The Effect of Niobium Addition on the Microstructure and Tensile Properties of Iron-Nickel Base A286 Superalloy

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.