Mechanical properties deterioration and its relationship with microstructural variation using small coupons sampled from serviced turbine blades

Turbine blades of three different service conditions made by a directionally solidified superalloy were investigated to reveal the reduction in the mechanical properties caused by the microstructural degradation in service. The tensile and low cycle fatigue experiments were carried out using the small coupons sampled from the turbine blades. The microstructural degradation reduces the tensile strength and the fatigue life of the samples from the turbine blades. A quantitative relationship between the microstructural degradation and the residual life reduction was proposed by defining a microstructural damage factor based on the strengthening mechanism of the Ni-based superalloy. The fatigue fracture behaviour was studied with the help of the scanning electron microscopy. The results showed that the eutectic microstructure and carbides are the critical positions where the cracks initiates and propagates. In addition, scanning electron microscopy and Vickers hardness measurements were adopted to characterize the microstructural variation at different positions of the turbine blades. The Vickers hardness has an obvious variation at different positions of the turbine blades after different service exposure conditions, which is closely related to the microstructural degradation including the coarsening, rafting and dissolution of the γ^' precipitates.