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The Effect of Niobium Addition on the Microstructure and Tensile Iranian Journal of Materials Science and Engineering, Vol. 18, Number 1, March 2021 RESEARCH PAPER The Effect of Niobium Addition on the Microstructure and Tensile Properties of Iron-Nickel Base A286 Superalloy Reza Soleimani Gilakjani1, Seyed Hossein Razavi1* and Masoumeh Seifollahi2 * [email protected] Received: April 2020 Revised: August 2020 Accepted: September 2020 1 School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran 2 Metallic Materials Research Center (MMRC), Malek Ashtar University of Technology, Tehran, Iran DOI: 10.22068/ijmse.18.1.7 Abstract: Niobium is a significant addition to superalloys to enhance high-temperature mechanical properties. The purpose of this study is twofold: (1) to investigate the η and γ/ phase precipitations along with (2) to identify the high-temperature tensile properties in A286 and Nb-A286, as a modified type. The heat treatment of both alloys was carried out in a two-stage aging procedure at 760 °C for 16 h and 820 °C for 2 to 30 hours, then characterized by optical and scanning electron (SEM-EDS) microscopies, differential thermal analysis (DTA), and high-temperature tensile tests. The results showed that niobium addition increased the volume fraction of γ/ phase, from 10.7 % to 12 %, decreased its size, from 94 to 71 nm, and intensified the γ/-dissolution temperature from 987 °C to 1007 °C. Moreover, the γ/ to η phase transformation was sluggishly occurred in Nb-A286 due to the higher stability of γ/ precipitations. Furthermore, the Nb-A286 alloy demonstrates higher mechanical properties than the A286 one, approximately 100 MPa improvement, which was contributed to the much larger volume fraction and finer size of more stabilized γ/ phase. Keywords: A286 superalloy, Niobium addition, η and γ/ phases, Microstructure evaluation, Hot-tensile properties. 1. INTRODUCTION [9] showed that the phase did not form in the temperature range of 650 to 720 °C, precipitated A286 superalloy, as the most common iron base at higher temperatures in the range of 780 to superalloy, can be used in hot sections of gas 850 °C that its amount increased with turbine engines, because of its mechanical temperature and time. properties and phase stability at high temperatures Various studies on optimizing heat treatment [1, 2]. This superalloy with high strength and cycles, adding alloying elements to the chemical corrosion resistance at moderate temperatures is composition, and so on have been conducted to widely used in the manufacturing of land-based, control the phase [10]. marine, and air gas turbines [3]. According to some Studies on the effect of niobium on iron-nickel- studies, the formation of the phase during base superalloys show that the addition of this solidification, heat treatment, and operation reduces element, by dissolving into the austenitic matrix of the volume fraction of strengthening phases such as superalloy, increases the strength through the solid γ' phase. Therefore, the formation of such phases solution mechanism. On the other hand, the Downloaded from ijmse.iust.ac.ir at 8:48 IRST on Thursday October 7th 2021 [ DOI: 10.22068/ijmse.18.1.7 ] can lead to a decrease in mechanical properties [4, increase of γ' volume fraction enhanced the alloy 5]. Also, according to some other studies, the stability, and consequently, improves the strength, precipitation of the phase at the grain boundary fatigue life, and creep resistance of the superalloys not only does not cause the embrittlement of the [11-14]. Rho et al. [15-17] showed that the addition nickel-base superalloys at high temperatures but of niobium to the A286 superalloy improved the can act as a barrier against boundary slip by fatigue life and creep resistance of the alloy by precipitation at the grain boundary [6]. delaying the formation of the η phase. After exposure to a temperature of about 730 °C So far, extensive research has been done into for a short time, this alloy becomes unstable and investigating the precipitation mechanisms of the η thus its γ' strengthening phase is transformed phase and the effect of this phase on the into a topologically closed packed (TCP) η phase mechanical properties of iron-nickel base with cellular or Widmanstatten morphology at or superalloys [3, 18-20]. However, few studies have within the grain boundary [7, 8]. Seifollahi et al. been conducted on the effect of niobium addition 61 R. Soleimani Gilakjani, S. H. Razavi and M. Seifollahi on the rate, morphology, and precipitation analyses the thermodynamic of the system. Using mechanisms of the η phase and its effect on the the thermodynamic properties of phases, phase tensile properties of the A286 superalloy. Besides, equilibrium is calculated by a Gibbs energy sometimes conflicting results have been reported. minimization process [21]. This study aims to add niobium to the chemical composition of the A286 superalloy and investigate 3. RESULTS AND DISCUSSION the effect of its presence on the η phase formation and mechanical properties of this alloy. 3.1. Microstructural Studies Fig.1 shows an SEM BSE2 mode image of A286 2. MATERIALS AND METHODS and Nb-A286 microstructure after aging heat treatment at 720 °C for 16 h. The study of the The A286 and Nb-modified A286 superalloys microstructures of both superalloys (Fig. 1) by were melted and poured in a vacuum induction Clemex software shows that there is an increase in melting (VIM) furnace at the pressure of 10-3 the volume fraction of the γ' strengthening phase Pa. The chemical composition of cast ingots was from 10.7 to 12 volume percent and a decrease in determined by the spectroscopy method. Table 1 the average size of this phase from 94 nm to 71 shows the chemical compositions of the two nm, due to the addition of niobium. Adding alloys investigated. Homogenization treatment niobium to the chemical composition of iron- of as-cast ingots was performed at 1180 °C for 6 nickel base superalloys leads to the occupation of hours and air cooled. The homogenized samples the interstitial spaces of the matrix austenite phase were subjected to a hot rolling process at by niobium atoms. Also, by adding niobium, a 1100 °C up to 50 % reduction. The wrought decrease in the solubility limit, and an increase in alloys solutionized at 980 °C for one hour the activity of Al and Ti elements in the matrix subsequently cooled in water. phase are observed. As a result, the nucleation The 5 × 5 × 5 mm3 specimens were subjected to sites and the volume fraction of the γ' phase are aging heat treatment in the temperature range of increased [22]. The dissolution of the niobium 650 to 900 °C for 2 to 30 h to evaluate the η element in the γ' phase increases the stacking fault phase. An electrical resistance furnace with a energy and delays the transformation of γ' to η temperature accuracy of ±5 °C was used for heat phase [23, 24]. The presence of niobium in the treatment. The specimens were etched with a matrix also reduces the diffusion rate in the matrix, glycerol solution after polishing. The Nikon and consequently, decreases the growth rate of γ' Epiphot 300 optical microscope and Philips XL40 phase and its size [22]. scanning electron microscopy (SEM) which is To evaluate the η phase precipitation, a two-stage equipped with energy-dispersive X-ray aging treatment was performed on samples. Fig. 2 spectroscopy (EDS) at a voltage of 30 kV were shows optical microscope images of A286 and used to study the microstructure and distribution Nb-A286 microstructures after two-stage aging of phases. Quantitative analysis of metallographic heat treatment at 720 °C for 16 h and then at images was performed by Clemex software. DSC1 820 °C for 8 h. Fig. 2 shows the twins, austenitic Mettler Toledo optical emission spectrometer was matrix, and η phases. As shown in Fig. 2, at Downloaded from ijmse.iust.ac.ir at 8:48 IRST on Thursday October 7th 2021 [ DOI: 10.22068/ijmse.18.1.7 ] used to determine the transformation temperatures similar aging times and temperatures, Nb-A286 of the η and γ' phases with heating and cooling superalloy had a lower η phase volume fraction rates of 10 °C/min. The temperature range of than the A286. Clemex software analysis showed heating and cooling of the samples was selected that the η phase volume fractions for A286 and between 350 and 1400 °C. The tensile properties Nb-A286 superalloys were 3.5% and 1%, of both alloys were measured at 650 °C using a respectively. Fig. 2 shows that the addition of computer-connected Instron Model 8502 testing niobium has prevented the precipitation of the machine. Then, JMatPro 7.0 software was used to Widmanstatten η phase. For η phase, two types of simulate the thermodynamic diagrams. This cellular (for lower times and temperatures) and software is based on the calculation of phase Widmanstatten morphology (for higher times and temperatures) are observed [25, 26]. diagram (CALPHAD) which mathematically 1 Differential Scanning Calorimetery 2 Back Scattered Electron 62 Iranian Journal of Materials Science and Engineering, Vol. 18, Number 1, March 2021 Table 1. Chemical composition of A286 and Nb-A286 superalloys (wt%). Elements Fe Ni Cr Al Ti Nb Mo Mn Si V C B Alloy A286 Bal. 25.3 15 0.45 2.25 --- 1.35 0.93 0.21 0.3 0.04 0.006 Nb-A286 Bal. 24.8 14.7 0.42 2.22 0.52 1.38 1 0.26 0.23 0.03 0.005 Fig. 1. SEM BSE mode image of γ' precipitations after aging heat treatment at 720 °C for 16 h (a) A286 and (b) Nb-A286.
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