr₂Nb precipitated or not depended on the entropy reduction degree and Cr/Nb residual amount of BCC solid solution caused by the reactions between nonmetals and metals.Vicker’s hardness of CON-1 to CON-4 were 9.5 GPa,9.4 GPa,11.2 GPa,9.3 GPa,respectively.The difference of Vicker’s hardness depended on the load transfer strengthening contribution of secondary phases.Moreover,the ceramic enhanced CrMoNbWTi RHEA has excellent high-temperature mechanical properties.The compressive yield strength/fracture strain at 1300°C,1400°C and 1500°C of CON-4were 1266 MPa/10%,553 MPa/>70%and 183 MPa/>70%,respectively.The superior high-temperature mechanical properties were attributed to the excellent thermal stability and high-temperature strength of the ceramic enhancements.