Microstructure And Properties Of In-situ Synthesized TiC/AlCoCrFeNi High Entropy Alloys

Since the concept of"high entropy alloy"was put forward in 2004,they have attracted wide attention for its unique design concept and excellent properties.Different from traditional alloys,they exhibited high strength,high hardness and excellent corrosion resistance properties.Among the reported HEAs,AlCoCrFeNi HEAs have been extensively studied.TiC has high melting point,high hardness and excellent chemical and thermal stability,which can be used as a strengthening phase to improve the properties of alloys.However,studies on the in-situ ceramic particle reinforced HEAs are quite limited.Therefore,it is of great importance to study the microstructure evolution and properties of the AlCoCrFeNi HEAs reinforced by TiC particles.In this thesis,using AlCoCrFeNi HEAs,pure Tiand Fe5C master alloy as matrix,titanium source and carbon source,respectively.The(AlCoCrFeNi)_100-x(TiC)_x(x=0,2,4,6,8)HEAs were successfully prepared by an in-situ synthesis method.The microstructure,mechanical properties,corrosion resistance and high temperature oxidation resistance of the alloy were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray photoelectron spectroscopy(XPS).The main conclusions are as follows:(1)Addition of Tiand C elements significantly alter the phase structure and microstructure of the(AlCoCrFeNi)_100-x(TiC)_xHEAs.The addition of TiC refines the microstructure of the alloy and acts as an effective nucleating particle to favor the precipitation of?phase in the alloy.They exhibit equiaxed crystal and consist of BCC+B2 phase for the TiC-free AlCoCrFeNi HEAs.However,they exhibit dendrite structure and contain a coexistence of BCC+B2+TiC+?phases for the TiC-containing AlCoCrFeNi HEAs.(2)The mechanical properties of(AlCoCrFeNi)_100-x(TiC)_xHEAs are associated with Tiand C contents.With an increase of TiC content,the strength increases,while the ductility decreases.The yield strength increases from 1282 MPa of TiC0 alloys to 1886 MPa of TiC8alloys,while the compression strain decreases from 23.34%to 13.60%.The addition of Tiand C reduces the density and increases the specific yield strength(SYS)of the alloy.And the SYS of the HEAs increases from 180.60 MPa·cm~3·g^-1to 296.26 MPa·cm~3·g^-1.The TiC8 alloys exhibit the optimized mechanical properties,and their SYS reaches maximum value.(3)The potentiodynamic polarization test shows that the corrosion resistance of(AlCoCrFeNi)_100-xTiC_xHEAs improves with the increase of Tiand C content.The corrosion potential increases from-266 m V to-222 m V.The corrosion current decreased from 0.1?A/cm~2to 3.16×10^-2?A/cm~2.(4)Electrochemical impedance spectroscopy(EIS)and X-ray photoelectron spectroscopy(XPS)tests show that the stability and corrosion resistance of the passivation film of(AlCoCrFeNi)_100-xTiC_xHEAs gradually increase.The formation of TiC leads to the decrease of Al content and the increase of Ticontent in the alloy passivation film,which improves the compactness of the passivation film and improves the corrosion resistance of the HEAs.(5)The high temperature oxidation resistance of(AlCoCrFeNi)₉₂TiC₈HEAs is studied,and its high temperature oxidation resistance mechanism was revealed.The oxidation kinetics curves of(AlCoCrFeNi)₉₂TiC₈HEAs obey almost the same tendency for alloys oxidated at800?,900? and 1000?,respectively.With the increase of temperature,their oxidation resistance decreases gradually.The oxidation rate constant K_pincreases from 1.97×10^-3Mg~2·cm^-4·h^-1to 1.51×10^-2Mg~2·cm^-4·h^-1.(6)XRD and SEM analysis show that the oxidation products of(AlCoCrFeNi)₉₂TiC₈HEAs after oxidation at 800?,900? and 1000? are mainly composed of TiO₂and Al₂O₃,together with a small amount of TiO_xoxides.Theoretical analysis shows that the lowest gibbs free energies for the Al₂O₃and TiO₂promotes the formation of Al₂O₃and TiO₂during the oxidation process,which is consistent with the results of XRD and SEM.