Effect Of Boron On The Microstructure And Properties Of Nb-Mo-Ti-Al Refractory High-entropy Alloys

The refractory high entropy alloy refers to a multi-principal alloy mainly composed of refractory metal elements and has the same matchable advantages as conventional Ni-based superalloys in strength,specific strength and service temperature,so it has great potential for engineering applications under extreme temperature and extreme load conditions.However,there are few literatures on the microstructure and properties of refractory high entropy alloy,especially on the influence of trace non-metallic element,like boron.In the present study,NbMoCrTiAl-1Si-XB?X=0,1?and AlMo_0.5NbTa_0.5TiZrB_x?x=0,0.02,0.06?refractory high-entropy alloys are prepared by a non-consumable vacuum arc melting furnace combined with a water-cooled copper mold.The effects of trace B on the microstructures,room temperature mechanical properties and high temperature oxidation behavior of the refractory high-entropy alloys are studied by X-ray diffractometer,scanning electron microscope,Vickers microhardness tester,universal testing machine and synchronous thermal analyzer.The results can be summarized as follows.The microstructure of as-cast NbMoCrTiAl-1Si-XB?X=0,1?refractory high entropy alloy has the characteristic of dendritic morphology mainly with Nb-Mo rich BCC phase.However,1at.%B addition causes Mo content increase in the BCC phase of the alloy,thin strip-like or short rod-like Nb-Si rich phase precipitate between the dendrites,and improves the room temperature hardness and compressive strength of the alloy.The microstructures of the alloy annealed at1200°C for 20h mainly contain three phases,namely,BCC phase,A15 phase and Laves phase.1at.%B addition makes Nb-Si rich phase with good thermostability survive in the annealed alloy and reduces the contents of A15 and Laves phase and the hardness and the compressive strength of the annealed alloy at ambient temperature.The microstructures of as-cast AlMo_0.5NbTa_0.5TiZrB_x?x=0,0.02,0.06?alloys are composed of two kinds of BCC phases individually with Mo-Nb-Ta rich and Al-Ti-Zr rich and Al-Zr rich compound.The addition of trace amounts of B?x=0.02 and x=0.06?also reduces the hardness and the compressive strength of the alloy.The microstructures of the alloy annealed at 1200°C for 20h are mainly composed of Al-Ti-Zr rich ordered B2 phase,Mo-Nb-Ta rich BCC phase and Al-Zr rich ordered intermetallic compound.1 at%B addition has no obvious effect on the ambient temperature hardness but lowers the ambient temperature compressive strength of the alloy.The oxide films with rutile TiO₂ and corundum Al₂O₃ structure are formed in the surface of as-cast NbMoCrTiAl-1Si?X=0,1?alloys isothermally oxidized at 900°C for 48h.However,the addition of 1 at.%B promotes the formation of Al-rich oxide layers in the surface of the alloy,which improve the oxidation resistance of the alloy.The initial oxidation temperature of as-cast AlMo_0.5NbTa_0.5TiZrB_x?x=0,0.02,0.06?alloys is 500°C.However,B addition obviously reduces the oxidation weight gain rate of the alloy during the temperature range from 500 to 1200°C and accelerate the formation of the complex protective oxidation layer at 800°C with Nb₄Ta₂O₁₅ and AlNbO₄ in during the long term oxidation?over 50h?process,which can effectively prevent the severe oxidation of the alloys.