Microstructure And Mechanical Properties Of Graphene Enhanced CoCrFeMnNi High Entropy Alloys

CoCrFeMnNi high-entropy alloys(HEAs)are considered as the most promising structural candidate materials since their outstanding mechanical properties,such as excellent plasticity,higher low-temperature damage limit and so on.However,the low strength and hardness of the HEAs limits their application in engineering field.Graphene is considered to be an ideal reinforcement because of its ultra-high strength,but the serious agglomeration problem and poor wettability between graphene and matrix affect the strengthening effect.In this study,nickel-modified reduced graphene oxide(RGO@Ni)powders prepared by electroless plating were introduce into the gas atomized CoCrFeMnNi HEAs powders to fabricate RGO@Ni reinforced CoCrFeMnNi HEAs by hot-pressing sintering,which was expected to both improve the dispersion of graphene and the interface bonding between grapheme and matrix.We mainly studied the effects of rapid solidification on the microstructure and mechanical properties of pre-alloyed powers and CoCrFeMnNi bluks prepared by hot pressing sintering.Meanwhile,the effects of RGO@Ni content on microstructure and mechanical properties of RGO@Ni-CoCrFeMnNi composites were systematically studied.The main conclusions of the study are as follows:1.Single FCC structured CoCrFeMnNi HEAs powders with uniform composition and nano-crystals were successfully prepared by gas atomization.After the powders were sintered at 1100^oC for 2h with a pressure of 60MPa,the relative density of the bulks reached 99.3%,indicating that the sintered samples basically achieved densification.2.The sintered bulks kept single FCC structure after hot pressing sintering,and the microstructure was composed of fine equiaxed grains with an average size of?16?m.Moreover,mass metastable structures with a size of 55?160nm formed in the grains.These metastable structures were inherited from the rapid solidification structure in gas atomized powders,which resulted in excellent comprehensive mechanical properties of the sintered bulks(yield strength of 222MPa,tensile strength of 564MPa and elongation of?37%).3.Graphene oxide and nickel sulfate was used as raw materials,after thermal reduction and electroless nickel plating(p H=13,reaction temperature 80?,Ni²⁺concentration of 0.01 mol/L),the graphene oxide was reduced to grapheme,while Ni particles with a size of 50 to 200 nm were overlaid on the graphene surface to formation Ni modified RGO powders(RGO@Ni).4.The Ni particles overlaid on RGO surface were dissolved into the matrix,while RGO were reacted with matrix to form submicron and micron sized Cr-rich carbides with a M₂₃C₆ structure that dispersed in CoCrFeMnNi matrix after hot pressing sintering RGO@Ni-CoCrFeMnNi composites powders.There is no specific crystallographic orientation relationship between Cr-rich carbide and HEAs matrix.5.With an increase in RGO@Ni content,the content(f)of Cr-rich carbides with M₂₃C₆ structure increases gradually,and the size(r)remains almost unchanged.The increase in yield strength(??)of the composites satisfies the linear relationship of??=31.5+762*(f^1/2/r),and the main strengthening mechanism is the dislocation bypassing the second phase.The 1.5wt.%RGO@Ni-CoCrFeMnNi composites exhibited excellent comprehensive mechanical properties(yield strength,ultimate tensile strength and elongation at break are 374MPa,722MPa and 12.4%,respectively).