Resistance to Viscoplastic Deformation of Ni-Based SX Superalloys with Bimodal Distributions of Gamma-Prime Precipitates

Ni-based single crystalSingle crystal (SX) superalloysSuperalloys have a well-established history of application in high pressure turbine (HPT) blades and vanes. Nowadays, these components are increasingly being employed in the first stage of the low pressure turbine (LPT). As a result, aircraft engine manufacturers are facing new challenges arising from the designDesign, manufacturing processes, evolving service conditions, and refurbishmentRefurbishment requirements. Conventionally, the capability of Ni-based SX superalloysNi-based SX superalloy to withstand the harshest environments is related to a homogeneous cuboidal γ/γ′ microstructureMicrostructure. However, future new applications have considered bimodal γ/γ′ microstructuresMicrostructure (presence of fine tertiary γ′ precipitatesPrecipitates 10–100 nm inside the γ-channels) resulting from either the manufacturing processes or from the routine use. This study investigates the resistance to the viscoplastic deformation of a 3rd generation Ni-based SX superalloyNi-based SX superalloy with a bimodal distribution of γ′ precipitatesPrecipitates. TEM observations showed presence of dislocations after heat treatmentHeat treatment to achieve the bimodal microstructureBimodal microstructure. Even without plastic strain, dislocations were identified surrounding the secondary γ′ precipitatesPrecipitates. Stress relaxationStress relaxation and creep propertiesCreep properties at 750 °C and 850 °C were very sensitive to such bimodal microstructureBimodal microstructure. Specimens with a bimodal γ′ precipitationPrecipitation showed a creepCreep life five to six times lower than the reference samples. According to Norton’s type diagram, rate controlling deformation mechanismsDeformation mechanisms of the reference and bimodal microstructuresBimodal microstructure appear to be same at both temperatures and under different initial conditions (with and without prior plastic strain), but with a higher strain rateStrain rate for the bimodal microstructureBimodal microstructure.