Study On The Mechanical And Corrosion Properties Of Ultra-fine Grain CoCrMnFeNi High Entropy Alloy With FCC Structure

At the end of the 20th century,high-entropy alloys entered the vision of materials researchers.The single component design concept of traditional alloys could no longer meet the needs of social development.The unique performance of high-entropy alloys solves the performance defects of most traditional metal materials.It was an important substitute for current aerospace,military weapon equipment,biomedical equipment,oil and gas drilling,automobile manufacturing and other application materials.such as,typical The face-centered cubic structure(FCC)Co Cr Mn Fe Ni high-entropy alloy.The FCC structure Co Cr Mn Fe Ni high-entropy alloy has excellent mechanical ductility and corrosion resistance,but its low strength at room temperature greatly limits its engineering applications.Therefore,according to the strengthening methods of traditional alloys,the refinement of grains was used to improve the The properties of FCC structure high entropy alloy materials.However,the current research on the FCC structure Co Cr Mn Fe Ni high-entropy alloy mainly focuses on the coarse-grained structure,and there are few reports on the ultra-fine-grained(UFG)structure.Therefore,this study selects FCC structure Co Cr Mn Fe Ni high-entropy alloy as the research object,uses grain refinement strengthening methods to improve performance,and studies the mechanical properties and corrosion properties of ultra-fine grain FCC structure Co Cr Mn Fe Ni high-entropy alloy.In this study,Co Cr Fe Mn Ni high-entropy alloy with ultra-fine grained structure were prepared using cryogenic(liquid nitrogen,77 K)rolling followed by short time annealing.The deformation behavior and strain rate sensitivity(SRS)of processed material were investigated by tensile test at temperatures ranging from 77 K to 573 K.It was found that the critical twin stress of ultra-fine grained Co Cr Mn Fe Ni high entropy alloy decreases at 77K,which was attributed to the reduction of stacking fault energy with the temperature.In addition,as the test temperature increased from 298K to 473K,the ultrafine-grained specimens showed significant plastic instability,which was related to the obvious grain boundary relaxation in the ultrafine-grained high-entropy alloy.In addition,the strain rate jump test results show that as the grain size decreases,the SRS change of the ultrafine-grained sample was inversely proportional to the corresponding strength enhancement,and shows a stronger temperature dependence than the coarse-grained(CG)sample.Especially in the temperature range of 298K to 473K,the higher the temperature,the stronger the SRS dependence of the ultrafine-grained Co Cr Mn Fe Ni high-entropy alloy.The study found that it was mainly due to increased grain boundary relaxation,which was thermal activation process and depends on the strain rate and temperature.Based on the study of mechanical properties,the corrosion behavior of ultrafine-grain Co Cr Mn Fe Ni high-entropy alloy in 3.5wt.%Na Cl,1M/H₂SO₄,1M/Na OH solution was studied.The ultra-fine grained Co Cr Mn Fe Ni high-entropy alloy potential-dynamic polarization test and electrochemical impedance spectroscopy(EIS)results show that the ultra-fine grained Co Cr Mn Fe Ni high-entropy alloy has lower corrosion potential,higher corrosion current,and easier surface compared to coarse grains An unstable passivation layer is formed and it has a faster corrosion rate.In addition,the microstructure characteristics of ultrafine grains indicate that the Cr-rich Ni-depleted zone(?precipitation phase)is formed in the ultra-fine grain Co Cr Mn Fe Ni high-entropy alloy,resulting in electrochemical corrosion between the?precipitation phase and the matrix phase nearby.In addition,the Cr-deficient passivation layer formed on the sample surface also reduces the stability of the passivation film,thereby reducing the pitting corrosion resistance of the sample.The whole research results show that the ultra-fine grained Co Cr Mn Fe Ni high-entropy alloy has worse corrosion performance in 3.5wt.%Na Cl,1M/H₂SO₄,1M/Na OH solutions than coarse grains.