| Recently,NbMoTaW high-entropy alloy(HEA)has been considered as a new high-temperature structural material that can be applied to the aerospace industry.However,previous studies indicate that refractory HEAs have low plasticity at room temperature,since the matrix is mainly composed of single-phase BCC solid-solution phase leading to a limited machining ability.Meanwhile,this alloy,prepared by the traditional casting method,shows coarse grains causing low strength.Therefore,this work focuses on the effects of the preparation process and composition modification on the trade-off between strength and plasticity for refractory HEAs,which shows certain theoretical significance and application value.In this work,two refractory HEAs of Nb25Mo25Ta25W25 and Ti8Nb23Mo23Ta23W23 were designed and prepared via the combination of mechanical alloying(MA)and spark plasma sintering(SPS).The main research results are as follows:By discussing the ball-milling parameters,including process control agent,milling time,rotation speed and milling media-to-powder weight ratio,an optimized process combination was obtained to obtain a fully alloyed HEA powder.The results show that under room temperature and argon environment,acetone is used as the process control agent and the ball-to-powder weight ratio is 15:1.The milling speed and milling-to-stopping time are 400 rpm and 60 min/10 min,respectively.After 60 h of ball milling,the powder shows a single solid solution phase indicating the completion of alloying process.The research on spark plasma sintering process focuses on pressure,heating rate,heat treatment and sintering temperature.A set of optimal process parameters for the highest-density bulk high entropy alloy preparation is that the sintering temperature is 1600?,the pressure is 35 MPa,the heating rate of 450?-1200? and 1200?-1600? is 50?/min and 25?/min respectively and the holding times at 1200? and 1600? are both 8 min.Base on alloying process,phase composition and microstructure analyzing of Nb25Mo25Ta25W25 alloy,it is found that the alloying rate of each element is related to the melting point and brittleness of the element.The higher melting point element has stronger atomic bonding,which leads to the decrease of diffusion coefficient.Nevertheless,the lower plasticity the element has,the more easily it breaks during ball milling,which facilitates alloying.The alloying powder is in a single-phase BCC solid solution state,while the sintered bulk is composed of a Mo-W-rich BCC1 matrix phase and a Nb-Ta-rich BCC2 phase and the average grain size is 0.88 ?m.The grain refinement effect is remarkable.Excellent comprehensive compressive mechanical properties and a clear yield platform at room temperature have obtained,yield strength,maximum compressive strength,and fracture strain are 2460 MPa,3016 MPa,16.8%respectively,and vickers hardness is 7.78 GPa.The effects of Ti element on the as-sintered microstructure and mechanical properties of Nb25Mo25Ta25W25 alloy were investigated.It is found that the addition of Ti has no significant effects on the alloying process,phase composition and microstructure of the alloy,but to some extent refines the grains.Among them,Ti8Nb23Mo23Ta23W23 alloy has the best comprehensive compression performance at room temperature.Compared with Nb25Mo25Ta25W25 alloy,the fracture strain and maximum compressive strength of Ti8Nb23Mo23Ta23W23 increased by 56.5%and 10.7%,respectively,and the yield strength decreased by only 3.4%.At the same time,the alloy also shows an increased corrosion resistance. |
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