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JMEPEG https://doi.org/10.1007/s11665-020-05135-8

Effect of Hot Deformation Parameters on Grain Refinement of an As-Cast Ni-Based Superalloy Xinxu Li, Zhouhua Jiang, Zhipeng Wan, Yong Zhang, Chonglin Jia, Tao Wang, and Zhao Li (Submitted February 15, 2020; in revised form May 17, 2020) The grain refinement behavior in superalloys has been identified for a long time, while its exact underlying mechanism remains to be explored. In this study, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the fundamental mechanism governing the process of grain refinement for as-cast superalloys GH4720LI. The results show that grain refinement highly depends on the distributions of c¢ precipitates. In c + c¢ two-phase region, c¢ precipitates could promote the dislocation multiplication, hinder the dislocation motion and lead to the formation of highdensity dislocation substructure. Hence, the sub-boundaries and high-angle grain boundaries (HABs) are formed resulting from the gradual transformation of the dislocation substructures. In this process, the new grains are developed by quasi-continuous dynamic recrystallization (quasi-CDRX) and discontinuous dynamic recrystallization (DDRX). In addition, strain rate has little influence on grain size and DRX behavior at high strain rate and low temperature. While in c single-phase region, the dislocation within grains is partly consumed through continuous original boundary migration (COBM) at 0.01 s21. Moreover, in some deformed grains which are expected to possess high dislocation density, new grains can be formed by DDRX. With the strain rate increasing, the process of COBM was suppressed while DDRX was promoted. Therefore, the main dynamic softening mechanism of alloy is DDRX under the high temperature and high strain rate. Keywords

GH4720LI alloy, grain refinement mechanism, hot compression tests, microstructure evolution

1. Introduction GH4720LI is a precipitation strengthened nickel-based superalloy which was evolved by modifying the chemistry compositions of superalloys Udimet720, originally developed for land-based turbine blades. Now due to the excellent fatigue, creep, corrosion and oxidation resistance at high temperatures, it is mainly employed for long-term use at 650-750 C and short-term use at 900 C (Ref 1). The structural stability and high-temperature strength of GH4720LI alloy are significantly improved because it contains a remarkable volume fraction of more than 45% c¢ precipitates under specific heat treatment conditions (Ref 2). The complete dissolution temperature of c¢ phase in this alloy is around 1150 C (Ref 3). However, the high fraction of c¢ phase significantly reduces the hot workability of GH4720LI alloy, resulting in great difficulty for the control of its microstructure and mechanical properties.

Xinxu Li, School of Metallurgy, Northeastern University, Shenyang 110819, China; and Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Mat