Effects Of Nitrogen And Oxygen On Microstructure And Properties Of Ti₂ZrHfV_0.5Mo_x High-Entropy Alloys

As a new type of alloys developed recently,high-entropy alloys have great potential for development because it breaks the shackles of traditional alloy design.Body-centered cubic（BCC）structure high-entropy alloys have obvious advantages in terms of hardness,mechanical properties at room temperature and high temperature,and high temperature oxidation resistance.In traditional alloys,nitrogen and oxygen as trace addition elements have their own obvious advantages in improving alloy properties.This paper mainly explores the effects of nitrogen and oxygen on the microstructure and properties of BCC structure high entropy alloys.Based on the existing high-entropy alloys design model and practical experience,this paper designed Ti₂ZrHfV_0.5Mo_x（x=0,0.2,0.4,0.6,0.8,1）single-BCC solid solution structure high-entropy alloys.The Ti₂ZrHfV_0.5Mo_x high-entropy alloy with single-phase BCC structure was studied,and a small amount of nitrogen and oxygen were added to it.The following conclusions were obtained by studying the microstructure and properties of the alloy system:（1）XRD diffraction analysis and SEM analysis showed that the Ti₂ZrHfV_0.5Mo_x（x=0,0.2,0.4,0.6,0.8,1）alloy was a single BCC structure,which was consistent with the predicted results.However,with the additions of Mo element,due to the large difference in melting point between the elements,element segregation occurs in the grains,and with the further increase of Mo element,the segregation phenomenon gradually increases.（2）The mechanical properties of Ti₂ZrHfV_0.5Mo_x high-entropy alloy were tested.It was found that with the addition of Mo in the alloy,the strength and hardness of the alloy increased and the plasticity decreased.This is because the difference in atomic radius between the Mo atom and atoms such as Ti,Zr,and Hf are large,resulting in severe lattice distortion and hindering the movement of dislocations.Among them,Ti₂ZrHfV_0.5Mo_0.2 alloy has a single BCC structure,uniform structure and good comprehensive mechanical properties.（3）The influences of nitrogen and oxygen on the microstructure of the alloys are basically the same.The additions of nitrogen and oxygen change the structure and properties of the Ti₂ZrHfV_0.5Mo_0.2 alloy.The nitrogen and oxygen elements are added to the crystal lattice of the base alloy in a part of the gap solid solution,and the other part is precipitated as a dispersed precipitate.（4）The hardness of Ti₂ZrHfV_0.5Mo_0.2 alloy is obviously improved after adding nitrogen and oxygen.When the nitrogen content is not contained,the hardness value of the alloy is 346HV.When the nitrogen content reaches 4at%,the hardness of the alloy reaches 598HV,which is 73%higher.Solid solution strengthening and precipitation strengthening have an extremely significant effect on the compression properties of the alloy.The alloy exhibits a characteristic of decreased plasticity and increased strength.When the nitrogen content reaches 4at%（when the oxygen content reaches 3at%）,the alloy has completely become brittle.（5）The effects of adding nitrogen and oxygen on the friction and wear properties of Ti₂ZrHfV_0.5Mo_0.2 alloy are slightly different.The additions of nitrogen has little effects on the friction coefficient,but the additions of oxygen not only reduces the friction coefficient of Ti₂ZrHfV_0.5Mo_0.2 alloy but also improves the stability of friction coefficien.For the wear resistance of the alloy,the additions of nitrogen and oxygen can reduce the wear rate of the alloy.When the nitrogen and oxygen contents are 1at%,the wear resistance of Ti₂ZrHfV_0.5Mo_0.2 alloy is better than that of Ti-6Al-4V.alloy.However,the excessive nitrogen and oxygen content will cause the Ti₂ZrHfV_0.5Mo_0.2 alloy to become brittle and reduce the wear resistance of the alloy.（6）With the additions of nitrogen,the corrosion resistance and pitting resistance of Ti₂ZrHfV_0.5Mo_0.2 alloy are significantly improved.The main reason is that nitrogen enhances the stability of the passivation film on the surface of the alloy.Compared with the improvement of the corrosion resistance of the alloy by nitrogen,the increase of oxygen has little effects on the corrosion resistance of Ti₂ZrHfV_0.5Mo_0.2 alloy,and the corrosion resistance of the alloys are almost unchanged.