Precipitation behavior and chemical composition of secondary gamma ' precipitates in a Re-containing Ni-based single crystal superalloy

摘要

Three-dimensional atom-probe tomography and scanning electron microscopy were utilized for an in-depth examination of the formation of secondary gamma' precipitates and the composition difference between secondary gamma' precipitates and primary gamma' precipitates in a Re-containing Ni-based single crystal superalloy. It was found that the secondary gamma' precipitates was formed during the air cooling after primary aging of 1180 degrees C/4h and would grow up during the secondary aging at 870 degrees C. Al and Ta were mainly distributed in gamma' precipitates (including the primary and the secondary gamma' precipitates) and Mo, Cr, Co and Re were more distributed in y matrix. Compared with the primary gamma' precipitates, the secondary gamma' precipitates contained more gamma-forming elements, such as Re, Cr, W and Mo, while the content of Ta was less. The distribution of W in the secondary gamma' precipitates was much higher than that in the primary gamma' precipitates and was equal to that in y matrix. Based on the diffusion constant of alloying elements in Ni, the diffusion distance of various elements was as followed during the air cooling after primary aging: Re < W < Co < Mo < Ni < Ta < Cr < Al. The insufficient diffusion distance of gamma-forming elements was an important factor leading to their enrichment in the secondary gamma' precipitates. And the Re element might hinder the diffusion of other gamma-forming elements, such as Cr and Mo.