Study On Microstructure And Properties Of The As-cast And Annealed CoCrFeNiAl_xTi_1-x High Entropy Alloys

In this paper,CoCrFeNiAl_xTi_1-x（x=1,0.8,0.5）high entropy alloys were melted by vacuum arc melting method（Al1、Al0.8、Al0.5）,in order to study the effect of Ti replaced Al equally and annealing temperature on microstructure and property of alloy,XRD,SEM,EDS,micro-hardness tester and universal testing machine were used to analyze the microstructure,hardness,compressive property of the alloy,and corrosion resistance in 3.5%NaCl solution,0.5mol/L NaOH solution and 0.5mol/L H₂SO₄ solution through The electrochemical corrosion test.The study found that in As-cast alloy Al1 is composed of a single BCC solid solution,Al0.8 and Al0.5 are composed of BCC+FCC1+FCC2 solid solution.Element segregation aggravated,eutectoid structure appeared in interdendritic while FCC1 phase increased and BCC phase decreased with the increasing of Ti.hardness,strength and ductility of three alloys in As-cast were much higher than most of the high strength steels showing good comprehensive mechanical property.Among which Al0.8 is the best.There is no obvious passivation in the3.5%NaCl solution.passivation occurs in the 0.5mol/L NaOH solution,but passivation zone is narrow and the passive film is unstable.Obvious passivation existed in the 0.5mol/L H₂SO₄solution,passivation zone is very wide and passive film is stable.Al1 and Al0.5 have little difference in corrosion resistance in NaCl solution and NaOH solution,in consideration of economy Al1 is a better choice.The corrosion resistance of Al0.5 is the best,followed by Al1,Al0.8 has the worst corrosion resistance in H₂SO₄ solution,so Small amount of Ti can not offset the adverse effects of segregation and the double phase structure on corrosion resistance,only when Ti is more than al can improve the corrosion resistance.The Al1 is composed of a single BCC solid solution after annealing at 600℃,800℃and as cast alloy,the new FCC phase appeared after annealing at 1000℃,the hardness reaches the maximum value of 689HV and the yield strength reaches the maximum value of 1630Mpa after annealing at 1000℃.As cast Al1 has best corrosion resistance in 3.5%NaCl solution.In the0.5mol/L H₂SO₄ solution,Al1 existed passivation both in the annealed state and the as-cast state,the corrosion resistance is the best after annealing at 1000℃,because the passivation range is largest and corrosion current density and the self-corrosion potential are the most positive;The phase structure of Al0.8 in as-cast and annealed states did not change,and it is always coexisting for BCC+FCC1+FCC2.However,with the increase of annealing temperature,interdendritic transform to small lamellar-type FCC1 and BCC eutectoid structure,segregation improved.After annealing at 800℃,hardness,yield strength,and fracture strength reach the maximum,and the corrosion resistance is the best in the 3.5%NaCl solution.In the0.5mol/L H₂SO₄ solution,Al0.8 in annealed and as-cast condition has the passivation,and after800℃annealing,the corrosion current density is the smallest,the corrosion potential is the most positive,the passivation range is larger and the corrosion resistance is the best.Al0.5 in as-cast and annealed condition phase structure did not change,is still BCC+FCC1+FCC2three-phase solid solution structure composition;but with the annealing temperature increases,the number of eutectoid in the alloy improved,after annealing at 800℃it has the best hardness and yield strength and best corrosion resistance in 3.5%NaCl solution.In the solution of0.5mol/L H₂SO₄,both as-casted and annealed condition have passivation.combined with soaking test results,figure out the conclusion that corrosion resistance reach the best after 1000℃heat treatment.