Research On Microstructure And Strengthening Mechanism Of CoCrFeNi High-entropy Alloys

The birth of a concept,high-entropy alloy,expands the available range of chemical composition in the field of alloy design exponentially.High-entropy alloys usually can show remarkable properties derived from the microstructure characteristics.To design advanced metallic materials for applications from room to cryogenic temperatures has long been one of the amazing challenges in materials science.It is due to the long demand of mankind for cryogenic applications.Outstanding mechanical properties in high-entropy alloys inspire novel ideas for the development of advanced metallic materials.Co₂₅Cr₂₅Fe₂₅Ni₂₅(at.%)high-entropy alloys are prepared using cold-rolling,cryogenic-rolling,and cryogenic-drawing,while micro-alloyed CoCrFeNi high-entropy alloys are deposited using magnetron sputtering.The microstructure and strengthening mechanism are studied via synchrotron X-ray diffraction,on-axis transmission Kikuchi diffraction,Cs-corrected field emission transmission electron microscope,field emission scanning electron microscope,atom probe tomography,atomic force microscope,mechanical testing at room and cryogenic temperatures,and nanoindentation.The conclusions from the work are as follows,1)The work about cold-rolled CoCrFeNi high-entropy alloys shows that,during the cold-rolling,a nanoscale 9R transformation from the face-centered cubic structure occurs with the nano-twinning mechanism in the face-centered cubic structured CoCrFeNi alloys.The 9R structure nucleates from?3{112}_FCC incoherent twin boundary,and grows along{111}_FCC coherent twin boundary by the motion of partial dislocations.The cold-rolled CoCrFeNi alloy with a thickness reduction rate of?76.4%shows tensile yield strength of 1402 MPa and fracture strain of 17.0%at293 K,while it shows enhanced yield strength of 1924 MPa and fracture strain of 22.2%at 77 K.On the other hand,the CoCrFeNi alloy with a thickness reduction of?90.7%shows yield strength of 1567 MPa and fracture strain of 7.5%at 293 K,as it shows enhanced yield strength of 2083MPa and fracture strain of 9.3%at 77 K?2)The work about cryogenic-rolled CoCrFeNi high-entropy alloys shows that,during the cryogenic-rolling,nano-twinning mechanism is more active than that during cold-rolling.The stacking fault energy decreases with decreasing temperature.As a result,decreased temperature promotes more 9R transformation.The cryogenic-rolled CoCrFeNi alloy with a thickness reduction rate of?76.4%shows high yield strength of 2065 MPa and fracture strain of 15.8%at77 K.3)The work about cryogenic-drawn CoCrFeNi high-entropy alloys(true strain:0.42,0.75,1.68 and 2.22)shows that,during the cryogenic-drawing,in addition to 9R transformation,the deformation mechanism evolves from nano-twinning to hierarchical nano-twinning.Hierarchical nanostructure can be formed in the alloys with a considerably large true strain.The width of its 9R structure can be the same as the nanotwin boundary spacing.The cryogenic-drawn CoCrFeNi alloy shows ultrahigh yield strengths of 1946 MPa at 293 K and 2578 MPa at 77 K,and considerable fracture strain.The hierarchical nanostructured CoCrFeNi high-entropy alloys show advantages compared with the currently advanced metallic materials.4)The work about Mn,La,Nb micro-alloyed CoCrFeNi high-entropy alloys deposited using magnetron sputtering shows that,the microstructure is fine-tuned by the addition of 1.82 at.%Mn,1.41 at.%La,or 1.00 at.%Nb,respectively.In so doing,the mechanical properties are enhanced,e.g.,the hardness can be increased by about 25%to 35%,e.g.,up to 11.44 GPa.Microalloying technology combined with non-equilibrium preparation is an effective strategy for fine-tuning the microstructure of nanocrystalline high-entropy alloys.5)Long-range structure,e.g.,9R,can be formed in face-centered cubic high-entropy alloys using plastic deformation or non-equilibrium preparation.It is an effective strategy for designing high-strength high-entropy alloys.Further work can be carried out in this emerging research field to maximize the mechanical properties of face-centered cubic high-entropy alloys.