PhD Thesis Colloquium: Ms. Srilekshmi M (16/10/24)

Thesis title:

Evolution of oxide layers in heat-treated/deformed CMSX-4 in different atmospheres

Faculty advisor(s):

Prof. Surendra Kumar Makineni

When?

16th October, 2024 (Wednesday), 3:00 PM (India Standard Time)

Where

KPA Auditorium, Department of Materials Engineering

Abstract:

Superalloys are special class of high temperature alloys that are used for making components in gas-turbine engines, catalytic reactors which are exposed to highly corrosive atmospheres. These alloys have exceptional oxidation resistance, creep resistance, high temperature strength as well as ductility. These alloys are primarily based on Ni, Co, Fe and Ti because of their high melting point (>1400°C). The efficiency of an engine primarily relies on higher inlet temperatures. The higher melting points of these alloys and good oxidation resistant enable higher working temperature for the engines.

Ni-based superalloys are widely used in aerospace (aircraft engines, turbine blades) as well as power-generation industries (steam turbines). Ni does not undergo phase transformations and retains its FCC structure until melting. Additionally, it can form uniform coherent precipitates, Ni3(Al, Ti) with ordered L12 structure in the FCC γ matrix. These are also stable at the application temperatures imparting the required strength and creep resistance. Moreover, the alloying elements (Ta, Re, W, Mo) help in improving the creep strength, oxidation resistance and increasing the solvus temperature. Other elements such as Co, Cr and Hf are also present in CMSX-4, one of the most popular Ni-based superalloys.

Aircraft fuels contain hydrocarbons, olefins, paraffins, naphthenes, aromatics and cycloparaffins. Air is also sucked in from the atmosphere to enable combustion of the fuel. Therefore, the turbine blades are exposed to a very harsh mixture of corrosive gases at high temperatures. Since CMSX-4 consists of ten elements, the process of oxidation is highly complex. For improving the alloy’s oxidation resistance, it is important to understand the oxidation process.

In the current study, the evolution of oxide layers during the exposure of CMSX-4 at 900°C in the presence of four different gases (air, oxygen, carbon dioxide and sulphur dioxide) is studied. These gases are generally present in the combustion products and are expected to corrode the turbine blades. The CMSX-4 samples with four different thermal and mechanical processed conditions having different microstructural features are studied in this work. These are:

  1. Solutionised state (without γ’ precipitates)

  2. Solutionised and aged at 900oC (standard CMSX-4 with γ-γ’ microstructure)

  3. Aged at 900°C and creep deformed at 800°C (γ-γ’ with stacking faults, micro twins, and having increased dislocation density)

  4. Aged and creep deformed at 1000°C (γ-γ’ with a high density of γ-γ’ interfacial dislocation network and antiphase boundaries)

These four conditions allowed for exploring the understanding of the effect of microstructure and defects on oxidation resistance.

The initial part of the study involved the design of the experimental setup to neutralise CO2 and SO2 gases used for oxidation. Experiments were conducted in a controlled setup of a three-zone vacuum furnace. The oxidised samples were characterised by XRD, SEM, EPMA and TEM to identify the oxidation products. Thermogravimetric analysis (TGA) was also performed to quantify the mass gain associated with oxidation.

The samples that crept at 800°C had the best oxidation resistance under air, oxygen and SO2 atmospheres. The standard CMSX-4 (aged sample) had the highest oxidation resistance only under CO2 atmosphere. The solutionised samples were found to have the lowest oxidation resistance in all atmospheres. It is found that the resistance to air and oxygen atmospheres increases when the conditions are favourable for the diffusion of aluminium to the oxide-metal interface. The oxidation under CO2 is found to be a balance between the adsorption and reaction kinetics. The stability of NiSO4 under the reaction conditions decides the oxidation mechanism under SO2. Overall, with this study, it was possible to characterize the different oxides formed in different microstructures under various atmospheres. Additionally, the mechanisms for oxidising different microstructures under four different atmospheres were proposed. These results can be utilised to further optimise the microstructure of the alloy according to the reaction conditions (presence of carbon, sulphur and oxygen).

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