introduction

The recrystallization phenomenon of single crystal nickel-based superalloys during solution heat treatment leads to high costs of investment in the single crystal casting industry. The nucleation of recrystallization is usually related to the migration of existing grain boundaries. In the absence of grain boundaries, 20-25% of severe deformations such as notches or compression trigger recrystallization are required, usually amplified by the presence of strain-concentrated regions such as carbides. Recrystallization was also observed during thermomechanical fatigue, nucleating at the intersection of deformed twin bands. These possibilities will not appear in cast single crystal superalloys because of its low potential strain and the absence of carbides in the design. So, how exactly are moving large-angle grain boundaries formed in a single crystal material undergoing moderate strain? One strategy to control recrystallization is to reduce the deformation during cooling by changing the ceramic mold and core material, reducing the strength of the mold, but increasing the risk of failure or deformation. Another method is to eliminate or reduce the nucleation of recrystallization.

Introduction to Achievements

Recently, Catherine MF Rae (corresponding author) of the University of Cambridge and others provided for the first time evidence of nucleation in the surface layer of single crystal castings, and showed that under the right conditions of very moderate strain, they can develop into large moving parts. Sizable grains of angular grain boundaries. In this study, two sources of surface nucleation in CMSX-4 alloy have been identified. Experiments have shown that the growth of microcrystalline grains on the surface causes complete recrystallization in areas where there is sufficient strain in the bulk material. By eliminating these surface defects, recrystallization can be completely alleviated. Etching the surface of the casting proved to be an effective way to achieve this. The research results were published on Acta Materialia under the title "Nucleation of recrystallisation in castings of single crystal Ni-based superalloys".

summary

In summary, the following conclusion can be drawn: γ'microcrystal grains with large-angle grain boundaries are formed in the surface eutectic layer toward the top of the casting. Even in the absence of a critical strain to produce recrystallization, the microcrystalline grains can still withstand standard heat treatment. In the lower part of the casting where there is no surface eutectic layer, the metal can locally adhere to the mold, and the subsequent separation produces a locally deformed surface area with a rotation angle of up to 20° relative to the bulk single crystal. After the heat treatment, the deformed region forms different crystal grains through repeated twinning, and has a highly mobile misorientation angle relative to the bulk. Experiments show that in the presence of critical deformation in the block, etching the surface of the casting can eliminate recrystallization.