Powder Metallurgy Fabrication And Hot Deformation Behaviors Of Ultrafine-grained MoNbTaTiV Refractory High Entropy Alloy

Refractory high-entropy alloys(RHEAs)exhibit many characteristics of simple microstructures,excellent high-temperature thermal stability and excellent high-temperature strength and have great advantages in extremely high service temperature conditions.At present,the fabrication methods of bulk RHEAs are mainly focused on the vacuum arc melting method.However,the segregation of components and coarse grains resulted by the melting and solidification process will reduce the strength and deteriorate the plasticity of the alloy,which hinders the application and development of the RHEAs.Therefore,this paper based on a Mo Nb Ta Ti V RHEA,and proposed a combination method of mechanical alloying(MA)and vacuum spark plasma sintering(SPS)to fabricate bulk RHEA with uniform microstructure and ultra-fine grains.On this basis,the properties of the hot deformation and thermal stability were further investigated to provide the scientific and theoretical basis for the formulation of the hot processing parameters of the ultra-fine grained RHEAs.Firstly,based on the effect of different ball milling processes on the degree of alloying and powder output,the optimal ball milling process parameters were determined.Nanocrystalline Mo Nb Ta Ti V RHEA powder with a single body centered cubic(BCC)solid solution phase and uniform component element distribution was obtained.Powder size,alloying behaviors and evolution of microstructures were analyzed during the optimal ball milling process paramaters.The mutual effect law of microstructure evolution and macroscopic change behavior of powder size was revealed.The crystal structure of the alloy powder exhibited excellent thermal stability,which was found through the vacuum heat treatment in the range of 1073 K?1473 K.The effect of different Ti content on the phase formation law,powder size evolution and alloying process were investigated.With the increase of Ti content,all of Mo Nb Ta Ti_xV RHEAs powders form single BCC solid solution phase.However,the powder size is increased,which results in the extension of alloying completion time.Secondly,bulk Mo Nb Ta Ti V RHEA with uniform microstructure and ultra-fine grains was obtained by SPS.The alloy fabricated under the best sintering process parameters had a room temperature hardness,yield strength,compressive strength and plastic strain of 542 HV,2208 MPa,3238 MPa and 24.9%,respectively.The strengthening mechanisms of RHEA were analyzed.Compared with the as-cast RHEA,the enhanced strength of the sintered Mo Nb Ta Ti V RHEA is mainly attributed to the contribution of interstitial solid solution strengthening mechanism and grain boundary strengthening mechanism.In addition,the evolution of microstructures and properties of the Mo Nb Ta Ti_xV RHEAs were elaborated with different Ti content.On this basis,uniaxial isothermal compression tests were used to analyze the deformation behaviors of the sintered Mo Nb Ta Ti V RHEA at different deformation temperatures and strain rates.The hot deformation and dynamic softening behaviors were investigated.At lower temperature and higher strain rate,the dominant deformation mechanism and dynamic softening mechanism of the sintered alloy are intragranular dislocation slip and discontinuous dynamic recrystallization,which are changed to be grain boundary gliding and continuous dynamic recrystallization with the increase of deformation temperature and the decrease of strain rate.The effect of precipitation phase on the hot deformation and dynamic softening behaviors was analyzed.Finally,the thermal stability of the sintered Mo Nb Ta Ti V RHEA in the range of1373 K?1573 K and 1 h?8 h was analyzed through vacuum heat treatment tests.The evolutions of the precipitation phase and matrix grain size were clarified.The growth behaviors of the precipitation phase and matrix grain were analyzed.The growth laws of the precipitation phase and matrix grain were accorded to the modern Ostwald maturation theory and kinetic equation D_n-D₀ⁿ=kt,respectively.The steady-state flow stresses at 1573 K and strain rates of 0.005 s^-1 and 0.0005 s^-1were only increased from 55 MPa and 30 MPa to 67 MPa and 43 MPa after heat treatment at 1573 K and 8 h.