Investigation On The Effect Of Silicon On The Formation And Transformation Of Carbides In GH3535 Superalloy

Generation IV nuclear energy systems including the molten salt reactor(MSR)have attracted more and more attention due to the serious energy shortage and the the security problem of conventional nuclear reactors.In January 2011,the Chinese Academy of Sciences(CAS)launched the strategic pioneer science and technology project: thorium molten salt reactor nuclear energy systems(TMSR).To obtain the critical components(core vessel and loop pipes)for TMSR,one Ni-based superalloy(GH3535)is developing in China on the basis of Hastelloy N alloy,which was designed by Oak Ridge National Laboratory(ORNL)in 1950 s.Some previous studies have proposed the obvious effects of Si addition on the microstructures and properties of Hastelloy N alloy and GH3535 alloy,which indicate plenty of room for optimization of Si content.However,the Si content is not the only variable in these studies mentioned above.The potential effect of different melting methods should not be neglected.No systematic investigation,covering the full practical range of Si contents,has been reported in Hastelloy N alloy to date.The existing results cannot provide enough for the development of GH3535 alloy.Also,there is lack of reasonable interpretations about the microstructure evolution after Si additions based on thermodynamic and kinetic considerations.In this study,seven GH3535 alloys with different Si contents were prepared,and the morphology,microstructure,composition and phase transformation of the carbides were investigated in the cast state,solid-solution state and thermal exposure state,respectively.The site preference of Si,and its effect on the structure and stability of M6 C carbides have been also investigated using synchrotron radiation technologies and first principles calculations.The results can be outlined as follows:(1)The increase of Si content inhibits the formation of M2 C carbides and then promotes the precipitation of M6 C carbides in as-cast GH3535 alloy.A critical Si content(0.319 wt.%)has been identified completely suppress the M2 C carbide.Based on classical nucleation theory,the high nucleation rate of M6 C carbides supports their preferential formation against M2 C carbides from residual liquid phase.M2 C eutectic carbides were crushed in the hot rolling and decomposed in subsequent solution treatment,while the more stable skeleton-shape M6 C eutectic carbides were crushed into strings along the hot rolling direction and remained in subsequent solution treatment.The quantity of M6 C carbide strings increases with the increasing of Si content.(2)Under different thermal exposure conditions,different types and sizes of second carbides form at grain boundaries in the alloys with different Si content.In the low-Si GH3535 alloys(0.0514 and 0.188 wt.%),fine granular M2 C carbides precipitate at almost all grain boundaries at the initial stage of the thermal exposure.With a further thermal exposure,M2 C carbides were decomposed and transformed into plate-like M6 C carbides.In the high-Si alloy(0.319,0.562 and 1.01 wt.%),M6 C carbides nucleated and grow into granular during the thermal exposure.In the alloy with 1.01 wt.% Si,M2 C carbides precipitate at grain boundary and coexist with M6 C carbides after long term exposure at 750 oC.The large account of M6 C carbides and the serious enrichment of Si in primary M6 C carbides lead to the low matrix Si content in the alloy with 1.01 wt.% Si,and are responsible for the abnormal formation of M2 C carbides.(3)The eutectic M12 C carbides and hot cracking form at grain boundaries along the rolling direction in 0.46 wt.% Si alloy when heated to 1335 oC or a higher peak temperature at a heat rate of 20 oC/min and then water quenched.The quantity and size of the eutectic phase and hot cracking increase with the increasing peak temperature.There are only few and small hot cracks forming in 0.06 wt.% Si alloy when heated to 1365 oC.The constitutional liquation occurs at the M6 C carbides-matrix interphase and lead to the formation of the eutectic M12 C carbides and hot cracking.(4)The site preference of Si,and its effect on the structure and stability of M6 C carbides have been investigated systematically in GH3535 alloys using synchrotron radiation technologies and first principles calculations.Si atoms preferentially occupy Ni(16d)sites.Provided multiple doped Si atoms at Ni(16d)sites,formation energies and site occupancy energies of Ni48-xSixMo48C16 structures(x=1,2,4,and 8)decrease with the number of doped Si atoms increasing.From DOS analyses,the hybridization of doped Si-s band takes place with s,p and d bands of the nearest-neighbor Ni(32e)and Mo(48f)atoms.The population analyses and Si K-edge XANES spectra of M6 C carbide prove the charge transfer between Si atoms and the nearest-neighbor Ni(32e)and Mo(48f)atoms.It can be concluded that the stability of Ni48-xSixMo48C16 structures derived from the attractive interactions between Si(16d)with the nearest-neighbor Ni(32e)and Mo(48f)atoms.Based on the reults mentioned above,we have evaluated comprehensively the effects of Si addition in GH3535 alloy from three perspectives: workability,weldability and mechanical properties.Then,the optimized Si content are recommended to be ~0.319 wt.%.The site preference of Si,and its effect on the structure and stability of M6 C carbides have been investigated systematically in GH3535 alloys using synchrotron radiation technologies and first principles calculations,which are universally applicable to explain the similar experimental phenomenon in high speed steels,cemented carbides and other superalloys.