Dendritic morphology evolution and microhardness enhancement of rapidly solidified Ni-based superalloys

The mechanisms of microstructure evolution and dendritic growth of Inconel alloys in a drop tube were investigated in this paper. The Vickers microhardness of the solidified alloy droplets was also measured to explore the effects of grain size and precipitated phase on the mechanical properties. From XRD results, the rapidly solidified Inconel 600, 617, 625, and 718 alloys are characterized by gamma phase solid solution with fcc structure. Further analyses based on EDS and SEM reveal that Laves phase precipitates at the grain boundaries of gamma phase in Inconel 718 alloy. With the decrease of droplet diameter (D), the dendritic morphology experiences a transformation of orthogonal long dendrites -> irregular stubby dendrites -> equiaxed grains in Inconel 600 alloy, orthogonal long dendrites -> stubby irregular dendrites in Inconel 625 alloy and orthogonal long dendrites -> equiaxed grains in Inconel 617 and 718 alloys. Although the dendrites become coarse locally, the dendritic grain size obviously reduces with the decrease of droplet diameter, leading to the increase of Vickers microhardness. For Inconel 718 alloy, the Vickers microhardness firstly decreases, when the droplet diameter decreases from 1000 to 900 mu m, and then increases linearly with the decrease of droplet diameter. It is found that the Vickers microhardness of Inconel 625 alloy is the largest among the four alloys.