Iranian Journal of Materials Science and Engineering, Vol. 18, Number 1, March 2021 RESEARCH PAPER 61 The Effect of Niobium Addition on the Microstructure and Tensile Properties of Iron-Nickel Base A286 Superalloy Reza Soleimani Gilakjani 1 , Seyed Hossein Razavi 1* and Masoumeh Seifollahi 2 * hrazavi@iust.ac.ir 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 A286 superalloy, as the most common iron base superalloy, can be used in hot sections of gas turbine engines, because of its mechanical properties and phase stability at high temperatures [1, 2]. This superalloy with high strength and corrosion resistance at moderate temperatures is widely used in the manufacturing of land-based, marine, and air gas turbines [3]. According to some studies, the formation of the  phase during solidification, heat treatment, and operation reduces the volume fraction of strengthening phases such as γ' phase. Therefore, the formation of such phases can lead to a decrease in mechanical properties [4, 5]. Also, according to some other studies, the precipitation of the  phase at the grain boundary not only does not cause the embrittlement of the nickel-base superalloys at high temperatures but can act as a barrier against boundary slip by precipitation at the grain boundary [6]. After exposure to a temperature of about 730 °C for a short time, this alloy becomes unstable and thus its γ' strengthening phase is transformed into a topologically closed packed (TCP) η phase with cellular or Widmanstatten morphology at or within the grain boundary [7, 8]. Seifollahi et al. [9] showed that the  phase did not form in the temperature range of 650 to 720 °C, precipitated at higher temperatures in the range of 780 to 850 °C that its amount increased with temperature and time. Various studies on optimizing heat treatment cycles, adding alloying elements to the chemical composition, and so on have been conducted to control the  phase [10]. Studies on the effect of niobium on iron-nickel- base superalloys show that the addition of this element, by dissolving into the austenitic matrix of superalloy, increases the strength through the solid solution mechanism. On the other hand, the increase of γ' volume fraction enhanced the alloy stability, and consequently, improves the strength, fatigue life, and creep resistance of the superalloys [11-14]. Rho et al. [15-17] showed that the addition of niobium to the A286 superalloy improved the fatigue life and creep resistance of the alloy by delaying the formation of the η phase. So far, extensive research has been done into investigating the precipitation mechanisms of the η phase and the effect of this phase on the mechanical properties of iron-nickel base superalloys [3, 18-20]. However, few studies have been conducted on the effect of niobium addition [ DOI: 10.22068/ijmse.18.1.7 ] [ Downloaded from ijmse.iust.ac.ir on 2021-11-27 ] 1 / 10