Research On Microstructure Evolution And Thermal Stability Of Nanocrystalline Nickel-based Alloy

In this thesis,cold rolling was performed on a γ’ precipitates strengthened Ni alloy to achieve a nanocrystalline Ni alloy sample.Through isochronal and isothermal aging experiments,microhardness and microstructure evolutions of the nanocrystalline Ni alloy under different aging conditions were studied.Microhardness test,scanning electron microscope,transmission electron microscope,scanning transmission electron microscope and energy dispersive spectroscopy were performed to characterize the mechanical behavior and microstructure of the Ni alloy samples subjected to ageing.The mechanical property,thermal stability and grain growth of the nanocrystalline structure subjected to high temperature exposure were explored.The grain growth kinetics were analyzed by calculating the grain growth activation energy.The main conclusions are as follows:(1)After 90% cold rolling,the microstructure of the coarse-grained nickel alloy was greatly refined to a nanocrystalline structure with an average size of ～19 nm.During the rolling process,the spherical γ′ precipitates originally existing in the alloy were completely sheared.Compared with the coarse-grained sample,the microhardness of the nanocrystalline sample is significantly enhanced.(2)Upon isochronal aging,the microhardness of the nanocrystalline Ni alloy gradually increases to reach a peak value,which is then followed by a rapid drop.The microhardness reaches a peak value of 818 Hv at 700 ℃.The grains grow up slightly at ageing temperatures below 800 ℃.When the aging temperature reaches 800 ℃,fractional abnormally grown grains are discernible.As the temperature rises,certain amounts of abnormally grown grains are apparent,while the majority of the grains remain to be in the nanometer scale.(3)The thermal recovering extent of γ’ precipitates damaged by the rolling process increases with aging temperatures,which leads to the peak microhardness at the aging temperature of 700 ℃.When the ageing temperature is above 800 ℃,the appearance of large grains result from abnormal grain growth leads to a rapid decrease in the microhardness.(4)During the isothermal aging at 700 ℃,the microstructure of the nanocrystalline Ni alloy remain stable up to 450 h treatment.After aging treatment of 450 h,the average grain size is still less than 60 nm,and no abnormal grain growth is detected.As a result,a high microhardness is maintained.When the nanocrystalline Ni alloy was subjected to isothermal aging at 800 ℃,fractional grains tend to grow rapidly with a 4 h aging.Nonetheless,certain amounts of stable nano-grains are reserved in the sample subjected to 450 h treatment,indicating the superior thermal stability of present Ni alloy sample.(5)During the aging process,the γ’ precipitates recover rapidly prior to the significant grain growth.The pinning effect on the nano-grain boundaries exerted by the γ’ precipitates stabilizes the nano-grains.Due to the inhomogeneity of defects and the second phase in the structure,grains with concentrated defects or lack of γ’ precipitates pinning are more prone to abnormal growth.The coarsening of the γ’ precipitates weakens the pinning effect on the nanograin boundary.The activation energy during the growth of the fine grains and the coarse grains is calculated to be 245 k J/mol and 301 k J/mol,respectively.The energy of the grains lacking the pinning of the γ’ precipitates exceeds the activation energy required for the abnormal grain growth during the aging process,so the grains grow rapidly.