Experiment And Multiscale Simulation Of The Effects Of Isothermal Aging On Microstructure And Properties Of Cobalt Based Claddings

Cobalt-based alloys have been widely used in aerospace,nuclear power,water conservancy and biomaterials due to their excellent mechanical properties,corrosion resistance and thermal stability.However,it often operates in high temperature and high stress environment for a long time,so it is vital to predict and evaluate the stability of the microstructure and properties of Cobalt-base alloy at high temperature.In this paper,three layers of Stellite 6 alloy with total thickness of 12 mm were deposited on the surface of 304 stainless steel by plasma arc cladding,and followed by the aging treatment of 700 ℃×(0～1000)h.The room temperature microstructure of the as cladded and as aged coatings were characterized by light microscopy,scanning electron microscopy,X-ray diffraction and transmission electron microscopy.The hardness and wear behaviors of Stellite 6 coatings at room temperature/700 ℃ were also measured.In addition,the interaction behavior between stacking faults and solute atoms(so called Suzuki effect)was simulated by thermodynamics,first principles and lattice kinetics Monte Carlo methods,the influences of solute atoms on Suzuki effect were analyzed both qualitatively and quantitatively.The as cladded Stellite 6 alloy consisted of γ-Co matrix and network eutectic structure.After aging treatment,γ-Co gradually transformed into ε-Co phase(also called as martensitic transformation),accompanied by the fine carbides precipitation and the decomposition of eutectic carbides,and some carbides tend to precipitate at the stacking fault influenced by the Suzuki effect.Through establishing the isothermal martensitic transformation kinetics model and comparing with the experimental datas,it presented a direct relationship between martensitic transformation and precipitations.Owing to the transformation of ε-CO and the increase of the total amount of carbides,the room/high temperature hardness of Stellite 6alloy improved significantly,and the carbide contributed most in hardness improvement,which was consistent with the first-priciples simulation results.With the increase of aging time,the high-temperature wear resistance of Stellite 6 increased continuously,and the wear mechanism changed from adhesive wear to abrasive wear.The improvement of wear resistance was mainly resulted from the comprehensive effect of hardness improvement,timely removal of oxide layer and the obstruction of dispersed M7C3 eutectic carbide.Multiscale simulation indicated that stacking fault could attract Cr atom and repel Fe,W and C atoms.The interaction energy was closely related to the formation and vanish of chemical bonding between the atoms in stacking fault.The solute atoms tend to segregate towards stacking fault,and M23C6 type carbides were gradually formed around the stacking fault.The relationship between the transformed fraction of M23C6 and the isothermal time presented as a roughly "S" curve,and there existed a proportional relationship between the amount of clusters and the stacking fault density.Additionally,the concentration around stacking fault didn’t change obviously owing to the strong interaction between carbon and metallic atoms.The above simulation results are consistent with the experimental results.