Development and optimization of the properties of gamma/gamma’ co-based superalloys for high temperature applications

• Autores: Marta Cartón Cordero
• Directores de la Tesis: Mónica Campos Gómez (dir. tes.), José Manuel Torralba Castelló (codir. tes.)
• Lectura: En la Universidad Carlos III de Madrid ( España ) en 2019
• Idioma: español
• Tribunal Calificador de la Tesis: Elena Gordo Odériz (presid.), José Antonio Sicre Artalejo (secret.), María Teresa Pérez-Prado (voc.)
• Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
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• Resumen

  • Nowadays, aircraft industry demands new efficient and clean engines to move the new generation airplanes. The engineering developments passes through improving the properties of the material at high temperature, especially in engine blades localized on the first step on high pressure turbine and on the structural material disposed on combustion chambers. Rises the resistance to temperature is going to improve directly the final engine efficiency and consequently reduce the costs per flight.

    During the last decades, the Ni gamma/gamma´ superalloys have been used thanks to their outstanding properties under those rough conditions. Thermal and dimensional stability, higher performance under creep conditions and oxidation resistance are the main requirements. However, Nickels monopoly was interrupted thanks to the irruption of Co-based alloys. Traditionally, those alloys were hardened by solid solution and carbides precipitation. However, Sato et al [1] developed in 2006 a ternary system Co-Al-W in which it was possible to develop a gamma/gamma´ microstructure. This phase was found to be similar to the gamma´ in Ni-based superalloys, since both provides stable and stoichiometry L12 structure which is coherent with the fcc-matrix.

    The main counter back was founded on W content in gamma´ phases whose hardening promoted induced poorer creeps’ behaviour. As a solution, Ta and Ti was used for Suzuki et al. [2] to rise the mechanical properties under compression at high temperature. Thanks to the alloying elements, an anomalous flow stress behaviour was obtaining, leading into a material, which could even overcome Ni-based superalloys.

    Commonly, Co-based superalloys were produced by conventional ingot metallurgy which is associated to inherent defects (such as macrosegregation, microsegregation, porosity, shrinkage solidification and second-phase inclusions). This Thesis proposed an alternative route based on powder metallurgy (PM) as new proposal to manufacture the Co-based superalloys.

    Co-based superalloys development passed through two different PM routes, studied in this research. The first one is based on elemental powders mechanical alloyed by high-energy milling and consolidated by FAHP (Field assisted hot pressing). The second one is focus in gas atomized powders followed by SPS (Spark Plasma Sintering). To realize both studies, two compositions were selected according to the Co-Al-W ternary diagram [1], Co-9Al-9W and Co-8Al-8W (both in at.%). Besides, using the gas atomized powder, Ti and Ta, were added by high energy milling to promote gamma´ precipitation.

    Both routes resulting in a massive precipitation or gamma´ phase after solubilisation or homogenisation at 1250 °C during 24 h and ageing at 900°C during 24 h treatments. Controlling gamma´ amount, size and shape passes through optimize the thermal treatments.

    In case of superalloys consolidated from the atomized powder, the gamma´ precipitation was revealed during consolidation process without subsequent heat treatments. The gamma´ precipitation without heat treatments has not been reported until the publication of this research. The gamma´ precipitation, grain size reduction, solid solution, carbides precipitation or intermetallics formation are the different hardening mechanisms found in the Co-based superalloys obtained by PM, which contributed to keep the properties at high temperature.

    Evaluating mechanical properties at high temperature were performed by compression tests. The reported results for the composition Co-9Al-9W obtained by PM routes reported three times higher values than the alloys produced by ingot metallurgy [2].