Influence Of Heat Treatment And Cryogenic Treatment On The Microstructural Evolution And Corrosion Resistance Of Metallic Glasses

The bluk metallic glasses?BMGs?have many excellent performances and broad engineering applications with the continuous improvement of forming technology which make the manufacture of large-size components easier.Corrosion resistance is one of the key factors considered in the application of BMGs.There are still many difficulty and challenges in the research of the corrosion behavior in the salt solutions.In present work,this subject takes Ni₆₂Nb₃₃Zr_5,Ni₅₉Nb₃₃Zr₅Co₃ and Pr₆₀Cu₂₀Al₁₀Ni₁₀0 BMGs as the research objects,which were fabricated by copper mold casting method.The influence of heat treatment and cryogenic treatment on microstructural evolution and corrosion resistance in 3.5 wt.%NaCl solution of Ni-based and Pr-based BMGs have been investigated by XRD,DSC,SEM,electrochemical workstation,electrochemical impedance,and XPS.Moreover,based on the correlation between phase structure and corrosion resistance,the corrosion mechanism is explored.The main research results obtained are as follows:?1?It occurs the structure relaxation,partial crystallization and complete crystallization for the the Ni₆₂Nb₃₃Zr₅ BMG during the annealing process from 400?to 470?.The final crystallization products are Ni-rich FCC-type phase,Ni10?Zr,Nb?7 and Nb oxide phases.Through SEM and EDS analysis,the grains with long strip and irregular shape are Ni-rich FCC-type phase,and the matrix is mainly Ni₁₀?Zr,Nb?₇ phase.The electrochemical corrosion analysis shows that the appropriate annealing treatment is beneficial to improve the general corrosion resistance of Ni₆₂Nb₃₃Zr₅ BMG.Combined with the corrosion parameters and electrochemical impedance results,470?-annealed sample shows the optimal comprehensive corrosion resistance.The corrosion morphology and EDS analysis present that the amounts of corrosion pits on the surface of the passivation film decrease and the peeling phenomenon becomes weak with increasing the annealing temperature.The annealed sample at 470?has more uniform and dense passivation films.The improvement in corrosion resistance after annealing can be attributed to the formation of Ni10?Zr,Nb?7 phase that facilitates the formation of the passivation film.In addition,the Nb oxides as the in situ phase induces the passivation film more homogeneous and dense.Ni₆₂Nb₃₃Zr₅ BMG presented partial crystallization after cryogenic treatment at-20?,and the cryogenic products are main amorphous phase and a small amount of Ni-rich FCC-type and Ni?Zr,Nb?phase.The XRD diffuse peak is smoother,indicating that the characteristics of amorphous phase become more obvious when annealed at-100?.According to electrochemical corrosion and impedance tests,the appropriate cryogenic treatment is beneficial to improve the general corrosion resistance of Ni62Nb33Zr5 metallic glass,when annealed at-100?has optimal comprehensive corrosion resistance.According to the corrosion morphology and EDS component analysis,the stability of the passivation film at-100?is better,and the number of corrosion pits is reduced.Because the-100?cryogenic treatment reduces the interatomic distance and atoms bound tightly,forming a more uniform and stable amorphous phase structure.?2?After heat treatment at 550?,700?,787?and 847?,the Ni₅₉Nb₃₃Zr₅Co₃ BMG exhibits structure relaxation,partial crystallization and complete crystallization.The final crystallization products are Ni₁₀?Zr,Nb?₇ phase,Co?Zr,Nb?phase,FCC-type Ni-rich phase and Nb oxide phase.The addition of Co significantly increases the crystallization temperature of Ni₆₂Nb₃₃Zr₅ BMG,indicating that it contributes to the improvement of thermal stability.According to electrochemical corrosion tests,the appropriate heat treatment is beneficial to improve the general corrosion resistance.Combined with the corrosion parameters and electrochemical impedance results,when annealed at 847?has optimal comprehensive corrosion resistance.The corrosion morphology and EDS analysis present there are fewer corrosion pits on the surface of the sample at 847?and the passivation film appears more uniform and stable with poor peeling phenomenon.Ni₅₉Nb₃₃Zr₅Co BMG after-20?cryogenic treatment presents few Co?Nb,Zr?,Ni₁₀?Zr,Nb?₇ and FCC-type Ni-rich phase.The samples treated at-100?basically maintained the amorphous phase structure.Through the electrochemical corrosion test,the appropriate cryogenic treatment is beneficial to improve the general corrosion resistance,and-20?-annealed sample shows better comprehensive corrosion resistance.The corrosion morphology and EDS analysis present that the passivation film stability of-20?sample was better and the number of pits was reduced.In conclusion,the Nb oxide phase formed after annealing treatment can be used as an oxidation film to resist corrosion,and the addition of Co can enrich Nb and Zr to some extent,improve the passivation film properties and the pitting resistance.Cryogenic treatment can make the atomic configuration more stable and disordered homogenization,and the volume shrinkage at low temperature can promote the formation of stable crystal structure,also improve the general corrosion resistance of Ni₅₉Nb₃₃Zr₅Co BMG.?3?It occurs the structure relaxation,partial crystallization and complete crystallization for the the Pr₆₀Cu₂₀Al₁₀Ni₁₀0 BMG during the annealing process from 120?,150?,180?to350?.Annealing at 120?and 150?undergo structural relaxation,but exhibits relatively complex crystallization products when annealed at 350?,which mainly is PrAl_xCu_y phase,PrAlxNiy phase,Al2O3 phase and Pr oxide phase.Combined with the corrosion parameters and electrochemical impedance results,the 120?-annealed sample shows better comprehensive corrosion resistance.According to the corrosion morphology and EDS component analysis,there are fewer corrosion pits and less peeling of the passivation film on the surface of the sample at 120?,and forms more uniform and stable passivation film.There is structural relaxation occurs after annealing at 120?,that forms a more stable and uniform amorphous phase structure,thus improving the comprehensive corrosion resistance.