Surface Fatigue Behavior of Laser Peened Single Crystalline Turbine Blades—A Multicycle Nanoindentation Study

Turbine blades are one of the most critical components in a modern gas turbine engine as they convert high energy combustion products into mechanical work and drive the main turbine shaft. Although they are remarkably resilient, the blades are prone to damage initiated at the surface from mechanisms such as stress corrosion cracking, fatigue, and foreign object damage. Additionally, the growing need for higher-efficiency combustion engines requires higher operating temperatures which only accelerates the kinetics of crack growth and other deleterious effects. Fortunately, methods of fortifying the surface through localized cold working techniques show promise at significantly delaying crack growth, creep rate, and corrosion; thereby enabling longer service lives. In this work, one such method known as laser peening (LP) was applied to single crystal Nickel-based superalloy turbine blades. Sections of the turbine blades were cut into flat plates and LPed on a face approximately parallel to the [001] plane. The peening was conducted over 4 consecutive layers with an irradiance of 7 GW/cm² to maximize the depth of plastic deformation without causing reverse plastic straining and consequently degrading the mechanical properties. Following LP, one set of samples was placed in a furnace at 700 °C for 300 hours to gauge the retention of LP-induced microstructural modifications under thermal exposures similar to those experienced in gas turbine engines. Cyclic nanoindentation fatigue tests were then conducted to provide insight into the surface-level energy absorption of CMSX-4 and how that behavior changes after LP and thermal exposure. Indentation tests were performed with a load of 500 mN at 300 cycles using a Berkovich tip within dendrite core regions of each sample to reduce error associated with anisotropic mechanical properties. The LPed specimen showed a dramatic reduction in indentation depth in comparison to baseline sample which is likely due to the compressive residual stresses and work hardened microstructure impeding dislocation motion and limiting the depth to which the indenter can induce plastic deformation. Interestingly, the thermally-exposed samples presented an increase in indentation depth compared to the LPed specimen but did not regress all the way to the baseline depth, which is a testament to the thermal stability of LP-induced dislocation structures under high-temperature loads. Additionally, optical profilometry was used in tandem with indentation data to gauge the indent topography following the fatigue test. It was observed that significant pile-up occurred on the LPed samples following thermal exposure which could stem from the reconfiguration of dislocation into subgrains and a consequently change in dislocation mobility to promote pile-up, though this mechanism is not well understood. Overall, the surface fatigue tests performed in this work show the promise of LP as a post-processing treatment for the extension of single crystal Ni-based superalloy turbine blades lives which can be of great use in the creation more sustainable air travel and power generation.