Effects Of Si,Cr And Hf Alloying On The Microstructure And Properties Of Refractory High-Entropy Alloys

Refractory high-entropy alloys have excellent high-temperature mechanical properties,corrosion resistance,radiation resistance and higher melting points,and have great development potential in the aerospace field.In this paper,NbMo_0.5Hf_0.5TiZrAl,NbMo_0.5Hf_0.5TiZrAlSi_0.5and NbMo_0.5Hf_xTiZrCrAl（x=0,0.25,0.5,0.75 and 1,x is the molar ratio）are prepared by vacuum non-consumable arc melting method.The effects of addition of Si,Crand Hf content in the presence of Cron the phase composition,phase structure,room temperature hardness,high temperature compression performance and high temperature oxidation resistance of refractory high-entropy alloys are studied,and the following conclusions are drawn:NbMo_0.5Hf_0.5TiZrAlalloy is composed of a single BCC1 solid solution phase.After adding Si,the alloy is composed of BCC1 solid solution phase and 5-3 type silicide.After adding Cr,the alloy is composed of BCC1,BCC2 and Laves phases.NbMo_0.5Hf_xTiZrCrAl（x=0,0.25,0.5,0.75 and 1,x is the molar ratio）refractory high-entropy alloys are all composed of BCC1,BCC2 and Laves phases.The addition of Hf has no effect on the phase composition of the alloy,but it will promote the formation of the BCC2 phase in the alloy.Adding Siand Crcan improve the room temperature hardness and high temperature compressive strength of the alloy.The Rockwell hardness of NbMo_0.5Hf_0.5TiZrAlalloy is 76.6 HRA,and the high temperature compressive strength is 97.9 MPa.After adding Si,the Rockwell hardness of the alloy is 80.8 HRA,and the high temperature compressive strength is 168.5 MPa.After adding Cr,the Rockwell hardness of the alloy is 79.8 HRA,and the high temperature compressive strength is 198.5 MPa.With the increase of Hf content,the Rockwell hardness of NbMo_0.5Hf_xTiZrCrAl（x=0,0.25,0.5,0.75 and 1）alloy shows a gradually increasing trend,while the high temperature compressive strength of the alloy gradually decreases.All alloys showed a parabolic oxidation weight gain after oxidation at 1200℃for 50 h.The outer layer of the oxide film of NbMo_0.5HF_0.5TiZrCrAlalloy is composed of（Ti,Nb,Zr）O₂,（Zr,Hf）O₂ and Al₂O₃,and the inner layer is composed of Al₂O₃,（Zr,Hf）O₂ and various transition oxides rich in Ti,Nband Zr.The main oxidation product in the internal oxidation zone is（Zr,Hf）O₂.After adding Si,the silicide with good high-temperature oxidation resistance are formed in the alloy,which can effectively hinder the diffusion of oxygen into the alloy matrix,thereby improving the high-temperature oxidation resistance of the alloy.The outer layer of the oxide film is composed of（Ti,Nb,Zr）O₂,（Zr,Hf）O₂,SiO₂ and Al₂O₃,and the inner layer is composed of SiO₂,Al₂O₃,（Zr,Hf）O₂ and a variety of transition oxides rich in Ti,Nband Zrelements.The main oxidation product in the inner oxidation zone is（Zr,Hf）O₂.The compactness of the oxide film decreases with the addition of Cr,which results in the decrease of the oxidation resistance of the alloy.The outer layer of the alloy oxide film is composed of alternately distributed（Zr,Hf）O₂ and Al₂O₃ oxide layers and CrNbO₄ and（Ti,Cr,Nb）O₂oxide layers,and the inner layer is composed of（Zr,Hf）O₂,Al₂O₃ and a variety of transition oxides rich in Ti,Nband Crelements.The main oxidation products in the inner oxidation zone are（Zr,Hf）O₂and Al₂O₃.The outer layer of the oxide film of NbMo_0.5TiZrCrAlalloy is composed of ZrO₂,CrNbO₄,（Ti,Cr,Nb）O₂ and Al₂O₃,and the inner layer is composed of ZrO₂,Al₂O₃ and various transition oxides rich in Ti,Nband Crelements.The oxidation products in the inner oxidation zone are mainly ZrO₂ and a small amount of Al₂O₃.After adding Hf,the outer layer of the oxide film formed alternately distributed（Zr,Hf）O₂ and Al₂O₃oxide layers and CrNbO₄ and（Ti,Cr,Nb）O₂ oxide layers,and their thickness,continuity and compactness vary with Hf content Increase with the increase.The continuous and dense layered structure can more effectively hinder the diffusion of oxygen into the alloy matrix,thereby significantly improving the high-temperature oxidation resistance of the alloy.The inner layer of the oxide film is composed of（Zr,Hf）O₂,Al₂O₃,and a variety of transition oxides rich in Ti,Nband Crelements.The oxidation product of the internal oxidation zone is mainly（Zr,Hf）O₂.