The ? Structure Stability And Properties Of Multi-element Zr/Ti/Hf-Nb Medium/High Entropy Alloys Based On Cluster Structure Model

Body-centered cubic(BCC)refractory high entropy alloys consisting of early transition metals(Ti,Zr,Hf,Nb,Ta,W,Mo,etc.)generally have exhibited great potential in high temperature applications,which are ascribed to it's high strength,hardness,excellent corrosion resistance and good high temperature performance.However,there are indeed easy to exist two BCC phase in these BCC refractory high entropy,that is BCC phase separation due to segregation of element,which is not conducive to the improvement of alloy properties.For the light-weight refractory BCC-based HEAs and MEAs consisting of Ti/Zr/Hf and Nb/Ta,Many existing works have demonstrated that the equimolar mixing of Ti,Zr,and Hf with other BCC-stabilized elements of Mo,Nb,Ta,and V could induce a phase separation of two BCC phases.Moreover,when the amount of BCC-stabilized elements is not enough,some metastable phases of??'and??''would like to precipitate from the BCC matrix in some Zr-Ti-Hf-based HEAs,leading to a adverse change in mechanical properties.Thus,it is necessary to stabilize the BCC phase structure through appropriately match the composition of alloy elements.In this paper,a series of Zr-Ti(Hf)-Nb alloys were designed according to cluster composition design method.Among them,a series of BCC based alloys were selected for mixing with O element.All alloys were prepared by suction-cast processing.And then microstructures and properties were characterized comprehensively by X-ray diffraction,optical microscopy,scanning electron microscopy,transmission electron microscopy,microhardness test and tensile test,damping test and so on.The conclusions are as follows:(1)For[Zr-Zr₁₄](Zr,Nb)₃series alloys,the phase structure transforms from HCP-??to BCC-??,accompanied by the precipitation of metastable phases(?',?)with increasing the Nb content.A minor amount of?? phase still precipitates on the BCC matrix of the[Zr-Zr₁₄](Nb)₃alloy,which indicates that it is difficult to eliminate the metastable?? phase only by adding Nb element.(2)The Ti addition with an appropriate amount of?5.56 at.%could inhibit the precipitation of?? nanoparticles and retain the BCC-??structure in T1-[Ti-Zr₁₄](Nb₃)alloy,while excessive Ti would reduce the?structural stability,resulting in a large amount of?? precipitation to induce an embrittlement in T3-[Ti-Zr₁₄](Ti₂Nb)alloy.(3)A combination of Zr,Hf,Ti,and Nb with a near-equimolar mixing could render the H3-[Ti-Zr₈Hf₄Ti₂](Nb₃)alloy with a single BCC structure.The BCC-??MEAs exhibit prominent mechanical properties with??_YS=540?662 MPa,?_UTS=588?754 MPa,? =14.2?15.6%and E=57?71 GPa,among which the H2-[Ti-Zr₈Hf₆](Nb₃)alloy has a much higher strength with??_YS=662 MPa and??_UTS=754 MPa due to a minor amount of?? nanoparticles precipitated in the BCC matrix.(4)The addition of O element could improve the strength and plasticity of H3-[Ti-Zr₈Hf₄Ti₂](Nb₃)alloy at the same time,where H3-1.8O alloy exhibits excellent mechanical properties(?_YS=966 MPa,?=15.1%)and damping properties(tan??_max=0.024),which may be due to the interstitial strengthening of O,the formation of ordered complex,low elastic modulus and Snoek effect caused by O atoms.