The effect of rhenium and cobalt additions on the microstructure and gamma prime phase stability of modified nickel-based superalloy, grade MGA1400 by means of a vacuum arc melting method with different contents of rhenium (Re) and cobalt (Co) was investigated. After being subjected to heat treatment, the size of the gamma prime particles was slightly reduced with increasing rhenium content. Then, the alloys were exposed at a temperature of 1173 K for 1800 ks. The results showed that increasing the rhenium addition leads to a slower coarsening rate of the gamma prime particles. Furthermore, the shape of the gamma prime particles changed to a more cubic shape. However, an excessive rhenium content (>1.21 wt.-%) promotes the formation of a topologically close-packed phase after a 360 ks exposure at 1173 K. It can be concluded that an increase in rhenium content can result in a lower coarsening rate of the gamma prime particles. The cobalt addition is beneficial for impeding or slowing the precipitation of topologically close-packed phases at an elevated temperature.

Morphological evolution of gamma prime precipitates was governed by an elemental partitioning ratio between the gamma and gamma prime phases. In the present study, Ni-based MGA 1400 alloy was modified by adding 4 wt% of Re and 6.4 wt% of Co by vacuum arc melting. The effects of Re and Co additions on lattice parameters of gamma and gamma prime phases were studied by TEM and XRD techniques. Calculated lattice misfit values obtained from XRD and TEM techniques were used to identify the shape and morphology of gamma prime precipitated particles after long-term exposure at 1173 K for 1800 ks? After long-term exposure, gamma prime particles became much coarser. The addition of Re retarded the coarsening rate of the gamma prime precipitates, resulting in smaller gamma prime particles at the same annealing times. Even though, both Re and Co atoms are strongly partitioned in the gamma matrix, the effect of Re and Co additions on lattice misfits was contradictory. The addition of Re produced rounded corner cuboidal morphology of gamma prime precipitates due to higher magnitude with negative lattice misfit (−0.3857%), while Co addition provided a more rounded shape of gamma prime particles due to the lattice misfit value approach to zero (+0.0936%).