Effect Of Sulfur On The Microstructure And Mechanical Properties Of Nickel-based Single Crystal Superalloy

The purity is an important factor affecting the performance of nickel-based single crystal superalloys.Among the many impurities,the harm of non-metal element sulfur cannot be ignored.Since sulfur was studied in superalloys in the 1970s,most of the research objects are polycrystalline and deformed superalloys,and the effect of sulfur on single-crystal superalloys has rarely been reported.In this paper,10 ppm,100 ppm,and 1000 ppm sulfur were added to a second-generation single crystal superalloy to prepare four sets of single crystal rods with different sulfur content.The differential scanning calorimetry(DSC),optical microscope(OM),scanning electron microscopy(sulfur EM),X-ray diffraction(XRD)and transmission electron microscopy(TEM)were used to study the role of sulfur in nickel-based single crystal superalloys.The effects of sulfur on the solidification characteristics of alloys mainly include the following aspects.Sulfur is mainly segregated in interdendrites region.When the sulfur content is 60 ppm,sulfides begin to appear in the alloy.The main components of sulfides are S,Cr,and Hf,which are distributed near the carbides and are ellipsoidal or irregular.The increase of sulfur content makes the?/??eutectic content in the alloy gradually decreased,and changed the eutectic morphology,at the same time increased the volume fraction of MC carbides between dendrites,and increased the precipitation temperature of MC carbides.Sulfur also reduces the segregation of W and Al,and the segregation of Re first increases and then decreases with the increase of sulfur content.The formation of sulfide reduces the initial melting temperature of the alloy.In addition,sulfur will increase the primary dendritic distance and reduce the porosity of the alloy.After heat treatment,the??phase of the four experimental alloys are all cubic.As the sulfur content increases,according to statistics,the??phase size in the alloy gradually increases after heat treatment,and the volume fraction is gradually increasing.Through long-term aging,the coarsening rate of??phase increases with the increase of sulfur content.In addition,in the heat-treated structure,the alloy dendrites with a sulfur content of 810 ppm precipitated the TCP phase.The TTT curve further verified that the addition of sulfur would promote the precipitation of the TCP phase.The addition of sulfur will also promote the decomposition of MC carbides from larger lumps to fine particles,and reduce the volume fraction of carbides after heat treatment.Hf and S are extremely easy to combine,and the precipitated sulfides will consume Hf elements,reducing the stability of MC carbides and causing carbides to decompose.In addition,after heat treatment of the alloys with sulfur content of 60ppm and 810ppm,irregular M₂₃C₆ carbides are observed in the dendrite cores,which are rich in Cr,Mo,Re,and S elements.The addition of sulfur has a complicated effect on the creep properties of the alloy at1100?/137MPa.The creep life of the alloy decreases with the increase of the sulfur content.Sulfur promotes the degradation of MC carbides into granular M₂₃C₆ carbides,which is beneficial to increase the creep life.However,more TCP phases in high-sulfur alloys consume a large amount of solid solution strengthening elements and reduce the strength of the alloy.In addition,sulfur causes severe oxidation of the alloy under high temperature creep and increases the distance between the dislocation network.The above factors lead to the creep life of alloys gradually decrease with the addition of sulfur.Sulfur is also extremely unfavorable to the creep performance of the alloy at 760?/800MPa.The creep life decreases rapidly with the increase of the sulfur content.The fracture mode shows obvious brittle fracture characteristics.The carbide and sulfide are torn,and the cracks propagation mainly follow the interface between the carbide and the matrix.