Strain-rate sensitivity of tensile behaviors for nickel-based superalloys GH3044 and GH4033 at room temperature

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1 Indian Journal of Engineering & Materials Sciences Vol. 23, October 2016, pp Strain-rate sensitivity of tensile behaviors for nickel-based superalloys GH3044 and GH4033 at room temperature Changying Wang a, Qinglong An a *, Qiulin Niu b & Ming Chen a a School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai , PR China b College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan , PR China Received 18 May 2015; accepted 16 august 2016 Nickel-based superalloys are of interest for high temperature applications, e.g. aircraft engine, due to their excellent high temperature mechanical/physical properties. The strain-rate sensitivity of the tensile behaviors at room temperature of nickel-based superalloys GH3044 and GH4033 are investigated concerning mechanical properties, microstructures and fracture surface morphologies. The experimental results showed that the yield strength and ultimate tensile strength of GH4033 alloy are approximately 35%-40% larger than that of GH3044 alloy. With the increase of strain rate, the yield strength values of the two superalloys increased, while the configuration of the stress strain curves is independent of strain rate. In addition, grain refinement phenomenon is observed on the microstructure of GH4033 alloy under a higher strain rate. It has a limited influence on the microstructure of GH3044 alloy, which can be attributed to the solid solution strengthening effect for the latter. Besides, the fracture analysis demonstrated that the dimple-ductile fracturing is the main tensile fracture mechanism for both GH3044 and GH4033 alloys. Keywords: Nickel-based superalloys, Quasi-static tensile, Strain rate, Microstructure, Fracture analysis Nickel-based superalloys have been widely used in aerospace and power generation industries to produce modern aero engines and gas turbines 1,2 due to their excellent high temperature strength and oxidation resistance 3,4. As wrought alloys, the physical and mechanical performance of nickel-based superalloys is quite sensitive to a number of factors, such as the changes of microstructure, the chemical inhomogeneity and some inclusions 4. The reason is that those factors have important effects on high temperature structural properties of nickel-based superalloys 5,6. Understanding the mechanical behavior of superalloys at different strain conditions is vital in order to obtain microstructure and mechanical properties to meet the application. The mechanical properties and microstructural evolution is strongly sensitive to the processing parameters like deformation temperature, strain rate and strain. So far, numerous researchers have introduced the tensile properties of these superfluous alloys through solution treatments at different temperatures or environments However, the literatures concerning the strain-rate sensitivity of tensile properties and microstructures of nickel-based superalloys are still *Corresponding author ( qlan@sjtu.edu.cn) limited. During the tensile deformation process, Liu 9 introduced that as strain rates increased, the yield strength of GH4169 nickel-based alloy increased after being treated by the electrical field. The focus of the present study is to investigate the strain-rate sensitivity of the tensile behaviors of nickel-based superalloys GH3044 and GH4033 at room temperature. Microstructure characterization is discussed to study the microstructural evolution under different tensile strain rate levels. Experimental Procedure In this study, two typical nickel-based wrought superalloy, i.e., Ni-Cr based solid solution strengthening wrought superalloy GH3044 and Ni-Cr based precipitation strengthening wrought superalloy GH4033, were investigated. Main chemical compositions of GH3044 and GH4033 superalloy are listed in Table 1 14,15. The test samples were cylindrical tensile specimens with a diameter of 5 mm and an original gauge length of 25 mm according to the International Standard ISO : 2009, as shown in Fig. 1(a). A group of quasi-static tensile tests were carried out on an electronic universal testing machine WDW-100D combined with the electronic extensometer at room temperature. The tensile samples were separately

2 WANG et al.: NICKEL-BASED SUPERALLOYS GH3044 and GH Table 1 Chemical compositions of GH3044 and GH4033 superalloys (wt%) Material C Cr W Mo Al Ti Fe Si Ni GH Bal. GH Bal. Fig. 2 Stress-strain curves of GH3044 and GH4033 alloys under different strain rates Fig. 1 Specimens used for the tensile tests (a) dimension of the specimens, (b) specimens of GH3044 after the tensile tests and (c) specimens of GH4033 after the tensile tests pulled at nominal strain rates of s -1, 0.01 s -1 and 0.1 s -1, corresponding to the load speeds of 1.5 mm/min, 15 mm/min and 150 mm/min, respectively. During the test, displacement and other deformation conditions were recorded automatically by the computer system. The test samples after the tensile tests are shown in Fig. 1 (a) and (c). After the tensile tests, these samples were treated by wire-electrode cutting and polishing to observe their microstructure and fracture morphology. In terms of observation devices, a metallographic microscope equipped with a Zeiss AxioCam MRc5 digital camera was employed for the microstructure observation. The fracture morphology of the samples were investigated by using a scanning electron microscope (SEM) HITACHI S Results and Discussion Mechanical properties The true stress-strain curves of the nickel-based superalloys GH3044 and GH4033 are plotted in Fig. 2, in which the strain rate ranges from s -1 to 0.1 s -1. It is observed that the flow stress increases with increasing of strain rate. It can also be seen that the curves can be divided into three regions: (i) elastic zone, the stress and strain have a linear relationship in the elastic zone, the material will be yielded when elastic limit reaches; (ii) hardened zone, after the material is yielded, there appears work-hardening with the strain increasing, and the hardened zone covers a great part in whole tensile fracture behavior; (iii) necking and fracture zone, the flow stress decreases rapidly with strain increasing. There is a local necking deformation of the material until specimen fractures. Experimental parameters and results such as yield strength, ultimate tensile strength and elongation are summarized in Table. 2. There is no obvious yielding platform in the strain-stress curves. It is quite obvious that the stress-strain curves of GH4033 alloy are above on those of GH3044 alloy. In fact, the influence of strain rate on the ultimate tensile strength is rather small. For example, the ultimate tensile strength at the strain rate of 0.1 s -1 for GH4033 is MPa, while the ultimate tensile strengths at the strain rate of 0.01 s -1 and s -1 are MPa and MPa. The difference is only about 1%. The ultimate tensile

3 338 INDIAN J ENG. MATER. SCI., OCTOBER 2016 Material Stain rate (s -1 ) Elastic modulus (GPa) GH3044 GH4033 Table 2 Mechanical properties of GH3044 and GH4033 superalloys Yield strength (MPa) Ultimate tensile strength (MPa) Elongation (%) Reduction of area (%) Fig. 3 Metallographic structures of GH3044 and GH4033 alloys after the tensile tests under different strain rates strength at the strain rate of 0.1 s -1 for GH3044 is MPa, while the ultimate tensile strengths at the strain rate of 0.01 s -1 and s -1 are MPa and MPa, respectively. The difference is less than 1%. It is very small. Therefore, we can conclude that the strain rate has little influence on the tensile strength. The average yield strength (0.2%) and the average ultimate tensile strength of GH4033 alloy are about 520 MPa and 1030 MPa, respectively. While the average yield strength of GH3044 alloy is only about 380 MPa and the average ultimate tensile strength is about 750 MPa. The average yield strength and the average ultimate tensile strength of GH4033 alloy are respectively 35% and 40% larger than those of GH3044 alloy. The results reveal that precipitation hardening and aging treatments play significant roles in improving the strength of nickelbased superalloy. However, the average elongation and reduction of area of GH3044 alloy are higher those of GH4033, demonstrating a stronger ductile behavior of GH3044 alloy. On the other hand, the yield strength exhibits a general increase for the two superalloys with the increase of the tensile strain rate, while the strain rate has almost no influence on the ultimate tensile strength of the two superalloys. It is also indicated that the configuration of the stress strain curves is independent of strain rate. Besides, the elongation and reduction of area decrease with the increase of the tensile strain rates for both superalloys which can be attributed to the heterogeneous ductile deformation. Metallographic structure evolution Figure 3 presents the metallographic structure evolution of the fractured surfaces for nickel-based superalloys GH3044 and GH4033 under various strain rates at room temperature. At normal state, the metallographic structures of GH3044 alloy are composed of single-phase of austenite, a small amount of carbide (mainly M 23 C 6 ) and WCr solid solution. While the metallographic structures of GH4033 alloy are composed of single-phase of austenite and a small amount of TiC, TiN and Ti(C, N). As observed, the metallographic structure of GH3044 alloy after tensile tests mainly consists of the matrix single-phase of austenite and other phases (e.g., WCr solid solution).

4 WANG et al.: NICKEL-BASED SUPERALLOYS GH3044 and GH As the strain rate elevated, no obvious variations are observed. However, for GH4033 alloy, there is a trend of grain refinement phenomenon observed due to the strain-hardening effect. The evolution clearly indicates a decrease in overall grain size with increasing strain rate indicating the progressive occurrence of dynamic recrystallization aided structural evolution. The average overall grain size of GH3044 alloy observed at s -1 is about μm, while the overall grain size reduced to μm as the strain rate increased to 0.1 s -1. The above observation is consistent with the earlier reported studies on nickel based superalloy 16 where in the volume fraction of recrystallized grains increases with increasing strain rate at higher strain rate. Fracture surface morphologies Figure 4 shows SEM fracture surface morphologies of tensile-fractured specimens of GH3044 and GH4033 alloys obtained from the tensile tests under different strain rates at room temperature. It is observed that apparent necking occurs during the tensile tests. As shown in Fig. 4, greater plastic deformation occurs in the macrofracture surfaces of GH3044 alloys than that of GH4033 alloys in the uniaxial tension, which is demonstrated by more serious necking in GH3044 specimen. Besides, the result indicates that the ultimate tensile strength of GH3044 alloy is less than that of GH4033 alloy once again. The fractures of specimens can be mainly divided into three typical regions, i.e., fiber region, shear lip region and radial region. In addition, with the increase of the strain rate ranging from s -1 to 0.1 s -1, the macrofracture surfaces perpendicular to the tensile axis show that the radial region has somewhat expanded, which is directly associated with the Fig. 4 Fracture surface morphologies of the tensile-fractured specimens of GH3044 and GH4033 alloys after the tensile tests under different strain rates

5 340 INDIAN J ENG. MATER. SCI., OCTOBER 2016 increased strength of the alloy caused by the increase of the strain rate. Compared with the radial regions of the fractures, GH4033 alloy is larger than GH3044 alloy on account of higher strength of GH4033 alloy. Typical ductile fractures are observed on all of the fractures surface morphology. Dimple-ductile facture surfaces are observed for GH3044 and GH4033 alloys. Dimple pattern of GH3044 alloy is more significant than that of GH4033 alloy. It is indicated that ductile property of GH3044 alloy is better than that of GH4033. The microanalysis for fractures also shows that the fracture mechanism is dimple-ductile fracturing for GH3044 and GH4033 alloys, especially for the GH3044 alloy. Conclusions In the present work, tensile behaviors under different tensile strain rates (at room temperature) of two typical nickel-based superalloys GH3044 and GH4033 were investigated concerning mechanical properties, microstructures and fracture morphologies, respectively. Based on the results, the following conclusions can be drawn: (i) The yield strength and ultimate tensile strength of GH4033 alloy are respectively about 520 MPa and 1030 MPa, which are 35%-40% higher than those of GH3044 alloy, respectively. The results reveal that precipitation hardening and aging treatments play significant roles in (ii) improving the strength of nickel-based superalloy. It is indicated that both the yield strength of GH3044 and GH4033 alloys increase with the increase of the strain rate, while it has little influence on the ultimate tensile strength. (iii) Grain refinement is observed in the metallographic structure of GH4033 alloy under higher tensile strain rate. (iv) It is found that the dimple-ductile fracturing is the main fracture mechanism for GH3044 and GH4033 alloys according to the fracture analysis. Acknowledgments This work was supported by the National Natural Science Foundation of China (No & ) and Important National Science & Technology Specific Projects (2016ZX ). References 1 Chen X M, Lin Y C, Chen M S, Li H B, Wen D X, Zhang J L & He M, Mater Des, 77 (2015) Dongxu Z, Zhixun W & Zhufeng Y, Rare Met Mater Eng, 44 (2015) Arunachalam R & Mannan M A, Mach Sci Technol, 4 (2000) Pollock T M & Tin S, J Propul Power, 22 (2006) Hrutkay K & Kaoumi D, Mater Sci Eng A, 599 (2014) Kaoumi D & Hrutkay K, J Nucl Mater, 454 (2014) Ro Y, Koizumi Y & Harada H, Mater Sci Eng A, 223 (1997) Nazmy M, Künzler A, Denk J & Baumann R, Scr Mater, 47 (2002) Zhang X, Jin T, Zhao N, Wang Z, Sun X, Guan H & Hu Z, Mater Sci Eng A, 492 (2008) Kim D, Sah I & Jang C, Mater Sci Eng A, 528 (2011) Yang G, Xu Y, Jiang L & Liang S, Prog Nat Sci, 21 (2011) Kumar A L, Chaitanya N B, Kumar B S, Nath V S & Singh P, Procedia Mater Sci, 5 (2014) Zhang H, Zhang K, Zhou H, Lu Z, Zhao C & Yang X, Mater Des, 80 (2015) Wang L, Dong J, Tian Y & Xie X, Rare Met Mater Eng, 35 (2006) Wang H Y, Zuo D W, Sun Y L, Feng X & Zhang D, T Nonferrious Met Soc, 19 (2009) Kumar S S S, Raghu T, Bhattacharjee P P, Rao G A & Borah U, J Alloy Compd, 681 (2016) 28-42