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In this research, the effects of the partial, full and partial + full solution heat treatments followed by aging at 900° C for 10 h, on the microstructure of cast Ni-based IN100 superalloy were assessed. It has been found that, the alloy in the partial + full solution treated condition had the optimal combination of γ' morphology, volume fraction and size. In this condition, the alloy possesses a cubic primary γ'with an average size of 470±10nm and 45% volume fraction. Discrete M 23 C 6 and M 6 C carbides were formed at the grain boundaries and the morphology of the cubic MC carbide was changed to the spherical shape. In addition, the volume fraction of γ'/γ eutectic phase dropped to half of its value, compared to the as-cast alloy. During partial solution treatment followed by aging, discrete carbides were formed at the grain boundaries. This treatment without full solutioning was not an effective method to provide an optimal volume fraction and arrangement of γ' and MC carbides morphology. Full solutioning alone, changed the cubic morphology of the primary γ' and the blocky MC carbides to the spherical shape.

In this paper, the shear deformation behavior of A286 Iron-based superalloy was studied with an emphasis on the influence of η phase on shear strength. The η (Ni3Ti) phase precipitates at high temperature heat treatment or during services at the expense of gamma prime phase. According to the microstructural features, no evidences of η phase were found at 650 and 720°C. η phase precipitated at 780 and 840°C and the amount of it increased with an increase the time and temperature. Because of using the alloy as fasteners, investigation of shear properties and the influence of η phase on it are indispensable. The shear strength of the alloy with different volume fractions of η was examined. It was found that, with an increase of η volume fraction, the ultimate shear strength decreases. The shear punch fracture surfaces were also examined by the scanning electron microscopy. The fracture surfaces of sheared samples indicated that low and high volume fraction of η phase result in interior cracks and grain boundary decohesion, respectively. In fact, the fracture of weak grain boundary films (η phase) produces this kind of decohesive cracking.KEY WORDS: A286 superalloy; η phase; high temperature phase transformation; shear punch test; fracture surface.

The Time–Temperature–Precipitation diagram of M23C6 carbide formation was established for a Nimonic 105 Ni-based superalloy by means of microstructural observation and mathematical-thermodynamic analysis. The results showed that during heat treatment at 750 °C, M23C6 carbide usually forms at grain boundaries due to the decomposition of MC carbides, indicating that the grain boundaries promote M23C6 carbide precipitation. Also, by increasing temperature or time, diffusion rate increases and new diffusional paths (such as γ matrix and twinning) are provided leading to the nucleation of these phases inside the matrix by means of γ→M23C6 + γ′ reaction. According to thermodynamic analysis, the interactions between M23C6 formation Gibbs free energy and diffusion activation energy of elements, especially carbon, defines M23C6 precipitation behavior. The peak of the Time–Temperature–Precipitation diagram was calculated to be about 900 °C based on experimental results, and about 927 °C based on thermodynamic results. These two temperatures can be considered as approximately equal.