Dissolution behavior of the secondary γ' precipitates in nickel-based PM FGH97 superalloy during high temperature pre-precipitation treatment

The comprehensive mechanical performances of nickel-based PM superalloys are greatly influenced by the morphological characteristics of the secondary γ' precipitates. To understand the dissolution and coarsening behavior of the secondary γ' precipitates during high temperature pre-precipitation treatment, a series of tests were carried out using a dynamic dilatational machine with a temperature of 1150 °C and holding times ranging from 5 min to 240 min. The results show that with the increase of the holding time, the secondary γ' precipitates dissolved initially and then coarsened. Large-sized dendritic secondary γ' precipitates dissolved at the top of them, and gradually changed into a cuboidal shape during the early stage of dissolution. Meanwhile, the small-sized cuboidal secondary γ' precipitates split into different shapes from the γ/γ' phase interface, including Doublet, L-shaped, Triplet and Quartet. Compared to the large-sized γ' precipitates, the small-sized ones dissolved faster. As the secondary γ' precipitates coarsened through neck connections caused by diffusion, the large-sized cuboidal secondary γ' precipitates started to split and dissolve from the center. Nanohardness results indicated that there was a relationship between hardness and the size of the secondary γ' precipitates. As the size of secondary γ' precipitates decreased, the average hardness increased.