Effect Of N Addition On The Microstructure And Mechanical Properties Of FeCoCrNiMn High Entropy Alloy

FeCoCrNiMn high-entropy alloys(HEAs)with face-centered cubic(FCC)is a typical high-entropy alloy which shows excellent plasticity and superior low temperature performance.However,its strength is relatively low.In this thesis,to simultaneously keep the high plasticity and improve the strength of the HEA,different N contents(N=0,0.05,0.1,x as the molar ratio)were added to FeCoCrNiMn alloy and the alloys were prepared by vacuum induction levitation melting furnace,subsequently processed by forging or rolling and finally annealing.Thus,the microstructure,properties and oxidation resistance of FeCoCrNiMnNx HEAs were investigated,the performance micro-mechanism of FeCoCrNiMnNx HEAs about the influence of the interstitial N were explored.The results show:FeCoCrNiMnNx HEAs through forging and annealing still exhibit a single FCC phase,contain a large amount of annealing twin and the ?3 grain boundary proportion is about 38%.Meanwhile,an increase in N contents from x =0 to x = 0.1 can strengthen the HEAs without dramatically sacrificing the uniform elongations to fracture.Specially,the uniform elongation to fracture of N0.1 alloy is 60% and its strength reaches 712 MPa,which is 35.56% higher than the N0 alloy.Processing with cold rolling and subsequent annealing has significant effects on the microstructure and mechanical property of FeCoCrNiMnNx HEAs.With the rolling deformation increased from 40% to 80%,grains were elongated along the rolling direction and the grain boundaries became fuzzy,accompanied with a hardness increased.After cold-rolling and annealing at 800?,the fine Cr2 N precipitations appeared,a large amount of annealing twins form and the ?3 grain boundary proportion is about 50%.The amount of the precipitations increased with the increase of rolling deformation.Moreover,with the increments of N content and rolling deformation,the gain sizes in the HEAs get small.After 80%cold-rolling and annealing,the average grain size of the N0.1 alloy is refined to ~ 3.46 ?m,which is one third of the N0 alloy.Rolling and subsequent annealing process can strengthen FeCoCrNiMnNx HEAs.The strength of the cold-rolled and annealed HEAs increased with the increase of N content.For N0.1 alloy after 80% cold-rolling and annealing,the hardness and tensile strength reached the maximum values of 247.4 HV and 930 MPa,respectively,while elongation of the alloy are still about 35%.For N0.1 alloy after the 60% cold-rolled and annealing,the strength and elongation reaches 893 MPa and 39%,respectively.After 60% cold-rolling and annealing,the elongation and tensile strength of the N0.1 HEA are almost equal and 1.37 times of N0 alloy,respectively.FeCoCrNiMnNx HEAs at 700?-1000? possess has good oxidation resistance and belong to a level of completely antioxidant.The oxide of the alloys are mainly composed by Mn2O3 at700? and 800?,by Mn3O4 at 900? and by Cr2O3 at 1000?.