Study On The Thermal Stability Of Nanocrystalline Alloys Decorated By High Entropy Grain Boundary

Nanocrystalline materials have excellent comprehensive properties and wide application prospects.However,nanocrystalline materials are unstable and tend to grow because of poor thermal stability,which seriously limits the engineering application of nanocrystalline materials.High entropy grain boundary decoration can improve the thermal stability of nanocrystalline materials by reducing the grain boundary free energy of nanocrystalline materials.Nanocrystalline Ni-Zr_xNb_xMo_xTa_x alloys,which decorated by high entropy grain boundary,were prepared by mechanical alloying and spark plasma sintering in this work.The isothermal annealing and compression experiments were finished in our work.Microstructure,grain size and thermal stability of nanocrystalline alloys were studied by XRD,SEM,TEM and DSC.The thermal-kinetics stabilization mechanism and the strengthening mechanism of mechanical properties of nanocrystalline alloys were elaborated,and the following conclusions were drawn:(1)The grain size of nanocrystalline alloys was refined with the increase of ball milling time and solutes content.The grain size of nanocrystalline Ni-Zr_xNb_xMo_xTa_xalloys refined to less than 10 nm under 30 h ball milling time,and solutes are evenly dissolved into Ni matrix to form high entropy grain boundary.After annealed at 200~1000?,it was found that the nanocrystalline grain size increased to 25~48 nm but remained stable.(2)Based on the thermodynamic extremum principle,normal solution approximation and steady-state diffusion assumption,the thermal-kinetics model of multicomponent nanocrystalline alloys grain growth under the interaction of solutes was deduced.Based on the second phase precipitation and intragranular solution of solutes,combined with Cahn solute drag model and Zener pinning effect,the kinetics model of grain growth under the coupling effects of intrinsic drag,solute drag and second phase drag was deduced.It can be confirmed that:the solutes segregated to the grain boundary and formed high-entropy grain boundary(T<800?),which can significantly enhance the thermal stability of nanocrystallines by reducing grain boundary free energy and enhancing solute drag effect.At T>800?,the solute Mo diffused into the grain and soluted,while solutes Zr,Nb and Ta precipitated at the grain boundary and formed second phase precipitations,which reduced the grain boundary mobility,but the effect of high entropy grain boundary was weakened and the thermal stability of grain size changed from thermodynamic stability to kinetics stability.The stability of high entropy grain boundary structure is the key to the thermal stability of nanocrystals.(3)Composite micro-nano construction of bulk nanocrystals was achieved by SPS sintering.The grain size was maintained at 50~100 nm,the microhardness of nanocrystalline alloys was up to 393~456.5 HV,the compressive strength was up to1427.0~1647.3 MPa,while the plastic deformation was down to 7.8~2.97%.With the increase of solutes content and second phase precipitates,the material changed from ductile fracture to brittle fracture.