Investigation On Microstructure And Mechanical Properties Of Refractory High Entropy Alloy

Refractory high-entropy alloys are considered as potential candidates for high-temperature structural materials due to their excellent high strength,radiation resistance and high-temperature softening resistance.However,the development of refractory high-entropy alloys is hindered by problems such as high density and poor room temperature plasticity.At the same time,investigation on microstructure,mechanical properties and internal deformation mechanism is relatively lacking.Therefore,three low-density plastic refractory high-entropy alloys were developed in this paper.The microstructure and mechanical properties of the alloys were systematically studied.The specific contents of this paper are as follows:1.Three NbTiAl-based refractory high-entropy alloys with excellent room temperature compressive plasticity and high specific strength have been developed.The structure,mechanical properties and deformation mechanism of the alloys have been analyzed.It is found that Nb₄₂Ti₂₅Al₁₅Zr₅Hf₅Ta₅Mo₂W₁（NbTiAl-1）Nb₃₉Ti₂₃Al₁₅Zr₁₀Hf₅Ta₅Mo₂W₁（NbTiAl-2）and Nb₃₇Ti₂₀Al₁₅Zr₁₅Hf₅Ta₅Mo₂W₁（NbTiAl-3）three refractory high entropy alloys have excellent room temperature compressive plasticity,excellent medium and high temperature compressive strength and low density characteristics（～7.4 g/cm³）.And the phase structure of as-cast refractory high-entropy alloy is nanoscale B2 phase and antiphase domain boundary.During the deformation process of the NbTiAl-3 alloy at room,the dislocation is dominated by plane slip,and the dislocation band channels with orientations of{110}and{112}are generated.The dislocation continuously cuts the B2 phase,the dislocation resistance is reduced,and there is a softening trend.The interaction between the{112}plane slip system and the{110}plane slip system promotes the subsequent hardening behavior.With the increase of strain,the generation of microcracks at grain boundary causes the transition from strain hardening to strain softening and finally fracture.The alloy has excellent specific yield strength(109.8 MPa·g^-1·cm³)at 1123 K compression.The deformation is dominated by dislocation mixing.A large number of intragranular dislocations are annihilated,and the dynamic recrystallization at grain boundaries consumes a large number of dislocations,resulting in a further increase in dislocation strain softening effect.Therefore,dynamic recrystallization and dislocation annihilation are the main causes of strain softening during 1123 K deformation.2.The microstructure and mechanical properties of NbTiAl-3 refractory high entropy alloy after thermomechanical treatment and effect of B element on microstructure and mechanical properties of the alloy have been investigated.It is found that the compressive yield strength of thermomechanical NbTiAl-3 refractory high-entropy alloy aged at 923 K-30 h is 1843 MPa,which is 427 MPa higher than the as-cast compressive yield strength（1416 MPa）at room,and the specific yield strength is as high as 249MPa·cm³/g.Through TEM analysis,it is found that the increase in strength may be due to the formed modulation structure during aging has a nano-scale two-phase interface with high volume fraction,hindering the deformation.At 1023 K aging,the alloy yield strength decreases obviously,and the fractures of the alloy are intergranular fracture after compression after aging at 1023 K-30,120 h.Therefore,a large number of rod-like Zr₅Al₃phases are distributed along the grain boundary after aging at 1023 K,which is the main reason for inducing intergranular fracture.The Zr₅Al₃ phase in the grains would also causes stress concentration during deformation and causes transgranular fracture.The NbTiAl-3 refractory high-entropy alloy has been added 5000 ppm B element before thermo-mechanical treatment 923 K-30 h aging treatment.It is found that the precipitated phase is formed in the grain interior and the grain boundary,and the distribution is uniform.The compressive plasticity is obviously improved.During compression,the interaction between the precipitated phases and the deformed bands and the pinning effect greatly improves the strain hardening ability.The softening of the alloy is improved,and the transformation from softening to hardening is achieved.