Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718
Introduction
Superalloy 718 is a nickel-base alloy strengthened predominately by ordered gamma double prime (γ″-Ni3Nb) precipitates. Because of its good mechanical properties at elevated temperatures up to 650 °C, superalloy 718 has been widely used in gas turbine and related high temperature applications. Better control of the thermomechanical processing is of great importance for superalloy 718 to obtain a superior performance. It has been reported that dynamic recrystallization (DRX) is the responsible mechanism for hot restoration during high temperature deformation of superalloy 718 [1], [2], [3]. The microstructural characterization and recrystallization behavior in hot deformed superalloy 718 have been extensively studied using hot torsion or hot compression experiments [4], [5], [6], [7], [8], [9]. Mathematical models to predict the recrystallized grain size and recrystallization fraction as well as the processing maps were well developed in the past several decades. However, informations on the micromechanisms of DRX during the hot deformation in superalloy 718 were still limited [10].
As we know, a fundamental understanding of DRX is essential for modeling the microstructure evolution and analyzing its effect on flow behavior. The key to the comprehension of DRX lies on the physical knowledge of the nucleation mechanisms which are closely related to the microstructure evolution during deformation. Researches on single-phase polycrystalline materials have demonstrated the existence of different nucleation mechamisms of DRX, which was suggested to be strongly dependent on deformation variables. Meanwhile, different nucleation mechamisms of DRX may be happened simultaneously in a certain deformed material. Therefore, systematical investigation on the microstructure evolution under different deformation conditions, especially the nucleation mechanism of DRX for superalloy 718, is quite needed. Electron backscatter diffraction (EBSD) technique is a powerful method to obtain the detailed information on the orientations of grains and subgrains. Many researchers have studied the deformation microstructures and the nucleation mechanisms of DRX for different alloys by EBSD [11], [12], [13], [14], and it was also in application to the orientation analysis of superalloy 718 in the last few years [10], [15], [16]. The aim of the present study was to investigate the evolution of DRX microstructure in hot deformed superalloy 718. Misorientation measurements were systematically carried out to examine the nucleation as well as the progress of DRX. Particular attention was paid to the role of continuous subgrain rotation and the twin boundaries evolution with regard to the nucleation of DRX during hot deformation of superalloy 718.
Section snippets
Experimental procedure
The chemical compositions (wt.%) of superalloy 718 used in this investigation are as follows: Cr, 18.09; Fe, 17.69; Nb + Ta, 5.43; Mo, 3.07; Ti, 0.97; Al, 0.46; Co, 0.18; Si, 0.078; Mn, 0.065; Cu, 0.065; C, 0.040; S < 0.001; P < 0.007; Ni, balance. A 12.5 mm thick sheet was cut from an as-received wrought billet (252 mm in diameter and 152 mm in thickness) in the direction of cross section. Cylindrical specimens, with a diameter of 8 mm and a height of 12 mm, were machined from the center part of the
Flow characteristic and microstructures
True stress–true strain curves of superalloy 718 obtained at strain rate of 10−1 s−1 and at various temperatures from 950 °C to 1120 °C are shown in Fig. 2. The flow stress curves exhibit the similar features, i.e. a single peak at a critical strain followed by a strain softening stage and then sometimes a steady stage at high strain zone. The characteristics of the flow stress curves are the typical ones observed in many alloys, which implies the happening of DRX phenomenon during hot deformation
Flow softening and new grain evolution
The flow stress curves of superalloy 718 under hot deformation conditions, as shown in Fig. 2, are similar to that of superalloy 718 reported by other investigators [4], [5], [6], [7], [8], [9]. The increase of deformation temperature brings about an increase in mobility of grain boundaries and dislocations, which leads to the increase in the fraction of DRX grains (Fig. 3). A power exponent relationship between σp and DR is obtained for superalloy 718, which was reported to derive from the
Conclusions
Superalloy 718 was hot deformed to different strains at temperatures from 950 °C to 1120 °C with a true strain rate of 10−1 s−1. The relationship between the stable dynamically recrystallized grain size and the peak stress can be expressed by a power law function with an exponent value of −0.75. Microstructure analysis substantiate that the nucleation of DRX in superalloy 718 can hardly be considered to only one mechanism. At higher temperature (T = 1100 °C), new grains were formed by bulging of
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