Research On Microstructure And Mechanical Properties Of Co-Cr-Fe-Ni System High Entropy Alloys And Their Composite Materials

In this paper,by changing the ratio of Fe and Cr in Co-Cr-Fe-Ni high entropy alloy,Co Cr₃Fe₅Ni was selected as the matrix for strengthening and toughening research.The effects of cooling rate on the solidified phase structure of Co Cr₃Fe₅Ni and tensile process at 300K were further studied by molecular dynamics simulation.The effects of alloying elements C,Al,V,Ti and Y on the phase structure,microstructure and mechanical properties of Co Cr₃Fe₅Ni high entropy alloy were systematically studied.The variation of microstructure and properties of high entropy alloy with composition was expounded,and the mechanism of strengthening and toughening of high entropy alloy by adding alloy elements was expounded.In addition,based on Co Cr₃Fe₅Ni high entropy alloy,carbon containing ceramic particles such as Ti C,Si C and Nb C were formed by in-situ reaction.The effects of these ceramic particles on the microstructure and mechanical properties of the matrix were studied,and the strengthening mechanism was discussed.The main results were as follows:According to the calculation results of mixing entropy and electronegativity of Co Cr_xFe_8-xNi（x=1,2,3 and 4）high entropy alloys,it is proved that Co Cr_xFe_8-xNi alloys tends to form stable solid solution phases,which belong to the category of high entropy alloy.The Fe/Cr ratio has an important effect on the structure of Co Cr_xFe_8-xNi.Co Cr₃Fe₅Ni has dual-phase structure and excellent tensile strength and ductility,which is used as the matrix for the study of strengthening and toughening.The solidification process of Co Cr₃Fe₅Ni HEAs was simulated by molecular dynamics.The phase structure of Co Cr₃Fe₅Ni high entropy alloy was dominated by FCC and contains a small amount of HCP and BCC structure.The tensile process of Co Cr₃Fe₅Ni HEA was simulated at 300 K,and the tensile fracture process at atomic scale was characterized.The effects of C,Al,V,Ti and Y on the phase structure,microstructure and mechanical properties of the Co Cr₃Fe₅Ni high-entropy alloy were studied.With the addition of carbon atoms,the crystal structure became the FCC structure,and the eutectic structure was introduced and the grain boundary is strengthened,which help achieving the purpose of strengthening and toughening.The addition of Al,V and Ti elements increased the proportion of BCC phase and the microstructure changes into a unique lamellar structure of Widmanstatten structure,which increased the strength but decreased the plasticity.With the addition of Y and Al at the same time,the nano phase was precipitated in Y_0.01Al_0.4Co Cr₃Fe₅Ni,resulting in excellent tensile strength（1002.5 MPa）and elongation（25.9%）.The effect of cooling conditions on the microstructure and mechanical properties of Ti C-reinforced CoCrFeNi composites was studied.During slow cooling,rodlike Ti C was formed at the grain boundary.During rapid cooling,Ti C was evenly distributed in the grain boundary and matrix.With the increase of Ti C volume fraction,the hardness and ultimate tensile strength of the sample increased steadily,but the elongation decreased.In the Co Cr₃Fe₅Ni matrix,carbon-containing ceramic particles include Ti C,Si C and Nb C were generated through in-situ reactions.As part of the C element was solid-dissolved into the matrix,the matrix changes from the FCC/BCC dual-phase structure to the FCC phase.These ceramic particle reinforcements could increase the strength of the composite materials at the expense of plasticity.The addition of Ti C and Nb C at the same time will introduce（Nb,Ti）C particles into Co Cr₃Fe₅Ni alloy through in-situ reaction.（Nb,Ti）C is a solid solution based on Ti C or Nb C in the growth process.Both Ti C and Nb C can be used as the nucleation center of（Nb,Ti）C carbides.