Effects Of Ni On Microstructure And Properties Of The Co-Al-W Base Superalloy

The Co-based superalloy has been widely used in the field of aerospace,marine vessels,nuclear industry,petrochemical and other advanced power propulsion system of hot end components with its excellent orrosion,oxidation and thermal fatigue resistance.In conventional Co-based superalloys,the strength of the alloy depends only on solution strengthening and the precipitation of carbides,so the high-temperature strength and bearing temperature ability is significantly lower than that of relying on γ’-Ni3Al phase strengthening of Ni-based superalloy,hindering its application under the condition of high temperature.In 2006,a stable ternary γ’-Co3(Al,W)phase with L12 structure has been found in a Co-Al-W base alloy by Sato J.et al,which can be stable at 900℃,providing a new pathway for Co-based superally.In this study,we optimized the composition design with different Ni from 15%-30%(mass,%)based on the existing research,and with the different heat treatment,we study the microstructure,hardness,tensile strength and hot corrosion resistance of the four alloys to provide the data basis for the development of the alloy.It is found that γ’ phase dissolution temperature increases with the increasing of Ni content,from 894℃ of the 15Ni alloy to 957℃ of 30Ni alloy,but the solid phase line temperature is stable at～1400℃.With the increase of aging time,the size of γ’ phase continues to grow,while the volume fraction of y’ phase firstly increases and then decreases.The volume fraction peak of γ’ phase appeared at 850℃/100h and 870℃/72h,precipitating of about 70%of the volume fraction of γ’ phase,but the latter’s γ’ phase size is greater than the former.Without γ’ phase precipitating,the hardness of the as-cast alloy is the lowest.After aging,due to the precipitatiton of γ’ phase with strengthening effect,the hardness increases,and the hardness value increases with the increase of γ’ phase precipitation.The hardness and yield strength is lower at 870℃/72h than 850℃/100h,because at high temperature coherent strengthening decreases with a increasing in y’ phase size and a decreasing in γ’/γ phase misfit.The alloys have ductile fracture at room temperature with obvious intergranular fracture along the dendrite,but they have intergranular fracture at 800℃.The yield strength increases with increasing Ni content,the strength of the alloy matrix strength and y’ phase is ehanced by the Ni added in the alloy.The corrosion experiment of the alloy at 900℃ in 75%Na2SO4+25%NaCl molten salt was carried out.The results show that the surface corrosion products of the alloys are CoO,Ni,Ni3S2 and Co9S8.After corrosion for 20h,the corrosion layer of the alloys is divided into two layers:the outer layer is CoO layer,the inner layer is mixed corrosion layer with Al,Cr,S,Co,O.With the increase of Ni content,the depth of the corrosion layer becomes thickness and corrosion resistance degrades.The reason is that due to the increase of Ni content,more and more Ni·Ni3S2 eutectic are generated,damaging the integrity of the corrosion film,leaving the corrosion resistance decreased.Considering the mechanical properties and hot corrosion properties,the content of Ni in the alloy is not more than 25%.