Effect Of Mo On Microstructure And Properties Of FeCrAl Alloy

FeCrAl alloy is a new type of accident-tolerant envelope candidate,which has received extensive attention in recent years because of its excellent mechanical properties,good high-temperature tissue stability and corrosion resistance.Alloy elements play a very important role in the microstructure and properties of the alloy.However,systematic studies of Mo elements on FeCrAl alloys still need further investigation.Here,systematic studies of Mo content（0,1,2,3,wt.%）on the microstructure,mechanical properties and corrosion resistance for FeCrAl alloys were carried out using X-ray diffraction,Scanning electron microscopy,Kelvin probe force microscopy and first principles calculations.The average grain size decreases with increasing Mo content,indicating that increasing Mo content hinders the recrystallisation of FeCrAl alloys at 800°C,850°C and 900°C.The recrystallisation activation energy of 0Mo,1Mo,2Mo and 3Mo is 278.76,286.66,312.11 and318.51 k J/mol respectively,The increase of Mo content increases the recrystallization activation energy of FeCrAl alloy.Meanwhile,the addition of Mo significantly increases the volume fraction of Fe₂Zr.First principles calculations show that the cluster formation energy of Zr atoms in the Fe-Zr system（-0.22 e V/atom）is significantly higher than that in the Fe-Zr-Mo system（-0.25 e V/atom）,indicating that the increase in Mo content promotes the precipitation of Fe₂Zr.These Fe₂Zr are effective in pinning grain boundaries,thus hindering the recrystallisation of the FeCrAl alloy.Increasing Also,Mo content also increases the dislocation density of the FeCrAl alloy matrix.During the initial stages of reversion and recrystallisation（within 1 h of holding at 900°C）,the yield strength of the alloy decreases.3Mo alloys have a significantly higher yield strength than 0Mo alloys.This is mainly due to the reduction in dislocation density during the initial stage,which is dominated by the recovery phase,resulting in a reduction in yield strength.The high dislocation density of the 3Mo alloy leads to a higher yield strength.In fully recrystallised FeCrAl alloys,the yield strength increases significantly with increasing Mo content.Strength is mainly increased by solid solution strengthening,grain boundary strengthening and precipitation strengthening.With increasing Mo content,these strengthening effects increase significantly.Compared to the 0Mo alloy,the three strengthening mechanisms in the 3Mo alloy contribute 93.2,20.9,126.2 and 12.1 MPa greater strength increments respectively.Owing to the enhanced strengthening by the addition of Mo,the yield strength of 3Mo is 0.36 times higher than that of 0Mo.Adding Mo slightly increased the grain size of the FeCrAl alloy at 1200°C and 1300°C.This is mainly due to Mo increases the size of Fe₂Zr,which weakens the pinning effect of Fe₂Zr in the 3Mo alloy and then increases the grain size.Meanwhile,these coarse Fe₂Zr leads to the brittle fracture for FeCrAl alloy.With increasing Mo content,the corrosion resistance of FeCrAl alloy in 3.5%Na Cl showed a trend of increasing and then decreasing.The self-corrosion currents I_corr（0Mo）,I_corr（1Mo）,I_corr（2Mo）and I_corr（3Mo）were 5.232×10^-6、2.387×10^-6、3.715×10^-6、4.471×10^-6 A/cm²,respectively.The pitting potentials E_pit（0Mo）,E_pit（1Mo）,E_pit（2Mo）and E_pit（3Mo）were 0.175、0.235、0.226、0.193 V vs.SCE,respectively.In LiOH（7000,700,70,ppm）,the variation of Mo content on the corrosion resistance of FeCrAl alloy showed identical trends.Mo is mainly present in the passivation film as Mo O₃ to protect the matrix.As the Mo content increases the stability of the passivation film is significantly increased.The corrosion resistance of the alloy is improved.With a further increase in Mo content,the volume fraction of Fe₂Zr in the matrix increases significantly.Then,the Volta potential difference between Fe₂Zr and the matrix（17-45 m V）results in galvanic corrosion and thus accelerates the corrosion of the matrix.Therefore,the higher the Mo content,the worse the corrosion resistance of the alloy.