Effects Of Silicon And Thermal Exposure On Microstructure And Mechanical Properties Of GH3535Superalloy

The Molten salt reactor is one type of six candidate reactors for Generation IVreactor system recommended by GIF (Generation IV International Forum). In themolten salt reactor, the structural materials have to suffer from a complex operatingcondition of high temperature, strong irradiation and serious fluoride salt corrosion.Hastelloy N superalloy is the most promising candidate structural materials inventedby ORNL (Oak ridge National Laboratory) specifically for molten nuclear reactorsystem in the1960s. The long-term thermal stability of Hastelloy N superalloy underhigh temperature is one key problem which affects the service life of reactors.Furthermore, the type and morphology of carbides can be controlled by adjusting thecontent of silicon. Therefore, effects of silicon additon on thermal stability andmechanical properties of GH3535alloy deserve further investigation.In this thesis, X-ray diffraction (XRD), scanning electron microscopy (SEM),transmission electron microscopy (TEM), tensile tests, DSC, Thermal-Calccalculation were used to study the effects of silicon additons on the type andmorphology of carbide in alloy, the effects of silicon additons on precipitation anddecomposition behaviors of M6C carbide, the effects of silicon addition and long-termthermal exposure on the tensile properties. The main research contributions of thisthesis are as follows:It is found that the M2C carbide almost disappears in the investigated cast GH3535alloys with the silicon content increase. Granular M6C carbides formed at the grainboundaries in the heat with0.46wt%Si. The quantity and morphology of M6Ccarbides remained unchanged in the exposure treatment at700-800℃up to1000hours. The EDS analyses indicated the enrichment of Si in the M6C carbides. While inthe non-Si heat, the carbides formed at grain boundaries and the M6C-matrix phaseboundaries were identified as M2C type by TEM at the initial stage of the exposure treatment. With the exposure time increasing, the fine M2C particles transform intocoarse plate-like M6C carbides. It can be concluded that silicon additions can lead tostable and beneficial grain boundary carbides in the Ni-Mo-Cr superalloys.Silicon increases the amount of undissolved M6C carbide particles obviously inGH3535superalloy. Composition analyses revealed that primary M6C carbides in the0.46wt%Si alloy are highly enriched in silicon. The addition of silicon widens thetemperature range of M6C carbide existence to1335°C, while the M6C carbides innon-silicon alloy were completely decomposed when heated to1260°C. According tothe calculation results using Thermal-calc software, the Gibbs free energy of M6Ccarbides decreases with the silicon content, and the silicon-riched M6C carbide aremore stable than that in non-silicon heat samples. In tensile tests, it is found thatsilicon-rich M6C carbide particles act as cracking origin sites, and further give rise tothe degradation of the high temperature tensile properties of standard heat samples ascompared to non-silicon heat ones.The coarsening of grain boundary carbides occurred in both alloys withincreasing exposure time. In addition, the coarsening kinetics of grain boundarycarbides in the non-silicon alloy is faster than that in the standard alloy. The tensileproperties of both alloys improved after exposure for100h due to the formation ofnano-sized grain boundary carbides. The grain boundary carbides are coarsened moreseriously in non-silicon alloys than in Si-containing alloys, resulting in a moresignificant decrease in the tensile strength and elongation in the former case. Siliconadditions can effectively inhibit the severe coarsening of grain boundary carbides andthus avoid the obvious deterioration of tensile properties after a long-term thermalexposure.