Evaluation of Hot Workability of Powder Metallurgy Ni-Based Superalloy with Different Initial Microstructures
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TRODUCTION
TURBINE disks are one of the aero-engine components exposed to extreme environments, and polycrystalline Ni-based superalloys that exhibit balanced mechanical properties, such as high creep resistance, high tensile strength, and low cycle fatigue at temperatures of 700 C and higher are commonly used to produce these disks. Conventionally, the superalloys used for turbine disks have been processed by the cast and wrought (C&W) route.[1] With an increase in the demand for higher-temperature resistant alloys for aero-engines, higher-strength disk superalloys have been developed by increasing alloying element concentrations and c¢ volume fractions.[2,3] In order to form such high-strength superalloys into complex shapes, it is necessary to improve the processing technique as well as the alloy development toward the practical application. The powder metallurgy (P/M) route allows for the formation of fine microstructure which helps to improve hot workability of such high-strength superalloys. The
MASAYA HIGASHI and NAOYA KANNO are with the Technology & Intelligence Integration, IHI Corporation, 1 Shinnakahara-cho, Isogo-ku, Yokohama 235-8501, Japan. Contact e-mail: [email protected] Manuscript submitted July 22, 2020; accepted October 24, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
conventional P/M processing route consists of the powder consolidation by hot isostatic pressing (HIP), and subsequent hot extrusion followed by isothermal forging.[4] In the case of conventional C&W route, a hot forging is usually performed as the final forging process, instead of an isothermal forging, suggesting that a certain degree of heat loss is acceptable. In contrast, P/ M superalloys are usually isothermally forged using a die heated to temperatures higher than 1000 C at a low strain rate, suggesting that the processing window for high-strength P/M superalloys is strictly limited because of the low hot workability. It is important to select the appropriate deformation conditions to obtain the desired level of hot workability. The hot deformation behaviors of superalloys have been investigated extensively through isothermal compression tests in both C&W and P/M superalloys. The deformation conditions, such as the temperature and strain rate, have a significant effect on the flow behavior and microstructural evolution, which are largely dominated by dynamic recrystallization (DRX).[5–8] Further, based on the obtained results, the processing conditions for various superalloys have been optimized. However, most of these studies were performed using samples with the same initial microstructure, and there have been little studies on the effect of the initial microstructure of P/M superalloys on the hot workability. Isothermal forging utilizes the superplasticity of superalloys. For superplastic deformation to occur, the microstructure
should be sufficiently fine, and the deformation conditions should be appropriate (low strain rate and high temperature).[9] Therefore, it is important to understand the effects of the deforma
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