materials Article Thermal Processing Map and Microstructure Evolution of Inconel 625 Alloy Sheet Based on Plane Strain Compression Deformation Yuelin Song 1 , Jiangkun Fan 1,2,3,*, Xudong Liu 1, Peizhe Zhang 1 and Jinshan Li 1,2 1 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China; [email protected] (Y.S.); [email protected] (X.L.); [email protected] (P.Z.); [email protected] (J.L.) 2 Innovation Center, NPU·Chongqing, Chongqing 401135, China 3 National & Local Joint Engineering Research Center for Precision Thermoforming Technology of Advanced Metal Materials, Xi’an 710072, China * Correspondence: [email protected] Abstract: Plane strain compression tests were used to study the deformation behavior of an Inconel 625 alloy sheet at various temperatures and strain rates. The peak stress was selected to establish the constitutive equation, and the processing maps under different strains were drawn. The results show that the effective stress–strain curve of Inconel 625 has typical dynamic recrystallization (DRX) characteristics. With the increasing deformation temperature and the decreasing strain rate, the softening effect is significantly enhanced. The parameters of the constitutive equation are calculated, and the average error of the constitutive equation is 5.68%. Through the analysis of the processing ◦ −1 map, a deformation temperature of 950–960 C with a strain rate of 0.007–0.05 s were determined as the unstable region, and obvious local plastic-rheological zones were found in the unstable region. ◦ −1 Citation: Song, Y.; Fan, J.; Liu, X.; The optimum deformation condition was found to be 1020–1060 C/0.005–0.03 s . Through electron Zhang, P.; Li, J. Thermal Processing backscattered diffraction (EBSD) characterization, it was found that both the increase of temperature Map and Microstructure Evolution of and the decrease of strain rate significantly promote the recrystallization process. At a low strain Inconel 625 Alloy Sheet Based on rate, the main recrystallization mechanism is discontinuous dynamic recrystallization (DDRX). It is Plane Strain Compression expected that the above results can provide references for the optimization of the rolling process and Deformation. Materials 2021, 14, 5059. microstructure control of an Inconel 625 alloy sheet. https://doi.org/10.3390/ma14175059 Keywords: Inconel 625; constitutive equation; processing map; recrystallization; microstructure Academic Editor: Frank Czerwinski evolution Received: 4 August 2021 Accepted: 31 August 2021 Published: 3 September 2021 1. Introduction γ Publisher’s Note: MDPI stays neutral Due to the stable matrix elements and the large extent of the ” phase, Inconel 625 has with regard to jurisdictional claims in excellent high-temperature strength and structural stability [1–4]. In addition, Inconel 625 published maps and institutional affil- has good machinability and weldability [5,6]. Therefore, Inconel 625 can be easily processed iations. into various components such as plates, bars, pipes, wires, and strips [7]. However, due to its complex composition, strong deformation resistance, and narrow range of hot-forming parameters, it is easy to segregate during preparation, and there are some problems in rolling processes, such as uneven distribution of stress or temperature, and fracture [8]. In order to solve various problems in processing, some scholars have studied the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. deformation behavior of Ni-based superalloys recently [9–12]. Chen et al. [13] studied This article is an open access article the hot-deformation behavior of GH4169 under different strain modes. They found that distributed under the terms and when the strain rate remains high in the first stage and then becomes low in the second conditions of the Creative Commons stage, the true stress decreases with the sudden decrease of the strain rate. Compared with Attribution (CC BY) license (https:// the equivalent constant strain rate, there is no difference in the final stress. Both constant- creativecommons.org/licenses/by/ strain-rates and changed-strain-rates GH4169 have the same recrystallization mechanism, 4.0/). while the recrystallization grain size and volume fraction of the former is between those Materials 2021, 14, 5059. https://doi.org/10.3390/ma14175059 https://www.mdpi.com/journal/materials Materials 2021, 14, 5059 2 of 18 of the latter. Jia and Tao et al. have carried out uniaxial compression and high-speed compression experiments on cylindrical Inconel 625 respectively [14,15]. Under different temperatures and strain rates, the real stress–strain curve shows an obvious steady-state flow rule, and the adiabatic heating effect decreased with decreasing temperature and increasing strain rate. Moreover, many new models have been developed to describe the high-temperature rheological behavior of nickel base alloys, such as the improved Arrhenius model [16], constitutive model based on physical mechanism [17], artistic neural network (ANN) model [18], deep belief network (DBN) model [19], multigene genetic programming (MGGP) [20], and so on. All of those have good prediction effects with different accuracy. Some scholars have also studied the microstructure evolution and recrystallization behavior of the alloy [21–23]. By combining T-EBSD with transmission electron microscopy (TEM), Sun Fei [24] accurately studied the microstructure evolution of recrystallization process of U720Li disc superalloy. They found that with the increasing rate of strain, the slip dislocations tended to gather into the wall and form subgrain boundaries. Then, the dislocation wall was connected with the local primary γ0 and combined to form subgrains. Kumar et al. [25] found that as for the advanced P/M Ni-based superalloy, the DRX reached the maximum at a higher deformation temperature. While at a lower deformation temper- ature, the DRX was limited and the structure highly deformed. Sun et al. [26] developed the effect of microstructure on recrystallization evolution of Inconel 625. Through high- resolution EBSD and transmission electron microscopy, they found that the δ phase can promote the progress of CDRX. In addition, Lamellar carbides (NbC) have a pinning effect on dislocations, which limits the growth of DRX grains in DRX process. Jiang et al. [27] simulated the microstructure evolution of the 690 alloy pipe-forming process by using the finite element method (FEM). The results show that the established numerical model can accurately simulate the dynamic recrystallization, subdynamic recrystallization, and grain- growth behavior of the 690 alloy pipe-forming process. Sun et al. [28] developed a model for predicting the microstructure evolution of nickel base alloy during hot deformation by 2D cellular automaton (CA). This method uses deterministic or stochastic element evolu- tion rules, which are not limited in time and space and do not need to establish complex differential equations. The method can easily simulate the nucleation position, orientation, and growth process of grains, and provides great help for the study of recrystallization. At present, the research on the hot deformation of nickel base superalloys mainly focuses on the uniaxial compression of cast cylindrical specimens, but there is little on the plane strain compression of rolled plates. Limited by the single deformation mode of the former, it is difficult to reflect the deformation characteristics of the alloy in multiple modes, and it is even more difficult to optimize the processing technology. With the increasing demand for plate and strip, the simulation of rolling behavior has become a part that cannot be ignored. However, the cylinder compression experiment cannot reflect the deformation behavior of the actual sheet. Due to the effect of friction, it is prone to uneven deformation and a “bulging” phenomenon. In addition, when the strain is large, it may lead easily to the abnormal increase of flow stress, which has an impact on the measurement of flow stress and the observation of microstructure [29]. In contrast, the geometric profile of the deformation zone during plane strain compression is closer to that of flat rolling. This method can well reflect the deformation state, heat conduction, or other information in the rolling process, and the measurement of rheological stress is more accurate [30]. In order to study the rolling deformation behavior and microstructure evolution of the Inconel 625 plate more accurately and systematically, the plane strain compression of a rolled rectangular plate at minimal strain rate was carried out, and some novel results were obtained. This complements the related work of observing a rolled Inconel 625 plate under different compression conditions and minimal strain-rate range. It is conducive to a more accurate and in-depth understanding of the rolling process of Inconel 625 sheets, which is of great significance for guiding production, reducing cost, and subsequent research. Materials 2021, 14, 5059 3 of 18 2. Experimental 2.1. Plane Strain Isothermal Compression Test An Inconel 625 industrial hot-rolled sheet was selected in this experiment, and its chemical composition is shown in Table1. Table 1. Chemical composition of the Inconel 625 used in this work (wt.%). Ni Cr Mo Nb Fe Si Al Ti Mn C S 61.00 21.50 9.00 3.60 2.00 0.20 0.20 0.20 0.20 0.05 0.001 After annealing at 1150 ◦C for 50 min, the sheet was processed into a rectangular specimen with a