Effect of hot forging on the microstructure and superplastic properties of the powder nickel base superalloy EP741NP

A.A. Ganeev, V.A. Valitov, M.I. Nagimov, V.M. Imayev show affiliations and emails
Received: 11 December 2019; Revised: 16 January 2020; Accepted: 23 January 2020
This paper is written in Russian
Citation: A.A. Ganeev, V.A. Valitov, M.I. Nagimov, V.M. Imayev. Effect of hot forging on the microstructure and superplastic properties of the powder nickel base superalloy EP741NP. Lett. Mater., 2020, 10(1) 100-105
BibTex   https://doi.org/10.22226/2410-3535-2020-1-100-105


Fine-grained (top figures) and ultrafine-grained (bottom figures) microstructure of superalloy EP741NP after superplastic deformation.The work is devoted to the study of the microstructure and superplastic properties of the nickel base powder superalloy EP741NP subjected to different hot forging procedures, which were performed after preliminary heterogenization treatment of the initial HIP-ed material. Condition 1 of the superalloy was obtained through three-stage uniaxial canned forging with intermediate recrystallization annealing at a temperature slightly lower than the γʹ solvus temperature (Ts − 30°C, where Ts is the γʹ solvus temperature). Some of the forged workpieces were subjected to additional low-temperature uniaxial forging under isothermal conditions at the temperature of Ts − 230°C (condition 2). The produced superalloy conditions were studied by scanning electron microscopy, in particular, using electron backscatter diffraction (EBSD) technique. In condition 1, predominantly a fine-grained microstructure of the duplex type was obtained. Condition 2 contained an ultrafine-grained constituent with a size of γ grains/γʹ particles less than 1 μm resulted from recrystallization processes at a lower temperature. It was revealed that condition 2 demonstrated superplastic elongations (δ = 200 –1320 %) at temperatures Т =900 –1000°C (έ = 5 ×10−4 –10−3 s−1), whereas condition 1 showed superplastic elongations (δ = 200 – 630 %) at significantly higher temperatures (Т =1075 –1125°С, έ = 5 ×10−4 –10−3 s−1). The decrease in the superplastic temperatures and the increase in the superplastic elongations in condition 2 (in contrast to condition 1) was caused by: i) a larger length of high-angle grain and interphase boundaries that was favorable for operating grain boundary sliding, which is the main mechanism of superplastic deformation; ii) higher thermal stability of the microstructure providing small changes in the γ and γʹ sizes during superplastic flow; iii) a smaller fraction of twin boundaries, which are unfavorable for grain boundary sliding.

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