Microstructure And Mechanical Properties Of NbHfTiVC_x Refractory High Entropy Alloys

As a new research frontier in the metallic material filed,high entropy alloys(HEA)have motivated rapidly increasing investigations due to their unique composition design strategy,as well as its high mixing entropy,serious lattice distortion and sluggish diffusion effect.Herein,the refractory high entropy alloys(RHEA)which primary composing elements are refractory metals,adopt the design concept of HEA and possess exceptional high-temperature mechanical properties,which have great application potential in thermall-harsh environments(e.g.aerospace and nuclear industry).In this paper,a refractory high entropy alloy NbHfTiV was selected as master alloy and carbon element was introduced to synthesize the novel NbHfTi VCx RHEAs.The microstructure,mechanical properties and corresponding heat treatment,hot rolling process were studied and analyzed.(1)NbHfTiV RHEA is comprised of single BCC phase,while the mixture of BCC phase,primary and nano-precipitate FCC-MC carbides are observed in NbHfTiVCx RHEAs(x=0.1,0.2,0.3).With the addition of carbon,the grain size is greatly refined from 537 to 25 μm,and the microstructure of primary carbides evolve from solid spheroid to sandwich-like pattern,which is related to the peritectic reaction.(2)The yield strength of NbHfTiVCx RHEAs increases from 958 MPa to 1115 MPa as the molar ratio of carbon increases from 0 to 0.3.The NbHfTiVC0.1 RHEA exhibits the optimal mechanical properties,and the yield strength is enhanced to 1048MPa as well as the fracture strain remains more than 50%,which outperforms the most of reported dual-phase RHEAs.(3)After a 24 h heat treatment at intermediate temeprature(700～900℃),some Laves phases rich in V and Hf elements precipitated in matrix and along the grain boundary.At low temperature,the precipitates are granular and needle-shaped with the size of 200-600 nm.At higher temperature,the precipitated phase coarses with the size of 1～5 μm.As the heat treatment temperature higher than 900℃,no precipitation is produced in alloys,and the microstructure characteristics were consistent with the as-cast state.(4)Heat treatment in intermediate temperatures(700～900℃)can produce phase decomposition,thus introducing the strong effect of precipitation strengthening in alloys.In particular,after a heat treatment for 24 h at 700℃,the compressive yield strength of NbHfTiVC0.1 RHEA increased from 1048 MPa(as-cast)to 1563MPa,which mainly due to the nanoprecipitated phase.(5)At the same hot rolling temperature,the strength and plasticity are simultaneously improved with the increase of rolling reduction.Meanwhile the carbide phase tended to be broken and refined,and some fine carbide particles fall off and disperse in the matrix along the rolling direction.In addition,a<101>(111)G/B(T)texture is formed in matrix phase under large rolling deformation,and partial recrystallization occurred.(6)At the same hot rolling reduction,the plasticity of alloys are significantly improved with increasing rolling temperature.When the rolling temperature is lower than 850℃,the work hardening in the alloy is obvious,and the yield and necking stage do not appear in the tensile stress-strain curve.When the tensile temperature is higher(up to 1050℃),recrystallization,yield and necking stage will occur during hot rolling.