EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

High pressure turbine blades are the most challenging parts due to severe operating conditions in a jet engine. Single crystal nickel based superalloys are extensively used for this application due to high creep and thermal fatigue resistances. Mastering directional solidification during investment casting is one of the main challenges when manufacturing them, as defects such as recrystallization are seriously detrimental to the mechanical performances of the final part. The occurrence of this defect is related to the amount of plastic strain, which appears when the molten metal solidifies during casting. Plastic strain is however insufficient to trigger by itself recrystallization: thermal energy is required for new grains to nucleate and grow. The heat treatment that follows casting to homogenize the microstructure is the step where recrystallized grains appear. In order to predict accurately the occurrence of recrystallized grains in a turbine blade, a representative description – through finite elements calculation - of stresses and strains formed, during solidification and cooling, is necessary. Therefore, an anisotropic elastic-viscoplastic behavior law of the alloy in the as-cast state is required. In the current work, the methodology to identify parameters of such a behavior law for as-cast CMSX-4 by inverse analysis is provided. Experimental data are obtained with tensile/relaxation tests performed with a resistive heating machine, using contactless instrumentation. The mechanical behavior is found strongly correlated with the presence - or absence - of γ’ hardening precipitates. The γ’ phase fraction in CMSX-4 samples at elevated temperature can be estimated either with electrical resistivity measurements in tensile test machine or in situ XRD measurements.
The impact of deformation temperature, strain rate and plastic strain on recrystallization taking place during heat treatment is studied using EBSD. Results suggests that the presence of γ’ precipitates plays a predominant role in the occurrence of recrystallized grains.