Study on the surface modifications of single crystal Ni-based superalloys during industrial processing at high temperatures

Spathara, Dimtira ORCID: 0000-0003-4861-0388 (2019). Study on the surface modifications of single crystal Ni-based superalloys during industrial processing at high temperatures. University of Birmingham. Ph.D.

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Abstract

This work investigates the mechanism of surface modifications on the Re-rich single crystal superalloys CMSX-10N and CMSX-10K due to elemental sublimation. When the alloys surface is exposed at high temperatures, mainly during solution heat treatment, modifications occur on the surface layers. Observations and characterisation analysis shows polycrystalline layers of γ' matrix with precipitates of TCP phases on a single crystal substrate, rich in γ forming elements. It is found that they become more pronounced at high temperatures and for longer exposure time. Thermodynamic studies with the method of Knudsen effusion mass spectrometry were performed. The activities of Ni, Co, Cr and Al were determined, leading to the thermodynamic description of the reaction of elemental sublimation of these four species when in equilibrium. A model has been developed to predict the microstructural evolution on the surface modified layers, for specific exposure time, at a given temperature with the CALPHAD type thermodynamic and kinetic databases. The model was validated with experimental measurements and observations. The results were compared with simulations on the CMSX-4 and the newly developed CMSX-4®Plus. Lastly, the reverse process; i.e. vapour deposition instead of elemental sublimation was studied for the CMSX-10K alloy during a 24-hour heat treatment at 1350°C. After this heat treatment, observations did not show any TCP phases forming on the surface layer.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):