Studies On The Microstructure And Properties Of （CoNiFe）₉₅Al_xTi_5-x Medium Entropy Alloy

The design concept of high entropy alloy points out a new direction for the research of metallic materials.At present,there are still many problems to be solved in the research field of high entropy alloy,such as structural characteristics,deformation mechanism,processing and forming,etc.At the same time,the medium entropy alloy with less principal alloying elements and excellent properties has gradually been valued.The strengthening mechanism of medium entropy alloy can be further regulated by adding specific trace alloy elements.And hence,the properties of the alloy can be improved and the application of medium entropy alloy in advanced engineering fields such as automobile,aircraft and aviation system can be expanded.In this paper,the（CoNiFe）₉₅Al_xTi_5-x（x=1,2,3,4）medium entropy alloy plates were prepared by vacuum arc melting combined with hot forging and cold rolling;following and then,different temperature and time parameters were set to anneal the plates.The effects of different Al and Ti alloy additions,forging and rolling processes on the microstructure and properties of the alloy were studied by utilizing of XRD and SEM.The effects of different annealing temperature and time on the microstructure and properties of the（CoNiFe）₉₅Al₂Ti₃medium entropy alloy were compared and discussed.It is found that the as cast（CoNiFe）₉₅Al_xTi_5-x（x=1,2,3,4）medium entropy alloy still has a single fcc structure after the addition of Al and Ti elements.A small amount of L2₁precipitated phase appears in the as-rolled alloy.The work hardening phenomenonof the alloy is obvious,with high strength and hardness.With the decrease of Al content and the increase of Ti content,the corrosion resistance of the alloy is improved slightly.Among them,the comprehensive properties of as rolled（CoNiFe）₉₅Al₂Ti₃ medium entropy alloy are better,its tensile strength and elongation are 1384.6 MPa and 15.6%respectively.In addition,the corrosion potential of the alloy is the most positive,the corrosion current density is the smallest,indicating that the corrosion rate is the slowest,and the corrosion resistance is the best.Under different annealing conditions,the（CoNiFe）₉₅Al₂Ti₃ medium entropy alloys are still fcc structure,and the annealing teratment results in the increased content of L2₁ precipitated phase.When the annealing temperature is low,the microstructure of the（CoNiFe）₉₅Al₂Ti₃medium entropy alloy is changed slightly compared with that of as rolled alloy.The recrystallization temperature of the alloy is between 600℃and 800℃.With the increase of annealing temperature,the grains of the annealed alloy gradually recrystallize and grow up,resulting in a dramatic change of grain size.When the temperature is high,there are large grains with the size of 100μm in the alloy,but the main structure is small grains with more annealing twins.The prolongation of annealing time is beneficial to the homogenization of microstructure,especially at high annealing temperature.Compared with as rolled alloy,the tensile strength and hardness of（CoNiFe）₉₅Al₂Ti₃ medium entropy alloy first increase and then decrease.When the annealing temperature rises to 900℃or above,the work hardening of the alloy is completely eliminated and the plasticity is restored or even enhanced.After annealing at 800℃for 10 hours,the tensile strength of（CoNiFe）₉₅Al₂Ti₃ medium entropy alloy is 1005.2 MPa and the elongation is 19.9%.When the annealing temperature is increased to 1000℃,the strength of the alloy decreases,but the plasticity is greatly improved and the elongation is increased to59.4%.As compared to the annealing temperature,the effect of annealing time on the mechanical properties of the alloy is smaller,especially when the annealing time is shorter（≤1h）.The electrochemical properties of（CoNiFe）₉₅Al₂Ti₃ medium entropy alloy are significantly affected by annealing temperature and time.When the annealing time is extended（≥1h）,the annealing temperature of the alloy to obtain the most positive corrosion potential is 800℃.