Superalloys 2024: General Session 3: Disk Alloy Mechanical Behavior I
Program Organizers: Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346
Monday 6:00 PM
September 9, 2024
Room: Exhibit Hall
Location: Seven Springs Mountain Resort
Session Chair: Mark Hardy, Rolls-Royce Plc; Nicholas Krutz, GE Aerospace
6:00 PM
Inhomogeneous Distribution and Coarsening of γ″ Precipitates in a Ni-based Superalloy and their Effect on Creep: Chang-Yu Hung1; Stoichko Antonov1; Paul Jablonski1; Martin Detrois1; 1National Energy Technology Laboratory
Modifications to INCONEL® alloy 725 (IN725) with higher Nb or Ta and high Ti/Al ratio previously revealed the formation of γ'' precipitates as the sole precipitate strengthening phase which was accompanied with significant increases in elevated temperature strength and resistance to creep deformation. In this study, the γ'' alloys (named M725-Nb and M725-Ta) were chosen to observe the microstructural evolution of γ'' precipitates during creep. Following aging, γ'' was found to be the main precipitate strengthening phase with no detectable γ' in both M725 alloys. After creep, the grip/gage sections of the crept M725-Nb/Ta exhibited preferential coarsening and inhomogeneous distribution of γ'' platelets, and sandwich-like structures, comprising of cubic γ' precipitates with small γ'' discs on each facet. This γ'' variant selection behavior taking place in the grip section with no stress applied was different from the typical stress-induced variant selection reported in previous literature. However, creep tested specimens with prior high temperature exposure to promote varying γ'' characteristics prior to deformation demonstrated a similar creep behavior when compared to aged specimens tested under identical conditions. This suggested that the coarsening and inhomogeneous distribution of γ'' precipitates was not the determinant factor for creep life in bulk M725-Nb/Ta alloys. The failure of bulk M725 was linked with the development of cracks along grain boundaries (GBs), where the emergence of Nb-rich δ and/or Ta-rich h phases at the GB markedly depleted the γ'' strengthening phase in the surrounding GB regions which led to localized weakening.
6:25 PM
Spark Plasma Sintering of Nickel-based Superalloys: A New Route to Produce Dual-alloy Turbine Disks: Emmanuel Saly1; Patrick Villechaise1; David Mellier1; Pierre Sallot2; Amélie Caradec1; Jonathan Cormier1; 1ENSMA - Institut Pprime - UPR CNRS 3346; 2Safran Tech
Spark Plasma Sintering was used to join powders of two nickel-based superalloys containing over 40% volume fraction of γ' phase and considered very difficult to weld. After one minute of sintering, a very high relative density was reached, and both alloys were successfully solid-state joined without precipitation of detrimental phases in the diffusion-affected zone. Solutioning of samples showed that a dual-alloy-microstructure, composed of a coarse-grained alloy in the rim and a fine-grained alloy in the core, could be produced by this technique. In both as-received and fully-heat-treated states, assemblies exhibit tensile strengths at least higher than the weakest parent material. The heat-treatment sequence enables modulating strain partitioning during tension, leading to a more homogeneous deformation at very high stresses. Thus, failure occurs in the weakest parent material, far from the weld, with no alteration of the ductility, suggesting a sound joint has been produced during the short sintering time. The study on the viscoplastic response by means of tensile stress relaxation tests shows that a dual-alloy disk technology has a high potential, resulting in improved creep properties, chemical stability, and corrosion resistance in the rim section.
6:50 PM
Viscoplastic Behavior of the Grain Size Transition Zone in a Dual Microstructure Turbine Superalloy Disk: Fabio Machado Alves da Fonseca1; Denis Bertheau2; Loïc Signor2; Julien De Jaeger3; Patrick Villechaise2; Jonathan Cormier2; 1SafranTech/Institut PPrime; 2Institut PPrime; 3Safran Tech
This study focuses on the challenging characterization of the microstructures and the viscoplastic behavior (creep, relaxation) within the transition region of dual microstructure disks made of γ/γʹ Ni-based superalloys. This transition microstructure, consisting in spatial variations of grain sizes and/or γʹ precipitation, poses difficulties in providing insights for effective modeling strategies in aero-engines. To characterize viscoplastic behavior at intermediate temperatures, stress relaxation tests were employed using AD730TM superalloy. A single-crystalline version of AD730TM was used to isolate the effects of different precipitation states (ranging from 15 to 300 nm in size), at 700 °C and 800 °C. The results reveal a significant impact at 700 °C, while no substantial impact was observed at 800 °C. Additionally, precise specimen machining was conducted in different radial zones of a dual disk to explore the influence of various grain size gradients, on viscoplastic behavior at both 700 °C and 800 °C. The findings indicate a progressive viscoplastic behavior along the gradient of grain size for both testing temperatures. Finally, the viscoplastic behavior of transition specimens was analyzed within the framework of a grain-sensitive phenomenological model. The study provides valuable insights into understanding and modeling the complex behavior of dual microstructure disks in aero-engine applications.