Effect of sintering temperature and heat treatment on microstructure and properties of nickel-based superalloy

Abstract A nickel-based superalloy was prepared through powder metallurgy (P/M). In order to balance their microstructure and properties, heat treatments were carried on the alloys. The microstructures of the alloys were characterized by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The solidification process in the alloys during the sintering process was calculated using Thermo-Calc software. A good quality nickel-based superalloy with a homogeneous microstructure and fine grain size can be obtained at the sintering temperature of 1140 °C. The alloys mainly consist of irregular polygon CrB particles, irregular morphology Ni3B particles, hexagonal M7C3 particles, (γ-Ni + Ni3B) eutectics and bright γ-Ni solid solution matrix. The calculated results show that the solidification process in the alloys during the sintering process is liquid → liquid+γ-Ni → liquid+γ-Ni + M7C3 → liquid+ γ-Ni + M7C3+CrB → γ-Ni + M7C3+CrB+(γ-Ni + Ni3B). The calculation results are helpful to understand the microstructure evolution in the nickel-based superalloy and agree well with the experimental data. The microstructure of the alloys after solution treatment is mainly composed of supersaturated γ-Ni solid solution and M7C3. During the aging treatment, the supersaturated γ-Ni solid solution decomposed and many small particles were uniformly precipitated in the γ-Ni solid solution. As the aging temperature increased, the decomposition process of supersaturated γ-Ni solid solution was accelerated and the precipitated particles became coarsen according to Ostwald ripening theory. The hardness of the alloys increased when aging treated at 300 °C and 500 °C due to the second phase strengthening effect, and then the hardness decreased when aging treated at 700 °C due to the coarsening of precipitated particles.