Microstructure And Corrosion Behavior Of Three High Entropy Alloys

The design of traditional alloys has been mainly based on one or two principle elements with adding other elements to improve the properties,such as steel,copper-based,aluminum-based and titanium-based alloys.Recently,the researchers show great interest in the so-called "high entropy alloys" which are composed of at least five major elements with equimolar or near-equimolar concentration between 5 and 35 at.%.High entropy alloys have numerous alloy systems,and have outstanding mechanical properties and better corrosion resistance.High entropy alloys could be used in a number of applications in the future.Three high entropy alloys-Co1.5CrFeNi1.5Ti0.5Mo0.1,AlCoCrFeNiSi0.1 and TaNbHfZrTi were prepared by arc melting and copper mold-casting.Their microstructures and microhardness have been investigated by a combination of optical microscope(OM),X-ray diffraction(XRD),scanning electron microscope(SEM)and microhardness analyser.Corrosion behavior of the three high entropy alloys in different corrosive environments at 25 ? and 300 ?have been studied by electrochemical methods.In this thesis,commercial 304 stainless steel(SS)and alloy 690 are used as contrast materials.The results show that:(1)CO1.5CrFeNi1.5Ti0.5Mo0.1 high entropy alloy possesses a FCC crystalline structure with the lattice parameter of a=358.3±0.3 pm determined by least squares and extrapolation method.The value of hardness is 334.1 Hv.The OM images show that the chill crystals at the sample's edge,followed by columnar crystals,and finally terminating with an equiaxed dendritic grain stucture to the center.The equiaxed dendritic zones in the center contain two phases,the dendrites(DR)and the interdendrites(ID),and there are plate-like precipitates(?-phase)embedded in the interdendrites.Energy dispersive spectrometry(EDS)results demonstrate that the DR is Cr-Fe riched,the ID matrix is Ni-riched,and the ?-phase is riched in Ti and Ni.It is conjectured that the ?-phase is(Ni,Co)3Ti.The potentiodynamic polarization curve of Co1.5CrFeNi1.5Ti0.5Mo0.1 high entropy alloy in dearated 3.5%NaCl solution exhibites passive corrosion behavior,and the alloy has a wide passive region(AE = 1083 mV).It is interesting to note that secondary passive behavior was observed at 819 mV.The cyclic potentiodynamic polarization curve shows that Co1.5CrFeNi1.5Ti0.5Mo0.1 alloy is susceptible to pitting corrosion in 3.5%NaCl solution.SEM images display that there are some pits formed on the surface of the alloy after potentiodynamic polarization.The corrosion of the Co 1.5CrFeNi1.5Ti0.5Moo.1 alloy took place mainly in the interdendritic areas.The potentiodynamic polarization curve of Co1.5CrFeNi1.5Ti0.5Moo.1 alloy in dearated 0.5 mol/L H2SO4 solution exhibites active-passive corrosion behavior,and the alloy yielded a wide passive region(?E = 1168 mV).The alloy is not susceptible to pitting corrosion in 0.5 mol/L H2SO4 solution.The form of corrosion in 0.5 mol/L H2SO4 solution is uniform corrosion.The corrosion took place mainly in the interdendrtic regions.Electrochemical tests at 300?indicate that the Co1.5CrFeNi1.5Ti0.5Mo0.1 high entropy alloy repassivated after scratching.According to the cBV value,the stress corrosion cracking susceptibility of the Co1.5CrFeNi1.5Ti0.5Mo0.1 alloy is lower than that of 304SS and alloy 690.The different corrosion behavior of different phases in various corrosive medium may ascribe to the chemical inhomogeneity.(2)AlCoCrFeNiSi0.1 high entropy alloy has a BCC crystalline structure with the lattice parameter of a=286.7± 0.3 pm determined by least squares and extrapolation method.The hardness value is 526.1 Hv.The OM images show that the chill crystals at the sample's edge,followed by columnar crystals,and finally terminating with an equiaxed dendritic grain stucture to the center.The equiaxed dendritic areas with apparent coring in the center comprise the dendrites and the interdendrites.The interdendrites are characterized by an intertwined stucture with a period of?100 nm in the width.The dendrites are characterized by Cr-Fe-rich precipitates of spherical or cuboidal morphology.The potentiodynamic polarization curve of AlCoCrFeNiSi0.1 alloy in dearated 3.5%NaCl solution exhibites active corrosion behavior.The cyclic potentiodynamic polarization curve shows that AlCoCrFeNiSi0.1 alloy is susceptible to pitting corrosion in media containing chloride.The corroded surface shows that the major type of corrosion is mainly pitting corrosion.The corrosion of the AlCoCrFeNiSi0.1 alloy took place mainly in the dendritic areas.AlCoCrFeNiSi0.1 alloy exhibites active-passive corrosion behavior in dearated 0.5 mol/L H2SO4 solution,and the alloy yielded a wide passive region(AE = 1350 mV).Notably the secondary passive bahavior was observed at-228 mV.The alloy is not susceptble to pitting corrosion in 0.5 mol/L H2SO4 solution.The form of corrosion in 0.5 mol/L H2SO4 solution is general corrosion.The corrosion took place mainly in the dendrtic regions.Electrochemecal measurements at 300? indicate that the AlCoCrFeNiSi0.1 high entropy alloy repassivated after scratching.According to the cBV value,the stress corrosion cracking susceptibility of the AlCoCrFeNiSi0.1 alloy is lower than that of 304SS but higher than alloy 690.The corrosion behavior and corrosion products of the dendrites and interdendrites are different.The different corrosion behavior of different phases in corrosive,medium may ascribe to the chemical inhomogeneity.(3)TaNbHfZrTi high entropy alloy has a BCC crystalline structure with the lattice parameter of a=339.9± 0.1 pm determined by least squares and extrapolation method.The hardness value is 311.3 Hv.The cast microstructure is a typical dendritic structure.Ti?Zr and Hf are slightly higher in the interdendritic regions,Ta and Nb are higher in the dendritie regions.The potentiodynamic polarization curve of TaNbHfZrTi alloy in dearated 3.5%NaCl solution exhibites passive corrosion behavior,and the alloy has a wide passive region(AE>1171 mV).The cyclic potentiodynamic polarization curve shows that TaNbHfZrTi alloy is not susceptible to pitting corrosion in 3.5%NaCl soution.TaNbHfZrTi alloy exhibites active-passive corrosion behavior in dearated 0.5mol/L H2SO4 solution,and the alloy yielded a wide passive region(?E>1242 mV).The corroded surface shows that the major type of corrosion is uniform corrosion.TaNbHfZrTi high entropy alloy is also not susceptible to pitting corrosion in 0.5 mol/L H2SO4 solution.Electrochemecal measurements at 300?indicate that the TaNbHfZr high entropy alloy repassivated after scratching.According to the cBV value,the stress corrosion cracking susceptibility of the TaNbHfZrTi alloy is lower than that of 304SS and alloy 690.A continuous and compact oxide film formed on the alloy surface after immersed in the high temperature high pressure water for 24 h.The size of the spherical oxide particle is about 100 nm.TaNbHfZrTi alloy shows outstanding corrosion resistance in these corrosive medium,it has huge potential to be used in many fileds.(4)As mentioned above,no matter in 3.5%NaCl,0.5 mol/L H2SO4 or high temperature high pressure primary loop water,TaNbHfZrTi high entropy alloy has the best corrosion resistance among these three high entropy alloys,followed by Coi.sCrFeNi1.5Tio.sMoo.i alloy and then the AlCoCrFeNiSi0.1 alloy.