Effective Microstructural Homogenization and its Influence on the Localized Mechanical Properties of Single Crystal Ni-Based Superalloy, Cmsx-4

Single crystals of Ni-based superalloy, CMSX-4 is an important candidate for various niche applications owing to the attractive set of properties. However, fabricating the singles crystals lead to presence of not only expected micro-scale inhomogeneities i.e., γ/γʹ structure but formation of undesirable macro-scale heterogeneity i.e., the dendritic structure. These certainly influence the mechanical performance of the superalloy negatively. The present study targets to optimize the fabrication condition to achieve microstructural homogeneity along with uniformity in the mechanical properties. It is highlighted that sixteen-fold enhancement in the withdrawal rate to 720 mm/h does influence the microstructural heterogeneity of the superalloy, drastically. The combined effects of not only the withdrawal rates but subsequent customized multiple-step solution annealing and aging heat treatments on the microstructure, elemental distribution and mechanical property evolution are thoroughly scrutinized further. Significant homogenization of macro and micro-scale non-uniformities are noted following the specialized heat treatment with complete elimination of the dendritic structure after two step ageing for this alloy. In addition, uniformity in the cuboidal γ/γʹ structure are also noted in the aged alloy, irrespective of the withdrawal rate. Interestingly, the superalloy single crystal fabricated with higher withdrawal rate showed improved mechanical properties along with better tendency for post heat treatment homogenization. Finally, size dependence of mechanical properties is explored, that showed least effect for annealed alloys owing to homogenization. Subsequently, the gradual transition from the elastic to the plastic deformation under nanoindentation is analyzed for the complex superalloy upon employing nanoindentation with blunt spherical indenter tip. The systematic study is therefore extremely beneficial for identifying the optimized fabrication condition including the withdrawal rate as well as specialized heat treatment schedule. This will enable the complex multi-component superalloy single crystal system to achieve homogenized microstructure devoid of dendritic structure along with uniform mechanical properties.