Microstructural rejuvenation in a Ni-based single crystal superalloy

Abstract

In the modern turbine blade industry, one of the most significant innovations is the successful application of single crystal (SX) Ni-based superalloys, which push the parts in the hot section running in a harsher environment with longer lifetime. During service under combinational impacts of thermal and stress, the SX superalloy might fail and reach its life owing to the microstructural degradation/damage. Two kinds of degradation microstructures, caused by tensile creep and compression at various temperatures, are obtained to investigate the possibility to rejuvenate their initial well-defined microstructures and still keep SX in nature. Defect and microstructural evidence on every processing step have been carefully analyzed to understand the rejuvenation mechanisms of the alloy with the help of advanced microscopes. In all rejuvenation processes, the γ/γ′ two-phase cuboid microstructure can be restored by solid solution plus aging treatment. The specimen crept at 1050 °C with tensile strain ∼1.5% can be fully restored, and SX nature remains, but not for all compressed alloys, where SX nature loses owing to recrystallization. For the compressed alloys, the heat treatment method with additional recovery annealing is successful to surpass the recrystallization in samples with ∼1.5% compressive strain. The mechanism for the additional recovery annealing to overcome recrystallization is justified based on defect analysis in nanometer scales. Our results may provide microstructure/defect guidelines to rejuvenate the Ni-based SX superalloys by preventing the formation of the polycrystalline microstructure.